75A-3 AGC

[K9AAL]

I have a 75A-3 that has a forty (40) cycle tone on the AGC line when receiving local AM broadcast stations or loud AM stations on 75 Meters.  Also, I can generate the tone with an unmodulated carrier.  I have substituted NOS tubes for each of the IF Amplifiers and put a .01 mfd cap in parallel with C-15, C-61, C-68, C-73, and also changed out C-81, C-97, C-94 and C-98.  Nothing seems to eliminate that tone and of course it causes bad distortion with loud AM signals. The receiver otherwise works well. 

The caps are all ceramics which I understand are unlikely to leak. 

Any help would be appreciated!!

Thanks, George, K9AAL

[W3RSW]

Yeah, something's motorboating on carrier overload at 40cps.  You mentioned that the receiver works well otherwise, low am sigs, cw, etc. but is the AGC working on the lower level signals.?

Did you measure all the resistors in the agc line too? Check your operating voltages?

Also try a new V8, 6AL5 det./avc tube and V9, 12AX7 AVC amp./af amp, etc.
Disconnect avc line from pin 1, V9 12AX7 and see what happens. This turns the avc off.

Be sure contacts on CW/AM/'FM" switch are ok, (S-3).

[W3RSW]

Also check C82 (.01uf) which feed backs the plate through a 100k resistor to grid of the 1/2 12AX7 AVC amp, V9.

There are some interesting time constants of R/C combinations in the grid and plate of V9, e.g., R33, C81, R34 and the above mentioned feed back C82, R35.
TC alone of C82/R35 is 0.1 sec or a freq. of 10 cps.

[K9AAL]

I won't have time to work on the receiver for a couple of days.  However, I did substitue good tubes for V-8 and V-9.  R-33. R-34 and C-81 are all correct value as are C-82 and R-35.  I will need to check the resistors in the AGC line.  The bias voltage off the power supply is -66Volt.  Other voltages will need to be checked.  C-97 and C-83 are also okay. 

The AGC voltage does vary with the intensity of the input signal.  Reciever works nicely on CW, sounds distorted on loud AM signals unless RF gain reduced to about "7".  Actually does well on SSB too, using the RF gain control.

[W3RSW]

Possible first converter V2 overload is a possibility.  RF amp V1 too.
Same R, C, and voltage checks may be required for those stages.

Since your using classic bfo injection for SSB and CW demod. , you have to back down the RF gain manually to reduce distortion or even get demodulation on strong signals.  This is the nature of the design without a product detector or AVC action on CW and SSB signals.  The RF gain simply grounds the AVC line from -50 volts to '0' by varying degrees in the "CW" position of S3.

[KM1H]

Any tube in the RF/IF path can cause that problem if they have cathode leakage causing the grid voltage to go out of spec and even positive. V1, 2 and 3 are particularly suspect. Late production went with a 6DC6 in V-1 to greatly improve overload.

Carl
KM1H

[K9AAL]

I have done some voltage checks and other checks:

V1--Pin 1 = -.3volts
V2--Pin 2 = 0 volts (should be .9 volts, but this is crystal oscillator input, so doubt it is a problem)
V4--Pin 2 = -.5 volts (should be -1.7 volts, but this is the pto input.  It also has 10K resistor to ground on pin 2, should be 100K per my schematic)
V5--Pin 7 = 0 volts (this is the catheter, and it should have a 150 ohm cathode resistor to ground in my schematic and voltage should be .5 volts.  The cathode is directly grounded in my receiver. The grid voltage is -1.4 volts, so it is relatively too positive)

Other cathode and grid voltages on V1, V2, V4, and V5 are all okay. 

Any suggestions??  The resistors on pin 1, V1 are okay (R1 and R2)

One other IF amplifier is associated with the mechanical filter box, and I am not sure how to get at the wiring in that part!!

Of course I am still getting the 40 cycle sine wave on the AGC with the loud signals!!

Thanks!

[w3jn]

Any of those oil tub capacitors in the AVC should be suspect.  I had a dual section oil tubber in a 51J-4 that was leaking section to section, which completely upset the AVC.

Anyway, specific to your problem - you say C-94 and C-97 are "OK", how do you know?  You need to make sure you have the polarity right if you've replaced them.  You can't assume that new capacitors are "OK".  And those handheld VOM/cap testers are suspect, due to the fact they don't test ESR nor leakage.  A low frequency oscillation points to areas of the circuit expecting a high value capacitance that's not seeing same.

First make sure ground points have good conductivity.  Cinch down the screws and nuts on the ground points.  The IF can shields would be a prime suspect here.

Then, I'd get at it with a scope.   Inject a nice strong signal from a sig gen, look at the AVC line, and bridge C-94 and C-97 with a 10 uF or so capacitor and see what happens.  If the oscillation level or frequency changes drastically, I'd replace those capacitors.

If that doesn't work, inject a 455 KHz signal at the 2nd converter, and try removing toobs one at a time from the front end backward and see if that kills the oscillation.  You can thereby determine whether the oscillation is dependent on the front end and variable IF and focus your attention accordingly.

[W3RSW]

Several of your findings are a result of Collin's Service Bulletins, e.g. the grounding of pin 7, V5  1/22/54.
http://www.collinsradio.org/archives/manuals/75A-2_3_SB_1.pdf
http://www.collinsradio.org/archives/manuals/75A-2_SB_3B.pdf

Many of the A2 mods are actually for the A3 too.
As mentioned, later series used a diff. RF amp tube, etc.

Lots of good info on the Collins collectors assc. site.

I agree with JN's comments, particularly the tub caps and bad grounds.

[K9AAL]

Update on the AGC:  Loosened and tghtened all the ground  contacts I could find including the IF cans--no change

Bridged across C94 and C97 with a 10 uF cap--no change.

Injected 455 KC signal into pin 7 of V4 and could not reproduce the 40 cycle hum.  However, injecting a 3mV signal with 400cps modulation resulted in a 400 cps sine wave on the AGC at about 50 mVp-p.  At .1mV signal modulated with 400cps, the AGC was "clean" without 40 cps hum.  Increasing to .3mV signal modulated with 400 cps produced the hum on the AGC line.  Finally, by decreasing the RF gain to about 7, the hum disappeared from the AGC line. 

So there are two symptoms.  First is the 40 cps hum with loud signals injected into the antenna input.   Second is the  hum on the AGC line that is the same as the audio modulation.

I did one other "experiment" and compared the AGC to the audio output when on an AM station, using two channels on the scope.  The two signals looked very similar!  This was true with the RF gain all the way up and I believe it disappeared at about RF gain of  7.  I also noticed that wiggling V-8 around caused the S-meter measured gain to jump up and down about 10 to 15 dB.  I had cleaned the tube pins and socket with deoxit, so I am beginning to wonder if the socket is defective causing my problem. 

This is a quite a learning experience! 

[KM1H]

DeOxit itself may be the problem unless all residue has been flushed away immediately after use.

I use a hand wire brush on the tube pins and then several socket insertions until its stable.

When I have to use DeOxit on anything it is with a very tiny drop on a toothpick and then flush; that has worked well on switches, moving catacomb coil racks, etc.....just got thru with a NC-240D for a customer

Carl
KM1H