[ARC5] BC-348-Q Capacitor Questions

[arc5 at ix.netcom.com]

----- Original Message ----- 
From: "Mark K3MSB" <mark.k3msb at gmail.com>
Subject: [Milsurplus] BC-348-Q Capacitor Questions


> ...My 348 is stock ( and unbutchered ) 
> and it has the heavy anti-fungal sprayed on coating.
> 
> Does the sealing property of the (MFP) spray have 
> any effect on component longevity...? 
> Should I suspect the tubular caps,  or are they like
> mica caps that rarely go bad with age?

I can't answer on the sealing properties of MFP.
I'd doubt it, since it fractures on most finishes I've seen.

I gave up "shotgunning" capacitors years ago, because 
I was doing damage to replace what were, in many cases,
serviceable caps.  I recommend isolating the B+ busses
and doing some tests to determine what actually needs
to be replaced.

I just pulled-out my 348 J,N & Q manual and had a look
at the diagrams.  Here's a procedure I would respectfully 
suggest to isolate problem caps in an unhacked 348Q.
It will not check every cap, but will concentrate 
on those caps most likely to fail.
If your 348 is hacked, you can follow along on the original 
schematic and you'll get the idea.

You'll need:
An external B+ source; 100-200 volts at a few mills.
If you don't have that, you can pull all the tubes
and use the internal supply.
A three or four good insulated clip leads.
A 1-or-2 k-Ohm, 1 or 2 watt "test" resistor.
A good voltmeter.
Very careful fingers- or ya gonna git bit. ;-)

CW Osc. ON
Xtal Filter OUT
Remove any "TR muting" short you have between
pins 2 and 6 on the back connector.
Disconnect both B+ and B- at the dyno 
or other power supply.

Disconnect one end of these resistors:
R 87-4, the 1 M-Ohm at the base of the 2nd IF tube.
R 87-3, the 1 M-Ohm on the terminal strip 
             next to the fuse bracket.
R 94-2, the 80 k-Ohm on the terminal strip
             on the side wall near the headphone jacks.
(This is a good time to test these hi-Ohm resistors)

Connect  B- to ground.  Use the clipleads to connect 
B+ through the test resistor to pin #2 on the back
power connector.  If you measure any voltage
across the resistor, you have capacitor leakage somewhere.
Ohm's Law will tell you how much.  If you're using a 1K 
test resistor and you measure 1 volt,  you've got 1 milliamp
of leakage.  What level of leakage you should worry about 
is a matter of many opinions.  Minor leakage in bypasses 
is not, IMHO, a big deal, but what's "minor?"
 If you're leaking 1 mA at 250 volts B+, 
I wouldn't worry about it.  
If you're leaking 8-10 mils, I would.  
Or if you're leaking 1-2 mA at 100 volts test B+, 
it will be much worse at the working voltage of 250 V.
I hope some people smarter than me will give their opinions
on what level is "acceptable leakage" vs. 
the time, trouble and damage of replacing the guilty caps.
If you have significant leakage current to pin #2,
here are caps that will need a closer look:
24-2    60    70-A    61-5    
There are grid caps that can cause the problem, 
but we'll cover them later.

Next, use the clipleads to connect 
B+ through the test resistor to pin #6 on the back
power connector.  If you measure any voltage
across the resistor, you have capacitor leakage.
Caps to check:
61-2    61-3    61-10

To check for grid cap leakage:
Connect pins 2 & 6 on the rear connector together
and apply B+ at this point (B- still to ground).
Clip the 1 kOhm test resistor from the Grid -1 
contact on each tube socket to ground.
If you measure voltage across this resistor,
the grid cap is leaking and has gotta go.
The caps:
1st RF    47-1
2nd RF   61-1
1st Det/Osc.  26
2nd IF    34

There are other "hi-threat" caps that will need to be disconnected
and check individually:
All of them in the dyno base plus-
70-B     61-6    61-7    61-9    63-1  63-2

As I said- this won't check every cap, but it will 
check the ones most at risk for failure.
The point is to find out what's actually bad, rather 
than assume and end up with a big mess.
The idea can be applied to any BA.
Hope this is helpful

73 Dave AB5S