M8650 async board problem - test-drb@ccmp.vtda.org

22 Dec 2001

On Dec 22, 23:15, Tony Duell wrote:

...
  As the fault wasn't on the M8650, why did they cut
that track? Did they
 just never want interrupts? 
I've no idea.  It wasn't cut by the previous owner; he doesn't know why
either.

...
  > However the fault, for those who're still
reading, wasn't really the M8650
...
  > itself.  There were two problems.  At some point,
I had unplugged the
 > serial cable from the Berg connector, so there was noise on the serial
 > input, which upset the diagnostics.  The second problem was that the PSU
...

 And adding an 'antenna' (as in the serial cable) actually helps this? 
The proper cable has a jumper in the Berg connector to enable the EIA part
of the circuit.  It connects the 1489 output to the first gate in the
receiver section.  Without the jumper, the input to that gate floats, and
hence picks up all sorts of noise.  I wouldn't be surprised if I could pick
Radio 2 up on that :-)

...
   +5V was down
to about 4.5V, which isn't great for TTL.  I'd checked the 
 This has caught me so often when repairing DEC machines. The PSUs set up
 fine on no-load and drop to 4V to 4.5V when loaded. And this causes the
 most _amazing_ faults. 
Yes, and I should have known better.  I realised something of the sort was
amiss when I found that moving cards between front and rear backplanes made
a difference.	With everything in the front, the +5V went down to less
than 4.3V.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York