A3000 Scan Doubler problem.

[Castellen]

Are you in fact turning the pot correctly?
It can be tricky to get at if you don't have the right shaped screwdriver.

Don't forget it's a 10 or 20 turn pot, so try the video throughout the complete adjustment range of the pot.

Obviously you need to have the rear switch in the "enabled" position for the scandoubler to work.

[Castellen]

You shouldn't really worry about using a non-metallic screwdriver as the brass screw of the pot should be electrically isolated from the circuit.
There's no high frequency RF circuits in that region which will be effected by a metallic tuning tool either.

They probably suppled a plastic tool becuase they're cheaper, and to minimise electrostatic discharge risks inside the computer when you go poking a metal screwdriver in there.


@Crusher

No idea what you mean by "coalface".  Maybe carbon??

The modern multi turn pots are usually made of stuff called cermet which is kind of like conductive ceramic.

As for tuning it, normally you find a "peak" in the tuning range.

So you keep turning it one way until you start to see the image getting better.  Now keep turning while counting the number of turns until it starts to degrade, then go back by approx half the number of turns you counted to put you roughly in the center of the tuning range.

If the picture remains crappy, make sure the monitor cable is securely plugged in.

[Castellen]

Sorry, I forgot to mention that it is possible the pot is damaged.

If so just replace it.  Any decent electronics supplier will have them.

Any generic 1k ohm, 25 turn horizontal mount pot should be fine.
Take the original part with you to get the physical dimensions and footprint.

[Castellen]

@ Noster:

Thanks for the info.
Haven't personally had that problem with any of the A3000s I've worked on.

As a radio communications engineer by trade, I usually have plenty of ceramic tuning tools about, but I have used metallic tools before on the A3000 and haven't noticed anything obvious.

Looks like the video circuit in that section is quite sensitive after all.  It's possible that the stray internal capacitance between the pot's isolated screw and the rest of the circuit is enough to cause problems in your case.


In which case a ceramic or plastic tool would be a far better option for other A3000 users as well.