[time-nuts] HP 3458A ISOLATOR DTACK err @ pwr up

[David Forbes]

Brooke Clarke wrote:
> Hi Ed:
> 
> Yes, all the supply voltages are within spec, but the error message 
> stays the same.
> 
> The Outguard supply visually looks OK and the voltages are centered in 
> the spec.
> 
> Yes the transformer has seven wires feeding the outguard p.s.
> 
> See: http://www.prc68.com/I/HP3458DVM.shtml#Bot
> You may need to <shift><reload page> to see it.
> 
> Have Fun,
> 
> Brooke Clarke

Brooke,

This poor instrument does look like it suffered overvoltage on the AC input. 
Because of that, it's likely that one or more digital ICs took an overvoltage 
hit, causing localized damage that produces the interesting symptom.

Unfortunately, there's enough programmable logic and other hard-to-find parts on 
the A5 CPU board that you likely will have to replace the board to fix the unit.

The DTACK error is a clue if you want to track it down. You may be able to find 
that fault by triggering a scope on the DS0* or DS1* signal being at 0V for 
longer than a microsecond or two. Then look at the address bus to see what 
address is being accessed, and consult the documentation (if you can get it) to 
see what device is at that address, and go from there.

It's a good activity for someone with lots of free time.

--David Forbes