The AM Forum

Hi!

I have talked, on several recent occasions with Chuck, W3FJJ... when he was using his new Retro 75. It sounds very nice.

I was looking at the schematic of the transmitter, and I found two places that constrain the low frequency response.

The first is C33... the 100 uF capacitor that provides DC blocking between the output of the audio amplifier and the input to the modulation transformer.

According to the Retro 75 manual, this transformer is nominally 3.2 Ohms:48 Ohms... so the turns ratio is approximately 1:3.9.

If I assume that the B+ on the IRF530A is 12 volts, and that the average current is 0.3A at carrier (i.e. 3.6 watts DC input and ~2 watts rf output)... then the modulation resistance of the rf stage is about 40 Ohms. Thus, the impedance, looking into the modulation transformer is approximately 40 x 3.2/48 Ohms ~ 2.7 Ohms.

The question is: at what frequency is the impedance of C33 (100uF) equal to 2.7 Ohms. The answer is 589 Hz.

Even if the impedance looking into the modulaton transformer is somewhat higher, the low frequency cutoff is at too high a frequency.

I would suggest increasing C33 to 1000uF (or at least 470 uF)

The second low frequency bottleneck is C24. The gain of the audio preamplifier is 10 at medium frequencies (e.g. at 500Hz), and 20 at frequencies above 1kHz; but, at the frequency where the impedance of C24 increases to 10k Ohms, the gain is 3dB lower than the medium frequency gain... and rolls off, as the frequency gets lower, at 6dB per octave.

The frequency at which C24 has an impedance of 10k Ohms is 159 Hz. I would suggest increasing C24 from 0.1uF to 0.33uF

Stu

Stu,

 With all do respect to your analysis, I'm sure the low frequency pole by design has something to do about increasing intelligibility to give a Retro-75 a fighting chance to make a contact with only 2 watts AM. I would hesitate to embrace East Coast Audio principles when the carrier is only 2 watts.

  On a side note, the power level of a Retro75 is a perfect match for a Central Electronics 600L single 813 broad band linear amplifier. That 813 will do 100 watts AM linear mode with a little blushing. Once a Retro75 is up to 100 watts or so, then increasing the low frequency response might be more desirable.

Just my opinion,
Jim
WD5JKO

Thanks
I've been saving my nickels for a retro-75. I've avoided temptation and passed on other QRP kits out there.

I have yet to hear one on air but I like the idea of increased low freq resp.

I recently journeyed down to middle NJ and picked up a 1957 handbook based 813 transmitter / modulator rack project. It was on Craigslist and no one was biting.
I'll post picks if anyone is interested.
It might be interesting to bumb it with the Retro-75.

re: the 600L, It does seem like a match made in heaven.
( in a non-denominational way  ;) )

http://www.eham.net/reviews/detail/3872

Stu,
I'll have to give those mods a try.
Bob

Using mine with a D-104 feeding a K7DYY mic processor. I switch the same mic/amp to all of my transmitters. In the case of the Retro 75 I had to install an inline attenuator as the K7DYY output was a bit too high. I also installed a micro reed relay for PTT function as my station's switching system directs both PTT keying and the mic audio.

 Really enjoying the little bugger and aside from the PTT I have no present intention of any further mods. The concept is quite sound and works well. I have no inclination to encroach on the elegance of the design by trying to make it sound like a 2 watt broadcast transmitter. This is indeed a real piece of amateur radio gear.

   Stu, I retract what I said earlier in this thread. I did some testing of my Retro75 last night, and it becomes apparent that the whole speech amp including the recommended microphone are designed to work together.

    The C33 issue is interesting. That little tiny 100uf cap besides being low in value, also likely has high ESR. If you disable the automatic modulation control, and increase C33, a low frequency instability occurs, even when driving with speech. The problem appears to focus around the modulation transformer which seems to saturate is the frequency drops below 500 hz. To me this is Deja Vu back to the Gonset G50 where the Heising choke saturates below 500 hz.

    If I just drive the modulator by itself (no preamp), I still see the effect. Increasing C33 also requires increasing the +12V decoupling capacitor to > 1000uf, and when doing that things get better noticeably. With C33 at 1000 uf, C112 bridged with 1000uf, and C110 bridged with 330 uf, the modulator is flat 500-5000hz and will very cleanly modulate 100%. Going below 500 hz you see the waveform distort as the % mod lessens, and the power supply current increase, i.e. core saturation.

   So the trick is to Taylor the speech amp to lessen the energy below 500 hz to stay away from saturation. After the mods, I had the following behavior with a nice modulated envelope pattern (driving U6 directly from empty socket at U5-1):

150hz, 25%
200hz 40%
300hz, 70%
500hz - 5000hz, 100%
5000hz - 8000hz, 90%

   In stock form the modulation limiter prevented a lot of the bad behavior by limiting the modulation percentage. In my case with a sine wave drive, I am limited to 30% modulation with an asymmetrical waveform (distorted). Speech will hit higher peaks though.

   I need to take a fresh look at the preamp stage. With the changes I made to the modulator (U6), I see instability (low frequency)* if I run audio as intended into U5. I think there could be a ground loop or some other form of regenerative feedback. Still all the bad behavior comes about below 500 hz.

   It might be easier to eliminate the preamp, and use line level input directly to U6 with a gain pot added. I have an external speech amp ready for that task.

   Been thinking of modified Heising to get the DC current out of the mod transformer.

* exceed a certain level at < 500 hz, and the waveform increases in value suddenly with asymmetrical distortion. Bring C33 back to 100 uf and all that weird stuff goes away.

This is a very interesting little rig. I am amazed at Small wonder Labs that they are producing this little rig for such an inexpensive price. A lot of thought and work went into it. I can see a lot of fun ahead with this nifty little rig.

The mod transformer used is a Xicon 42TU04C-RC, rated 48/8 ohms at 460mw with a low end down 3db at 300hz. A possible alternative is a Hammond 106R is also 48/8 but at 1500mw with a low end down 1 db at 170hz.

http://www.mouser.com/catalog/specsheets/XC-600133.pdf
http://www.hammondmfg.com/102.htm


Jim
WD5JKO  

Jim

Also reminiscent of the Johnson Ranger....

