Advice about repairing an IBM 5151

Vlad Stamate vlad.stamate at
Tue Sep 1 13:27:52 CDT 2015

On Sep 1, 2015, at 1:57 PM, tony duell <ard at> wrote:

>> Hmm, you are right, I am not seeing any spikes in voltage there. On
>> the collector of the TR23 all I am getting as DC voltage is around
>> 110mV. The oscilloscope tells me I am getting Vpp of about 4.4V with
>> Vmax at 2.6V and Vmin at -1.8V and an average of 105mV (which matches
>> what my multimeter tells me too).
>> That sounds too low doesn't it?
> That is far too low. 
> Firstly, there should be a 12V or so feed to one pin on the flyback primary
> winding (the winding that links to the collector of TR23). Is that present and 
> correct?

Yes it is. I am getting 15V on pin 4 that connects to the collector of TR23.
> What about other components connected to that winding.
There is a diode and a capacitor. They both measure 300mV or so. The diode seems to be working. I will desolder one leg of the capacitor and check it too. I do not have an ESR meter so I will do normal checking.

> Are you sure TR23 is good. The horizontal output transistor in many monitors
> often fails.
I am not sure. I assume I have to desolder it to check it right?

> -tony

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