[Milsurplus] BC-342/BC-344 Question
Robert Kilian
robertkilian at ostechnologies.net
Wed Nov 6 11:00:22 EST 2019
Joe --
Yes. And those are the wax/paper units (black, rectangular, angled
corners) so must be replaced for good measure (once you've gone to the
trouble to removing those cans, you might as well, if not just to avoid
having to repeat the process later). There are other Micamold branded
capacitors in the set that are a mica-based (not wax/paper), those are
usually red & square and problem-free for the most part.
The manual for -342 (312, 314, 344) lists all of the capacitors, their
composition, values, and locations which is very handy when overhauling
one of these radios.
Rob Kilian
K6RBK
On 11/6/19 7:56 AM, Joe Connor via Milsurplus wrote:
> Thanks, Miles. One question: isn't there one Micamold carrying B+ in
> each of the under-the-chassis RF boxes? Those concern me.
>
> Joe Connor
>
> On Wednesday, November 6, 2019, 11:40:34 AM GMT-4, k2cby at optonline.net
> <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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