retro 75 board is complete

[ka3zlr]

  Here she be:

[ka3zlr]

I have a question about c15 it calls for a .001 uf poly I had a .01 uf I don;t know what happened.

73
Jack.

[W1RKW]

Jack,
C15 should be a .001uF poly type marked 102J, red, according to the parts list.  I have that value in my C15 capacitor position.  Maybe the parts count was off and it ended up there.  If not, you may have exchanged it with another part on the board.   My parts inventory was right on the money and everything ended up (at least I think) where it was supposed to go.

My board is complete too.  I'm waiting on a few other parts to assemble it into an enclosure.  I should have it operating by this weekend. Then I start on the single ended amp.

[W1RKW]

Jack,
I compared my board to your photo.

A couple of things I noticed. The C15 cap is nothing like what I have on my board.  So maybe that is an incorrect part that was missed by Dave.  

Also, I have 2 crystals for 3885 and 3880.

Despite those to things it looks like your board is identical to mine.

How are you going to mount or install your board?  I was thinking of mounting it on a piece of finished wood and using plexi for the controls and switches.

[ka3zlr]

Hey Bob  

 I'm really impressed with this little guy the ease of building it
I did the coils last sometimes they can be Tom drudgery so I saved
them for last. any ways I cleaned out an old White Face case
down to the bare necessities I bought a shack 3 amp reg supply
that'll fit right on the tail of the chassis I'm gona mount the board
close to the crystal bank and I'm thinking of using that for 5 channel
operation so that is my plan and with Vol and Squelch controls at
the ready I changed them out so that covers Off/On Vol an Tune.
Also I have a 4 inch fan for circuit air on demand. Also I was considering
a small step start circuit..


Whatya think of my plan..? Fairly Retro ...

73 OM

Jack.

[ka3zlr]

Jack,
I compared my board to your photo.

A couple of things I noticed. The C15 cap is nothing like what I have on my board.  So maybe that is an incorrect part that was missed by Dave.  

Also, I have 2 crystals for 3885 and 3880.

Despite those to things it looks like your board is identical to mine.

How are you going to mount or install your board?  I was thinking of mounting it on a piece of finished wood and using plexi for the controls and switches.

Hi Again Bob,

 I called Barno electronics he has one so off I go to Mckeesport Pa road trip 
get outta the house for a little bit Yaaaaaaaaaa.

My brother is ridding me over. cross my fingers I'm taking the Condenser Checker
just in case.

73

Jack.

[W1RKW]

Jack,
Once everything is done and a maiden voyage, I'm going to run Micro GORT as a PW AM field day station in June and run it off of a 6A 12v solar panel and a 12v backup battery.  It should be interesting.

[ka3zlr]

 ..Heck ya Bob I didn't think of that kool idea.

Today we're working on fitment of the DC supply
it's going good and getting the control pots in place
This is a blast  like miniature everything..

73

Jack.

[W1RKW]

I'm working on the wiring as well.  Got all the pots, jacks and switches yesterday.

[WU2D]

What a nice exciter. GO HIGH POWER 75M MOBILE!

Now you need a pair of water jacketed 4CX250's LINEAAAR in the glove box off the radiator- woops I mean 12 pill Doherty in the cupholder piped into the AC.

[WD5JKO]

Jack,
Once everything is done and a maiden voyage, I'm going to run Micro GORT as a PW AM field day station in June and run it off of a 6A 12v solar panel and a 12v backup battery.  It should be interesting.

  I ran natural power during FD back in 1976, ran 40w 2m FM uplink to Oscar 6. One guy pointing the Yagi, and listening on 10m for the pass through. The other guy pedaled a 10 speed bicycle with a Chrysler alternator hooked to the rear rim with a large V-Belt. We had a great signal, but not a single contact. 

