Help with a busted MSV11-L?

[Jon Elson]

On 12/07/2014 07:22 AM, Noel Chiappa wrote:
> Hi, all, I'm trying to fix a broken MSV11-L card (M8059-KF). The symptom is
> that it always reads back with the 020 bit set, but otherwise appears to be
> fine. (E.g. you can write a number in 0, and a different number in 020, and
> each reads back OK, with the exception that the 020 bit in each is always
> set; i.e. the addressing is working fine, as is the QBUS generally.)
>
> So poking around with my 'scope, I find that the latched D04 signal ("LAT D04
> H") is always around 2V - not good! From the prints:
>
>
Power the system off for 10 minutes, then power on and read 
various locations.
If data bit 4 shows 1's and 0's, then the reading is OK, and 
it is the writing path
that is stuck.  I think this may be the case, as you say the 
bit is stuck when you
scope it, I assume when writing to the memory.  That may 
narrow down which
chip is the culprit.

Jon