Wednesday, October 21, 2009

Finishing the Bi-n-Tic, and A Couple of Oopsies

I finished up the panel wiring last night. Here's the results:

Before I did the last of it, I reviewed the existing work and I found three mistakes. The first two were caused by my misreading of the CGS Web page on how to wire up the "excite" input, which takes divided-down waveforms from the counter and allows them to be injected back into the filter input. For some reason, I looked it and saw a four-position rotary switch, selecting one of the Q4/Q5/Q6/Q7 counter outputs, going to the switch contact on the external input jack. But in fact, it's a five-position rotary switch, and the jack is wired to the fifth position. So that will teach me to look more closely. I had to re-do the wiring of the jack, and of the first position on the rotary switch (fortunately, I had not soldered the rest of them yet). I was worried about the rotary switch since it's a plastic bodied type, but it appears to have survived. Here's how that came out:

Note all of the unused contacts. This is actually a 12-position switch, but I've got the stop set at position 5. To the right of the switch is the pot that controls the exite input level.

The other big booboo I made was that I put the banwidth pot in the panel where the resonance pot is supposed to be, and vice versa. In the picture below, the big double-gang pot (the bandwidth pot) is supposed to be at the far right:

That wouldn't be hard to fix except for one thing: remember me talking about those locator pins on the pots, a couple of episides back? Well, the big pot has its locator pin in a different place than the small Alpha pot. So in order to put these two controls where they are supposed to be, I'd have to drill two additional holes in the panel for their locator pins. At this point, I really do not want to be drilling on the panel and getting metal shavings all over the board. I can still detach the board, but the wiring makes it difficult to get it more than a few inches away. So instead, I'm going to "fix" it by putting new legending on the panel. If reality doesn't conform to one's pre-conceived notions, then change reality!

I waited until the end of the build to insert the ICs into the board. I wound up wishing I had done that before I did the panel wiring; some of the socket locations were difficult to get to, and I bent a few pins. But eventually I got them all in. I'm glad I took a photo of the bare board before I started assembly, because with the sockets in, it was impossible to see the silkscreening that shows which IC goes where. But with the photo, I was able to figure it out. The kit substituted TL071 opamps for the LF356s indicated on the silkscreen; apparently the LF356 is no longer available in a through-hole version.

The photo below shows the pads for the panel wiring to the resonance and bandwidth pots. The silkscreen didn't indicate which pads were supposed to be connected to which terminals on the pots, so I had to follow some of the traces away from the pads to see where they went, and compare to the schematic. Here's the photo:

The three pads at the right (note that we're looking at the board upside down, relative to the silkscreen) go to the resonance pot. As near as I can figure, they go to the pot's terminals in the same order as they are on the board: left (blue) goes to the left terminal, center (yellow) goes to the wiper, and right (orange) goes to the right terminal. We'll see when I try it; it may be that I've wired it so that the pot works "backwards". If so, I'll switch the left and right at the pot. To the left of these pads, there are two pairs of pads which go to the two gangs of the bandwidth pot. In each pair, the one on the right goes to the wiper terminal; the other goes to the left terminal. (I've connected the wiper and the right terminal together, as shown on the schematic, but I don't really know that that does anything.) Note the two pads to the left of the bandwidth pads, labeled "in". These are filter inputs that don't go through the input level pot. (The one that does go through the input level pot is elsewhere.) I've used one to connect the exite signal from the excite level pot, and the other is unused.

Here are a few more detail shots of the assembly. I wound up running four grounds. The filter signal input got its own ground. The other inputs share a ground, and the outputs share a ground. The jack wiring:

The grounds connected at the pads that are intended for a Eurorack 16-pin power connector. Since I'm building this for a 5U configuration, I'm using the MOTM-style power connector above and to the right, and the six ground pads on the Eurorack area made a handy place for bringing grounds to. Note there are four here; the three I mentioned above, and a fourth that goes to the excite input level pot.

The switch that I installed for the capacitor switching mux mod:

Finally, I put the knobs on (not indexed yet, just sitting there) to show what it's going to look like when it's finished:

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