Help!

[Piero Andreini]

Hello Tony,
I saw that you have a perfect knowledge of HP9816 and I thought to ask you for help because I have a problem that is making me crazy.
In the past, my 9816 started to occasionally give the error "HP-IB Fail" at power on, even without any device connected. It disappear just switching it off than on.
Now the error is permanent. I tried to replace the TMS9914 and the two buffers 75160 and 75162 but without success.
I got the same error even if I turn on the machine without 75160 and 75162 or without TMS9914.
No error, instead, if I move switch 7 as a slave device instead of a master.
What do you advise me to check?
Thanks in advance,
Piero