[Collins] No Grid Drive

[Bob McGraw - K4TAX]

Doc Jerry is right.   Just about every tube and circuit is involved with the 
grid drive issue.  As a start, if you have one or can find one, a good 
transconductance tube tester is needed and simply test every tube in the 
transmitter.  That's a good start.  Then replace those that are known bad or 
show low emission.  Likely, everything will come back to life.

If not then a circuit by circuit, stage by stage trouble shooting method 
should be employed.  Voltage readings and RF reading using a scope is in 
order.  The manual gives voltage and resistance readings.  Yes, those 
trimmer caps have been known to fail.  Rebuild kits or replacements are 
available from various sources such as Surplus Sales of Nebraska for one. 
Switches do get dirty and should be cleaned.  You own solution of "witches 
brew" contact cleaner i.e. what ever you want to use is OK.  Some cleaners 
have a problem with plastics and some, I would caution, that more is not 
better.

You want to clean the switches, not drown them.  Then there's the relays. 
They too can be cleaned.  My "witches brew" for relay contacts is a strip of 
index card dipped in Carbon Tet and pulled between the manually closed 
contacts. Of course I doubt that one can buy Carbon Tet these days although 
I still have a small amount left over from years ago.

Good luck.  These are great sounding transmitters.  I know, I have one on 
the desk.

73
Bob, K4TAX


----- Original Message ----- 
From: "Dr Gerald N Johnson" <geraldj at storm.weather.net>
To: <collins at mailman.qth.net>
Sent: Tuesday, December 11, 2007 7:36 PM
Subject: Re: [Collins] No Grid Drive


> On Tue, 2007-12-11 at 16:53 -0600, J. Dave Mayfield wrote:
>> Guys my 32S1 has no grid drive, any ideas where I should start?
>>
>
> Check in tune or key lock position. Check for plate current and power
> output. PA plate current is a much more sensitive indication of drive
> than grid current.
>
> Virtually EVERY stage is involved and loss of gain in any (most often a
> tube or dirty switch contact) kills grid drive.
>
> Band switch contacts are known to not make contact well or long if not
> exercised. That can be true of the grid current or ALC meter switch too.
> Rocking the band switch back and forth a couple bands above and below
> the band of interest often brings back drive and power output.
>
> The ALC detector is a tube too and if it stops working the ALC meter
> won't indicate.
>
> Besides tube and switch failures, the ceramic trimmers do give troubles
> with going open.
>
> -- 
> 73, Jerry, K0CQ, Technical Advisor to the CRA
> All content copyright Dr. Gerald N. Johnson, electrical engineer
>
> ______________________________________________________________
> Collins mailing list
> Home: http://mailman.qth.net/mailman/listinfo/collins
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:Collins at mailman.qth.net
>
>