[R-390] Working on the R-392

[David Olean]

Hi Larry

Well, the project isn't quite over.  I have noticed that the AGC sags as 
the set warms up. A -25 dBm BC signal will drive the AGC up to about 
-2.7 volts, but after an hour or so, it only rises to about 1.7 volts. I 
checked two receivers, and both are doing the same thing.  It must be a 
leaky cap. I need to buy some freeze mist to find it. If the AGC sags, I 
start to hear a bit of distortion on strong signals.

Dave K1WHS

On 1/17/2020 2:03 AM, Larry H wrote:
> 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 
> <mailto: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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