[Milsurplus] BC-342/BC-344 Question

Mon Nov 11 10:39:04 EST 2019

At this point, the first attempt at production will take place via 
turning on an old Atlas lathe.

Rob Kilian
K6RBK

On 11/9/19 4:29 AM, John Hutchins wrote:
>
> Robert -
>
> "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."
>
> I have a few BC348 sets that use the same trimmers - How will you 
> procure/produce these replacement collars ?
>
> I my have to do the same for my sets.
>
> Thanks
>
>
>
> On 11/6/2019 10:09 AM, Robert Kilian wrote:
>> 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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>>>
>>
>>
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