Repairing an old GEC GEM Drive

Ok, so I've been asked to post the report for my latest job. It is a GEC GEM Drive 24V8400/10. This one was a pain to do but I couldn't have asked for a better machine to fix than one with a discrete logic controller. There were a couple of surprises with the power section with a diode looking every part a normal diode but it had broken to a zener when driven and two others had gone resistive. Those took a while to find and the first time I have actually seen them do that. Well here is the body of how it came to be fixed with the help of Ross and Warrick. Hope it helps with diagnosing a fault with your drive or at least be interesting reading.

Report for GEC GEM Drive 24V8400/10

The power board was inspected and all the snubber caps appeared to be damaged or aging so they were replaced and the PCB cleaned. The appropriate links were setup on the terminals and lamps were attached to the motor output. The B bridge gates were disconnected to prevent shorting of the SCR's. The drive was powered and noted that the gate firing LED's were not lighting. Tracing through the circuit proved they were being driven by logic but the bridge driver power supplies revealed some feedback into the disabled bridge supply. Because the bridge was disconnected the SCR's did not short and the load could be driven in one direction. TR1 and TR2(PNP switching darlingtons used to select each bridge) were checked for faults with a multimeter but no obvious faults were found. They were replaced and the fault remained. The pulse transformer insulations were checked with a megger meter on 500V but they all showed high resistance. A 1M resistance was measured across three of them but the cause was believed to be due to other circuit(eg. The armature feedback). The pulse transformer diodes were checked with a multimeter on diode and resistance setting in forward and reverse bias. A diode showed low voltage and resistance in both directions and it was removed. It measured 343R in both directions external to the circuit so it was replaced. Another diode showed 11k in reverse and was replaced. The drive was powered but one bridge still showed feedback. The diodes relevant to this bridge were measured while active and they all showed voltage feedback but one had a higher voltage feedback indicating that it was likely to be the problem. This was replaced and the bridge selection worked correctly.

The phase control was not working so 0-15VDC was ramped with a pot directly into the phase control IC's(IC5 and IC14). The output phased correctly so after some circuit checking TJ1's(a JFET) drive was checked as it was used to disable to the control. Further investigation found that the interlock was not enabling it although the signals were present. AND gate (c) of IC29 was not operating correctly and the chip was replaced. When the drive was turned on the lamp load flashed and the drive turned off. Measurement of the SCR's showed one had an o/c gate(This is a rare event says Ross, because they usually go short but I found a spare of the same brand that someone didn't check and it also had an open circuit gate. Maybe this brand has that problem). This was replaced and the ramp control from the external speed input worked correctly.

A motor load with 40R 1.5kW(c/o Warrick) series resistor was attached to the drive. It was driven with one set of bridge gates attached and then the other bridge alternately. As a precaution all the diodes in the power section were replaced and the PCB cleaned. New screws were used for the SCR's and the monitor module PCB was cleaned. The series resistor was removed and the motor driven in both directions to the motor's maximum voltage(180V) with the external pot. Checking of the bridge supplies and motor output concluded the testing and verification of the drive. The drive was returned.

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There was a lot wrong with this one but at least I had the circuit diagrams. The beauty of discrete logic is that it can be easily interpreted, found and replaced. Other jobs will no doubt be posted in the future so keep an eye on this site to learn more.