[R-390] RE (R390/A) AGC, etc.,

[hamfish at comcast.net]

Haven't posted for a long time, here goes. In a general reply to Gary
Biasini, you have several posts and I'm trying to find the start of your
issues with a R390/A? Are these posts for the same receiver? Please list the
restoration efforts at this point to help this long distance R390/A service
dept. 

 

First thoughts are, these beasts are Ferrai autos, not VW's. Better tools
and parts are needed. Under the list of tools: Good working VOM, Signal
Generator, Tube Extenders, Tube Tester, soldering iron/station.....comes to
mind. Oh yes...Y2K R3!

 

1st: Remove all decks from the chassis, replace paper caps. These old caps
will haunt you until they are replaced. I can send you a list if needed!
While the decks are removed, check connectors, resistors, switches, etc. On
the RF deck are many RF transformers. They plug into the deck with pins, use
Deoxit on the pins. Want more punishment? The trimmers caps have been
twisted for decades and metal has transferred to the other half of the
capacitor. A real small clip on the bottom holds the two sections together.
Slide to one side, remove top half, clean with a Q-Tip and a very small drop
of Deoxit. It works, trust me. Same with the 2nd IF section, (going by
memory????). Audio deck, replace the cap that rots, check resistors, etc.
Clean switches in crystal deck. Power supply usually is in better shape, but
look at everything. The IF deck, replace paper caps (all of them), C551 &
C553. The only replacement cap I've found for C551 is made by NTE and is a
mylar cap. 

 

I forgot the impedance adapter recommended to be used for alignment, get
one. It makes a difference! Going through the Y2K R3, please note the
mechanical alignment sections is FIRST. If the mechanical alignment is
wrong, the electronic alignment will be wrong. At this point, having crossed
all T's & dotted all I's, the R390/A should be somewhat working. Now the
hard part, those nasty little bugs. What isn't quite right? Checking
resistance and voltages at each tube might turn up some clues. Using a
sig/gen to inject a signal, following said signal with tube extenders and an
o'scope is one method. This is fun once you have a R390/A that is 100%!

 

You mentioned a possible AGC issue. Long ago I chased an AGC issue and
replaced all those ceramic capacitors, about 2 dozen. Ouch! By memory, I'm
older than dirt, they all are 5000pF +- 15% 1000VDC all along the AGC line.
AGC issues and replacing mechanical filter are a pain in the backside, been
there.

 

Regards,

Craig