Tuesday, October 22, 2019

Final repair of Telex 8020 drive B power supply


I was in error believing that the -12V supply was not working - I apparently checked the wrong pin. I couldn't see any common parts other than the AC transformer and it was working properly. However, the +12V supply is the only one malfunctioning.

The supply uses an LM723 regulator IC with support circuitry to control current, set the regulation point, and to protect against overvoltage with a crowbar circuit using an SCR. The two obvious candidates for the failure are the regulator chip or the SCR. I removed the supply from the enclosure to let me bench test it more easily.

The only complication was the current configuration which was set for 208V input. It was just easier to take it out to the garage and do my debugging there.

Lo and behold, with the supply disconnected, all voltages were working. I connected it and again the +12 disappeared. I finally traced this to the Dump card and transistor circuit that will shunt off power through the huge 300W resistors whenever the +45 and/or -45 went too high.

With my reconfiguration of the main supply power transformer, I was getting essentially spot on voltages with my household line levels. I don't expect them to surge and therefore I pulled the Dump card and will run this way will I look into the cause of the problem, It is either on the card itself or one of the two transistors which are mounted on the rear of the card cage. Both of these can be serviced with the supply enclosure bolted back in the drive.


Using the diode mode of my multimeter, I see that the suspect bad triac has no diode action between the gate and MT1 terminal, unlike the working triac with has diode action both from g-MT1 and g-MT2. A replacement triac is already on order.


However, I may not have to repair this part of the power supply, because I have a second such triac switch in the other power supply, the one mounted in the base of drive cabinet A. If that one works, I can simply use it and ignore this one. After all, I only have a single vacuum/blower assembly and will only be using one of the two drives.


With the +12V working, sans Dump card, and postponing the motor triac function decision until I test the other cabinet's power supply,  and everything else working perfectly, I closed it up and installed it back into the drive B cabinet.


  1. I think the dump regulation is important when the motors are running. There is a lot of energy in the spools during high speed seeks, and this has to go somewhere when the motors stop. I suspect the idea of the dump circuit is to dissipate this energy to avoid excessive voltage building up in the smoothing capacitors.

    1. After I found the maintenance manual for the tape drives, I could read the principles of operation for the power supply. They intended the dump circuit to absorb energy when the voltage is above 47.5V as a result of high speed rewind. You were correct.

  2. That is reasonable - would explain why a shunt regulation supply would use the term 'dump' instead of 'regulator'.