Heavy Metal from the 1960s

Any time you're working with older electronics, there's a risk that some of the components may have failed as a result of age. This is especially true of wet electrolytic capacitors. Given that the museum's CDC 161 Typewriter is at least 61 years old (there's an inspection sticker from 1964 in it), the sheer mention of a power supply problem led to a decision to pull the thing out for at least a visual check of its internals. We were also looking to find the source of a loud buzzing sound coming from within the cabinet.

The power supply in the CDC 161 hangs from the underside of the cabinet's top like a bat, held in by four hefty bolts. I took photos of the wiring before disconnecting everything and stowing the cable bundle out of the way. I tried to loosen all the bolts evenly to make it easier to remove, but when the first bolt on the left side came out the second on that side broke off. I found out later that it was badly rusted, and may have been shearing rather than unscrewing. It's fortunate that I'd put some packing material to help support the power supply, because otherwise it would have crushed my hand -- it weighed 40 to 50 lbs!

It seems the CDC 161 had suffered some water exposure, as some of the screws holding the power supply's cover were rusted. With sufficient persuasion, I was able to unscrew all but one of the screws. We removed the head of the last screw with a Dremel and a cutting disc.

With the cover removed I could see why the thing weighs so much. Take a look at the three huge transformers in this thing! That's some heavy iron.

Despite being told repeatedly that switching power supplies didn't exist in the 1960s, this COTS beast from the late 1950s is indeed a switcher.

Operated on the bench it produced clean, steady DC power on all three of its outputs with only a slightly audible 60 Hz hum, even when operated under load (note the 100W load resistor on the left side of the picture). Scoping the filter capacitors showed essentially no ripple, confirming that they were still doing their jobs.

But what about the overvoltage lock-outs I'd been experiencing?

To minimize the effect or voltage losses in the wiring between the power supply and the load, this power supply has separate voltage sense inputs (the top three connections on the terminal block shown in this pic). These allow the supply to sense the voltage at the load rather than inside the power supply. The problem with this scheme is that a poor or open connection in these sense leads will cause the supply to think it needs to increase the output voltage.

While inspecting these connections I discovered that the faces of the ring terminals were corroded under the terminal block screws. My theory is that this lead to the overvoltage condition, and it correctly shut down the supply when the output voltage rose to an excessive level.

At least that's my story, and I'm sticking to it!

After reassembling the power supply, I was faced with the challenge of getting it back into the cabinet. I decided the simplest way to do this was to flip the cabinet onto its top, supported by some cribbing. This allowed gravity to work in my favor while I fitted the bolts.

But if the loud buzzing wasn't coming from the power supply, where was it coming from?