Fluke 199C Scope dead chn A

[Simon Spiers]

I have a fluke 199C that was given to me a few years back as it had failed cal and was written off.
When CHN A is enabled Channel B trace vanishes from screen and the scope is useless. I can use chan B on it's own as long as A is disabled.
Enabling chn A only shows no trace at all.
My question is does that sound like the A-D  convertor has failed or something more serious? 

[Assafl]

Sounds to me like a logic issue.

Both channels go to the S-ASIC for sampling, and then go to 2 ADCs. After that they go to the D-ASIC for display. Since they are parallel channels - If one channel would fail - why would it inhibit the other? That is why I am thinking some switching logic (or else turning on Channel A causes a problem elsewhere).

There is a version of the service manual online with full schematics.