When you remove some of the high frequency bottlenecks in the audio chain and the modulator... you discover that some of those bottlenecks are also "band aids" that the engineers used to control oscillations associated with nonlinearities in the high level portions of the audio chain + the audio feedback circuit. Then you have to fix those oscillation problems (a different way) as well.

Stu

Usually the cause is/was that the decoupling caps between the audio stages were too small. People often forget to change those when doing audio mods.

A Johnson T'Bolt linear would also be a perfect companion. A grid driven pair of 4-400s. Find a CB op willing to part with one.

Some Retro75 updates.

   Since last week I finally got back out in the shack since the temp was in the 60's until a little while ago. Boy it was cold just a few days ago.

   Anyway, I made a few discoveries.

That Hammond transformer I linked to costs ~ $40 whereas the Xicon that comes stock sells for about $2. Big difference! Looking at the schematic I thought it would be easy to disconnect the mod transformer primary from C33 (-), and then connect C33 (-) to the secondary C.T.  This was easy. The idea was to use the secondary as a tapped Heising reactor. The results were interesting.

Stock 2.45:1 mod tranny:
The stock tranny used as intended is 48-8 ohms, or 48/8, square root of 6 = 2.45 turns ratio. So about 5 v peak from the modulator will provide ~ 12v peak to the RF amplifier to 100% modulate it. The modulator with 12V on it, idles at 6V, and is capable of almost +/- 6V to swing 12v P-P.

Same tranny used as a Heising reactor:
Since I am driving the audio to the C.T., I expected the turns ratio to be 2:1 which would lower the maximum modulation percentage to ~ 82% if I had 100% before. That is pretty much what happened but with a big surprise! The waveform distortion below 300Hz limiting the low frequency max mod % got better, and by a lot. Also the modulator current was lower than before, and the low frequency instability was much better, but still could be coaxed to occur, but at a lower freq (200hz) and input higher level than before.

Searching for the low frequency instability when using the mic preamp turned out to be easy. At Pin3 of U5 we pre-bias the (+) input to 6V from the U2 regulator. Chopping that trace (near TP1) and inserting a 4.7K resistor, along with replacing C108 (0.1uf) with a 10Uf cap seemed to completely eliminate the low frequency instability. I later found it to return somewhat when putting the mod tranny configuration back to stock, but still much better than before adding the R-C.

So I did some audio tests driving U6 only as I did before. With the mod tranny as a C.T. Heising choke, I could modulate cleanly at 85% from below 300Hz to above 5 khz. At 1 khz, sine, triangle, and square wave envelope patters were darn good at 85%, and near perfect at 50%. Overdrive to 100% was possible with clipping. Looking at U6 output it was clipping at 6V peak both up and down as you'd expect. To modulate higher without clipping, the turns ratio needs to be higher. That would mean moving the C.T. away from center.

When driving the mic input through U5, I did some tests. Remember my last post (driving U6 only) where C33 is 1000uf, and I bridged 1000uf across C112, and replaced C110 with 330 uf.

Max undistorted mod % when driving the mod tranny at the C.T. (Heising):

100hz - 20%
150 - 50
200 - 80
300 - 85
500 - 85
1000 - 85
3000 - 85
5000 - 80

As above, but Mod tranny back to stock:

100hz - 10%*
150 - 20*
200 - 50*
300 - 85
500 - 100
1000 - 100
3000 - 100
5000 - 90

* increasing level from here causes LF instability where every odd cycle peak has more amplitude than the even cycle peaks with a ratio of almost 2:1.

  I tried three microphones (2 dynamic and D104). The audio polarity seemed backwards with the dynamics, and monitoring myself (always subjective) sounded pretty raspy to me since the audio peaks were easily clipping the baseline. Reconnecting the modulator AGC corrected that problem, and the audio sounded decent. The D104 output was low and tinny sounding. Using a crystal mic requires a higher load impedance, and with that a level control.

Edit: A frequency sweep from Mic input as modified was 200-4000 at -3db referenced to level at 1500 hz (peak). This was done at 50% modulation looking for the .707 level on the scope. The freq response was the same with either mod transformer configuration.


All in all a fun evening in Retro75 land.

Jim
WD5JKO

Hi Stu,

I've heard a few Retro 75s on the air, and immediately noticed the constrained low frequency response.  They may have done this "on purpose" (because it's low power), but I personally would have made the rig full fidelity (I also wouldn't have used a transformer, but that's another discussion!), and then either given an option to switch in a low cut filter, or something - as long as the rig was CAPABLE of high fidelity audio.

I'm wondering out loud if the modulation transformer can pass the low frequencies without saturating.  We will see  ;)

Regards,

Steve

Steve

Hi!

One nice things about these "little" transmitters... whether they are using solid state or vacuum tube devices... is that they are easy to tinker with... safe to tinker with... and we learn a lot by tinkering with them.

Even a "high end" audio transformer, operating at these power levels isn't going to cost very much ... and there is great satisfaction in tinkering with the design to achieve "broadcast quality" reports from a rig that was never intended to produce broadcast quality AM.

I must admit that I have occasionally "drawn the line" with respect to leaving a rig in its "as designed" configuration. I very much enjoy operating (once in a while) my Heathkit SB-10 sideband adapter in SSB or DSB with varying amounts of inserted carrier.

Stu

Steve, There is no doubt that the transformer is saturating below 500 Hz. This effect is countered with a pretty steep High Pass roll-off in the AC drive to the transformer, mainly C33 being 100uf. Then there seems to be a tendency towards a low frequency instability that is also held in check by modulator AGC loop.

That transformer is rated at 460 mw without any DC current flowing through the 48 ohm winding. We need a couple of watts of audio at least for 100% upward modulation. I have been scouring the internet looking for a suitable replacement, and short of a DIY wind job, all I see is the Hammond 106R that costs about $40 whereas the one used on the Retro75 (Xicon TU048-R) costs about $3 each.

