Power supply failure and debugging on the Calcomp 565 plotter

RESTORING LAST BIT OF CIRCUITRY WHEN AN ANOMALY OCCURED

Having previously restored the carriage left/right servo to full operation with the replacement power transistor installed, I turned my attention to the pen up/pen down circuit. This operates a solenoid in the pen that either holds the pen up in the air off the paper or allows a spring to push the pen down for drawing. 

I had discovered a failed power transistor in the circuit and when the spare part arrived, I installed it. As I was probing around investigating the operation of the circuit, I installed a similar solenoid onto the carriage and hooked it up. Apparently Calcomp also made a cutter solenoid that would work on the same plotter mechanism, but place a knife on the paper/plastic/fabric so that patterns could be cut with .01" precision. 

It is almost identical to the pen solenoid, which is why I scooped it up on eBay, planning to convert it into the pen. It has the same inductance and method of attachment to the carriage. It was in place and I began checking voltages, when I saw that the -24V level shot down to -41V. It may have been something I did duriing the probing or it may have been a coincidental failure. 

I turned it off, then when I powered up later the voltages were now all near or at zero. The fuses were good, so it appears that something failed in the power supply. 

FIRST FAILED PART DISCOVERED - A WIRE

I verified that the AC output was coming from the transformer, but there was essentially zero coming out of the bridge rectifier. Checking all the diodes did not find any that were shorted, open or otherwise malfunction. Just zero volts coming out and I had confirmed it was NOT due to a short that the rectifier was feeding. 

After some continuity and other testing, I discovered a defect in the wire which connects one of the transformer secondaries to two diodes in the bridge. The wire was not a wire. It is one of a bundle of wires that are laced together and routed through the power supply, with no obvious sign of damage or overheating. It was just an open circuit. 

ADDED A NEW WIRE BUT THE VOLTAGES WERE NOT GOOD

I soldered in a new section of stranded wire between the terminal board where the transformer secondaries attach and the junction of the two diodes. With continuity, I powered up and began to check the voltages being produced by the supply. The line that should be -24V (or a bit more negative since it was unloaded) was at -41V, the same as what I observed just before the wire committed suicide. 

I checked a couple of the other power rails and they were all off significantly. Something else failed in the supply besides the wire. I ran out of time to find the issue when I had to leave the workshop.

OVERVIEW OF THE POWER SUPPLY

The power supply of the Calcomp plotter is interesting, using just one s et of transformer secondaries and no regulators yet producing -24, -9, -7.5, +1.5 and +3V rails. It depends upon a chain of diodes to generate the proper voltages, a voltage divider taking the approximately 27V produced by the rectifier bridge under load and turning it into the rails above.

There is a small additional part of the circuitry that is connected to the bridge rectifier before the filter in order to have pulsing DC, used for the fast stepper modes that drive the servos at 120 steps per second. It is not salient to the issue we are debugging so I didn't include it in the pseudo-schematic above. 

For me to see 41V instead of the 34V I previously observed, the most likely failure is that the Zener diode has shorted. I tested all the other diodes with a VOM in diode mode and found them all to have a good diode voltage and not pass any current in the reverse direction. However, I couldn't check a Zener with that method.

The acid test for a Zener is a transistor curve tracer, where carefully limited current and controlled voltages can be introduced and the results observed on an oscilloscope. The diode should be high resistance at voltages up to 7.5 then exhibit a nearly vertical slope to the current flow allowed by the tracer setting. 

When I get back to the shop, I will remove the Zener diode and test it on my transistor curve tracer. If I need a replacement I must be sure to get one with a high enough power dissipation capacity. The schematic lists it is a 1N3016A diode so that would be the perfect replacement, or a substitute with the same or higher specification of 1W capacity. Interestingly, the spec sheet does not show the voltage threshold as 7.5V, instead listing it at 6.8V.