Analogue monitor board repair

Adrian Graham witchy at
Mon Oct 31 16:29:39 CDT 2016

On 31/10/2016 22:29, "tony duell" <ard at> wrote:

>> 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

That's the pot I replaced, it goes through a 22ohm resistor.

> 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.

Yep, lines sloping both ways is exactly what happens. Perhaps bizarrely this
also seems to affect the contrast.

> 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
> IC.

Pin 8 goes right back to a 74LS04 up near the RAM/ROM section of the
motherboard. I'm going to see if it's possible to assemble the whole thing
outside of the big plastic housing and still hold the screen as securely as
possible, it's a complete pain to get the screen in its swivel top and
connect up everything without risk of breakage.

> 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.

Watch this space :)

Binary Dinosaurs creator/curator - the UK's biggest private home computer

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