So I have been moving forward trying to get the most out of the Xicon. In my last post I reconfigured the tranny as a tapped Heising reactor where the audio drives the C.T. That made a huge improvement in the LF response and linearity where it can be used down to ~ 200hz @ 80% modulation. The big drawback though was the turns ratio went from 2.45:1 to 2:1 which reduced the peak (midband) modulation from 100% to 85%.

So tonight drawing from a Timtron trick, I used the 8 ohm winding in series (phased properly) with the Heising reactor which should change the turns ratio from 2:1 to 2.8:1. Bingo!!
I can now modulate 120% upward midband with very clean waveform. Using voice with sharp audio peaks I see more.

In my last post I stated that the frequency response was 200 - 4000 at the -3db points with a deliberate rise to a peak at 1500 hz. Using the 8 ohm winding for more modulation didn't seem to change the response. Remember I stated earlier that C33 is now 1000 uf.

Next I need to control the negative modulation since that clips at the baseline very easily. While searching the 2, 3, and 4 diode negative peak limiters, I stumbled upon a circuit in my computer download file that I found somewhere. I will post it here. Anybody know who did this?

Edit: Added picture of 120% modulation at 1500 hz.

Edit2: added second picture with two diode clipper in action. I used a 1n5818 to handle the B+, and instead of 3 diodes to the keep alive supply, I am using a 68 ohm resistor in series with an LED. For the keep alive supply, I use U2 which provides 6v already.

Played some MP3's looking for that special male voice. Elvis Presley did the deed. No AGC action connected, peaks going to ~ 130% and occasionally more if overloaded. Listening on the receiver (Icom R75 wide open) it sounded pretty good considering that LED was blinking pretty regular. No shortage of lows..

So I am getting about 2.75W of carrier (power at 12.6V), and around 12W PEP.

Jim
WD5JKO

 I believe the supplied Xicon output transformer is listed at 48:3.2  ohms (475mW) . Regardless, that would make a Hammond 106Q (48:3.2 ohms @1.5 Watt)perhaps a better choice ?  Another possible , perhaps more suitable package is the Hammond 146B , a 48:3.2 ohm but rated at only 700mW. Even at the 700mW rating it is a lot beefier than the Xicon unit and the price is only $17.
 Either way, a 3.2 ohm primary should present a more efficient load for the 2003 audio IC output.

  

I ordered the $40 hammond xfmr to try. I'll try the 146 too.

  It does say 48/3.2 in the Retro instructions, but the Xicon model (link above in my earlier post) is mentioned in the parts list which is 48/8 where both windings have a C.T.,

 Take that 8 ohm C.T winding, and common to C.T. is 3.2 ohms. I did verify by voltage ratio that we do indeed have a 2.45 to 1 tuns ratio, which is same as 48/8 = 6, square root 6 = 2.45.

  I am very excited to hear how the Hammonds work out. Still for those of us on a budget (tight wads) reconfiguring the Xicon as a tapped Heising reactor seems to work pretty well and gives you a higher turns ratio (2.8:1) so that 120% upward modulation (midband and  up) is easily achieved.

  If you want strong full lows below 300 hz though, the Xicon won't cut the mustard. The possibility of using the Hammond as a tapped Heising reactor also exists.

  I hope to take a picture of how I rewire that Xicon, but that will have to wait a few days before I can get to it.

   Remember the other issues I've posted such as the +12v decoupling caps, C33, and the low frequency instability that popped up on mine as soon as I increased C33. All had solutions, and need to be addressed as a package, in my opinion.

Jim
WD5JKO

I guess my main point was that the previously mentioned alternative, the Hammond 106R was also an 8 ohm unit . I was suggesting that 3.2 ohm alternatives may be more suitable.(Notably the 106Q) One of the 3.2 ohm alternatives was quite a bit lower in cost as well.
  Of course the Xicon being an 8 ohm unit , using the CT could explain a better match as the modulator IC is very likely also a 4 ohm nominal output.(6 watts @ 4ohm, 10 watts @ 2 ohm loads)

  Fun stuff! That 48/3.2 ohm tranny has a 3.9:1 turns ratio compared to the 2.45:1 in there now. The 2003 amp (U6) can drive a low Z as you say, but also realize it is a linear amp with almost no heatsink. Asking it to put out low volts at high current will mandate some thermal management since it might get spit boiling hot. :-)

  With the stock turns ratio, 5v peak drive gives you 12v peak to 100% modulate. Change the turns ratio to 3.9:1, and 3v peak gives you 12V peak on the secondary. Chip U6 can swing +/- 6V peak when B+ to box is ~ 13v. The 2 ohm resistance of the 48 ohm winding drops nearly 1 volt going to the FET.

Let me know how it goes, please.

EDIT: With a solid state AMP like this TDA2003, the source impedance is close to zero, so matching impedance's is not the route you need to go. The amplifier being class AB will achieve greatest efficiency when swinging as close to rail to rail as possible just prior to clipping. I prefer to use this idea to find the modulator turns ratio that gives me the peak modulated voltage I need, and at the same time swing rail to rail at the modulator side. If that were to come out to 2:1 as an example, then choose a ratio a little higher (maybe 2.2) to gain headroom. Keep in mind that the higher the ratio beyond what you need, the lower the modulator efficiency will be, and therefore it will get hotter. 

If you use the Hammond 48/3.2, the TR will be high, maybe too high unless you intend to modulate close to 200%. That 48/3.2 might be perfect though as a tapped Heising inductor phasing the 3.2 ohm winding like I am doing with the Xicon 48/8 ohm transformer.

