[ARC5] BC-348-Q Capacitor Questions
Mike Feher
n4fs at eozinc.com
Mon Aug 23 06:57:26 EDT 2010
I just love my 428Bs. That is why I have about 6 of them. Sometimes you get
them with broken meters or probes, or no probes. I manages to get 6 of them
together, most from ebay. One of the handiest pieces of test equipment
around, even if by today's standards it is rather bulky. 73 - Mike
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960
-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of Dennis Monticelli
Sent: Monday, August 23, 2010 1:36 AM
To: Discussion of AN/ARC-5 military radio equipment.
Cc: milsurplus at mailman.qth.net
Subject: Re: [ARC5] BC-348-Q Capacitor Questions
I find that it is easier and less invasive to use an old HP 428 clip-on
current meter to check caps. You just power the gear the way you normally
would and use the clip-on probe to check the caps in question. Occasionally
there isn't enough room for the probe to encircle the wire and so one just
improvises. On its most sensitive range you can read 1mA FS on the large
meter. This means you can check paper caps as well as electrolytics. The
428 also has a BNC output that drives an oscilloscope, which allows you
to view AC waveforms up to about 400Hz. This is extremely handy in checking
and troubleshooting power supplies. You can see the capacitor ripple
current, the full current waveform of the inductor, and the current through
the rectifiers. I got my 428 on eBay.
Dennis AE6C
On Sun, Aug 22, 2010 at 1:41 PM, <arc5 at ix.netcom.com> wrote:
>
> ----- 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
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
More information about the ARC5
mailing list