[Collins] 75S3 PROBLEM

[Dr. Gerald N. Johnson]

I didn't find any subs. I'd think the 12BA6 would have better than 
average heater to cathode leakage because it was used a lot in AC/DC 
radios with series string heaters so it could have up to 120 volts AC to 
the cathode.

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

On 2/7/2016 8:26 PM, james.liles at comcast.net wrote:
> 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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