[R-390] Working on the R-392

[Larry H]

Dave, Nice work.  Thanks for letting us know what and how you found it.
It's good to hear the blow by blow story.  Sadly, I have been relegated to
fix the noisy bath exhaust fan.  I'd rather by R-390(x)ing.

As Charles said, we are seeing more and more silver micas bite the dust.

Regards, Larry


On Wed, Jan 15, 2020 at 11:17 PM David Olean <k1whs at metrocast.net> wrote:

> 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
>
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