ADM-3A question

Charles charlesmorris800 at
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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