 The funny thing is that with all the efficiency hits, getting 40w out meant the bicyclist was good for about 3-5 minutes before a cardio event neared. The biggest, toughest guys were reduced to a sweating mass of flesh left in the fetal position afterward. It didn't help that this was in the full sun on a hot humid day.

  Back to the Retro75. I bet many CB type linear amplifiers will work on 75m. I'm referring to the ones using push-pull class B transistors with a broadband circuit. Remember unlike SSB, we don't need a bias source with this circuit with AM, so zero bias class B works fine. The issue however is that most of these AMPS I've played with have too much gain, so with 2-3 watts drive they saturate to max power (60-70W) BEFORE modulation. AMPS like that need about a 6 db resistive pad at the input.

  I bet a Retro75 with an appropriately modified CB type linear would be an effective combination. Then the power would be about right to drive a SB-220!

Jim
WD5JKO

[KD6VXI]

I run a 16X2879 Messenger amplifier mobile.  1.6 to 2.5Kw PEP output.

It does GREAT from 40 meters to 10.

On 80 it's about half power, and the transformers begin running warmer.

160 it's way below half power.

Other CB amplifiers have fared better.

You DO need bias on it, or you have switching distortion.  A simple diode clamp will work.  Remember, most of the bipolars in RF service need about .7 volts to turn on, in a PP device that's 1.4 volts of drive that ISN'T BEING AMPLIFIED, and when your talking about 4 or 5 volts swing at most, that's 25 percent of the drive power that ISN'T being linearly amplified.

I've tried it both ways.  Most people want to "volt" the devices, I built a regulator that gives me a regulated 6 volts now (had a 12, didn't like the regulation under full power), and I pull bias from there.  That's allowed me to be able to run to the limit of the 2879 WITH bias (18 volts).  At 18 volts, PEP > 3 Kw.....  But I don't have a way to measure purity, so I don't run it there.

--Shane

[WD5JKO]

You DO need bias on it, or you have switching distortion.  A simple diode clamp will work.  Remember, most of the bipolars in RF service need about .7 volts to turn on, in a PP device that's 1.4 volts of drive that ISN'T BEING AMPLIFIED, and when your talking about 4 or 5 volts swing at most, that's 25 percent of the drive power that ISN'T being linearly amplified.

Shane, you are absolutely correct when you need to linearly amplify SSB. But with AM, the carrier itself overcomes those thresholds, or so it seems. Playing with these amps on AM, adding the bias only reduces efficiency a smidge. On the scope, with and without bias the outputs are identical. A spectrum analyzer plot would be interesting to look at.

There is also a concept for linear amplification with AM called the Class BC Push Pull Linear amplifier. As I recall, the non linearity of this approach increased the depth of modulation. It went something like 70% modulated excitation cranked out 100% after the class BC PP amplifier. Yes that is some distortion, but compared to the CB approach that saturates the amplifier prior to modulation, this class BC stuff is hi-fi!

Jim
WD5JKO

[KD6VXI]

I've done A/B comparisons, and you can hear distortion when switching from grounded base to having bias on the base.  I uploaded the .mp3 file once (for a demo of computer generated audio).  It was taken at 100 miles away, with me at a MAX of 7 S Units.  2.5 to 3 S units of carrier.

I'm VERY familiar with the BC approach.  They also call that a "Modulator".  Run a class c amplifier and increase the carrier to the point it is equal going in and out of the class C amp.  Any amount of modulation causes the transistor to conduct, GREATLY increasing the PEP of the signal. 

The only thing I've discovered that makes EITHER method above actually work is finding the SWEET spot of carrier power....  Generally on 2 transistor amplifiers this is about 2-2.5 watts of carrier INPUT.  The self-rectification of the BE junction will then apply enough turn on of the base that you don't get the inherent switching distortion.  This ONLY works with a carrier that has no shift to it, though...  As soon as you start to shift the carrier power, you end up with non-linear amplification as the transistor goes from B to C.  The same thing can be done with SSB...  You find that Pin that allows for linear amplification, set the ALC to limit your PEP output to that limit, and then your "davemade" is all of a sudden a SSB linear.

