REPLACED ORIGINAL RELAY IN VOLTAGE TEST BOARD
I had mentioned previously that the voltage testing SMS card had one of its relays removed and replaced with some IC-like substitute. I found the original reed relay in the parts box, tested it and found it worked just fine. I removed the substitute and put back the original on the board. No idea why it was changed, although it was the one to validate +6V, the same rail whose regulator had failed.
REPAIRING THE 6V REGULATOR
I didn't have a bench supply able to delivery more than 20A of 6V power. The unit installed in the machine as a substitute, a $12 part, wasn't going to be used if I could help it. I pulled the regulator and began to trouble shoot the unit.
This supply uses a set of six power transistors in parallel to handle the +6V current. One of them was dead shorted and causing the regulator to pop its breaker. I removed that one transistor, leaving five that could handle 21A out of the 24A original capacity. This is a short term situation while I acquire a replacement transistor to mount in the supply.
I set up the supply on the bench, fed it with 9V power and set its output to 6V. I hooked up my electronic load, which is only able to sink up to 5A of current. I don't have large resistors or other means of driving the supply beyond this level, but it did perform well and stay right on voltage through that level of load.
I had to verify all the wiring surrounding the 6V supply. The terminal block was missing a couple of metal bridges that tie pairs of terminals together, pretty critical to the operation of the regulator. With all of them in place, the other ends of all the wires verified, and the regulator back in the machine, I was ready to adjust all the rail voltages.
CHECK REGULATED DC VOLTAGES
With all the raw DC and AC power looking good, it was time to validate the controlled voltages for the SLT logic produced by three regulators. I switched on CBs for +6V, +3V and -3V one by one. After powering up, I checked the levels at the wiring going to the rail connectors on the main logic gates. I was tweaked all of them to the middle of the valid ranges.
With this done, the next step was to connect the power to the logic circuitry and power up to begin the debugging. I still have an issue with the 7.25VAC supply for the panel lights, but the rest of the machine is good as far as power.
I powered up the system, which then lit the RUN lamp indicating it is executing some code. I flipped the rotary mode switch to Single Step (SS) which extinguished the light as it should. Once I have the lights working I can check to see why the RUN condition is active. There are two relatively innocuous reasons - a hot interrupt or a hot cycle steal (DMA access) request from a peripheral device that is not currently connected. Other defects could cause this too.
BREAKER TRIPPED AGAIN ON THE +6V REGULATOR
The regulator tripped causing the machine to power down exactly per its design. There are two possibilities here - the breaker may be weak or the regulator may have additional power transistors failing. The breakers on similar supplies used on the IBM 1401 systems do sometimes start tripping well below their target current level, requiring replacement with a proper breaker.
I can quickly determine whether we had a new transistor failure by resetting the breaker and powering up again. If the system comes up for a short time, then the issue is the breaker. If it trips immediately, we have more repair to do on the regulator itself.
BEGINNING TO INVESTIGATE THE LACK OF PANEL LIGHTING POWER
I put in fuse F7 for the panel lighting power supply, then checked the AC inputs to the supply which should have 115V present. The lines were not energized. I began testing continuity of the wiring back to the sequencing power box which houses the fuses. I ended for the day before I completed this, but with the problem sitting on the AC input side, I am confident I will have this back in operation shortly.
No comments:
Post a Comment