[Milsurplus] BC-342/BC-344 Question

[k2cby at optonline.net]

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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