ADM-3A question

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 connector 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. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus