[Collins] 75S3 PROBLEM

[james.liles at comcast.net]

Gerald,

I should have asked, is there a sub for the 12BA6 to mitigate hum?

Regards Jim

-----Original Message----- 
From: Dr. Gerald N. Johnson
Sent: Sunday, February 07, 2016 7:25 PM
To: collins at mailman.qth.net
Subject: Re: [Collins] 75S3 PROBLEM

Heater leaking to cathode. Later receivers and transmitters used a 7543
which my RCA tube manual says is the same as a 6AU6A but with "a
controlled hum characteristic."

The cathode is floating in that oscillator and is where the frequency
shift for sideband change happens.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 2/7/2016 7:12 PM, Ed - K9EW wrote:
> Problem solved.
>
> The culprit was a bad 6AU6 in the VFO.  I want to put the tube on an
> analyzer to see what's shorted to what, but the solution turned out to be
> as simple as installing a new tube.  Didn't have to touch C59.
>
> Thanks to those who responded.  Your combined replies guided me to the
> solution.
>
> 73,
> ed - k9ew
>
> On Wed, Feb 3, 2016 at 3:06 PM, Ed - K9EW<k9ew57 at gmail.com>  wrote:
>
>> I have a 75S3 with a raspy-sounding audio tone when receiving CW.  It
>> looks like it must be from the 4.4Vpp of ripple on the B+ line, so that
>> means replacing the triple-electrolytic C59.  (I've already replaced the
>> other electrolytics in the hope that one of them may be the culprit, but
>> the raspy CW note persists.)  It looks like a tedious job to replace it, 
>> so
>> I'm wondering if someone knows of an easier way to do this than removing
>> several parts just to get to the terminals of C59.
>>
>> Do you replace C59 with three discrete electrolytics?  Where do you put
>> them / how do you mount them?
>>
>> Or can you rebuild C59?
>>
>> Or, is there a direct replacement for C59?
>>
>> Thanks in advance for any suggestions.
>>
>> 73,
>> ed - k9ew
>>
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