The AM Forum

 I have allways been interested in small solid state receivers and transmitters the RETO 75 with vfo almost fills the bill. I have a old HW8 and would like to mount the 75 in it, use the vfo knob and meter and loading controle. Has anyone come up with a way to put a smeter and install a loading controle on the out put of the 75. And the rf gain and audio controle would not be bad either.

John W9BFO

John,

   There are several ideas at the Yahoo Group for the Retro-75. For packaging, I like the converted Heath kit sixer into a "75 ER". Also, a fellow named Robert Walker has added an RF gain control, audio derived AGC, and a RF preamplifier. These are all in the photos section. I added a file in the files section called "Retro75 Hints and Kinks" as well. I hope to revise this document in the coming weeks since I've made significant headway in the audio department since that file was written.

Good Luck!
Jim
WD5JKO

How about putting a Kenwood 950 xcvr and an Alpha amp into the Retro 75 case ?  ;D

As Jim/JKO has stated, I have an RF amp on the front end of the Retro with an RF attenuator on the front of the RF amp.   Was using an audio derived AGC but dumped it.  I'm currently working on an IF derived AGC and an S-meter.  Also, just finished working on a 40watt after burner which seems to work OK.  It went on its maiden voyage last weekend.

  Ok, I built my VFO today for the Retro75. It worked first try, and only needed a tuning touch up. The RF seems a little hotter to me although I did not measure it. There is a slight frequency offset between RX and TX where the TX is about 900 hz higher than receive. Since the RX bandpass is a little sharp, I offset the tuning on Rx anyway to bring in some highs. So tuning a AM signal on the low side for optimum reception also seems to put the transmitter on a near zero beat. Not a bad setup, I like it.

   I am attaching a picture of the new PC board, and it also shows my new modulation transformer. This is a Bendix mid 60's car radio audio output autotransformer with a 3:1 turns ratio: perfect match to the Retro75 modulator / final.

Jim
WD5JKO

Jim, I noticed that I had a slight shift between RX and TX.  what did you do to compensate?

I have a Hammond 106R mod transformer I want to try.  I have 3 days for radio. I may integrate it into the REtro.

Jim

WOW! That's a very busy board

Al

  Al, my unit was the test bed for a lot of changes. There are other Retro75's out there that look so much nicer! The recommendation is to run the wires underneath...

   But mine does 150% positive peaks, has a negative peak limiter, adjustable audio AGC, and full response down to 100 hz at 100% modulation, and 70hz at 50% modulation. The high end goes too high. Drives my CE 600L to ~ 75 watts carrier real nice..


 I tune low on receive about 1 khz, and that makes it about right on transmit. Thoughts of an RIT, and BFO injection are staying just thoughts...I'd love to hear how the Hammond 106 works out.

Jim
WD5JKO

Hammond 106R installed with cap mods. See comments at: http://amfone.net/Amforum/index.php?topic=24493.50

Bob: How are you doing on the RETRO 75 agc and s meter work, I cant do much now till I get my test bench setup. I dont even have a 75 to experment with, but would like to be ready to go when the time comes.

John W9BFO

Hi John,
If you get a Retro75 I'll bet that test setup will come together quick!   :D

Still working the modeling/simulating phase on a couple of circuits that will work off the 6 MHZ IF.  A couple more weekends and I'll prototype and integrate them into the Retro.  It's slow going but progressing.

Ok, as for modification, for those that installed a Retro Helper VFO kit, we have an unused switch that once selected between two crystals. What can we use that switch for:

Possibilities:

* Maintain 3880 xtal as a calling frequency. Preset PCB to bias select diode for 3880 xtal, and route VFO and XTAL RF through switch via tiny coax, and from there to C36.

* Use switch to key VFO SA612 only (not cascode amplifier) to use as a spot switch, or BFO

* Select between line level Mic input or low level (stock) mic input

* Have two thresholds for the automatic modulation control, or NO AGC / AGC selection.

Just some ideas...

Jim
WD5JKO

 I am wiring mine using a center position for VFO control and the other two positions to select between the 2 crystals.

I eliminated the switch all together and used the hole for a mic jack so the xtal positions are open.  I don't think I'll use them for anything but I was thinking of making a small BFO for receiving CW and SSB.

  I have built three of them now. All have a microphone jack with PTT in the standard location. I figure the crystals are already there so it is nice to have the option of using them. Allows to offset the RX tuning a bit while maintaining a stable TX frequency.

(http://inlinethumb50.webshots.com/26033/2424665320032728487S600x600Q85.jpg) (http://good-times.webshots.com/photo/2424665320032728487AgXpLD)

  This sounds great, but the implementation makes me think you needed a DPDT center off switch and a SPDT relay. Then again maybe I'm having trouble visualizing how you DID, or how you Intend to do this. I'd love to see a schematic of the change as it seems very worthwhile to do.

