Sunday, October 27, 2019

Converting the Telex drives to different speeds


Mainframe makers had long pursued a strategy to build a peripheral device with multiple speeds of operation and optional features, allowing them to win business with more price sensitive customers while protecting the prices and margins for those who valued the highest performance and added features.

The device was designed for its top performance and features, then methods are developed to slow it down. This can be by changing the size of a pulley on a mechanism, or adding delay logic. An advantage of having one design with slowed models is that a field upgrade can be offered, allowing a customer to pay the market price difference and receive the benefits of the faster model.

A sizeable price difference may be implemented by a pulley or jumper, with the cost of parts changed way out of proportion to the price changed. It is most visible in processors with slowed speeds, where the CE key, used to change metering of hours used between the customer and the service meter, also increased the machine to its full performance.

With the IBM 3420 tape drives, IBM offered models with three different speeds - 75, 125 and 200 inches per second. The Telex 8020, a competitive drive that was program and plug compatible with the 3420, offered models at 75 and 125 ips.

In looking through the schematics for the tape drive, I found that the same PCB design would support both speeds, differing only in the value of some components on the board. If my drive was a lower performance model, I could easily modify the machine to give myself a field upgrade.

Of course, if my drive was already set up to run at 125 ips, there is no need for changes. I pulled a board (the capstan drive pre-amplifier) from slot 7 and looked for the parts that vary based on speed. I discovered that Telex had been a bit clever, making it harder for a customer to upgrade the drive themselves.
Board 7 schematic where parts values determine speed of the drive
The differences in speed for this board are set by the values of six resistors and two capacitors, as shown in this chart on the bottom of the schematic page. I looked over the board to find parts with numbers like R302 and C302 but quickly realized that they were all installed inside a pseudo integrated circuit U27 that had a DIP 16 mounting but was a big encapsulated block with an obscure part number.

Table of parts values to set capstan tape speed

Pseudo-IC containing the discrete parts
Using the pin numbers on the schematic, I checked the values of the embedded components and found that my drive A was the full 125 ips version. If it had not been, I would need to remove the DIP 16 block and wire up discrete components of the proper value on a DIP socket to plug into the board.

I pulled board 07 from cabinet B to check which speed it implemented. To my surprise, the pseudo-IC has been removed from the board. I can see that the board has a flush level socket into which the IC 'plugs' instead of being soldered. The good news is that I know the values and can build a DIP 16 socket with the parts.

Flush socket with removed pseudo-IC on drive B board 07
This cabinet had been destroyed by a forklift through the window of the front door and subsequent fall that was hard enough to bend the frame and break some welds. Typically in those cases the insurance carrier pays the owner or vendor and the machine is supposed to be scrapped.

The missing chip may be because the normal installation procedure at the customer site would have had the CE plug in the appropriate chip to set the speeds, but it is shipped sans IC. However, this board appears beat up and used, indicating that the chip was removed deliberately. Telex may instruct their CEs to remove these modules because they can be used to upgrade other tape drives in the field - before release the unit for scrapping.

Nothing else appears to be removed, although I need to look more carefully at all my boards just in case this was stripped for some parts before it was turned over to the scrapper. I definitely need to restore the drive in cabinet A first, as it is the most likely to have had a full complement of working parts.

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