Advice about repairing an IBM 5151

Vlad Stamate vlad.stamate at
Mon Aug 31 17:18:39 CDT 2015


I traced the 18Khz signal all the way from the moment it enters the
logic board until the video board (attached to the neck of the CRT).
The signal is there (it has different shapes but it is 18KHz). The
output of both transistors is 18KHz and so is the output of the
transformer. I traced it across resistors and capacitors as well. The
voltage (Vpp) does change as expected but the signal is present. Pin 6
and 7 have the 18KHz signal too on the CRT connector. However they
only measure 7 and 10V respectively.


On 31 August 2015 at 02:26, tony duell <ard at> wrote:
>> 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?
> -tony

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