Jim
WD5JKO

Bob: Have you had any luck on the RETRO 75 s meter, agc, and loading controle. I am real close to starting to work on one.

John W9BFO

Hi John,
I have a working S-meter. The down side is I'm still
working on a receive AGC and integrating it with the S-meter. I still have some
tweaking to do but will post a working circuit soon.

Be advised that this will only work with the RF front-end preamp I posted
earlier this year on the  Yahoo mail group.

Bob
w1rkw

Rumor says the Retro75 XCVR board kits are fast approaching "sold-out" status.

I'm curious why an RF preamp would be needed on 80 Meters.  I haven't built one yet and am not familiar with them.  Is the receiver that insensitive as designed, and if so, I wonder why?

   Good question! I have one here, and these receivers don't have any RF AGC, so they run wide open. If a strong signal is encountered, and audio AGC kicks in with some audible distortion. The cure is to cut the audio gain to clean up the audio some. Still it is quite listen-able with a round table. Some will come through louder, others weaker.

   The receiver is quiet. Then hook it to an antenna and signals pop out of the speaker. Sometimes though when the signal is weak, the audio recovery is very low..indicating a lack of sensitivity. Then maybe I will switch in a different receiver, like my Icom-R75. Here I find plenty of audio, but poor signal to noise ratio. The copy was no better on the Icom. The R-75 is a decent and well respected receiver among the SWL crowd.

   The Retro-75 receiver with its low noise, and crystal filter selectivity is a good 75m AM receiver considering normal atmospheric noise levels on that band. The audio detector is a FET based infinite impedance detector with very low detected distortion.  On 40m or 20m things would be different however.

  My best guess would be that the MDS signal level for this receiver is around 10uv when S9 is usually around 50 uv. My HP 606 RF generator is down for the count after sending out smoke signals last winter.. :-(

   This rig like many QRP transceivers use a broadly tuned series L-C tuned to the receive frequency. This is part of the TR switch. The convention is add a dual diode limiter at the L-C junction. On my Retro-75, the L-C resonance was off quite a bit, and I dialed the resonance in by making the 'C' adjustable. The received level came up about 6 db.

Jim
WD5JKO

My Retro 75 is deaf as can be without the pre-amp.  If I were working signals in the S9 range or 10 over and lower I would never anything.  Trying to increase the sensitivity in the stock setup has been unfruitful. Even on the most static filled nights the receive section just sits their quietly unless a 20 or 30 over signal comes in. And when a strong signal comes through it's startling.  So the pre-amp comes into the picture. With the pre-amp I can hear the noise floor and use it as a baseline.

Can anyone hear the noise floor on 75m as a stock setup on their Retro75s? I'd be curious to know.  

With the preamp I have sensitivity equivalent to my FT102.

I see.  I noticed that others have voiced a similar complaint.

The SA612, which is mis-identified on the schematic as 602, is spec'd as having a 17db conversion gain and capable of a -119dbm for 12db s/n/ at 45 MHz when driving the SA614 (not used here of course.)  That's approximately 0.4 microvolt sensitivity which by itself would virtually always be well below the practical noise floor on 3.5 MHz.

Thus I am thinking there needs to be more gain in the IF section.  An RF preamp will increase total system gain but at the cost of overloading the SA612 at some point, which will not be remedied by AVC control of the preamp.  I wonder why that wasn't noticed at the design stage, although I guess maybe for a simple little product I am expecting too much from it.  If you could hear as well as a $ 1000 new ham rig, maybe you would be frustrated with how few people could hear your 2 Watts?

 It seems the receiver "complaints" are a small minority. Not unusual for kit built electronics however.


The receivers in all three of mine work quite well.Are three were built to the original design. The very minor distortion on strong signals already mentioned is at an easily acceptable level. The stability, selectivity and especially the sensitivity are fine as is. I have favorably compared it to my Drake R8A on some very weak signals (Similar to reports from N8SDD when side by side comparing the Retro to his Yaesu FT-1000) .The Retro itself generates very little noise, refreshing when compared to noisy modern receivers like the JRC NRD-545 (Among many others) ! Theoritcal speculations aside,the little Retro indeed has a very capable receiver. If I can work them I can certainly hear them.
  A few minor tweaks and the modulation is at 100% with room for more. We have been able to stick with the original design concept, an elegantly simple but effective design. Two of the kits were built into the Tentec cases, one is residing in a homebrew brushed and laquered solid brass case. All three have both VFO and crystal control(3880/3885) TX ability.
  Ordering #4 this week. It will be installed in an ammo can with a LiPo battery pack, built in speaker , charger and antenna tuner.Along with space to carry the microphone and a very small freq counter. Perfect for weekend stays on Mackinac Island.

