Re: {Collins} 75S-3 S-meter Issue



Steve & Glen:

THANKS to you and others who offered suggestions

I seemed to have rushed into my CCA posting, without thinking this through.  It started with a bad R13, the S-meter zero adjust pot.  Zero would jump from close to zero, then up to S-8 or so.  Found a suitable replacement, brand new and installed it.  Zero adjust is now very stable.

Then I noticed how high the S-meter display was on most signals, and as already mentioned, a full scale needle position when the crystal calibrator was turned on.

I checked the R57 setting, it was 100% clockwise.  With the crystal calibrator on, I backed it counter-clockwise in steps, but S-meter remained at full scale deflection, all the way to full counter-clockwise.

I do have a good HP-5687B RF Signal generator, so I will first see if I can spray some DeOxit into the R57 pot, work it around then try the signal injection and see what happens.  Not sure what type of pot R57 is, will it have an opening to actually get any cleaner inside?  Will check pot continuity first too.

Also plan on checking the AGC, another potential issue.

Chuck W9KR




> On Aug 19, 2019, at 1:04 PM, Steve Berman via Collins <collins@xxxxxxxxxxxxx> wrote:
> 
> Chuck,
> 
> As Glen suggested, check the receive gain pot.  It may be dirty or defective.  The rec gain pot will interact with the S-meter zero adjust.  You'll need to inject a -70dbm (50uv) signal at
> 14.1mhz and set the rec gain to S-9.  After that go back and reset the S-meter zero.  Basically what you're doing is balancing out the bridge circuit.  You may have to make this adjustment a few times until you get S-9 with -70dbm and have the S-meter perfectly zeroed.
> 
> Steve N6HK
> ________________________________
> From: Collins <collins-bounces+n6hk=hotmail.com@xxxxxxxxxxxxx> on behalf of Don Rasmussen via Collins <collins@xxxxxxxxxxxxx>
> Sent: Monday, August 19, 2019 8:28:27 AM
> To: Glen Zook <k9sth@xxxxxxxxxxxxx>
> Cc: Collins <collins@xxxxxxxxxxxxx>
> Subject: Re: {Collins} 75S-3 S-meter Issue
> 
> IS YOUR AGC WORKiNG ??
> 
> On Mon, Aug 19, 2019 at 11:23 AM Glen Zook via Collins <
> collins@xxxxxxxxxxxxx> wrote:
> 
>> Check R-57 the RF gain adjust potentiometer.  It is a 1K in series to
>> ground from the cathode of V-6.
>> Glen, K9STH
>> website:  https://k9sth.net
>> 
>>    On Sunday, August 18, 2019, 07:09:14 PM CDT, Chuck Curran via Collins <
>> collins@xxxxxxxxxxxxx> wrote:
>> 
>> Hello the Group:
>> 
>> I received a S-Line for repair, all items not having been on the air for 20
>> years.  I started to repair the 75S-3 receiver for the owner, and was
>> pleased with the success, after replacing electrolytic's and the Sangamo
>> coupling caps.  PTO was messed up, as far as end stops and pointer being
>> vertical, but that was resolved too.  Alignment followed, with great
>> success, all ceramic trimmers were NOT frozen, a real surprise!
>> 
>> OK, now on to the issue.  S-meter was jumping all over, the zero pot was
>> junk.  Found a suitable replacement and installed it after lunch today.
>> That totally cured the jumping problem, when trying to set zero.  However,
>> I now see that the S-meter response is way high.  The calibrator signal
>> PINS the S-meter, without a problem.  I checked pin voltages on V6 and V7,
>> both were within spec.
>> 
>> All on the air signals I have listened to are also WAY high, crazy high
>> S-meter levels.
>> 
>> All tubes were checked on my Hickok 752, three weak tubes were replaced,
>> V3, V8, and V11.  Supply voltages are to spec.
>> 
>> So, any suggestions on where to go next?
>> 
>> Thank You in advance,
>> 
>> Chuck W9KR
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