[Drake] PS-7 Problem

[ltaft at charter.net]

Many times a 2N5301 shorts, raises the output to 20 volts +/- so the crowbar SCR fires and blows the fuse.  The finals in the TR-7 do not like anything above 15 volts.  

I fixed one PS-7 that had several problems including 3 of the 2N5301s shorted, the SCR was open and there was a very large fuse in the holder.  When you plugged this baby in it put out over 20 volts.  

After replacing most of the semiconductors in the thing the SCR would fire at turnon.  Turns out the circuit board ground was to the middle compartment plate that is rivited in the case.  It was NOT ground as I discovered by making aeveral voltage drop measurements.  Must have been the anodizing was an insulator.  Fixed the problem by running an extra ground wire from the board socket pin to the big ground bolt on the rear panel.

AFAIK all PS-7s use the same circuit.  The early ones had the control circuit board hard wired in.  The later version used a molex pin socket for the board.

73, Larry  K2LT
drakerepair.com
800-687-9161

> From: AB3L1 at aol.com
> Date: 2005/02/02 Wed AM 01:07:49 GMT
> To: Drake at mailman.qth.net
> Subject: [Drake] PS-7 Problem
> 
> Let's try that again.
>  
>  
>  
>  
> A few years ago I had a problem with my PS-7 and it sounds the same. Bear  
> with me as it has been a while. There is a protection circuit that will drive  
> the output to ground. At least that is what mine did. I seem to remember a 
> heavy  amperage triac maybe?? When this thing clamped to ground it took out two of 
> the  2N5301 power transistors.
>  
> Mine was a very early model and the later units might have had it removed.  I 
> couldn't figure the reasoning behind it except to protect the TR-7 of course. 
>  But then there is that fuse there also. A lot cheaper than the  transistors!
>  
> Hope it is leading you in the right direction Kevin. 
>  
> Bob  AB3L
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