Advice about repairing an IBM 5151

Vlad Stamate vlad.stamate at
Tue Sep 1 08:57:34 CDT 2015

On 1 September 2015 at 00:54, tony duell <ard at> wrote:
>> Hi,
>> 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
> No, that is a different 18kHz signal. That is the video, which will
> have a significant component at the horizontal frequency. It
> starts from pins 6 (intensity) and 7 (video) of the DE9 connector

Oh, I think I understand now Video is different than horizontal board.

> You need to trace on from pin 8 to transistors on the main board,
> right up to the flyback transformer.
I did. The 18KHz signal is present when it enters the board from the
wire, it is also present on the collector of the following 2
transistors (the TR21 and TR22 which are part of the brightness
limiter and horizontal driver respectively), also at the output of the
T501 transformer and at the collector of the TR23 transistor (which is
labeled as Horizontal out). I then traced at every pin until the
flyback transformer input (pin 4). According to the schematic and the
board it seems that is the signal path from the moment it enters the
logic board until it reaches the flyback transformer.

However the flyback transformer is definitely not outputting the
voltages it is supposed to, as the manual tells me to measure 56.8V at
the cathode of diode D504 which is basically connected to the output
pin 6 of the flyback transformer and I only measure few mV.

Could a winding of the FBT be burned somehow?

> -tony

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