Sunday, September 13, 2009

Rebuilding a Super Psycho, Part 2

OK, I know it's been a while, but I ran into some employment difficulties. But that's all resolved now. Anyway, I've been working on the Super Psycho rebuild. The first step, as I documented in the previous post, has consisted of moving everything from the circuit board that was falling apart to a new board. Below is a photo of what I've gotten done:

A few things to note. I replaced all of the resistors along the top of the board with new ones from my stock, mainly because desoldering the old ones was going to be such a pain; that was an area of the old board where I'd had to add a lot of jumpers and kluges. Most of the capacitors were salvaged from the old board; I tested them all first. The two green 100 uF caps at the left are new from my stock because the old board was an earlier revision and it didn't have these. There's also a 10 uF cap above and below these; I replaced the one at the bottom because the original was borderline (measured 8.1 uF vs. a specified tolerance of 20%).

I've installed MTA-100 headers everywhere that panel wiring will attach, a la Dotcom. This should help considerably with the soldering mess that the old board had along the top edge, where 42 individual wires were soldered in. Look, it's much cleaner now:

(This photo was taken prior to washing, which is why it appears to have a lot of excess flux; it does. I used organic solder for this part.)

I wound up replacing the transistors and the IC sockets. Really, trying to salvage sockets just isn't worth the trouble; I destroyed one of them during the desoldering, and even though I got the other two out, they had distortions that made them very difficult to insert into the new board. Sockets don't cost much. I also decided to replace all of the transistors after two of them had the base leads break off while I was stuffing them. The replacements aren't the same type; the originals as specified by CGS were BC557, while the replacements are 2N4403. (Why those? Because that's what I had on hand.) All they do in this circuit is drive the LED indicators; almost any PNP type should work for that.

So the basic board is almost ready to go. There are four 470K resistors that go on the board just to the left of the 8-pin socket at bottom center in the first photo. The originals were butchered and I didn't try to salvage them. These resistors form a passive mixer for the four oscillators on the board that don't have the switchable waveform. I want to take those signals off the board at this point, so I need to get new resistors that I can stand up and have a long lead to solder a wire to. This is in pursuit of one of the two mods that I've decided to make: adding a second output bus with mix capability. Below is a block diagram of what I have in mind:

There are six of these, one for each individual oscillator. I'll pick off the four non-waveform-selectable ones from the board as stated above, and I'll intercept the other two at the waveform selection switch common terminal on the panel. The signal will come into this block at left. Each will have a pot to mix it to the B bus. There will also be a switch that allows the signal to be removed from the A bus (the on-board bus). To save some panel space, I decided to use pots with pull-out on-off switch capability to implement the A bus switching. I wasn't going to do that originally because of the cost, but I found some inexpensive ones from All Electronics. Only problem: The switch is SPST and the "on" state is when the knob is pulled out. I want the opposite sense, so to create it, I'm going to invert the switch signal and then use it to switch a bilateral switch that will feed the signal back to the A bus.

The other thing I'm doing, which I haven't drawn yet, is adding control voltage capability. I'll do this by putting a vactrol in series with the resistor that determines the frequency for each oscillator. There will be two CV inputs, an A and a B, and for each oscillator there will be a switch that switches it to the A input, the B input, or neither. I haven't decided yet if I'll make any attempt to compensate for the non-linear response of the vactrol.

I'll build all of the new circuitry on a piece of stripboard. I'll make an auxiliary panel to hold the bus mixing pots, the A/B CV input switches, and the extra jacks. I haven't figured out yet how I'm going to do this. One possibility is getting a blank MOTM-format panel with studs on the back for a board mounting bracket; that would allow me to mount the stripboard to the auxiliary panel without any screws coming through the front. Another possibility is using JB-Weld to glue standoffs to the rear of a panel and mount the stripboard that way, Dotcom-style. The third option is simply to mount the stripboard on the bottom inside surface of the case, behind the panels. However, that would make it harder to move the combined modules later.

I had to order some more 470K resistors from Mouser to replace the ones I messed up. I also had to order additional 100 nF (0.1 uF) polyester box caps; they are used for decoupling, and the rev B board uses more of them than the old board did. I've got these now and I'll be installing them next week.

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