Advice about repairing an IBM 5151

tony duell ard at
Mon Aug 31 01:26:35 CDT 2015

> Hi Tony,
> I have checked the horizontal signal from the MDA using an
> oscilloscope. It is pin 8 like you mentioned. I get a regular 18KHz or
> so signal out of that. Pin 9 gives the vertical signal at roughly
> 50Hz.

OK. Your PC and MDA card seem to be OK.

Now trace through the horizontail circuit in the 5151. IIRC there
is a driver transistor, the collector load of which is a transformer
which couples to the base of the horizontal output transistor
(sorry, I am doing this from memory, I don't have the techref
in front of me). Do you get the 18kHz on the collector of
the driver transistor? 

Are both transistors good? 


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