charlesmorris800 at centurytel.net
Fri Aug 16 19:46:18 CDT 2019
>I am still struggling with my ADM-5. The smoke I mentioned last time came
from a tantalum capacitor decoupling a -20V supply. After removing it, I
got back to the monitor showing a cursor.
Interesting you should mention that... early in the debugging process
(always start with the power supplies!),
I had discovered the +12/-12 power supplies weren't right. (+12 was very
low). The problem was a fractured solder joint on the PCB's male Molex
J3. Specifically, pin 4 (red/white) - the center tap of the transformer.
This is the worst pin to be open since the load on the +12 is much greater
than the -12, so the voltage across the unregulated supply soared on the
minus side to over 35 volts (should be + and - 20v, as you mentioned). The
electrolytic is rated at 35 volts, and of even more concern, so is the 2.2
uf tantalum bypass. Those match-head tantalum caps are known for
short-circuiting even when operated below their maximum rated voltage (as
any Tek owner can attest).
Fortunately I found the problem before the tantalum exploded and took out
the bridge rectifier with it.
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