Analogue monitor board repair

tony duell ard at
Mon Oct 31 16:29:04 CDT 2016

> Before I replaced the failed potentiometer (new one seen top left) the
> display looked like its horizonal hold had gone so I reasoned that's what

OK... The TDA1180 is the horizontal oscillator, etc, IC. It's well-known.

Start by getting its data sheet. Indentify the horizontal oscillator components
connected to pins 12, 13, 14, 15 of that IC. Typically you will find a pot
that controls the DC voltage on pin 15 (slider of pot to pin 15 through a 
resistor). That sets the free-running horizontal frequency

Now try adjusting it. If you can get the oscillator to run both too fast
and too slow (lines sloping both ways) that that part is most likely fine
and the fault is in the sync circuit. If not, then the oscillator components
have problems.

Are you getting a sync pulse at pin 8 of the IC? If not, trace back from
there to the connector to the logic board and if necessary to the video

Are you getting a flyback pulse at pin 6? The horizontal control circuit is
basically a phase-locked loop comparing the incoming sync pulses with 
flyback signal from the horizontal output stage.


More information about the cctalk mailing list