Inside my ancient Tandy resistor substitution box

This is my very old and very obsolete resistor substitution box. And I thought I'd made a video about this before, but I've searched for keywords. Can't find it anywhere. But that's okay. Let's make a new video. And uh we'll go super high resolution on the actual circuit boards in this. So, this came from Tandandy a very long time ago. And let me demonstrate what it does. If I bring in a meter and I turn it on and select resistance and hook it up to the leads on this little unit and then rotate it. So at the moment it's displaying a resistance of nothing. Excellent. That means it's not clicked into position. Uh so this is 6. 7K 6. 8K. 8K. Okay. And I turn it around to another position, say for instance 100K, 99. 9K. Uh, and turn it to its highest value, which is 1 megga. And it says. 998 9. I think there is actually a bit of resistance in the uh sliding contacts in these, but it's pretty good. It's uh it served me well. It's been thoroughly abused. That's 100 99. 9. That's good enough. But the idea is that you can clip this into your circuit, dial up the resistance you want. There used to be a little label in this with an arrow and a sort of omega shape. Uh but I've just had a bea put a little line there because that uh that label detached a long time ago. You know, I could 3D print something with that, couldn't I? I could. But anyway, I digress. Let's turn this meter off. Stick it out the way and whip the back off this. Open it up. It's worth mentioning the this these are little feet. Are they little rubber feet or they just molded in? Oh, they are rubber. They're spongy. They don't feel rubbery anymore. But this is literally decades old. It's a really old unit. So, let's zoom up in it. If I Well, I'll for a start I'll note that when you rotate it, the lead stays still. That's quite handy. It's worth mentioning that because I thought it worked in a different way. But let's take this screw out that is probably not the original screw. This has been a part before. It's been serviced. Probably may even of new resistors when I've smoked resistors in it. Not sure. Let's see if we can find out. So, here is uh here are the wipers. So, this is the bit that stays still. And we've got these wipers, which could do with a bit of a clean. And these are the ones that wipe as you rotate the circuit board. They actually bridge the contacts of the circuit board. And the circuit board itself, that's kind of like very sticky like Vaseline. That's probably something I've done. Uh, the circuit board itself has Well, I'll take a picture of it, right? And then we can have a closer look. One moment, please. That's better. So, here is the back of the circuit board with the contacts. It's got a large central contact there. And then this is all solder because that's actually resistor leads. Then these holes are important. There's one under every resistor. And on this uh board here that stays put, there is a spring and a ball bearing. And that's not the first time that ball bearing's tried to escape. It tries to escape every time I've opened this. This has been serviced many times, but uh we have these indent holes. So as you rotate it, it clicks round into distinct positions with a good break between each resistor so you don't accidentally bridge and get weird resistor values. And then it's got the actual uh resistor pads themselves on the outside of the circle. And if we take a look at the other side of the circuit board, it has the remnants of glue. Why does All right. Okay, I see why. There is a large hole and three small holes matched by this and it has been glued onto this. So that basically that once it's in there um solid that then as you rotate this it's rotating the whole resistor array around on the stationary contacts. But these are resistors. Do any of them look different? This one looks different. 47 ohm. I think that one may have been changed. This one's chipped, but it's not going to There were a lot that were chipped. How's that? This They shouldn't really be chipped, but uh this is the one that's obviously been changed because something terrible happened in the past. I wonder what happened. Probably involving mains voltage. Uh where is that one here? Is it particularly visible? I'm just looking. It's a bit further back from that. It's there. Does it look really It does have different soda joints. So yeah, I've definitely changed that resistor at some point, but it does make me think theoretically you could 3D

print something. You could make a circuit board in the same style as this um and then uh 3D print an enclosure because these things are not made anymore. This one, as I say, came from Tandandy and for a while there was one Australian distributor selling them, but um there's that ball trying to escape again. uh but then they kind of disappeared and it's a shame because they're actually a really good versatile unit. But the resistor values are kind of strange because it starts off with 5 ohm which is a strange value to have then 10 22 33 47 56 100 and you think it's going to go up in decades after that but then it's 150 220 330 568 21K and it goes up in strange uh increments but technically speaking you could make that just about anything you wanted. Uh but that's it. That's what's inside. It's uh very straightforward, very handy. Uh yeah, it's seen a lot of use. But there we have it. Um the Tande or Radio Shack um rotary resistor selector. Um it has seen an awful lot of use and I'm going to give it a bit of a service and stick it back together again. That will see lots more use in the future, too. It's very handy.