 This is an HP 4145A semiconductor parameter analyzer. It can be used to characterize semiconductors like transistors and diodes where you create voltage current curves. Now you can see that it uses a CRT and there are two controls on the front for the CRT called intensity and focus. And the focus adjustment works pretty well. I can rotate the control and the text goes out of focus and it goes back into focus, so I can adjust it very nicely. But when I try to adjust the intensity, this is what happens. The thing flickers and the intensity does change, but it's as if the potentiometer is very dirty. So I thought that I would play another round of Rob tries to fix a thing. So I've already taken off most of the screws and there's the front panel. It just comes right off and then there's the control board which contains all the buttons and some lights and that just unplugged. And these are the controls. Now they're held in by these nuts and this is the intensity control that is not working very well. Now I looked at it and it appears to be a pretty sealed unit. There's no wiper in the back. Everything is very nicely sealed. So what's going on is not just a simple matter of dust getting between the wiper and then just using some contact cleaner to clean it. I basically have to replace this. Well, I went online and looked up the brand of this and I could not find an equivalent replacement. And the worst part is that of the replacements that I could find that were approximately close to the value of this potentiometer, they were all imperial threads. This is a metric thread. So unfortunately that means that this piece is no good for the replacement potentiometer. Now here is the replacement potentiometer. It's obviously not perfect. It's kind of bigger in some sense and smaller in another and I will probably have to cut this shaft down. And of course, because it's not metric, this nut will not fit on top of it. So I had a good friend actually make a replacement nut, which is of the same size, of the same width, but it has the correct thread to fit on this potentiometer. So that means that I should just be able to put the potentiometer in back here and then screw the nut back on and have it work just like the other one. So I just need to desolder these connections and solder it to the replacement potentiometer and hopefully we should get a nice smooth intensity control. Now if we look a little closer at this potentiometer, it's probably going to be very difficult to make out, but on the side it says 1K ohm and there are three terminals in the back. Now two of those terminals will measure 1K and the other one is going to be the variable wiper. So it's important to get the connections correct. So there's a red wire, an orange wire and a brown wire and I'm going to check across the red wire and the orange wire to see what the resistance is and I'm getting 0.8K. Now if I check against the red wire and the brown wire, I'm getting 28 ohms and if I check between the brown wire and the orange wire, I'm getting 0.8. So I'm not quite sure which wire is which so I'm going to move the wiper to a middle position and I'm going to try again. So again, orange to red, that gives me 600 ohms, red to brown, that gives me 330. So that probably means that between brown and orange I'm going to get the measurement that I'm looking for. Okay, so this isn't exactly 1K, it's more 870 ohms. Now this is the replacement and if I go across, I get about 1K but the range is about the same so I think I'm happy with this replacement. So let's go ahead and replace it and in fact now that I've disconnected those two outer wires I can measure the potentiometer now that it's out of circuit and get its correct value which is 975 ohms, pretty close to 1K. Now because the terminals on the new pot are not solder terminals, they're actually PC mount terminals while these are actually solder mounts so you can mount the wires directly to it. Instead I'm just going to solder these wires on like this and I'm going to use a little bit of heat shrink tubing in order to protect them in case this happens to move and the wires sort of touch each other. That will really only be a factor when I install the potentiometer because once I do it should be fixed in place and the wires shouldn't move. Okay, and now that the wires are soldered on and the heat shrink is in place I usually like to shrink the heat shrink using a heat gun rather than soldering iron because that way the heat is more evenly distributed I feel and also there's no danger of desoldering the wires while they're inside the heat shrink. And it looks like I didn't actually have to cut down that shaft. I think as long as it stays within this nut it'll be fine. Well let's turn it on and see if I can adjust the intensity and not have it cutting out. Okay so it looks like we have a blank screen but if I adjust the knob Aha, there we go. Now I can adjust the intensity quite nicely and there's no cutting in and out. Great, well it appears that I've fixed something. It was a small thing but it was a nice thing to do. So thanks for watching. Rob tries to fix a thing.