[R-390] IF Deck Problems Revisited

[n4buq at knology.net]

As reported earlier, when I increase the IF gain past a certain point, the 
diode load voltage spikes to about 90 volts and the radio goes silent.  As 
Roy suggested, I checked the IF output and, sure enough, the thing was 
going into self oscillation.  The frequency counter indicated it was about 
410kc.

I pulled the IF deck and checked for loose grounds and bad solder joints 
and didn't find anything that I would suspect as a problem.  I put the deck 
back in place and the problem persisted.  I began pulling tubes trying to 
isolate the problem.  With only three tubes left in the deck and still 
seeing the problem, I figured I had it pretty much narrowed down and would 
pull the deck again and look closer in those areas.

All this time, I was running without the IF can covers in place.  I don't 
have holes in the tops of the cans and had been tinkering with the 
adjustments with them off.  I put the can covers back on and it appears 
that the problem has mostly disappeared.

I suppose a combination of the IF cans being pretty far out of adjustment 
and the can covers removed was allowing the signals in the coils to 
interact with other components and provide a feedback route.  Not sure 
about all that but it does seem to have cured the problem.

I think once I get the holes in the cans, get them properly secured, and do 
the complete IF alignment, this "problem" will no longer exist.

Thanks for all the input.  While my "problem" was probably more just 
stupidity than anything else, it did help me figure out what was going on.

Regards,

Barry - N4BUQ