[volt-nuts] A 7081 with a MAINS FAULT problem

[David C. Partridge]

This is the unit with the dead VFD.  I've borrowed the front panel and VFD from a 7071 to allow me to try to fix the other faults.

Initial symptoms MAINS FAULT followed by WAIT @F.

After replacing the blown +15V regulator on the floating board, all that I got was a MAINS FAULT message which cleared and was followed by a WAIT @F.

I tried substituting the earthy logic and earthy CPU boards from a known good meter, and that made no difference.   The link cable between the two boards tests good.

Replacing the floating CPU (IC801) with the one from the good meter results in a MAINS FAULT message being flashed at me at about 3 second intervals.  Sometimes I see a WAIT @F between the MAINS FAULT messages.

As far as I can tell, the ROM (IC803) and RAM (IC826) make no difference to the behaviour.

Scoping pin 12 (floating to earthy) of the floating CPU I see what may be one character being sent back to the earthy board at about the time the MAINS FAULT message clears. (-15V for 28uS, -10V for 28us, -15V for 36us, then back to -10V). 

Scoping the emitter of TR1 on the earthy board (or pin 11 of the floating CPU) appears only to show one 120uS long low pulse at about the same time.  

I'm not seeing any obvious DUMP NVM command being sent unless that single low pulse is the DUMP NVM command, and there's no sign of a reply.

Regards,
David Partridge