[Hammarlund] AVC problems on SP-600 JX-6

[James A. (Andy) Moorer]

Before you go blaming the bathtubs, you should check some voltages. I 
personally have done maybe 25 SP-600s and haven't found any bad bathtub caps 
yet. Other people have, I guess.

The RF gain pot should give a voltage on the middle pin between about -51 
(right off C160, the last bathtub cap) and -1. If it does, then the bathtub 
caps are not the problem - check R97. If you are not getting -51V on C160, 
then check the 10K resistors that go from one to the other. Sometimes they 
are open or burned. Have you tried replacing V20, the 6AL5 rectifier for 
the -51V supply? Plus there there are four resistors R86-R89 that you should 
check as well. If a bathtub cap is shorted, you should be able to locate it 
as the one without a big negative voltage. Note that R97, the 3.3 meg 
resistor around the MAN/AVC switch can be open as well, or could have a very 
high value. I routinely replace ALL resistors 500K or higher since they are 
almost always more than 10% higher than the rated value.

Oh yes - do check the MAN/AVC switch itself. Sometimes they go open as well.

Just my humble opinion . . .

James A. (Andy) Moorer
www.jamminpower.com

----- Original Message ----- 


I finally finished recapping the JX-6 (whoow!) although it now needs an 
alignment. Believe it or not I found the under chassis caps to be the 
hardest ones to access and replace.
Before I started and all the way through the AVC was marginal, on strong 
stations the meter wouldn't go down to 0 and the volume would not shut off 
completely. Last night I left it on for a while and came back to find the S 
meter pinned and it now has absolutely no AVC at all, it works normally with 
the AVC switch off and riding the RF gain but if I switch the AVC back on it 
immediately pins the meter and overloads the receiver to the point of no 
volume if there is any signal at all.
Someone had mentioned the bathtub caps as the likely culprit, would it be 
prudent to change them all or search out the one(s) that are causing the 
problem? Also any other ideas as to what could be causing this problem or is 
it a case of almost definitely the bath tub caps?

Bob Young
Analog, MA
KB1OKL