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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_______________________________________________?
?


SHARE THE FUN: JOIN THE COLLINS COLLECTORS ASSOCIATION TODAY !?

?http://www.collinsradio.org/membership/?

-paypal accepted-?
?

 NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific?

?SUNDAYS: 14.263 MHz-2000 UTC  +  AM Net: 29.050 MHz-1200 Central?

?1st Wednesday AM Net 3.880 MHz-2000 local (ET, CT, MT, PT)?
?



 







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