Tempest screen jitters - related to audio

westal_sage

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My Tempest has a problem with screen jitters. I was hoping that a new pot kit (the 8 pots) would resolve the problem, but it didn't. I have since noticed some things that I think could be some clues where to start:

1) The screen is super-stable until the game has been on for about 10 mins.
2) When the jitters start, they are closely related to the audio. Like if I just sit still and am getting only the audio of the enemies firing, the screen is pretty stable. But when a louder audio event happens, it gets way worse - it really freaks out during the audio of the "warp to the next level", for example.

Has anyone seen this before? Any ideas on what to start rebuilding? Thanks!
 
My Tempest has a problem with screen jitters. I was hoping that a new pot kit (the 8 pots) would resolve the problem, but it didn't. I have since noticed some things that I think could be some clues where to start:

1) The screen is super-stable until the game has been on for about 10 mins.
2) When the jitters start, they are closely related to the audio. Like if I just sit still and am getting only the audio of the enemies firing, the screen is pretty stable. But when a louder audio event happens, it gets way worse - it really freaks out during the audio of the "warp to the next level", for example.

Has anyone seen this before? Any ideas on what to start rebuilding? Thanks!

The first thing I would try is to lower the volume and see if that helps, sometimes with the volume way up it will do strange things.

You want to check your power supply too. I suspect you have a low voltage somewhere and the audio amp is pulling more current causing something else to suffer.
 
go to the monitor section,there is a post by smalltownguy2 that has a pdf on vectors.it talks alot about tempest/jitters.
 
Make sure the chassis mounted transistors are screwed down securely.

I had a couple loose ones once that would cause jittery vectors around the edges of the screen. Opened up the back of the machine and could see a little arc coinciding with the jitter.
 
Well..I tried turning down the volume, and sure enough it cleared up the screen jitters. So it seemed I was on to something, and to reinforce that I put the volume back about where it was and the jitters came back.

So then I thought....OK I'm gonna crank up the volume really loud and see if it makes the problem worse. Well, it didn't - the jitters weren't there at max volume. Put the volume back to original, and the jitters were still gone.

So I think maybe just a dirty volume pot has something to do with it. Seems a little weird to me how that would cause these pretty massive monitor image jitters. I still see a little bit of it happening here n there, but not near as bad as it was. I'll replace the pot and maybe that will be the end of it.
 
I did some convergence adjustments and really have this monitor looking fantastic. But those pesky screen jitters tied to the audio are still there. Very intermittent - I thought I'd fixed it but they still come back pretty often.

I replaced the volume pot - really didn't think that was the problem but worth a shot.

Most of my voltages look good. A little off on the 12 and the -5 (like 11.9 and -4.9). That's a question I have - the one trimpot on the AR board - does that adjust ALL of the voltages together, or only select ones, or what?

Brick Top - I did ensure those chassis xsistors are secure.

Tals7 - I couldn't find that particular thread by smalltownguy2 - do you happen to have a link to it?
 
Check the speaker wiring and if that's not it then replace the audio amp IC. It's probably bad and drawing so much current that it's causing other voltage sources to drop intermittently.

You can see that kind of garbage on the power supply outputs easily if you have an oscilloscope.
 
I have seen this where the smoothing caps on the 12V line are physically damaged/missing. These filter noise from the 12V line and without them the amp can oscillate, it will still amplify but it will run VERY VERY hot. While it does this it causes interferance to any nearby CRT, mainly shimmer but the intensity changes with the audio.

The filter caps are usually a large (physically) electrolytic somewhere between 1000uF and 3300uF and a single poly cap in the nanofarad range in parallel between the 12V line and ground. The big electro is often the larges cap on the board, it sticks up a long way and is often bashed clean off the board, or it has a leg ripped out which slides back inside the cap when it is straightenned out. The cap is then ruined but it looks ok unless you wiggle it and see one leg moves freely.

If the amplifier chip and heatsink is not burn-your-finger hot then you dont have this problem.
 
Thanks all! Looks like Brick Top nailed it. I can't believe I missed it, but it turned out that one of the chassis transistors was very loose. After tightening it down, this whole jitter thing seems to be cleared up for good.
 
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