Saturday, October 26, 2019

Final repair of power supply in Telex 8020 tape drive cabinet A, except for replacement fuse holder


I ordered the diodes from Newark, early in the day on the 22nd. Although the website had indicated that it would be shipped that day, USPS Priority Mail, tracking the assigned number  proved that it wasn't handed over to the post office until 9PM on 10/23. That delays delivery from the expected Thursday until Friday. Sellers are quick to fill out electronic pre-shipment and try to claim that as the mailing date, but they can dawdle in actually handing off the packages believing this is hidden.

As a backup I ordered two more diodes from a "Fast and Free" ebay seller whose promise was Monday the 28th. I had a credit on ebay from a past purchase with a guaranteed delivery date that wasn't met, so it was cheap insurance for the Newark order. Once I saw the shipment handed in the need for backup lessened, but that didn't come until late in the day past the promised ship date from Newark.

Once I had the diodes in hand, I soldered them into the circuit and put them back on the heat sink. My first test was to fire up the full wave bridge, sans capacitors or any other load, just to verify that the rectifiers are working properly.

The diode bridge worked, then everything still okay with the capacitors in circuit. Finally I put in the fuses and checked the outputs at the J5 connector. I didn't see +45, which turned out to be a broken cartridge fuse holder!. It had cracked during the removal of the enclosure from the tape drive cabinet.

The supply only need a replacement panel mount fuse holder to be put back into service. Everything works perfectly at this point. The darned holder is over $27, since it is a 600V, 30A spec holder for FQN time delay fuse cartridges. I should install it by mid next week, then the enclosure will be put into place and testing can move forward to active electronics throughout the drive.


Having proven that the vacuum/blower motor jack appears to have 240VAC even when off, I tested this by plugging the actual motor to the jack and then toggling the control pin 12 of J4 to verify that the motor goes on and off. Everything worked well.

The triac has a high resistor and 1uf capacitor installed across the two main terminals, which causes the 240 to appear on the socket but no power is delivered until the triac fires, gated by the control signal.


I fired this up as a closed unit, verifying the +45, -45, +12, -12, +5, +5 for operator control panel, +6.4, unregulated +8, and the 12VAC outputs were correct. It passed all tests, with the control pins 13 and 12 of J4 switching the AC for the 45 V supply and vacuum/blower motor.

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