Re: TECH: bench testing numeric displays for shorts or problems.

I grabbed some more zener diodes checked them against the old ones and
found on all the zener's they actually get a reading back though the
zener. I am guessing because these are 3.6V and the meter is using a
test voltage the same as the zener so it fails the standard diode
test. So the zener's turned out to be ok. Testing all other components
with the exception of the two 100uf 160V caps also gave me good
results.

As for transistors I am using 2N5401 PNP (MPSD52 Alternate) - Q2 and
N5551 NPN (MPSD02 Alternate) - Q4

Also using MJE15031 NPN - Q3 (SDS-202 Alternate) and MJE15030 PNP - Q1
(SDS-201 Alternate) with the legs rearranged for the base, collector
and emitter due to the different placement from the manufactures of
these legs.

Lastly I use 1N4004 vs 1N4001 which is only a higher voltage rating on
the 1N4004.

I know these are valid substitutes. This thing worked like a charm up
until the hex inverter installation.

I didn't have a 160V supply to check the two big axial caps, but just
checking the resistance within them I am getting varying values
between them and I see signs of heat on one. These caps run in
parallel to the circuit so a problem there could result in the blown
fuse. I am 99% convinced this is where my power supply HV problems
lie.

All I know is the only thing that changed was the hex inverters in the
equation when the HV section blew. I can only assume that the change
of states of the gates in the low voltage logic circuit opened up an
HV route to a short. My solder work is first class (no really it's
quite good as compared to some of the "professional" work I have had
done). I use a commercial grade Pace PRC 2000 workstation, and I
double check my work for connections, component orientation, and
shorts so I don't suspect it was the installation.

So as it stands now I replaced the fuse and it blows without any
output connections to the 100V supply. I have ordered new caps X 4
(backups or inventory) and a complete other HV rebuild kit from GPE
(great plains electronics. Ed is great!) and will do a full overhaul
of the HV in case any components got stressed in the chaos.

By the way this is my first System 7 I have worked on. Everything has
either been WPC or Sys 11 which I have worked on quite extensively,
and I only do it all as a hobby. I am reliant entirely on the internet
to keep me on track. Any input is of great help.
.

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