31 Oct
2016
31 Oct
'16
2:29 p.m.
On 31/10/2016 22:29, "tony duell" <ard at p850ug1.demon.co.uk> wrote:
That's the pot I replaced, it goes through a 22ohm resistor.
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.
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 :)
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?