Tempest PCB Corrupt Vectors - video inside

EclipseEye

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Hi

I've been trying to repair a Tempest boardset and have now managed to get the PCB running but with corrupted vectors;



I'm not sure where the problem lies, if in the digital VSM section or the analogue output section. I suspect the VSM, but not sure.

Another odd thing, when I run TEST mode the PCB constantly makes the same beeping noise every second and does not stop. I have verified RAM as good with a Fluke.

I've been over the board many times and getting ready now to shelve it.

I have replaced the entire analogue output section which includes the op-amps, DACs, DAC08, LF's and the MC1495s. Voltages including the +15v and -15v are also good.

Can anyone assist with possible solutions?
 
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I'd say it's a bad AVG chip, but of course Tempest does not use one. Those functions are built using about a dozen separate ICs as shown on the "Stack and Program Counter" section of the schematics (sheet 3 side A). I'd start there and follow the signal path.

As to the beeps, swap the two POKEYs on your AUX board and see if the behavior changes. You have reflowed the headers for the inter-board connector, yes?
 
I'd say it's a bad AVG chip, but of course Tempest does not use one. Those functions are built using about a dozen separate ICs as shown on the "Stack and Program Counter" section of the schematics (sheet 3 side A). I'd start there and follow the signal path.

As to the beeps, swap the two POKEYs on your AUX board and see if the behavior changes. You have reflowed the headers for the inter-board connector, yes?

Yes I thought the same thing, looks a lot like a failing AVG chip if it had one.

Thanks for the pointers, I will check the Counters and report back.

Yes reflowed the header pins and will try swapping the Pokeys.
 
Looks like it could potentially be DAC-related as well (or something else analog after that, one of the amps, etc), if it's not a counter.

Whatever it is, it's got to be late in the chain, as the game is playing, and not resetting.

I'd also swap out the aux board with a known good one, just to rule that out, if I were debugging this one, as bad PROMs can cause all sorts of weirdness. The distortion is a bit odd, as I can't say I've seen one that looks quite like that, but anything's possible (especially if both DACs are bad, which can happen).
 
I did think it was a DAC too, but I've replaced those with no changes.

I have confirmed that the AUX board is good. Test mode also shows corrupted vectors (even with AUX disconnected).

I was sure like yourself that the problem was in the final analogue stage, but cannot find any problems there.

Also, the constant beeping in Test mode is bugging me, clearly a problem there that is not analogue.


Looks like it could potentially be DAC-related as well (or something else analog after that, one of the amps, etc), if it's not a counter.

Whatever it is, it's got to be late in the chain, as the game is playing, and not resetting.

I'd also swap out the aux board with a known good one, just to rule that out, if I were debugging this one, as bad PROMs can cause all sorts of weirdness. The distortion is a bit odd, as I can't say I've seen one that looks quite like that, but anything's possible (especially if both DACs are bad, which can happen).
 
Oh wait, I forgot about the test mode beeping. (And sorry, I didn't see you already replaced the analog section. My bad for not reading better.)

So is test mode working normally, just with the beep? Or is it not completing the self tests? (I realize you can't see the test mode screens if they're corrupted, so there could be info there that you aren't seeing, error codes, etc.) Can you cycle through all of the test pattern screens also?
 
I have a Tempest board doing something similar, so I'd love to hear the fix for this issue.

Scott C.
 
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I was going to say you could also try replacing Q11 and Q12 for the drawing trouble, but....
If you still have beeping with a whole other AUX board, then it's not the POKEYs. They get their address and data signals for the inter-board connector. Since you've reflowed it, I was going to say you could check the buffers that drive it (H2, B/C1, and B3-F3) but since you have bad graphics even with no AUX board....
I'd check for activity on the outputs of the chips in the Vector Halt Latch section as well as the Vector Timer section (sheet 3A).
 
Well, i finally went back to this boardset after quite some time.

The same symptoms were still there, except this time I managed to solve the problem right away. The occasional 'beeping' was due to a failing RAM.

The video output....I've got a habit of shotgunning the analogue section of these vector boards, and I always replace the op-amps with new TL081/TL082 where applicable.

Turns out the TI TL082s I used on this board, which were imported from China incidently, are fakes.

So there you have it, except this time it took me 5 mins to solve.
 
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Yep, there are fake TL08x's out there. I got some myself years ago.

That's why it pays to get them (and anything else that is still made) from reputable sources. It just isn't worth the minimal savings. Things like the DAC's have been ok to get from China, but the 13201 switches and TL08x's can be obtained from most large electronics sellers these days.

Glad you got it figured out.
 
Well, i finally went back to this boardset after quite some time.

The same symptoms were still there, except this time I managed to solve the problem right away. The occasional 'beeping' was due to a failing RAM.

The video output....I've got a habit of shotgunning the analogue section of these vector boards, and I always replace the op-amps with new TL081/TL082 where applicable.

Turns out the TI TL082s I used on this board, which were imported from China incidently, are fakes.

So there you have it, except this time it took me 5 mins to solve.


Thanks for the follow up on this. It helps to hear what sources are no good. It's also interesting to hear that dacs are ok to get from China. They can be pricey.
 
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