[R-390] Lightning crashes in audio

[David Olean]

I had answered an e mail from Jacques yesterday, about silver mica caps, 
but a WIN 10 hiccup removed all evidence of it along with his e-mail.   
I have found at least two defective sm capacitors that were both testing 
fine in two different capacitance meters that both measured 
dissipation.  I had a 3rd meter that measured ESR and it checked OK on 
that one too. In each case the problem was thunderstorm static with no 
antenna attached.

Cap #1 was in the IF circuit of an R-392 receiver. The cap was located 
across the  2nd IF transformer winding, so it only had RF voltage across 
it. I definitely isolated the static noise to that area with an 
oscilloscope. When I removed the cap and replaced it, the noise was 
gone, but the only way I could find the trouble was to use my 0 to 450 
Volt DC variable power supply and put some DC voltage on one side of the 
capacitor, and a high impedance voltmeter (Fluke DVM) on the other side 
of the cap. If I kept DC voltage below 10 volts, there was no real dc 
leakage visible. The DC levels would drop down to a few millivolts.  But 
as soon as I increased the voltage above 12, I started to see 
significant leakage. The DC level started to soar across that 
capacitor.    A second silver mica cap was causing similar noise in an 
R-390A receiver. It happened on only one of the four rf bands in the 
R-390A RF section.  I think it was the 1-2 MHz band.  Again, the 
capacitor was across an RF coil, and it was causing static noise. It was 
also failing at a low voltage.

After seeing this, I am more appreciative of my brute force DC voltage 
testing with a high impedance volt meter. I trust it more than any low 
voltage testers. To be fair, my GENRAD RLC meter has a provision to 
apply up to 30 volts across any capacitors I may want to test, but I 
need a four lead test fixture to access that function.  My DC power 
supply is also great for testing electrolytic capacitors. I can see when 
the leakage current starts to rise.  I can also charge the cap up and do 
a relative test on how long it takes to discharge. The new electrolytics 
are so much better than old ones from even ten years ago.

73

Dave K1WHS