[MRCA] Working on the R-392

[J Mcvey]

 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

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