[Milsurplus] BC-342/BC-344 Question

Wed Nov 6 11:09:05 EST 2019

Another capacitor-related area to be careful with is re-alignment. The 
trimmer caps on the back of the RF/IF cans are held together by a 
press-fit collar on the shaft.  These collars are often cracked, and the 
slightest movement of the shaft will allow the spring tension (intended 
to hold the rotor & stator apart) to overcome the tension of broken 
collar, causing the rotor and stator to mesh, and shorting B+ to ground.

I intend to produce a run of new collars for these caps, as repair 
efforts I've tried on the collars to date just haven't been very successful.

On 11/6/19 8:00 AM, Robert Kilian wrote:
> 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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