Reference supply repaired I hope

My Digikey order arrived Friday. Today I replaced the two power transistors on the reference supply. I also changed the capacitor for a new one. When I first plugged it in, it was still not working, so I built my card extender so I could get it out. But, after some balancing and a warmup, the reference supply kicked in perfectly for both polarities. I'll see if it keeps working. There is a chance that there is a loose connection somewhere that is making it intermittent sometimes. But, it appears to be working fine, now.

I think I need to also let it run for an hour or so, then adjust everything. It may need some time to stabilize.

Front panel is much nicer - ordered parts

I finished the front panel restoration. I repainted the knobs and polished the aluminum. I replaced the dot with a red lacquer stick. The original was white, but I thought the red would look nicer. It's a massive difference from before. Everything looks new except the lettering in places has worn off. I don't really know a reasonable option for that without getting into silk screening.

The reference supply died again. I thought it might have just been a bad connection, but I think the power transistor is dead. I ordered new power transistors and I'm going to replace both with more modern parts. The amplifier works just fine. I tested it. I have three of the amplifiers installed and tested now as well. I need to get a function generator to properly test them, but I've not found a deal on eBay, yet.

I found a date of July 20, 1967 printed on one of the modules. I was 8 years old when this was built. It's amazing that so much of it appears to still work.

Cleaning up the front panel

I dismantled most of the front panel today. It was very dirty and scratched up in places. After some experimentation, I found that a good auto polish and the grinder with a buffing wheel does wonders for the paint. It doesn't put back where the lettering is rubbed off and I have no idea how I can do that. But, it looks much brighter and shinier than it did before.

The polish also works well on the modules. I have two amplifiers installed and working at this point, though I'll be down a while until I get the panel back together.

I also pulled the knobs. I put a coat of black paint on them to renew the black. It's not as good as the original anodized aluminum coating, but it's all I can reasonably do. After I am comfortable the paint is good and dry I'll put them in a drill and clean and polish the bare aluminum parts.

Reference supply is working now

I've been working on the reference power supply. It uses one of the dual amplifier modules that is pretty much identical to the rest of the ones in the machine and a module with a heated zener diode and a couple power transistors for current boost. The positive supply worked fine, but the negative was pegged at -15V. It should be -10V. I thought the transistor was bad, but as I was fiddling with it, it suddenly started working. I think it may have been a bad connection in the patch cords or in the adjustment pot. Now it locks on solid as a rock.

They are really fond of using little light bulbs as current limiters on output stages. They try very hard to protect everything from what a user might do like patching an output to a power supply.

I'll have to decide what to move to next. I'll keep an eye on this reference supply to be sure it continues to work.

Capacitor reforming

I did get the TR-20 completely dismantled. If they sit around for decades, electrolytic capacitors lose their internal insulating oxide. If you just plug them in, they will internally short, get hot, and potentially explode (not a big bang, but very messy).

But, they can often be recovered using a process called reforming. You apply voltage at the capacitor rating through a current limiting resistor for many hours and the oxide reforms. The picture shows the setup with lots of jumper leads. I'm reforming the 4 big 2500uF electrolytic capacitors in the power supply. I experimented with one and it was working well, so I set them all up and they are sitting there. They all appear to be recovering well. I will reform them for about 6 hours this way, watching the voltage carefully. There's some smaller caps on the power supply board, but I'll get these first. Those may be easier to just replace.

I want to get the power supply completely working first. Then I'll start moving in modules one at a time, starting with the reference supply, which uses the same precision amplifiers as the rest of the machine.

Bought a 1960's era EAI TR-20 Analog Computer

I managed to purchase a 1960's era Electronic Associates, Incorporated (EAI) TR-20 Analog Computer on eBay. These don't come around very often and I've put in snipes before, but never come close to winning one. This one was in West Lafayette, IN, which is in driving distance and was listed as local pickup, though he said he would ship in the ad. I think that impacted the price and I got it for less than people are paying for those horrid little Heathkit analog computers or the Compdynas. And, the computer museum has the owners and maintenance manuals online, so it's something I can work on. It has 14 amplifiers, 6 integrator networks, a multiplier, an x^2 diode function generator, the repetitive operations option, 6 precisions potentiometers, and 10 additional potentiometers. It has the card for the external display, but that was not included, nor was a power cord. It did come with quite a few patch cables and bridges.

It's one of the earlier ones, likely around 1967, so it's about 44 years old. It was the state of the art in that era for analog computation. It cost around $10,000 in the 1960's. I got it for a tiny fraction of that. It is, of course, completely obsolete, but it is a bit of history and I was fascinated with the analog computer they had at La. Tech when I was an undergrad.

It likely has been sitting around for decades or more, so I'm bringing up to slowly and hope to get it completely running. Many of the parts are pretty normal, but there are a lot of prevision parts and some things, like the chopper relays, may be very hard to replace.

I'm keeping my eye out for an XY plotter and scope to go with it.