Re: {Collins} 75A-4 loud static when noise limiter is turned off



I had a loud popping sound in my KWM-2A, which turned out to be carbon tracking on the end of a power supply filter capacitor. It would pop about once a second. The supply voltage would drop, then recover.

This can happen just about anywhere that high voltage exists. It might be possible to track it down by turning out the lights and looking for the spark -- I've never tried this with a radio, but it works with car ignition systems.

73
Bob

Tony Sokol wrote:
Hi Gang,

I have had "static and popping sounds" in my 75A-4 audio that would come and go (with the antenna shorted) twice now. Both times it turned out to be a noisy 6AQ5 audio tube. I replaced the tube and the problem was totally gone. Might be worth a try.

      73
Tony - W9JXN



George Babits wrote:
Yea, I get that too.  Been doing it for 25 years!

George
W7HDL


----- Original Message ----- From: "David Mcdonald" <jdmcd@xxxxxx>
To: <kkeller1@xxxxxxxxxxx>; <wb4jtt@xxxxxxxxx>; <collins@xxxxxxxxxxxxx>;
<cemilton@xxxxxxx>
Sent: Tuesday, January 13, 2009 3:08 PM
Subject: Re: {Collins} 75A-4 loud static when noise limiter is turned off


I get a popping speaker noise (pop - pop - pop etc,) when my 75A4, S/N 3777
heats up but it quits when warmed up, generally a minute.  Noise limiter
OFF.  Don't know what causes that but it goes away so why worry?

Thanks,

Dave
K4EWC
----- Original Message ----- From: <cemilton@xxxxxxx>
To: <kkeller1@xxxxxxxxxxx>; <wb4jtt@xxxxxxxxx>; <collins@xxxxxxxxxxxxx>
Sent: Tuesday, January 13, 2009 3:14 PM
Subject: Re: {Collins} 75A-4 loud static when noise limiter is turned off


 In addition to checking the micas as Kurt mentioned, it's possible a
carbon comp resistor is going high (or open) as it heats.? I've heard them cause "sizzle" and "static".? They appear to be o.k. from visual checks, but often a good light and magnifier will show a crack.? Sometimes shooting a cold spray on them temporarily stops the "sizzle".? Voltage checks on the
stage and resistance checks (before and after warmup) are helpful too.
73
Chuck
W4MIL







-----Original Message-----
From: Kurt Keller <kkeller1@xxxxxxxxxxx>
To: wb4jtt@xxxxxxxxx; collins@xxxxxxxxxxxxx
Sent: Tue, 13 Jan 2009 2:51 pm
Subject: Re: {Collins} 75A-4 loud static when noise limiter is turned off









I no longer have a schematic of this unit, but as I remember there
are two troublesome mica caps that go from?

the cathodes of the 6AL5 in the limiter circuit to ground. I had a
similar problem on a unit years ago and they were the cause.?
?

?


At 01:58 PM 1/13/2009, David Johnson wrote:?

Hello Everyone,?
      ?
My 75A-4 (s.n. 4187) sounds fine for about ten minutes after initial
turn on.  Then a very loud random static is heard (unaffected by the
presence of the antenna) which can be cured by turning on the noise
limiter, which causes the noise to cease even at the lowest (#1)
position.  The noise is not simply reduced but goes away completely,
leading me to beleive that the noise is not being "limited" but
rather is being eliminated because the ground has been lifted from
the bottom of the 220K cathode resistor R65 which is present when
the noise limiter switch is off.?
      ?
       Since the problem is warm-up/heat related, I thought the noise
limiter tube V12 (6AL5) was bad, but it tested OK.  I replaced it
with several NOS 6AL5 tubes that I have and the identical symptoms
remained.?
?
So then I assumed that the bypass capacitors must be at fault (I
still had the original "black beauties" present) and I replaced the
0.01 uf capacitors C97, C98 and C146 (I used orange drops because
that is what I had on hand).  But the exact same symptom remains.?
      ?
This kind of internally generated static is an unfamiliar type of problem for me. Can anyone suggest the likely villain here? Thanks very
much.?
?
       -- Dave WB4JTT?
      ?
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