[ARC5] BC-348-Q Capacitor Testing
Richard Knoppow
1oldlens1 at ix.netcom.com
Fri Jan 29 19:50:20 EST 2016
Polypropylene is probably easier to find these days than Mylar (AKA
polyester) caps. These types have very low leakage, very low ESR or
dissipation factor, relatively low inductance, and very low dielectric
absorption. The latter is of little importance in most circuits despite
the audiophool rant on it but comes with the other virtues. Life of
plastic film caps is very long. Low leakage is vital for coupling caps
because any parallel resistance will couple DC from one stage to the
next, typically a plate to a grid. These caps are cheap. Generally its
a good idea to get caps a of a little higher voltage rating than the
originals although paper caps were often over-rated to increase life
expectancy. The caps in metal sleeves are usually more moisture
resistant than those in paper or plastic cases and age more slowly but
we are looking at caps that are seventy years old or more, much beyond
their expected operating life. Electrolytics are more reliable than
generally thought but old ones are a gamble. Bad electrolytics do not
always cause high hum (although that is a frequent symptom) but can also
cause all sorts of puzzling parasitic responses in receivers. Best to
replace them.
Bathtub caps can be either paper or electrolytic, the markings will
tell you which. Bathtub paper caps are usually oil-filled (as in the
RCA AR-88). These had very long lives but many of them develop
degradation of the seals around the terminals allowing the oil to leak
out, they should be replaced. Its a lot of work to restuff these guys
but some people like to preserve the appearance of the originals. You
do NOT need to re-fill the oil filled ones.
Bathtub electrolytic caps, and some paper caps, are potted in the
case, that is, the case is filled with a mixture of resin and wax. The
purpose was the same as oil filling, to protect the capacitor windings
from moisture and oxidation. They are a PITA to restuff as are the
oil-filled kind but it can be done. Mostly I have replaced both kinds
with modern caps mounted on terminal strips or just point to point. May
not look as sexy but works fine. I am more concerned with performance
than looks although I like to keep things as original as possible.
Bad caps can cause failure of other components which may not be so
easy to find (like transformers) as well as affecting the performance.
Now, OTOH old mica and ceramic caps rarely go bad although they can.
They need not be changed as a routine. The most troublesom are silvered
mica caps because the silver plating tends to corrode and cause the
formation of silver whiskers. These short or partially short the cap,
the latter causing instability. Stacked micas rarely go bad and neither
to ceramics. The thin walled hollow dog bones in this rig are probably
temperature compensating ceramics. Dog bone resistors are nearly always
solid.
On 1/29/2016 3:45 PM, Sandy wrote:
> 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:
>>>
>>>
>>> http://www.k3msb.com/temp/bc348q_underside.jpg
>>>
>>>
>>> From previous discussions about the BC-348-Q it seems that these
>>> types of caps do not always need to be replaced. Instead of
>>> unsoldering each one, I think this can be done by considering the
>>> entire circuit as composed of several individual circuits separated
>>> by the various switch positions as shown below:
>>>
>>>
>>> http://www.k3msb.com/temp/bc348q_caps.jpg
>>>
>>>
>>> My testing will conducted with the Dynamotor and tubes removed and
>>> the AVC-OFF-MVC Switch in the OFF position.
>>>
>>> I plan on using my IT-28 Cap tester in "Leakage" mode as it provides
>>> current limiting.I’ll probably use a series millameter as well.
>>>
>>> So, here’s my proposed tests:
>>>
>>> Test 1 - Inject voltage into relay pin 6 to test caps on the Blue
>>> circuit.
>>>
>>> Test 2 - Inject voltage into relay pin 2 to test caps on the Red
>>> circuit.
>>>
>>> Test 3 - Inject voltage into non grounded end of 61-7 or 61-6 to
>>> test those two caps in the AVC circuit (Yellow).
>>>
>>> Test 4 - Caps 61-9 (near 28V input) tested individually
>>>
>>> Test 5 - Caps 63-2 (Audio Bypass) tested individually
>>>
>>> Test 6 - Cap 61-8 (Audio Filter, Relay Pin 1), needs to be
>>> unsoldered to test.
>>>
>>> Test 7 - Cap 61-1 (RF Module), needs to be unsoldered to test.
>>>
>>>
>>> I want to make sure I won’t inadvertently damage anything…….
>>>
>>>
>>> Comments welcome.
>>>
>>>
>>> 73 Mark K3MSB
>>>
>>>
>>>
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>>
>> --
>> Richard Knoppow
>> 1oldlens1 at ix.netcom.com
>> WB6KBL
>>
>>
>> ______________________________________________________________
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>
--
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL
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