[Collins] 75S-3B receiver restore

[Dr. Gerald N. Johnson]

Check the bias rectifier and bias circuit electrolytics. Low bias 
voltage will upset S (not SWR!) meter readings and fry the audio output 
tube and sometimes the audio output transformer. Some receivers used a 
selenium rectifier that is way past its useful life now.

Download or otherwise get the manual, there is a procedure for setting 
the electrical zero of the S-meter. It dependent on having good tubes 
and the right tubes in the 455 kHz IF stages and the setting of theIF 
gain pot. The S-meter circuit uses the DC voltages and currents of the 
IF stages to make a hugh impedance DC voltmeter to display the AVC 
voltage which is based on the detected signal level. Sinc in this 
receiver RF ain control is by injecting DC bias into the AVC circuit, 
the S meter moves upscale as the RF gain is reduced.

73, Jerry, K0CQ, technical adviser to the Collins Radio Association

On 11/19/2013 1:28 PM, Dick KF4NS wrote:
> Need a little help here on this RCVR that I was coerced into getting up and running again for a guy who
> is interested in selling it for an old non-ham friend.
>
> To make it short, I will just give the basics. It had been stored in unprotected area for about 10 years.
> What a mess! I managed to get everything cleaned up including all contacts throughout and the chassis.
> Not an easy job. It did not work when I first started on it except that the tubes lit up. All tubes were
> tested and 2 replaced. 90% of the trouble was the long resting period and corroded contacts.
>
> Right now, after alot of tweaking, the receiver is working darn good. One problem observed is the SWR
> meter which never reads UNDER 4. It appears to be giving a not too bad sensitivity reading on signals
> detected. It peaks on minimum rf gain.
>
> How do I adjust the low limit on the meter? It works fine mechanically with power off, going to zero..
>
> Any advice will be greatly appreciated.
>
> 73, Dick KF4NS
> St Petersburg, FL 33714 USA
> Keep The Glow!
>
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