Jim
WD5JKO

Someone please educate me on AM modulation and negative peak limiting, etc..  I was looking at Jim's scope images and see where he's getting 120% plus modulation out of his Retro75.  How does one determine modulation percentage beyond 100%?  And how does one determine it if the negative peak is clipped, soft clipped or not clipped?  The reason I ask is the images show the negative peak what I would consider clipped and almost soft clipped by the negative peak limiter he's experimenting with. To me it appears the negative peak flat-lined at baseline and that being the case, how does one determine that the envelope is actually at 120% in that condition?  Isn't there a subtraction occurring?  And if the baseline is clipped, what about the products produced?

I was messing around with a negative peak limiter myself but any differences I saw were very subtle and I wasn't seeing anything beyond what I considered 100% between non-modified Retro and modified Retro once the negative peak began to clip.  I was using just a signal generator and not a source that was asymmetrical.  Maybe that was my issue..

I would think if one is going to limit the negative peak, at baseline the sinusoid would be clamped and well rounded rather than what appears to be flat.

Bob-

I'm not sure why it took me nearly a year to find this forum!  Interesting conversation going on. :)

100% modulation occurs when the minimum waveform amplitude just reaches zero. Assuming you know the no-modulation value and the modulation is symmetrical, the voltage swing from 'no-modulation' to 'positive peak' corresponds to 100%. Crank up the signal by 20% and you've got 120% modulation. The carrier then vanishes for a portion of the modulation waveform cycle.

I'll wonder aloud how much benefit comes from a keep-alive circuit at these low power levels.  Once overmodulation has set in, the audio 'envelope' is clearly distorted (both images). As such, the result is additional spurious content (i.e., splatter). I doubt that splatter is a real issue with the stock Retro, but with an 'afterburner' it'd defiinitely be noticeable. As a relative newcomer to AM, I'll surmise that at higher power levels, the keep-alive is useful in keeping the AGC in listeners' receivers from 'pumping' between words.   

Gentle correction always welcome. <g> I'll be mulling over this whole thread and making changes to the Retro as appropriate. Good stuff! (and more thorough than my own work on this project). There were at least 4 different versions of the board over about 5 years, punctuated by a household move and a slow-motion family crisis.  At some point, the 'better is the enemy of 'good enough' philosophy took over- I wanted it released during my lifetime! :)

73- Dave, K1SWL

Hi Dave,
Welcome to the forum!  What took you so long  ;).

We've had several interesting and fun discussions on the Retro75 over the last year.  There are other threads lurking about too.

Several have contributed much in the way of potential mods and frequency response adjustments.

It's a fun little rig.  You've got a good design there. And it's perfect for experimenting too.

Bob

  I really like the little bugger pretty much as-is. The receiver alone is well worth the price of admission. If a better version comes along, I may consider updating. Really looking forward to the rumored 40 meter version . A stand alone receiver based on the Retro , perhaps with DDS tuning could be a real treat.

  Dave,  Welcome to AM Fone. Your little rig has been the topic of much discussion on this board. I wonder how many have been sold? I am sure having fun with mine.

  I finally got back to mine list night taking another look at the need for the 2- diode limiter with keep alive supply. Running 1Khz audio tone at 120% + modulation, the limiter is pretty transparent with the 2nd, and 3rd audio harmonics (as seen on Icom R75 looking at sideband harmonic levels), but I see benefit where the 5th harmonic is attenuated quite a bit. Adding a LPF to the modulated B+ seems to help reduce the 3rd harmonic as well. The LPF was 220 ohms in series with 5uf from the modulated B+ to ground. Still a work in progress.

  The modulation transformer rewiring into a tapped Heising reactor kind of nuked the AGC circuitry as designed, so I have been working on that. Currently have a clip lead storm sensing the modulated B+ through a 20v zener diode, another diode, some more resistors (divider) going into the LED. It seems to kick in at around +100% with a sine wave, and yet still allow 120% peaks with voice. Seems that momentary overload during the attack time is inevitable, and increasing the decay time makes the attack time issue worse on the first syllable spoken, but better thereafter. Still a work in progress.

  I hope to increase the maximum average modulation potential with voice from ~ 60% to 90%, and if successful that doubles the sideband power. On a QRP AM rig, a 3 db increase in sideband power is huge.

  I will furnish more data when I get closer to a final configuration.

Jim
WD5JKO

Ok, I just got off the air with my first QSO (4 stations) with the Retro75! Got decent reports considering the power level using my home made OCF Windom antenna. The receiver seems a little hard of hearing until the stations get on, then I was jumping to turn down the audio! I had a pleasant experience with this thing.

The audio is still a work in progress, and spilling out with clip leads, and a proto board. I need a good day to neaten everything up, and make a little perf board for the transmit audio AGC circuit. I have an adjustable pot to adjust the AGC threshold, and I can limit from about 80% to beyond 130%. The major darn with the circuit as-is is that the pot setting must change with supply voltage to maintain the same mod % AGC threshold.

I attach a hand drawn highlighted schematic, somewhat crude, and Rev. A. Any components with a value are either a change, or an addition. Also attaching a scope plot using speech while saying Yeaaaaaaaaaa where the upward peaks hit 130%.

I switched the dynamic mic phasing, and it was backwards before. The asymmetry is now bringing the positive peaks higher.  

Kind of fun watching the thing super modulate upward without seeing the baseline "white-line". The AGC limiting transistor Q10 acts as more of a clipper than a attenuator, so when I see the peaks clip from AGC action, it is not from the modulator clipping, it is Q10 at a low level clipping the positive peak. This might be bad, but it sounds decent, and clipping like this definitely brings up the average modulation percentage, which in a QRP rig is most desirable.

At 14V DC input, I see about 4W output, and ~ 20W PEP.

Been up all night, time to QRT.  ???

