Look what I’ve got…
The parts for our Resistor Substitution Box are in. I had two issues with the parts order, both related to the knobs. When the knob order arrived from Amazon, the envelope was empty. It was completely sealed all the way around, no rips or tears, but there was nothing in it. I called Amazon and they refunded my card and I reordered the knobs. The second issue is that the knobs don’t fit the potentiometer shafts. They are just a little too small. I looked at drilling them out, and was successful with a few, but I don’t think that’s a good solution. I also considered shaving down the shafts but can’t really recommend that either. I’ve reordered new numbered knobs from Mouser and they should arrive within the next few days. I’ll keep you posted.
Let’s get started on the build. It will be a lot easier if we solder the resistors and lead wires to the switches before we install them in the case. And we don’t want to manhandle them after the resistors are soldered on. So we’ll mark, drill, and test fit everything in the faceplate so that installation will be a breeze later on.
Marking Out the Top Panel
You are welcome to do it differently, but here’s how I did it. To start, I removed the faceplate and prepared it by applying blue painters tape. After applying two pieces of 3” tape side by side, I turned the faceplate over and trimmed the tape to size using a hobby knife. I’ll leave the tape on to protect the faceplate until I am ready to apply the labeling and install the switches.
Next, I arranged the switches and binding posts on the faceplate to get approximate locations and ensure everything would fit. I also made sure there was enough room between the switches and the side panels so there’s no risk of accidental shorts. Remember, I chose an aluminum case. Once I had an arrangement I liked, I took some measurements so I could transfer them to the faceplate itself. Here’s the dimensions I came up with. All of these are from the top and bottom edge and do not include the flange that slides into the case grooves.
Using a pencil and a straight edge, I transferred the measurements to the front panel. I marked the center of each rotary switch post and the binding posts. I also measured the hole sizes needed using my calipers. The rotary switches require a 5/8” hole. The binding posts (measure yours – I used some I had laying around and not what I recommended in the last installment) required a 3/8” hole.
Next, I moved to the drill press. I drilled 1/8” pilot holes for all of the components, switches and binding posts to get the centers correct and guide the larger drill bits. Then I worked up in drill sizes (1/4”, 3/8”, and finally 5/8” for just the rotary switch shafts).
Preparing the Rotary Switches
Once the holes were drilled, I cut the small nubs off (intended to keep them from spinning when turned) of the rotary switches. I didn’t want to drill additional holes for these and not having them will make it easier to align the switches so the top is at the 12 o’clock position. This also allows the switches to sit flush against the bottom of the faceplate.
Test Fitting the Components
Next, I installed all of the components to ensure they fit properly. They fit perfectly. I did find that without the little nubs on the switches I’ll need to tighten them down really well at the end.
At this point I attempted to fit the knobs so I could see how much I had to cut off of the rotary switch shafts. I don’t want the knobs sticking up too high with a lot of switch shaft showing below. That’s when the cursing started. The knobs are just slightly too small to fit on the shafts.
Next Steps
I’ve since ordered new knobs from Mouser (the numbered ones were about $6 per knob – ouch) and they should be here in a few days. In the meantime, there’s nothing to stop me from continuing the build by wiring up and installing the switches.
I’ll do that in the next installment which will be posted in a few days. Stay tuned.
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Cheers
Dominick
