[ARC5] BC-348-Q Capacitor Questions
Henry Mei'l's
meils at get2net.dk
Mon Aug 23 02:08:58 EDT 2010
Hello Dennis
How does this work?I don't have an HP 428 but is there some way I can rig up
something using standard VTVMs/VOMs &/or scopes to do the same thing?
Henry, Cph.
----- Original Message -----
From: "Dennis Monticelli" <dennis.monticelli at gmail.com>
To: "Discussion of AN/ARC-5 military radio equipment."
<arc5 at mailman.qth.net>
Cc: <milsurplus at mailman.qth.net>
Sent: Monday, August 23, 2010 7:35 AM
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
>>
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