Thursday, November 7, 2019

Restoration of power supply and bring-up of integrated 3803 equivalent control unit for Telex 8020 tape system


These tape drives are intended for use on an IBM mainframe type system, which uses input-output channels connected via a pair of cables called bus and tag. The IBM equivalent tape system was the 3420. A set of 3420 drives had a separate cabinet, the 3803 control unit, into which the bus and tag cables were connected.

The 3803 then had cables hooked to the 3420 tape drives which implemented a simple protocol, with lines to command specific mechanical actions and wires to transport 9 bits of data in and out. All the error checking, formatting and decoding was done in the 3803 control unit.

Telex designed the 8020 tape drives as a compatible alternative to the IBM 3420 system. To offer space savings as an additional inducement to customers, on top of a substantially lower price, Telex designed their control unit to fit inside the first tape drive of a string.

The Telex tape drives implement their own simple protocol, over ribbon cables, to command mechanical actions and read/write 9 bits of data. The control unit speaks this Telex protocol on the ribbon cables and uses the IBM channel protocol on the bus and tag connectors. It performs all the error checking, formatting and decoding too.


I began to move the big, heavy box out of my drive cabinet A to begin the restoration effort. It is more than a foot high, the width of the cabinet and spans from front to back. Coming out of the box are ribbon cables that run to the bus and tag connectors as well as to the various tape drives that would be controlled by this system.

Telex directly soldered them to the bus and tag connectors, thus removal of these would have to come from inside the control unit enclosure. I opened the top to get the lay of the land but the cable routing is quite dense.

I discovered that the entire card tray can slide out and then pivot up to give access to the backplane pins, thus there is no need to remove the box from the drive to do my debugging and restoration. I replaced the anchoring hardware and moved on to the first step of restoration - good power.


I can get access to the power supply from inside the front door of the tape drive, hopefully enough to do any repairs it might need. I first removed the front cover plate, but behind it was a solid heatsink wall so that went back in place. Next I removed the top plate and looked down at the innards including three large electrolytic capacitors.

Control Unit power supply electrolytics
I unscrewed one lead on each of them in order to test the capacitor's condition. I also pulled the connector from the power supply to the backplane in advance of applying first power assuming the capacitors check out okay.

I used my capacitance meter first and confirmed they were all at about 72,000 uf, appropriate for filtering the three supplies. Next I hooked up my ESR meter and confirmed that the equivalent series resistance of these is under 0.06 ohms. At the current of 1A that would only drop .06V when the circuit was fully pulling from the capacitor, much less for handling ripple. These are all good.
Capacitance meter used to verify electorolytics

Meter to measure equivalent series resistance (ESR) of capacitors
The power supply produces three voltages - +15V, -15V and +5V - thus the three filter capacitors we see. The control unit is built with TTL logic and op amps, thus determining the voltages required. Other components such as discrete transistors are designed to work within these voltage levels.

Another early check I applied was to test the jack from the controller logic for dead shorts across the three power supply rails, since I have already found two small filter capacitors on logic boards with dead shorts while restoring other portions of the system. All power rails exhibited the behavior of charging up the filter capacitors, quickly getting to acceptable resistance levels. As an arbitrary example, a 20 ohm total load for the 5V rail would demand 1.25 A from the supply.

Next up I wired up a plug for the controller. It uses an ordinary household style plug given its much lower consumption, so I needed to wire up a socket for that sized plug that would in turn have wires poked into the 240V socket. A bit sketchy but okay for testing purposes.
240V socket for controller (see sketchy wires pushed in bottom socket)
I plugged it in, pushed the on switch and watched for signs of life (or magic smoke escaping). Time to test the three voltages while plug J3 is unhooked from the logic drawer. I saw +5.0, +15.0 and -15.0, exactly what should be generated.

The next and probably final step at this time is to hook up J3 to everything else and turn it back on.
Since I don't have a powered-on tape drive nor legitimate channel cables hooked up, this won't be happy but I can at least watch the status LEDs for boot-up of the processor and indications that the two ends are unconnected.

The signs of successful startup of the controller are LEDs producing a repeating moving dot display on the processor board and two LEDs that indicate the channel adapter initialized properly.  That is exactly what I saw, suggesting that all is good. The processor runs self-diagnostics during power-up and didn't find anything wrong.
LEDs lighting to show successful startup of controller

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