[R-390] [MRCA] Working on the R-392

[David Olean]

I live in Maine and heat build up is not a great problem here. The joke 
here is that our summer runs from July 1 thru July 6. I have run my 
R-392s in their case and temps seem OK.  If I lived in SC or Florida, I 
might come to a different conclusion.  I actually had a MIL 26A7 SS 
replacement that I inherited from my army days. I sold it on EBay before 
I ended up with three R-392 receivers. Two of the R-392s had small 
diodes in the 12AU7 tube.sockets. I left them there except that I fitted 
gold plated pins from a DB-25 plug to each diode lead for a 100% 
positive fit to the tube socket. Maybe I will replace the audio amp at 
some point. You are sure right that it does get warm.

I saved the little 2000 pf mica cap for posterity.  It was a good 
learning experience for me. Looking at the circuit, it was in the 1st RF 
output stage. It had 28 volts DC on it, but the capacitor was floating 
and not referenced to ground. The only voltage across the capacitor was 
that developed by RF signals and the plate swing of the 1st RF tube.  
I'll bet there are plenty more of these silver mica caps getting ready 
to quit.

73

Dave K1WHS


On 1/16/2020 2:51 PM, J Mcvey wrote:
> The problem components always seem to hide in the most inaccessible 
> places, don't they?
>
> Did you solid state the 26a7 yet?
> The R-392 runs a lot cooler without the 15 watt 26A7 heater !
>
> I replaced the 12au7s with 1n4148 diodes also. Works fine. only one of 
> the 12au7 sections is used as a triode and that is the IF output 
> buffer for the front panel.
> I replaced that section with a fet.
> It was relatively easy to solid state the detector, audio section and 
> oscillators, but the IF and RF sections are a lot more challenging, so 
> I have let them alone for now.
>
> On Thursday, January 16, 2020, 2:17:36 AM EST, 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
>
> ______________________________________________________________
> MRCA mailing list
> Home: http://mailman.qth.net/mailman/listinfo/mrca
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:MRCA at mailman.qth.net <mailto:MRCA at mailman.qth.net>
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html