[Milsurplus] BC-342/BC-344 Question

[David Olean]

I would agree on the Micamold paper caps. I checked a bunch of them in 
my Bendix MN-26 and RA-10 receivers. All showed very high leakage. I did 
not try turning anything on, but replaced every capacitor before doing 
anything else.

Dave K1WHS

On 11/6/2019 3:39 PM, k2cby at optonline.net wrote:
>
> A number of years ago I did a complete overhaul of a BC-348R (similar 
> to the BC-342) and wrote up the results.
> I had no problems with either the “bathtub” capacitors or the actual 
> mica capacitors.
>
> Here are my observations regarding the “Micamold” (actually foil and 
> paper, having no relation whatever to mica) bypass capacitors:
>
> Eighteen .01 μF 500 Volt “postage stamp” paper capacitors are 
> scattered throughout the set and used primarily in plate, screen and 
> cathode bypass applications.
>
> Experience has shown that these have a nasty habit of deteriorating, 
> generally taking out any associated voltage dropping resistor. With 
> this in mind, a random sample of five was pulled out for testing.
>
> Subjected to an ohmmeter check with a Simpson 260, one showed a 
> resistance of 650k. The remaining four all tested above 3 Meg and one 
> went to infinity. While hardly spectacular, these results
>
> might be marginally acceptable in a non-critical application.
>
> An electronic capacitance meter showed values ranging from 21 nF to 75 
> nF (.02 to .075 μF), bearing little relation to the specified value of 
> 10 nF.
>
> An EICO Model 950B R-C bridge, on the other hand, showed a far 
> different result. None of the five tested would produce a null of the 
> bridge circuit. Four of the five showed unacceptable leakage
>
> above 50 volts, and the remaining one bowed out at 80 volts.
>
> The final test was to apply 250 Volts dc and measure the resulting 
> current. The result was nothing short of alarming. Two capacitors 
> showed a bit over 200 μA and a third 420 μA. The other two,
>
> however, produced current that steadily rose, reaching a peak of 40 
> milliamperes, when the test was terminated. (The entire BC-348 is 
> supposed to draw only 40 to 50 mA when operating.)
>
> /The only reasonable conclusion is to replace each and every one of them./
>
> The good news, if there be any, is that none of the offending 
> capacitors is located inside the Antenna, RF Amplifier, Converter or 
> HF Oscillator compartments, so these do not have to be removed.
>
> For the most part, they are on the back sides of circuit boards 271 
> through 274 and the two small circuit boards either side of the dial 
> lamp rheostat. It is easiest to clip the lead as it passes over the 
> top edge
>
> of the board and then physically yank the capacitor off its other lead 
> with a pair of pliers, being sure the remaining pigtail doesn’t short 
> to anything else. The capacitor can be replaced with a much smaller
> .01 μF 500-Volt ceramic capacitor wired to the front terminals.
>
> Miles, K2CBY
>
> Miles B. Anderson, K2CBY
> 16 Round Pond Lane
> Sag Harbor, New York 11963-3821
> Phone: (631) 725-4400
> FAX: (631) 725-2223
>
> e-mail: k2cby at optimum.net
>
>
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