The computer appears to be working electronically now. There are a few things that seem a bit wonky. The Reset/Hold/Operate switch may be a bit dirty, since it was not working reliably at first, but using it seems to be clearing it. Also, I can't get the meter to null pots when the voltage is negative, so there is something not right there that I'll have to fix.

But, it works and it computes. The circle to the left is what you get for y''=-y, where I'm plotting y in one axis and y' in the other. Since the answer to this equation is a sinusoid, you expect it to plot a circle, which it does. Interestingly, it will draw over that same circle over and over again with no apparent drift. After all these years it is rock solid. The plotter did start to have problems in one axis after a while, so I'll have to work on it.  But, the computer really does work.

It is all together other than the case, which I'm painting. I should get it installed this week. I'll look at that pot bus issue and tear back into the printer again to see why the X axis is binding up.

Back to the project

I can't believe it's been all of the way since May that I've had time to work on this project. Summer is a time of travel and projects on the house, then I had a really heavy load this fall with a lot of students, buggy software, and revisions to courses that I teach. I've just not had any time at all to work on this project. But, the semester is finally coming to a close and I am starting to have a bit more time available as I finish up projects for classes.

I've been working on two things lately. I finally got around to calibrating the quarter squared multiplier. I thought it had a problem with one section, since the bottom range was not working, but after I had finished the other sections and went back to it, it started working as I was moving pots. I think it had a dirty pot that was not making contact and I cleared it when I moved them. Anyway, it appears to be working now. I had to break the factory seals to adjust it and the adjustments were very small. It was almost in spec before I even started.

I'm also working on painting the case. I'm still debating about what to do. The original textured powder coat was pretty scratched up and was stained on top where people had put drinks and probably cigarettes (remember, this is from the 60's). I tried painting a coat on it to retain the texture, but I don't really like it. I'm debating between brown, the original color, or black, which I think would probably look nicer. I considered trying to get it repainted by someone who could duplicate the finish, but I don't really think that matters all that much, so I'm going to make it look how I like.

Multiply also works...

The last untested electronic component is the multiplication module. I plugged it in and it appears to work in all four quadrants. It did take a lot of patch cords to set it up, since you have to feed it both a signal and the negative of the signal to multiply. Hence, it took two amplifiers just to negate the inputs. But, it appears to work. I needs to be calibrated, of course. This is the last major module to plug in and the monster will be all working, though there is at least one amplifier that is still wonky. I won an auction for a function generator today, so I can test them all properly.

Note the simple service shelf I made.

I did get the x-square module calibrated and tested it a couple days later to ensure it was holding calibration. It works perfect and has been installed. I wrote a post about that at the time, but blogger ate it in the great crash they had and it has not come back.

Power inlet

Up till now I had been powering the TR-20 through a power cord I soldered to the back of the power inlet. It had an old 3-pin power socket and did not come with a power cord. I tried to find one of those cords. I'm pretty sure I've thrown a few away in the past. But, no such luck. So, I replaced the power inlet with a modern IEC style inlet. It looks much nicer now not to have the cord dangling from the back and it's probably safer as well.

X2 Function generator

I put power to the two sections of the x² function generator. In keeping with my good fortune so far, it works. It is out of calibration, of course. The largest error is about 3.5%, though it is closer to 1.5% over most of the range. It just needs adjusting.

It works by approximating the quadratic function with a 7-part piecewise linear function. They use switching diodes to determine the segment. Like all things on the TR-20, it works in conjunction with an amplifier section. This is why it needs 14 amplifiers. There are two x² sections, one for negative input and the other for positive. You can patch them both together to support bipolar input. Both work, though I did not test the tolerance in the position section, yet.

I hooked up my homemade extender and made a pass at calibrating it. It worked just fine for the negative side, though the first two pots are at the limit (but are in tolerance). The positive side would not work with an input greater than 7.5 volts. The diodes all tested okay. After some diagnostics, I figured out it worked perfectly on a different amplifiers, so amplifier 13 appears to have a problem with larger negative voltages. I'll work on it later on.

I also hooked up two of the precision potentiometers to the 10V supply and the inputs to the plotter. It works perfectly, though I don't have a pen, yet. You can adjust the pots to move the pen on the plotter. It's the worlds most complex and precise Etch-a-sketch.

Companion plotter

Two common output devices for an analog computer are an oscilloscope and a plotter. I picked up a 80's era HP 7015B XY plotter on eBay to go with the TR-20. It was filthy and the white surface was peeling up when I got it. I dismantled it and cleaned everything thoroughly. A bit of glue fixed the paper grip surface.

The white surface has a grid under it with 900 volts DC on it. When activated, it creates a charge the pulls the paper down and holds it in place.

The TR-20 I own originally came with a 4 channel XY scope. It even has the module to feed signals to the scope and the connector on the back for it. I doubt if many of these have survived. I may try to find a good use 4-channel scope to recreate that functionality later on. For now, my simple 2-channel Tektronix scope should work just fine.

All amplifiers are working

Amazingly, I have now cleaned up and installed all 7 amplifier modules. All 14 amplifiers work! They balance very well, don't seem to drift, and they amplify correctly. I still want to put some AC signals on them to test them more thoroughly, but I'm waiting to catch a function generator as a good deal. I've also put back in three of the potentiometers modules and the second dual integrator. Other than issues with the reference supply, everything is working. I've cleaned it all up substantially, so i looks much nicer as well.

Wish I could find some more of those nice EAI bottle plugs.