08-09-2008, 09:01 PM
That's a strange phenomenon, and I've run into it a few times. I use a Hickok 209(?) VTVM to read AGC voltage. I should be more suspicious about it, but I'm always in a hurry.
When these sets were new, there were no VTVM's, so they had you use an output meter on the speaker as an indicator. Tune for strongest output, and there you were. Doing that, you never had any notion of what the AGC was doing. But tou weren't putting any load on the AGC bus that way, either. The audio level wasn't a true indication of the detector's DC output, so that method isn't very good. The VTVM on the AGC has it all over the output meter.
Funny tale about one of my sets. It's an Airline (Wells-Gardner) set, about 1936 vintage. Very nice radio, as all of those W-G jobs were. Re-capped and aligned, for some reason, I accidentally shorted the AGC bus to ground. There was a small spark!
On the Hammarlund HQ 120, as I recaLL, THE S-meter is pretty sensitive, and reads AGC voltage directly off the detector. You don't see THAT, very often.
Think about it. If a set has an eye tube or a tuning meter, you have a built-in resonance meter. Just tune to max meter swing or minimum eye closure. You don't need a VTVM at all.
I once re-capped a Scott Imperial Allwave. It has 2 or 3 AGC systems in the RF-IF channel of the set. It also has possibly 3 I.F. stages. I did the alignment, and the set was so hot, you couldn't hold it down. A bit of checking, and the AGC tube for the I.F. strip was totally dead; just drawing filament current. A new one got the set going just fine.
Buit, reading the AGC bus is the best way to read your peaks, bec ause you're reading the true output of the detector. I seem to favor reading it at the AGC feed to the RF or mixer grid, and everything works just fine.
Please excuse my use of "AGC" in lieu of "AVC". It applies to radio and video either way. It saves brain wear and tear.
When these sets were new, there were no VTVM's, so they had you use an output meter on the speaker as an indicator. Tune for strongest output, and there you were. Doing that, you never had any notion of what the AGC was doing. But tou weren't putting any load on the AGC bus that way, either. The audio level wasn't a true indication of the detector's DC output, so that method isn't very good. The VTVM on the AGC has it all over the output meter.
Funny tale about one of my sets. It's an Airline (Wells-Gardner) set, about 1936 vintage. Very nice radio, as all of those W-G jobs were. Re-capped and aligned, for some reason, I accidentally shorted the AGC bus to ground. There was a small spark!
On the Hammarlund HQ 120, as I recaLL, THE S-meter is pretty sensitive, and reads AGC voltage directly off the detector. You don't see THAT, very often.
Think about it. If a set has an eye tube or a tuning meter, you have a built-in resonance meter. Just tune to max meter swing or minimum eye closure. You don't need a VTVM at all.
I once re-capped a Scott Imperial Allwave. It has 2 or 3 AGC systems in the RF-IF channel of the set. It also has possibly 3 I.F. stages. I did the alignment, and the set was so hot, you couldn't hold it down. A bit of checking, and the AGC tube for the I.F. strip was totally dead; just drawing filament current. A new one got the set going just fine.
Buit, reading the AGC bus is the best way to read your peaks, bec ause you're reading the true output of the detector. I seem to favor reading it at the AGC feed to the RF or mixer grid, and everything works just fine.
Please excuse my use of "AGC" in lieu of "AVC". It applies to radio and video either way. It saves brain wear and tear.