Jim
WD5JKO

Good story Jim,
  Thanks for the updates . Sounds like you have been pretty busy. My first evening of operation netted a roundtable rag chew Utica, MI to Knoxville TN to South Carolina to PA. I got excellent signal and audio reports . Over the course of the QSO I offered to increase the power and was told there was no need , copy was Q5 . My antenna is a 1/2 wave co-ax fed dipole @35'. Since then I have had over 100 QSOs  many of them at 200 miles plus. Quite a few were Retro to Retro. Quite a ball to sit out in my backyard shack running purely on battery power and carrying on a 1 hour rag chew with a ham buddy over 100 miles away !  The receiver in the Retro is indeed surprising. Same here on the low noise yet even the very weak signals seem to leap right out of the speaker.
  Nice thing about kits like this is the low risk factor regarding mods. I have beem content however to run it as-is for now. I have been getting the rated 2.5 watts @ 12.7 volts . The hottest running component is the modulator IC but it is well within a reasonable heat range. The PA is just a bit below that. I had considered a Hammond xfmr upgrade but I will save that for Retro #3. Retro #1 is here and I built #2 for my buddy Mike, N8SDD. Mike is also in awe of the performance of the very simple receiver.
 I am taking notes on your work and will let you know as we progress with #3. I already have the kit but taking time to work on some other projects.

  TNX again, Gary

Guys-

I'll introduce another 'wild card' into the ongoing discussion.    I just received the following message via e-mail:

>>Completed the Retro 75.  Traced a microphone/modulation problem to R28.  The bottom of R28 is floating and not grounded.  I inspected the board under a stereo microscope and appears that the pad goes nowhere.  I grounded R28 and now all is well.
<<

Uh.... he's right! R28 is flapping in the breeze.  It certainly would affect the ALC dynamics, but I haven't had a chance to look at it.  I did ask the gentleman for further information on the before-and-after performance.


with egg-on-face-  73, Dave- K1SWL

 Dave,

  Thanks for the heads up. I just checked the board on my yet un-built Retro #3 and R28 is indeed floating at one end, the other end of R28 going to Q 10 as it should. I don't have a stereo microscpoe but a back lit x-ray type view of the board agrees with an open ohm meter reading.
   I will check the other two (Both seem to be working quite well) tonight .  
No problem, this stuff happens and the little bugger seems fine. In any event the fix is quite simple.

I don't know what effect grounding R28 properly will have. Intuitively, it'll raise the dynamic threshold and thus increase the modulation percentages.

There was a resistor (R27) value change a while back on the SWLRetro-75 user group, now incorporated in the present configuration.  It had the effect of slowing the attack time, and probably is interactive with this R28 discovery.

73- Dave, K1SWL

In response to my question about R28, here's the response:

>>I checked the rf waveform on my scope and wasn't able to get full modulation.  At first I thought it was the Radio Shack microphone, so I went to an amplified mic, but increasing gain of the mic caused distortion, but no increase in modulation.  I was seeing a good waveform at the output of U5, but little audio at the junction of C27 and C28, so I pulled Q9 to no effect and then Q10 which did have an effect.  This led be to R28.  I suppose the magnitude of the problem would depend upon the leakage of C32.  I may just have been lucky enough to get a low leakage capacitor.
I now have full modulation without distortion.
<<

(Thanks, and a tip of the hat to Robert Sherwood for his followup)  73- Dave, K1SWL

All I can say, Dave, is thanks much for offering this kit.  It's certainly stimulated a lot of interest. 

It can't be easy designing the kit, buying the parts, and you're certainly not getting rich off the profit margin.  Kudos for a job well done!

Times two !
 It is amazing how many contacts I make with people who say "I have one of those kits. I just haven't put it toghether yet." Made a lot of Rtro to Retro contacts as well.
   Thanks to Dave and Robert for the updates.
And... Thanks to the AM Forum for providing a place to exchange information .

Although I haven't (yet) bought and built one, if this is your biggest error, Dave, you're doing GREAT!

--Shane
KD6VXI

Just waking up here getting ready for my shift..

  I know in my effort with my retro that the AGC is working, and at one point I clipped onto R28 (Q10 side) to add an R-C to ground to increase "decay time" without slowing down the attack time. It was clearly working, but I took it out. Maybe if the other side of R28 is floating, then the AGC behavior will be less abrupt once it is corrected.

  Thanks for the input, I will get to this in the coming weekend.

   For those who want to Hot-Rod their retro75, this thread still applies. The R28 issue once corrected (jumper) does sound like it validates the stock design. As for me I liked the V8 conversion to the Chevy Vega..  ;D

Jim
WD5JKO

Jim-

Thanks for all the good work on the audio response!

You asked a while back about the number of Retro-75s out there.   By today's count, I've shipped 375 of them so far.  Not bad for an 'obselete' mode!

When I first mentioned my plan to do an AM rig to a good friend, his reaction was "But there's only 4 guys on AM, and they're all running broadcast stations!"  'Au contraire!'  I'd have been satisfied if I'd used up the initial lot of 100 boards, but that didn't take long.

I now have a spectrum analyzer on order- I've been too long without one.  I hope to wrap up the 'Retro-Helper' in the next several weeks and need the S-A to verify some recent changes for spectral purity.   The little doodad mounts on the rear wall of the enclosure (that's what the spare holes are for). The transmitter will track the receiver tuning-  full VFO control.   :)

73- Dave

 Oh boy !  The retro 75 is a winner for sure. AM ops are naturals for building their own gear , kits or homebrew. The Retro Helper should be a huge seller as well. Is there still a chance for the 40 meter Retro 40 ?

The Retro-40 is at the top of my round-tuit list. I need to modify a spare Retro-75 and listen on 40M for a while.  I'm fairly sure the Retro will not stand up to 'hot' band conditions on 40M, but it may be fine for daytime use.  (It's a matter of IMD from SWBC 'powerhouses').

I did get started on one- the Tx put out 1.85W carrier, so that looks OK.

It looks like the IF would be 5.185 MHz-  I wouldn't even need to change the LO components or L2's turns count.

73- Dave, K1SWL

Dave,

Glad to see you've joined the forum.

I've been impressed with your product and still scratch my head when considering the cost of parts and how little you charge.  Your efforts are certainly appreciated here.

Thanks, OM.

Tom, K1JJ

Tom,
You'd be amazed at Dave's tech support too!

Dave defines the ole tried and true ham spirit.

