[R-390] Working on the R-392

[David Olean]

I finally figured out that my severe distortion was only affecting one 
band. Any strong station in the BC band between 1 and 2 MHz was 
distorted. It was interesting that stations between 500 and 1000 kHz 
were fine.  I had originally thought that my AGC circuitry must be bad. 
I traced the problem to some noise coming from the tuned coils after the 
first RF stage. I used a dual trace scope to ferret out the noise and 
used the R-392 RF stage test points for connecting the scope probes. The 
correct one with noise was Z-208. I took the tuning rack apart and 
removed the RF coil. Inside were a couple of silver mica postage stamp 
caps and the coil. I tested each cap with a Fluke DVM. each cap showed 
infinite resistance. I put some DC voltage across the 2000 pf postage 
stamp and saw almost all of the DC voltage show up on the other end. It 
was shorting out.  At first, I thought I hooked it up wrong. I couldn't 
believe my eyes. It was leaking so badly, it was almost a complete short 
at 50 volts. I then tried low voltage across it, and saw the cap worked 
fine up to about 8 volts DC. Then a little DC started to leak through. 
It was only when I ramped things up to 12 or 14 volts and then it broke 
down and allowed almost all the DC voltage to pass through! What was 
strange was that I could blow on the capacitor and the leaking voltage 
would drop to almost zero! This was shorting out when strong BC stations 
were going through the RF stage. No wonder I was having problems.  I 
only found it due to the static noise it was making occasionally as it 
was sitting there in the circuit.

So all the Sprague Vitamin-Q caps were fine, and it was a silver mica 
postage stamp type capacitor  inside an RF transformer that was giving 
me grief!


73

Dave K1WHS