[Milsurplus] [ARC5] BC-348-Q Capacitor Testing

[Kenneth G. Gordon]

On 29 Jan 2016 at 17:51, 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.

OK. But why? Is this a museum piece? If not, and if you intend to USE it, 
then doing so is far more trouble than it is worth, in my opinion.
 
> The tubular bypass and coupling caps are of the aluminum cylinder variety, as
> shown below:
> 
> http://www.k3msb.com/temp/bc348q_underside.jpg

Hmmm....I must admit to not being particularly impressed...

> 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

Sure. But even so, after examining your circuit diagram for only a few 
minutes, I discovered at least 6 capacitors, mostly bypasses, which would 
have to have the ungrounded end unsoldered in order to get an accurate 
leakage reading.

For instance, the "hot" end of one cap goes through the dynamotor to 
ground, another goes through the AF output transformer's winding to ground.

> My testing will conducted with the Dynamotor and tubes removed and the
> AVC-OFF-MVC Switch in the OFF position.

Other than certain internal connections (like from suppressor grid to cathode) 
tubes when turned off are completely inert to capacitors. I wouldn't bother 
removing the tubes.

Removing the dynamotor would clear ONE capacitor.

> 
> 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.

My comment is that if you intend to USE that receiver, and if after properly 
testing the caps, you find 30% of them leaking, replace all the paper and 
electrolytic caps with new ones. The papers with metalized film (usually 
polypropelene).

If you really want to go to all the trouble to restuff the caps, by all means go 
ahead. But I, most certainly, would not leave any 70+ year old paper or 
electrolytic caps in that receiver. The micas (if you can be certain they ARE 
micas) can usually be left alone. Trouble is, some of the flat-cased caps 
which LOOK like micas are, in fact, paper.

Ken W7EKB