[Test-Equipment] AN/PRM-10 Question Answered!

[Richard Knoppow]

     I found the cause of the dead spots was the failure of 
a brush type contact to touch the drive gear of the tuning 
capacitor rotor. The capacitor is a dual unit with a 
floating rotor, similar to a butterfly type but with the two 
sections side by side. The rotor is supposed to go to ground 
through a 470k ohm resistor through a brush on a spring 
which is supposed to contact the gear that drives the rotor. 
It was not touching. Pushing it down caused it to contact 
the frame of the capacitor shorting it. I found after some 
experimentation that I could bend the end of the spring to 
which the contact is fastened in such a way that it made 
contact with the rotor but not with the frame. The spring 
has a small, thick, pin fastened to the end which is the 
actual brush. It contacts the rotor gear through a hole in 
the side of the frame. I think it is marginally short. In 
any case the only way to be sure would be to see another 
part. I had thought of enlarging the hole but thought I 
would try some body english on it first. It must have worked 
originally and I don't like butchering stuff. Anyway, I got 
it to work. This cures the dead spots and the meter now puts 
out over its entire range. The oscillator tube, a 955, tests 
good but a new one might bring up the output on the highest 
band a bit.
     Frequency calibration is surprizingly good. I think the 
spec is 1.5% and its quite a bit better than that. I also 
got the internal oscillator going again although I am not 
sure what fixed it. I did resolder some contacts and lifted 
and resoldered a couple of resistors to test them, perhaps 
that's what did it.
     This is a very nice GDO, well made, and has all the 
featured you would want in such an instrument. The only 
shortcoming is that no one ever paid any attention to where 
the cables should go when its stored. They are just wound 
around and crammed in.
     I was able to find an original instruction book on ebay 
but not an original service manual. There is one, however, 
at the BAMA reflector. The illustrations in the operator's 
handbook are pretty awful, perhaps its a late reprint.
     I want to thank all on this list who were so helpful in 
dealing with this very frustrating problem and have posted 
what I found on the chance that it may be helpful to others.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com