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Old 02-01-2010, 06:49 PM
DaveWM DaveWM is offline
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Join Date: Feb 2009
Location: Orlando FL
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I use a scope to align mine you need to peak the 19khz pilot and the scope it the best way to do it. Also look around for any big .1 or 1uf caps in the path, I have had troubles there. I used a cheap rat shack MPX transmittier, a audio sine wave gen and a two channel o scope to look at L and R. you can feed one channel at a time from the sine wave generator to really see what is going on, its kinda fun after a while. There is a sep pot that balances the amount of L+R that it mixed with the L-R that is pretty critical, best not to mess with it.

One thing to really watch out for is the decoupling caps in the receiver that feeds the B+. I had a real tough dog that turned out to be an open filter (decoupler) can cap. No hum at all, so I just assumed it was ok, well it also decoupled the B+ so any audio signal riding along the plate circuit is bypass to ground. With out it you get a modulated B+ that can wreak havoc on the MPX (think L+R everywhere even in the phase inverter of the MPX).

So if you are having MPX issues and have NOT recapped every larger value cap (typically canned caps for that era), then try jumping a cap in and see if that has any effect. On mine the effect was a stronger R vs L and poor separation, no matter what I did with the MPX, and also a volume control that would not silence the audio from the radio (again the signal not being decoupled, riding on the B+ and going around the preamp tube).


all that being said prob best idea is to not adj anything unless your sure its been messed with, the caps are the most likely suspect.
My RCA's had some odd ball deemphasis on the MPX as well resulting in a more muffled sound, my guess is it was an attempt to quell the MPX "noise" that I guess was a problem, esp on a weaker station. I made a slight mode to the deemphasis caps and got a much crisper sound, if there is a weak FM station just go to mono anyway.

Last edited by DaveWM; 02-01-2010 at 06:58 PM.
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