tempest AVG troubleshooting help needed

Hi.

Hoping someone can help me. I'm trying the standard "trace the signal back"
methodology without much luck.

Have what I think is a Tempest board with a bad Y output.

Pressing reset (test switch on) and the test screen appears momentarily
(a little distorted but recognisable) and then collapses into a blank screen.
Spot killer is on.

Advancing to the checkerboard test screen (4th screen), Y direction is
"scrunched" into about a 1" area of the screen. Spot killer on.

Checking with a scope:

The output of the X multiplier (C12) looks good, pin 14 (output) is same as
pin 9 (input) but downscaled.

The output of Y 1495 (C13) looks bad, pin 14 (output) bears no similarily to
pin 9 (input).

I socketed C13 and replaced it (known good one), with no change.

There is what appears to be a noise? waveform on pin 18 of F10 (Y DAC).
There is no equivalent waveform on pin 18 of A10 (X DAC), rather a
straightline. On a good working board, don't see this output on F10,
straightline for both X and Y DACs.

I socketed and replaced F10 (Y DAC) with no change.

I don't understand the DAC/BIP circuit, the relationship between the IO
(pin 18 - is this the analog output?) of the AM6012 and the BIP circuit.

If anyone has some beginner level circuit descriptions and troubleshooting
advice, I'd appreciate hearing it.

Thanks

Tony

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Received on Tue Feb 4 00:46:10 2003