Tom-

Good to hear from you!  We lived in the NW corner of Colchester a while back. I'd breadboarded the receiver portion of the Retro before we moved, and needless to say, I could copy you really well. Wish I'd been further along with the project, or I could have easily worked both you and Bob, 'RKW.

73-  Dave

Dave - in terms of IMD response for your 40 meter version, a few possibilities come to mind -

1 - get rid of diodes D1-D4.  They'll definitely act as a mixer and create all kinds of undesireable crap.
2 - consider one more stage of bandpass filtering at the input if you get images
3 - a 3 dB or 6 dB attenuator can work wonders with probably minimal effect on S/N.  Perhaps this would be an option on the circuit board, or on a little daughterboard that a guy could add if he gets IMD.  The SA-602 gives pretty amazing performance for what it is, but a gigantor antenner could certainly overload it, considering the close proximity of blowtorch signals on 49 and 40 meter broadcash freqs.

Johnny-

Thanks for the thoughts!

D1-D4, along with L1 and C1, are doing the T-R switching at the front of the receiver.  There's 4 of them there to improve on the performance over a single pair of the diodes. Elecraft uses a slightly different scheme- a transistor conducting to ground on the positive half-cycle and using B-E breakdown on the negative half-cycle.  I don't remember the results I got with intercept point calculations (it was too long ago)- but I suspect the '612 wasn't far behind it as the next weak link in the chain.  For that matter, if removing the diodes makes a substantial difference, a small T-R relay would do the job. 

Yes- a few dB of attenuation up front might work wonders, since the intermod is a higher-order effect.   I'll know once I have a chance to give it a listen.    I'm not sure yet about any image issues.  It wasn't a concern with the 75M version, with the image at about 8.1 Mhz and therefore well down the attenuation curve on the output low pass filter.   

73- Dave, K1SWL

Dave,

Thanks again for this kit and all of the others you have done.  As you and I both know, selling kits can be very challenging and the challenge is mostly the "non technical"  part.

It was nice talking to you again when I bought the two kits several months ago. Keep up the great work.

Pat
N4LTA

  Dave,

  While I'm not a customer of any  of your products, I have looked at 
  the projects you have provided in the past...
 
  Seams to me a Retro-160 would be a better item than the 40 meter
  version.  Just my opinion however both the 40 or possibly 160 should
  reach a good audience.

  Logic says 160 faces less potent flamers than the 40 meter version.

Ralph-

Thanks for your comments.  The last time I was on 160M AM, I was in high school...and it was still the vacuum tube era.  From what I remember of operation on 160M, activity was pretty well distributed across the band.  The challenge with something as simple as the Retro would be to cover a big swath of the band without repeaking the tuned circuits.   

If AM activity's now pretty well centered around one particular frequency, that'd be a different matter.  :-)    73- Dave, K1SWL

Forty meters is a good band for fairly long distances on low power. Forty meter antennas are easily carried for portable use and a 40 meter RX on AM is handy for plain SWL use when ham activity is not an option.

I vote for 40!

Since most AM activity on 40 meters is during the daytime, broadcast stations are not much of an issue. The propagation and the antenna requirements favor low power operation over 80 or 160 meters. So, 40 would be my vote.

It's too bad because 40 is a great band early mornings with plenty of space between broadcast stations, but rarely any AM to be drummed up.

Update,

What I noticed though from the beginning when I first put a tone into my Retro-75 when 100% original, was that I was limited to ~ 30% modulation with a sine wave, and the waveform was not symmetrical once limiting kicked in. With speech I could kick up to maybe 60-70% on peaks.

   So last night I grounded the floating end of R28. In my case the AGC circuit is different than stock (see earlier post), and the result was not very apparent. Maybe that was because I already did have a voltage divider in my arrangement. That resistor R28 should discharge the peak hold capacitor (C32) at an RC rate, and without it the cap would stay charged to a voltage just shy of base current on Q10, and get a boost on audio peaks turning on Q10. In stock form, hooking up R28 as intended should delay the onset of AGC, and that is a good thing.

   So whether were stock, or as I have modified, the AGC system seemed prone to cause asymmetry of an incoming symmetrical wave, and also could be easily overdriven such that hard limiting could occur, and only occur on one polarity of the incoming signal. I mentioned this in my last update. Looking at Q10, and how it was used made me think about a soft clip instead of a hard clip. So with one of those 4 am inspirations, I clipped the collector of Q10 at board level, and soldered a resistor from the collector pin hanging to the trace the collector was formally hooked to. I tried a few values, and settled on 1.5K. I don't fully understand what happened, but all the bad AGC behavior I was mentioning just went away! I can run a sine wave fully symmetrical well into limiting now, and with speech I can see peaks to 150% before the TDA2003 modulator clips.

   I hooked the scope in X:Y mode configured for Trapezoid mode, and that RF power FET modulates with a perfect trapezoid up to 150%. So with 4W carrier (14V B+), 150% modulation, I'm up to 25w PEP now.

   In the circuit I posted (Rev - A) there are a few changes:

1.) C33 was installed as per silkscreen, but in my rewiring changes, that was backwards (+), and I changed it to 330 uf
2.) The R-C (270-10uf) across the modulated B+ was changed to 150 ohms - 47 uf. This is the passive keep alive supply that works well without needing a bias supply or additional diodes.
3.) I moved the adjustment pot for AGC threshold to the junction of R27 and D10.

  At this point, having control of the asymmetry of my voice (> 1.5:1) is essential. I have two Dynamic microphones, and both were phased opposite to what I need. Fortunately it was simple to swap the wires on the mic cartridge to fix this issue. I was thinking of how we can do this within the Retro-75. Well it appears that half of U5 (dual op-amp) is unused, so adding a simple X(-1) inverting amplifier could be added with a SPDT switch to select speech polarity could be used.

  I also have a condenser mic from the XYL's tape recorder. So I tried that, and added a 4.7K pull up resistor from 6V to the mic input. The mic worked fine, but the polarity was wrong again.  :(
Here I need the inverting amplifier since a condenser mic polarity cannot be changed.

