[Collins] HQ-160

[Dr.Gerald Johnson]

Somewhere in that high IF there's a connection with a hair line crack that 
has oxidized. Enough signal voltage will break down the oxidation 
temporarily. It could be a weld in a tube, (I've had that problem in 
molded integrated circuits), it could be a solidly wrapped connection that 
didn't get soldered well, it could be dirty switch contacts, could be dirty 
tube pins and socket. I've seen such a problem with IF transformers having 
built in fixed mica capacitors.

First apply a quarter drop or less of DeoxiT to the switch contacts. Then 
search visually for a solder lug or tube socket pin with several leads and all 
of the not enclosed in smooth solder. Apply Ersin multicore solder and heat 
to fix that. Clean the high IF tube(s) socket pins.

Then you may have to trace the signal. Don't blow it through with lots of 
signal from the generator, drive it gently so you don't burn through the 
oxide and either by signal injection find where the signal doesn't get 
through from or by signal tracing with the signal coming through the front 
end see where the signal is lost. This gets tedious because sometimes 
touching the circuit with a probe is enough to clear up this type of 
problem. Then if you can identify the part of the stage where the signal is 
lost you can check DC voltages to see if its a DC line or bad bypass.

Clean and DeoxiT first (switch and tube(s)) then look for the bad solder 
connection, then start with the hunting for the lost signal.

Sometimes alternate heat (hot air gun or hair dryer) and cold (freeze mist) 
will show up a temperature sensitive connection or part. But I don't think 
this one is temperature sensitive.

When you drive it hard with the generator, is it possible that you are 
peaking the high IF on the wrong side of the second mixer input?

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.