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-----Original Message-----
From: Keith Smith kahsmith@... [TekScopes] <TekScopes@...>
To: TekScopes <TekScopes@...>
Sent: Sun, Dec 14, 2014 7:34 pm
Subject: Re: [TekScopes] 7704A -just a quickie






Well, I'm getting some time in on this today. During the week I made a
spreadsheet to help me get a grip on all of the inter-connects for at least
the +50, -50, +15, -15, and +5v supplies.
That allowed me to work through it all fairly quickly and hopefully I
haven't missed too much in the process. I still have Magnus' "Do it
thoroughly." ringing in my ears.


I checked for continuity with the power supply removed entirely, starting
from the P20G connector pins to all of the other boards and found no
problems.

This may be contentious: I've acquired a couple of Huntron Trackers (1000 &
2000) that were thrown in with a couple of scopes some time ago. I've never
seen them mentioned here and would be interested to read any opinions on
their efficacy.
I used the Huntron 2000 for the continuity tests. I liked the indication of
whether the circuit being tested appears to be totally resistive or if some
other function is going on. I'm certainly not suggesting I possess any
expertise in its use.
All supply paths tested looked purely resistive, as expected.

Assuming that the PS is in tick mode due to an overcurrent problem, I
started looking for shorts.

I checked the resistance to ground (Pin 3 of P20G) of all the interconnect
points and added those values to the spreadsheet. Measurements were made
with a Fluke 87V.
The low R values of the +/-15 and +5 v supplies quickly indicated a
problem, I think, and were repetitive throughout the system ,but I followed
through for the sake of completeness.
Later, I checked R values from P20G-3 to the supplies on pins 1, 2 and 4,
while I disconnected the A10, A33, A34 boards and the entire Display unit.
The highest value I could get was about 400 Ohms (suspiciously close to the
in-circuit resistance of C2085).

I find the symmetry of the R values on both 15v supplies a bit curious.
Could be a clue?

I'm guessing the next thing is likely the C2081/83/85/87 caps, which all
look to be tantalum. I don't have an ESR meter, but I do have a Schmitt
trigger built from Alan Wolke's (W2AEW) video, so I could pull them and try
to get a rough ESR value on a scope. It might be quicker to just replace
them though. I can probably get them tomorrow at my local supply house. Any
suggestions on suitable replacements would be appreciated.

For what it's worth, 85 & 87 have nice fat ellipses on the Huntron, while
81& 83 look pencil thin even at 60 Hz.
Whatever is going on, I'm feeling it's probably on the A20 board or its
close cousins. Make sense?

Thanks again!
Keith

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