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 Post subject: Philco 49-1240 AGC Problem
PostPosted: Feb Sat 23, 2008 10:36 pm 
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I'm new to antique tv repair, and just recapped my first one. I replaced all of the electrolytics, and paper caps, but left the micas alone. The set works ,but an old friend who used to be a tv repairman during the 40's-50's came by to help me adjust it up. He said that the agc needs adjustment,[ has very strong blacks, with a buzz present in the sound] and we started to look for said adjustment, but couldn't find one. The schematic doesn't show one off of the agc tube, but there's a resistor in the cathode of the agc that appears to be where you'd expect to find one. What would anyone suggest to correct this problem?
My friend suggested replacing this resisitor R39 with a pot to allow adjustment. What do you think?

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thanks,
Bruce


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PostPosted: Feb Sat 23, 2008 11:13 pm 
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That will not solve your problem. You need to find the orig. problem. Could be bad micas, or, off value resistors.
Perhaps even a bad tube....
Bill Cahill

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PostPosted: Feb Sun 24, 2008 12:13 am 
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Are you running the set off an outdoor antenna, as it was designed to be, or are you using cable or another signal source? Many times those provide excessive signal strength and an attenuator at the antenna input will cure that problem.

In this set, the contrast control affects the "AGC" action depending on the position of the AVC switch M4. So the contrast control is actually the AGC control, but only when switched into that circuit.

However, this is a very simple direct feedback AVC circuit in the left hand position of switch M4, not like the keyed AGC found in more advanced designs. In the other position, apparently a negative voltage source not seen in the partial schematic is applied to the top of the contrast control, and through its wiper to to the AVC line.

You will likely need to use an attenuator as mentioned above or otherwise reduce the signal strength at the antenna input, it doesn't look like much else can be done with this circuit design.

I'd like to read the original Philco service manual for this set and any supplements to it, just to see if they mentioned this problem and any possible modifications to correct it.

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PostPosted: Feb Sun 24, 2008 3:36 am 
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Location: Monroe,NY
Dennis,
I think you're onto something. I used a cable signal with a 300-75ohm transformer. I'll try attenuating the signal with a pot where it hooks up to the tv and see what happens.
Thanks,
Bruce


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