I've also tried running grounded base and applying bias via voice...  ie, EBS.  It <<worked>> but didn't really accomplish much.  I could base modulate the amplifier, but getting ANY efficiency wasn't happening..  Scrapped the idea and went linear.

That's all I can say about it...  I can find and upload the MP3 if you'd like to hear an actual AB difference, difference being switched on the fly, no keyup.

--Shane

[W1RKW]

Jack,
You may want to do this. I figured if I have to take Micro GORT apart this will make it easy by connectorizing  the switches and controls on the Retro75.  If this interests you, order the part numbers shown below. 
I'm using these so if I have to disconnect anything I can easily do it.  The 1st 2 PNs have a header that you can solder to the PCB so you can disconnect the controls and switches. The same for the last 2 PNs but they are connectors that are part of a pigtail.  The PNs are from: www.allelectronics.com.

CON-242     2-PIN CONNECTOR W/HEADER
CON-243    3-PIN CONNECTOR W/HEADER
CON-30    3 CONDUCTOR CONNECTOR    
CON-20    2 CONDUCTOR MULTI CONNECTOR

[ke7trp]

Run one into a JB150

C

[KD6VXI]

Run one into a JB150

C

Think I'm scared???


--Shane

[WD5JKO]

Generally on 2 transistor amplifiers this is about 2-2.5 watts of carrier INPUT.  The self-rectification of the BE junction will then apply enough turn on of the base that you don't get the inherent switching distortion.  This ONLY works with a carrier that has no shift to it, though...  As soon as you start to shift the carrier power, you end up with non-linear amplification as the transistor goes from B to C. 

    Well there you have it, running real AM without a huge carrier shift works with a P-P S-S amp for AM. The example you give seems ideal for taking a retro75 from 2-3 watts to about 15-20 watts (carrier only). The little 11m amp I was playing with had too much gain, so instead of installing a 6 db pad, I added negative feedback from collector to base on each AMP transistor consisting of a series R-C. That worked like a charm. I played with a bias scheme, but abandoned it since it was not needed.

    It sounds like the CB scene has advanced over the years with hammer popping S-Meter swings with modulation. Sounds like a topic for another thread..

Jim
WD5JKO

[ke7trp]

scared no.. But I think it would work great with a few changes.

C

Run one into a JB150

C

Think I'm scared???


--Shane

[W9GT]

Jack,

I think your thread got hijacked!  Anyway...hope to hear you on soon with the little 2 watt wonder.  I sure have had fun with mine.  Also have heard Bob, W9RAN on with his, although we have not yet had a Retro75 to Retro75 QSO.

73,  Jack, W9GT

[W1RKW]

It would be interesting to know how many others have Retro75's. We should start a 24/7 net on 3872.   

[ka3zlr]

Settling mounting issues but case is working well.

I used circuit plugs to run operating controls makes it easier to service.

 
The two lights indicate step start is working yellow comes on first power
supply lights then 13.8 is provided to the on off switch. Also working on a
relay for xmit/rec.

[ka3zlr]

side picture:

[KC4VWU]

Looks good Jack. Will the enclosure slide up on it without hitting the board though?

I have several old Messenger I & II's sitting around, but I've thought about just doing conversions on them. One on 75 would be cool. With some tweaking, they can really sound nice.

Sometime, I'll have to tell you about the famous (around here) Johnson/ Browning ping mod project.

Phil   

[ka3zlr]

Hey Phil tnx for the reply OM..

 Yea it's been fun already I'm considering another build up.

 The case is perfect length an width the board is on stilts
right now till I get the fitment of all the parts done then I'll
down wind the screws and nip off the ends till I get it where it
looks best plenty of room in there.

73

Jack.

also I have come up with a mod for the Pa Fet for better cooling: