Monday, June 12, 2017

Finished all 1130 light panel boards, worked on 1401 system at CHM


I continued building the final large board today, completing all 96 triacs and resistors before breakfast. I continued with inserting all the lamp sockets and half of the signal pins before it was time to head over to the CHM to work on the 1401 systems. 

I did some testout of the resistors and signal pin wiring, confirming that the first 48 pins and their associated lamp test resistors were installed properly.  After I returned from the work at CHM and evening with the 6800 club at Holders, I finished up the board.
Big board completed
All boards check out, but the power on testing with the limiting resistor will be needed tomorrow.
Three boards in approximate relative position as they will sit inside 1130

One of the 1401 systems (Connecticut machine) was down since smoke poured out of the 1406 memory extension box last Wednesday. We removed the power supply and found the part that emitted the smoke.

The power supply has two SMS cards installed, one regulating the output voltage and the other protecting against overvoltage. If the output of the power supply goes too high, a silicon controlled rectifier is clamped across the output. This technique, called a crowbar, will cause a circuit breaker to pop.

In our case, the breakers did trip but took far too long, since the twin 3 ohm load resistors limiting current in the crowbar carried 10 amps each for enough time that they scorched the board underneath and burned insulation off of nearby wires.
Trace side of crowbard card

Component side of crowbar card
The cause of the crowbar activation was the failure of the regulator card, allowing the voltage to soar up to more than 40V, instead of the nominal 30V expected from the supply. We don't have any spares for this card type, thus will have to diagnose and repair the card before we can restore the 1401 system to operation. We have replaced one transistor so far but the card is not yet working properly.

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