  So does the asymmetric nature of the male voice vary a lot person to person as well as the dominate phase? With three microphones I am 0 - 3!  >:(

EDIT: Found my Shure Unidyne II dynamic. it too is phased backwards! So am I like the left handed guy in a right handed world? Now I'm 0-4 on mic polarity...

  I hope to make a PDF of these changes, and provide a link to my web space of these changes. That will take a while to do though.

  The modulation peaking 150% was reported as somewhat fuzzy by a few stations, while others said it was loud and clear. This is to be expected. I should probably limit to 120% by turning the pot. Still I held my own in a big Texas round table this morning with stations in west Texas, Houston, and Dallas all hearing me fine from the Austin area.
  
Jim
WD5JKO  

Since I dwell on a cliff now, and no way to monitor the bands, I wouldn't feel it a bad thing were you to call me on my shoe-phone (invoke mental image of Maxwell Smart) and say "hey, get on the air!" and I'd go get in the truck and make a mobile contact.

gang-

I need to get outside (on snowshoes, probably) and repair the antenna feedpoint. I'll put a Retro on 40M (I have the parts) and listen to band conditions for a few days.  If folks are good with the knowledge that 40M may be a lost cause at night, then I'd offer that version with a clear conscience.

If the activity on 40M is more concentrated at one frequency, the existing 'A/B' switch on the front panel could be used to switch an attenuator in or out.

73- Dave, K1SWL

In my experience (since 1976) the bulk (more the 90%) of the 40 meter AM activity I've heard was during the daylight hours. There has been some night time activity (you can find holes between the BC stations) but it's not the time or place for QRP.

Down low on 40M has been good at night.

7160Kc is free of international BC, but stateside signals are still well below the levels seen daytime on 40M, so yeah, running low power would be a challenge even when in the clear.

I should ask before proceeding further with 40M.  7290 kHz appears to be the classic AM frequency for 40M.  Is further down in the band (say, 7160 or 7175 kHz) now the preferred frequency? I have no preference, and would welcome inputs.   

73- Dave, K1SWL
 

7290 and 7295 would be a good start.

Hi gang, I finally got my Retro-75 modifications sorted out and documented. This is still a work in progress, but for now it is mostly at the following link:

Open the PDF document first.

http://pages.prodigy.net/jcandela/Retro75/

Jim
WD5JKO

Wow Jim ! That was a lot of work. Thank you for taking the time to present your findings and data .

Nice work Jim!

I just received a Hammond 106R transformer.  I'm going to give that a try in place of the stock xfmr.    Foot print is slightly different.  I will have to shoe horned in but not a big deal.  It's a potted transformer. Quite dense too.  It feels beefy for something so small.  Will let you know!!

And it can be used up to 50,000ft!

w4rkw
de w8znx
i waited
but
man
simply had had to build the radio

waiting to hear result

so does the Hammond transformer work
is it worth it

W8ZNX DE KG8LB

   Hello Mac. Good to hear from you. I was eager to hear about the Hammond as well but the little Retro does work pretty well in stock form. I have built three of them so far . The first two were in the SWL Labs enclosures but #3 is going in to a custom brass case, will post photos soon . The case is about done but the logos need to be engraved.  Need to stop by the bookstore and get an eyeball, maybe sked a  Retro to Retro local QSO , could be a three way with Mike, N8SDD as he has Retro #2 up in Shelby township MI.

  So far I have fixed the missing ground and added a PTT relay but otherwise as kitted.

guys, haven't tried the Hammond xfmr yet.  Been working on something else but it's in the que.

Nothing to do with freq response but Retro 75 #3 here at KG8LB is now in it's new homebrew brass enclosure. Still need to engrave the legends on the front and rear plates.
(http://inlinethumb45.webshots.com/46252/2424665320032728487S500x500Q85.jpg) (http://good-times.webshots.com/photo/2424665320032728487AgXpLD)

 When we hear a bit more about the great freq response issues we will delve again into old Retro #1 (In the background)

Gary-

*Very* nice!

Do you do your own engraving? Enquiring minds want to know. :-)

73- Dave, K1SWL

Thanks Dave,

  I do everything but the engraving...for now. I have been looking for an old fashioned pantograph to use as opposed to the CNC type routers.  I am lucky enough to have a very fine engraver about 1/2 mile away as well as Royal Oak Name Tag , a maker of just about any type metal label.
  

Gary,
You should call it 'Goldfinger'. 

Ahoy
  finished Retro works fine
best DX so far KA3PTX 438 miles
even better it was a Retro - Retro contact
had to work at it 3x3 each way

yes am thinking of building another
this time making my own box

have lots of other projects

Gary
very nice looking radio

thanks Dave
keep up the good work
have build DSW 40, SW-80,
the Retro,
and just finished a Rock Mite for 80

much fun

yours truly
mac

I have an update on my efforts to enhance the modulation capability with this transmitter. Since my last post, I have seen many instances of the occasional modulation transformer overload. It all depends on the voice, the microphone, level, and which syllable is spoken. The bottom line is that the modulation transformer is just too undersized if you want to pass any lows down to ~ 200 hz.  If you keep C33 at 100 uf, the modifications I made to the original transformer should be fine since the lows roll off pretty good below 500 hz.

Edit: Since writing this post I discovered that the 56mh choke I had across the mic input had opened up. This unloads the mic, and makes a dynamic have a bass boost. That choke used tiny wire, and had about 400 ohms dc resistance.This bass boost made the T3 saturation symptoms all the more worse, and obvious. I wondered why things had gotten worse.  :(
[/color]
So here is what I did:

I took a 10 watt line matching transformer that I had, and used it as a modulation transformer. The thing was to match a 25v line to 4, or 8 ohms with 5 primary taps to select the power to the speaker.

So I saw the 4, 8 ohm windings were fairly beefy (24 awg wire), and the winding was on the outside. What the heck! Apart it goes, and I unwind 47 turns from 0 to 4 ohms, and 25 more from 4 to 8 ohms.

Then I remove the tiny primary windings (extra tiny wire, maybe 36 awg) to get just a bare bobbin.

Back to the calculator, I got a spool of 28 awg (too small), but two strands of 28 side by side or twisted is ~ 25 awg. Close enough. I take the original wire (24 awg), and wind 50 turns, and bring out the wire. Then I wind 75 more turns (dual 28 awg twisted with drill), and bring that out. Then I wind 13 more turns, and bring that out.

So this gives me an autotransformer with a common, 4 ohm point (50T), a 25 ohm point (50T + 75T), and a 30 ohm point (50T + 75T + 13T).  The turns ratio from the 4 ohm tap to the 25 ohm is 2.5 : 1, and from the 4 ohm tap to the 30 ohm tap is 2.75:1.

So does it work? Well hell yes! I got lucky. All that saturation stuff is GONE. I still have C33 at 330uf, and with a midband peak at 1.5 Khz, going down I see the -3db point at 200 hz, -6 db at 175 hz, and -10 db at 150 hz. It looks like most of the roll-off is R-C stuff in the speech amplifier stages. The thing modulates clean at 100% down to 200 hz. That is really all the low I need, so good enough.

Edit: Looks like the transformer is saturating at 150 hz. I get a big tilt in a square wave drive going below 500 hz, but a sine wave is clean down to 200 hz. I need to ad more turns on the common to 4 ohm tap, and maintain a 2.5:1 ratio. An update to follow.[/color]

Looking at U6 only (remove U5, inject audio into U5 pin 1 socket), I can modulate a decent looking square wave at 1 Khz.

With voice, I think the 25 ohm tap is enough, and I still see > 130 % modulation peaks, but without the ragged occasional overload pattern.

The transformer mounts behind the case, and I'm using one of those two spare holes to hold 1 ear of the transformer.

Edit: The transformer I used was a Speco Model T2510.

http://www.amazon.com/SPECO-TECHNOLOGIES-T2510-LINE-TRANSFORMER/dp/B001MJI8S4

Looking around the net, maybe this one would work as well:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-040

The Speco was not potted, so I could remove the laminations with a small pocket knife to break them free. Somewhat time consuming, but not too bad. Count the turns used from common to 4 ohms, and increase that by about 10%, and figure the 2.5:1 ratio there where the ratio is (total turns) / (common to 4 ohm tap turns). The reason for a few extra turns is that sometimes you cannot get all the laminations back. I got all back except for 1 'E' piece. I did cross interleave, leaving no gap. So 400 ma of DC current seems to NOT cause core saturation, something I was afraid might happen.


Jim
WD5JKO

Jim, Here is a nice Shure xfmr : http://cgi.ebay.com/Shure-A102A-70V-Line-Transformer-4-Power-Taps-/170616119239?pt=BI_Circuit_Breakers_Transformers&hash=item27b9835fc7

  Looks like there are plenty of options among the 70V line transformers out there. Good work and very interesting indeed. Just looking for the balance between work, packaging and the return in performance. I am sure we will be making some mods soon.

  Dave, Hope there is still consideration for the 40M version, the upgrades kit etc ! Your little Retro is a screaming success from what I am seeing.


 Mac, Vy good stuff . Will have to get a sked here.  I now have a pair and a third one I built is up in Shelby Twp. at Mike N8SDD's station. We could pick a time and get on with the Retros. About 15 years ago you , Mike and I had a vy good time on a late night meeting up on 10M.

  BTW, I talked with Roy ,K8VWX a couple weeks ago and as a result the king of high power AM is now building a Retro .
 My bud Sam has taken "Two" and "Watter" and contracted that into "T'Watter". The name has kind of stuck around these parts but I don't think Dave is real keen on that monicker ???.

Installed the Hammond 106R into the Retro.  Overall frequency response is about the same as the stock but it does takes a bit more to saturate the transformer. It certainly can be driven harder unlike the stock mod transformer.  Modulation, without the modulation control connected, increased, that is, positive voice peaks in the 1kHz to 2kHz range stand up much taller.  I think with capacitance increases as noted by Jim and others will indeed make the low end response improve.  

Update:
Increased the cap values at C33, C110 and C112 as Jim suggested very early on.  Huge improvement in low end response.  No stability issues at this time.

Bob,

   Good news hearing of your success.  Getting that out of the way allows you to tweak out the rest of the modulation system. Your approach is probably better than mine so long as the Hammond shakes out like you want it to.

   So the audio AGC might need refinement. If you take Q10 collector, and cut the lead at board level, and then insert a resistor back to the trace, then you soften the AGC response which somewhat hides the asymmetrical AGC action of before. I used 1.5K, but 1K seems Ok too:

http://pages.prodigy.net/jcandela/Retro75/AGC_Mod.JPG

   My circuit is different, but as I let more bass through, the AGC ripple voltage (at Q10 base) would come through and modulate the audio signal...a bad thing. Increasing the AGC holdup capacitance will fix this, but also increase the AGC attack time. I did go up to 10 uf though.

I need to revise my Retro75 Hints & Kinks document. My Retro75 is a joy to operate, and I am sure yours is also.

Good Work!
Jim
WD5JKO

Hi Jim,
I'm using the modulator AGC system disconnected, it seems to work best for me. Using a scope connected to the Retro I can watch the negative peaks.  But let me tell you with your mods to the 3 capacitors and the larger mod transformer, what a big difference.  Frequency response aside, the modulation % is a lot better especially in the speech range.  Using an outboard processor takes care of the lack of on board mod control.  But if I go portable simply holding the mic away about 6 inches seems to provide good clean modulation.

I'm inclined to try some heising/auto transformer like you did.  With the new transformer mounted off the board by wires it should be easy to do.

Made some progress on my S-meter/IF-AGC mod.  Still tweaking but should have something soon. 

 I beefed up the heat sinks on the mod amp and RF PA.  While testing I could smell the heat.  I know some guys beefed them up so I did the same thing. 

Next will be to work on RCV freq response.