More 75A-2 issues: mechanical filter "bypass" and lousy AGC remedy.

[WB6VHE]

Greetings again to all!

Now that the A-2 is up and running, I have another couple of questions to
put out here:

First, I saw in some publication once (of course, I can't remember which publication
or when I saw it!) a simple resistor-capacitor combination that could be used to
couple across a mechanical filter socket if one wanted to effectively take the filter out
of the circuit.  Does anybody know how to do this, or remember that article I am talking
about?  Seems it was just a resistor across each side of the socket and a cap in series.

Second, the agc action is not the best.  Are there mods to improve this?

Thanks again to all in advance!

73, Ken

[KM1H]

Did you check all components in the AGC circuit? That includes resistors and any mica caps.

I believe ER had some AGC updates but that may have been for SSB.

Carl

[WB6VHE]

The caps in the agc circuit have been replaced; perhaps I ought to
check everything else in there! 

After some thought, I realize that the two sides of the mechanical
filter socket (input, output) probably have to be transformer-coupled
if the filter is pulled and you want to just couple straight between
the input and output i.f. amplifier stages.  I guess I've answered my own
question! 

[w3jn]

The AVC in the 75A2 is terrible.  You'll notice that on a perhaps S1-S2 signal, the meter will deflect backwards a bit as you tune across a signal.  Then, it won't deflect forward until you tune across a S-3 or S-4 signal.  It's not smooth, the attack is too slow and decay too fast.  I messed around with one I had for hours and never fixed that AVC delay problem with the meter, but putting a diode across one of the resistors between the AVC diode and the AVC line helped with the attack time.  I think I also replaced that series resistor with a high value - 5 meg? and that helped the decay also.  Still sucked though...

[KB5MD]

Try this, it is courtesy of W2DL, he sent it to me for the same thing on a 75A-3.
Place a 2.5 mh RF choke across the pins of the filter socket that went to the plate of the IF tube, put a 47k 1/2 watt resistor across the pins for the filter that went to the grid, etc. of the next IF stage, and then add a small capacitor (maybe 10 - 47 pf) from the plate connection to the grid connection, and there you have a broadband filter setup.  It works quite well.

[KM1H]

99% of Collins AGC designs sucked, at least for ham use and that includes all of the 75A series. The only good one Im aware of is the 75S3B.

Pick one of the better AM era AGC radios out there and copy their circuit. Maybe a NC-300?

[W3RSW]

Place a 2.5 mh RF choke across the pins of the filter socket that went to the plate of the IF tube, put a 47k 1/2 watt resistor across the pins for the filter that went to the grid, etc. of the next IF stage, and then add a small capacitor (maybe 10 - 47 pf) from the plate connection to the grid connection, and there you have a broadband filter setup.  It works quite well.

I guess that will work ok but don't forget your passing a 455kc signal not a HF one.  The bridging cap might be more like 500 to 1000 pf?   ..unless the RF choke and cap combo are resonant at 455kc?

The 75A2-A that I picked up from Gary, INR at Horseheads years ago has a similar lash-up.  A pi network consisting of a 15k, .004uf & 10k resistor.  Hmmm, I'll have to work out resonance for that circuit assuming similar source and loads as the plate and grid resistors given.
Pass band seems to be 9 to 12 kHz.

[WB6VHE]

If the cap is 50 pf, the choke/cap combo will be resonant at 455 kHz!  I didn't include the effect
of the resistor in the calculation...just a quickie.  This seems like it could be a cheap-and-dirty simulation
of tuned transformer coupling without the hassle of building a board/box to mount a 455 kHz
IF transformer on.  Verrry interesting!  I'm going to try this as soon as I get a chance (am
fixing busted o'scope right now...must get it done, I'm lost without the scope).

[WBear2GCR]

I seem to recall that I have read a number of articles and shorties in QST from the 50s and 60s addressing the AGC in the 75A series... fwiw. Dunno if any of them are worth following or not.

                      _-_-bear

[Don, W2DL]

Just a further comment on replacing a 6 KhZ mechanical filter with an RFC, a coupling cap and a resistor - The very small value of coupling capacitor between the plate of one IF tube and the grid of the next was set by the desire to keep the  gain of the overall IF strip the same with the normal mechanical filter or the simple network in use. Since the mech. filters have a considerable amount of loss one needs to match that loss in order to keep everything about the same gain and therefore audio output level to the speaker. A large "correct" value (500+ Pf) coupling capacitor will result in far too much stage gain in the IF strip, and consequent AVC/Smeter/volume problems. I used a 39pf, as I remember, and the signal strengths remain the same with or without the normal mechanical filter switched in. Works like a charm, and has for around 50 years in my receiver!!

[KM1H]

I believe the old filters had 12-15dB loss so a RFC and simple resistive matching pads might give what you need as well as a nice flat response. Gain can be increased in the IF stages if needed.

Carl