AGC resistor question

[electra225]

As a general rule, will a 9.3 megohm 1-watt AGC resistor that has drifted in value to 120K ohms cause a no audio/no video condition in a TV set? There is a raster with a full screen. Vertical and horizontal locking unknown.

[TC Chris]

Sounds like it might as well be zero ohms if it was spec'd at 9.3 meg. Why not plug any old higher value just to see what happens?

Off in storage I have the ca. 1952 Crosley TV that I had in grad school (i.e., two centuries ago). I was trying to improve it and pulled the AGC tube and there was no effect on the image. I was not in a position to investigate further so I plugged the tube back in and watched as-was.

I watched the moon landing on that TV (1969).

Chris Campbell

[electra225]

I need to be fairly positive of the outcome before I dig back into this TV chassis. Not only will it be a pain to pull then put back, but I don't want to have to do it more than once. I shouldn't have put it back in, but I was moving and had no other option. I know AGC difficulty will cause all kinds of maladies in a TV chassis. Weak picture, overdriven picture,buzz and, I think, no picture and no sound. I also need to order a good schematic from Steve the schematic man. Mine is a copy and not all that good.

[hermitcrab]

I know the AGC control on my old set would kill all reception at one end of the control ... maybe zero end?

[electra225]

This chassis does not have an AGC control, nor does it have an AGC stage, or at least a stage that is called an AGC stage. In fairness, there are control pots on the chassis that I haven't identified, so there is that. Thanks, Elton, I thought I'd had the same experience as you did. I don't want to have to pull the tuner. They literally built the instrument around the tuner. This is concerning my Magnavox Stereo Theater, just in case you were wondering.

[Motorola minion]

AGC is a feedback loop from the output (negative DC) of the video detector to the RF and IF tube bias voltage. Making this voltage on the IF tube grids even MORE negative results in total cutoff of these stages, white screen and no sound.

Ground the common return for the IF transformers (where other end of winding connected to input grids), which would otherwise be grounded if there was no AGC, to see if sound and picture returned. I will check my C36 schematic and advise on other suspected parts.

Keyed AGC uses a tube that is fed a negative-going horizontal sync pulse that cuts off the tube and allows the IF and RF amps to increase gain during blanking to better stabilize the sync pulse. I was a big deal for locations depending on distant reception, allowing you to watch a snowy, washed out picture that doesn't roll or go into horizontal lines due to a "noisy" sync pulse.
,
No surprise that a high value resistor has gone wonky in 60 years. Check the others...

[electra225]

Thanks, Dave. The other resistors checked good three years ago when I redid the chassis, but I didn't have the replacement capacitor yet for R43, the AGC resistor. I will check everything again when I have the chassis out. I have test sockets that I can use to measure voltages and inject a signal if necessary with the chassis in the set and operational. The IF tubes have permanent shields around them, hence the need for extension test sockets.