"MoiInAust" wrote in message
...
"Richard Knoppow" wrote
BTW, I downloaded a bunch of stuff from the VMARS site,
lots of R-390 and other books there and a military
handbook on the older Super-Pro models.
Good site, isn't it! Now Richard, stop salivating over
the R-390 stuff and solve my problem with the AR88!
I redrew the circuit. The AVC circuit is fairly
conventional. The AVC voltage comes from the detector and
the second half of the 6H6 is used to generate a "delay"
voltage to prevent the AVC from acting until the signal
strength reaches a certain minimum level. This is done to
maintain the RF and IF stages at full gain. There is a
negative bias supply obtained from a series of resistors in
the center-tap return of the rectifier. The RF gain voltage
comes from the RF gain control which is in this string.
About the only way I can see for a positive voltage to
appear on the AVC-RF gain bus is if this whole string is
being pulled positive. If that's so it should also show up
on the audio tubes which are biased from this source. Unless
you made a gross error in wiring I would be suspicious of
C-96 and C97, the first two filter caps since they are
returned to the top of the negative supply rather than to
chassis ground. C-98, the last cap, goes to chassis ground
so can't cause trouble. If one or both of the the filter
caps is leaky it could be pulling the negative supply to a
positive value.
I wouldn't worry about gassy tubes. Positive voltage
can come from one of the controlled tubes becoming gassy but
they are pretty well isolated in this circuit. I can't
account for the slight AVC action with the AVC switch in
manual but if the negative supply has been pulled positive
it might affect this.
The AVC switch essentially shorts out the AVC and puts
the controlled stages directly on the negative supply.
However, the whole AVC bus floats on the negative supply so
that the RF gain control works in both AVC and Manual.
I certainly agree with those who suggest going over
your work carefully. If you know another knowlegible person
have them look also since its possible for someone to
overlook the same thing over and over where someone else
will spot it right away. But, check the filter caps first.
--
Richard Knoppow
Los Angeles
WB6KBL