Well, I can hear the slightest QRN in my home, and when the noise from local stuff is absent, I do hear a low level hiss from the speaker that does mute when I disconnect the antenna. With my unit, the sensitivity is not great, but it does seem sufficient for most situations.

I have been in many round tables, all over the country using my Retro-75 using a transmit power booster. The receiver had no boost, and it does pretty well. There were times though, like 3 am over the winter where signal recovery was minimal, so I had the audio gain at maximum. Switching over to my ICOM (R75), I had plenty of audio, but at poor S/N. The copy on the Retro was actually equal or better!

Doing a receiver design without RF or IF AGC would dictate that there be sufficient dynamic range to avoid overload from signals 10uv to 1000uv. This would correspond to about S6 to S9+30db signals. The dynamic range need would therefore be only 40 db. Any extra gain would make overload a problem unless compensated for by either some form of AGC, or a manual gain control. Too much RF gain even with AGC would be bad since strong signals 5-10 KC away would overload the SA612, and yet no AGC would be provided due to the IF selectivity.

This is no $1000 receiver, but it is surprising to me how good it is for what it is, a transceiver kit for $69.

Jim
WD5JKO

How can one get more gain out of the IF of the Retro75.  I don't believe it's possible since the output of the SA612 goes right into the detector.

There is a 2200pF NPO (C7) on pin 7 of the SA612.  It's reactance is about 20ohms at 3.800MHz.  I wonder if changing the value so reactance is greater than 20 ohms would change the AC gain of the stage within the SA612 and increase the signal to the detector? ? ? ?

http://www.stanford.edu/class/ee133/datasheets/SA612A.pdf     See page 4 for internal SA612 schematic.
http://smallwonderlabs.com/Retro-75_Instr.pdf        See page 9 for schematic of Retro75.

I suppose you're right about people being frustrated trying to hear my 2 watts. But don't you think it would be prudent to have receive sensitivity so one could here what others are hearing so as to not interfere with the others?  I am surrounded by a couple of big guns and would hate to interfere with their communications because I could not here someone they are working.  My 2 watts would certainly cause a problem with these few big guns if they are working someone weak and I couldn't hear the weak station.

The RF preamp does cause some overload if run wide open but I have an attenuator on the front so the sensitivity can be adjusted.  It works just dandy.  Controlling the gain of the add-on preamp can be done using AVC/AGC derived from the SA612 IF.

   Bob, there is a dual 2N4401 cascade IF amplifier between the 612 and the detector. My RCVR was somewhat mute until I peaked the resonance of the series L-C in the TR circuit (the one with a quad of 1N4148 limiting diodes at the L-C junction). Figure the resonance for the values given, and then factor in component tolerances.

Jim
WD5JKO

Jim,
Are you talking about C1 and L1 at the front of the SA612?  Why wouldn't T1 compensate when peaked?

  Bob, Yes sir I certainly am. L1-C1 are resonant by themselves, as is T1 with C2. In my Retro I bridged C1 with 5 pf holding with wooden tweezers, and the receiver level dropped a fair amount. So I removed C1 and replaced with a 68 pf, and in parallel with that I put a 0-30pf PCB trimmer cap. The peak was sharp, and independent from the T1-C2 peak.

  I got the information from this web site:

http://www.qrp.pops.net/transmit.asp

   Look at similar circuits to the Retro-75, and see how they dealt with the TR switch series L-C with diode limiter circuits.

Jim
WD5JKO

Jim,
I dug out an NE602 receiver I once built and amazingly it still works.  It appeared in QST as the Neophyte receiver.  I remember it having good sensitivity on 75m.  Sure enough it is more sensitive than my Retro.  So I'll give your tip on L1 and C1 a try.

FWIW, K1SWL is coming out with a 40 meter version of the Retro75 and coincidentally named it the Retro40.

The Retro40 is now available from Small Wonder Labs. 

I built the RETRO 75 kit the other day and after several minutes after completion the transceiver was on the air. The kit was a gift from a friend who also purchased the factory housing which made assembly a breeze. I am having one small problem that I have not seen mentioned on any of the discussions on any websites. I have noticed that my RETRO 75 has a high frequency ringing sound when I turn up the gain whether the antenna is connected or not. It is present at all times with or without modulation. Has anyone experiencd the problem that I am encountering? I do not hear the ringing during transmit.

73's Liz

Good day,

Is the Transformer getting warm..? U-8 is a Notorious animal on these.
Oh yea I meant to say Dave won't be back till Monday they went out for the weekend

73
Jack
KA3ZLR