Wednesday, October 14, 2009

Bi-N-Tic Filter: Panel Assembly and Hackery

Didn't turn on the soldering iron tonight. This was the night for mechanical work on the panel. The first thing I had to do was decide what to do about the locator pins on the pots: cut them off, or use them? Using them meant drilling additional holes in the panel. What I decided to do was cut off the pins on the pots that are soldered to the board (they aren't going to rotate anyway), but use them and drill the additional holes for the pots that are panel mounted. Here's how it came out:

Another angle:

This doesn't show from the front; the knob body covers the hole:

So I went ahead and drilled the rest of them. Holding it up to the light:

Note the butchery on the second from the left. This pot is a double pot; two on one shaft, and I neglected to note that it has a larger body and so the locator pin is further away from the center. Fortunately, the knob will still cover the mess up. The hole in the very center is the one I added for the switch that will select which outputs from the counter will be routed to the capacitor-switching muxes.

I had been a bit up in the air about what knob I was going to use on the rotary switch, so I test-fitted the switch and knobs to the panel. Here are the candidates. First we had the knob that came with the kit; it is of course popularly known as a "chicken head" knob. It's a style from the 1950s. I like retro-modern as much as the next guy, but to be honest, I just don't know if I care for the chicken head knob on this panel.

Second up, we have a knob from my knob collection. I don't know where this came from, but it is closer to the style of the other knobs. However, it's rather large, and it covers up some of the panel legending above the jacks below the switch:

Here's an interesting one. It is one from a Radio Shack two-pack that I bought in 1980 for a guitar repair job. As it turned out, I only used one, and this one has been in the original baggie since then. Here it is:

And, just for fun, the bag it came in:

Yes, it says $1.89, and that was for two. Ah, those were the days. Of course, in 1980, I was a starving college student, and 1.89 was a week's worth of Kraft Macaroni & Cheese. Anyway... what I'd really like is the style of knob that uses on its rotary switches. Here's an example:

So now it's time to assemble the pots:

A close-up the resonance and excite pots. Note that the shaft of the resonance pot is shorter:

Next step was to get the rotary switch installed. I discovered, somewhat to my surprise, that it also has a locator pin:

At first I couldn't figure out how to set the rotation stop limit -- it's a 12-position pot, and the circuit only uses 5 positions. Then I realized: the way to do it is to pry up the little washer that you see at the very base of the shaft housing. It has a little tab that sticks into a hole, and that is what sets the stop. You pry it up, rotate it to the proper position (they are marked on the body face), and then push it back down.

Having done that, I decided what the heck, and I went ahead and drilled a hole in the panel instead of cutting the pin off. I was concerned about whether or not the knob body would cover the hole, due to the radius of the switch body, but I went for it anyway. Here's what it looked like as I was trying to install the switch -- I had to drill the hole out a couple of times with progressively larger bits in order to get it to fit:

Sure enough, the body of the chicken head knob didn't cover the hole:

But that was OK since I didn't plan to use it anyway. The body of the aluminum knob covered it just fine:

Next step was to fix the paint abrasion that I caused when I drilled out the hole for the new switch. This is easily done with a bit of black modeling paint on the tip of a paper towel:

I didn't put all the knobs on yet, but I did one to see what it will look like. From the front:

And from the side:

The bottom flange of the knob is up off the paned about 3/16", which seems a bit much. But I checked with some other modules I have, and they are all in that same range. I really don't want to have to get out the hacksaw and cut 1/8" off of the shafts, although I am concerned about the resonance pot with its short shaft. It'll look funny if the resonance knob is right down against the panel and the others are all standing off of it this much.

The last task for the night was to cut the locator pins off of the pots that are soldered onto the board. This turned out to be easier than I thought; applying sideways twist with a pair of needle-nosed pliers snapped them right off. The results:

And a close-up -- look at the right edge:

Next: install the jacks and start wiring up the panel.


krelnarb said...

This is so great, I'm building a Bi-n-tic at this very moment and popped on line for some info. Your blog really helps. Thanks!

Dave Cornutt said...

You're welcome!