[ARC5] BC-348-Q Capacitor Testing

[WA5CAB at cs.com]

As "they" say, different stroke...  Some will consider this sacriIige but 
generally I agree with Mark and only partially agree with Sandy.  The one 
practical rule to follow is to use testing parameters current when the 
capacitors were new.  There aren't any electrolytic capacitors in a BC-224 or 348 
but if there were, 100% of them would have failed 21st century spec tests the 
day that they were made.  And the paper capacitors may have.  Excepting any 
capacitor that, if it shorted, could take out a hard to find and replace 
coil or filter, and possibly any that are across tuned circuits, any capacitor 
that does no more than barely wiggle the needle of a Simpson 260 on the 
Rx10,000 scale is good enough to leave alone..  My personal experience in 
numerous WW-II vintage sets worked on over the past 55 years is the following.  
The odds against all of the capacitors in one set being bad are high but not 
infinite.  I ran across a BC-342 once in which every one of the capacitors 
that I tested was bad (leakage resistance even at low voltage  below 50K).  
But say there was a radio with 5 different type or value capacitors, and 10 of 
each type.  I test by type.  If the first 3 of a type are bad, I stop 
testing and start replacing.  if they aren't, I test all 10.  Exception - with 
multiple capacitors in the same can, if one is bad count all as bad. 
Generally, you will almost always find one here or there that needs replacing.  But 
sometimes, you don't find any.

Tip for testing filament bypass capacitors - Remove the tubes and 
disconnect from the power source.  Check the schematic for any shunt resistors and 
disconnect if found.  Test the entire filament circuit.as one.  If the entire 
circuit passes the 1 capacitor leakage criteria, they are all good.  

My preferred replacement type is metallized polyester instead of mylar.

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480

In a message dated 01/29/2016 17:46:11 PM Central Standard Time, 
ebjr37 at charter.net writes: 
> T^he biggest bitch about a recap job in the BC-348 is removing the 
> bandswitch modules!  If it were me, be  sure you replace ALL the paper capacitors 
> and ALL the "bathtub" capacitors! Mylar caps are a very good choice and 
> not expensive.  Will make an extreme difference in the receiver if it is 
> still even functioning.
> 
> 73,
> 
> Sandy W5TVW
> 
> On 1/29/2016 5:36 PM, Richard Knoppow wrote:
> 
> >>    To properly test a capacitor one end must be disconnected.  If you 
>> can manage that without unsoldering fine. I have not looked over the 
>> schematic you attached.  For the most part old paper caps will have some 
>> leakage, often a lot. Depending on the application leakage, which is parallel 
>> resistance, may be more important than series resistance.  Modern plastic 
>> film caps have vanishingly small leakage and generally better performance 
>> than paper caps.  if you want to actually _use_  the receiver I think you 
>> will be ahead by just replacing these caps.  I understand wanting to keep the 
>> thing authentic but if its more than a shelf queen you may have to 
>> tolerate some change. At least under the chassis.  Of course, this is just my 
>> opinion.  
>> 
>> On 1/29/2016 2:51 PM, Mark K3MSB wrote:
>> 
>> >>> I would like to keeps the inside of my BC-348-Q as stock as possible, 
>>> so I don’t want to replace caps if I don’t need to.
>>> 
>>>  
>>> 
>>> 
>>> The tubular bypass and coupling caps are of the aluminum cylinder 
>>> variety, as shown below:
>>> 
>>> 
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