Need help with a Tektronix 576 Curve Tracer! - Page 1
Forum section: electronics / repair

author: Stray Electron
author_rank: Super Contributor
timestamp: September 27, 2020, 04:53:28 pm
content: I picked up a Tek 576 a few days ago and I've been trying out all of the controls on it and I'm having problems with the Collector Voltage selector switch. The switch has an inner and an out shaft and an inner and outer knobs. When you pull out on the inner knob it lets you select the series resistance in the collector circuit and when you leave the knob in it's supposed to allow to change collector voltage. When you change the collector voltage you rotate the inner shaft (without pulling the knob out) and the mechanism inside then rotates the outer shaft and that moves the pointer that points to the voltage labels.Everything works fine when I pull the knob out and adjust the resistance however when I try to change the voltage the knob is very hard to turn, in fact, I have to use so much force that the knob slips on the shaft and the shaft never rotates. I replaced the factory knobs with knobs from a parts unit HP scope that uses larger set screws but even they slip on the shaft. The only way that I can rotate the shaft is to take the knobs off and use a pair of pliers but even that is difficult and it's chewing up the shaft. I have already opened up the chassis and lubricated both the detents (in both sections of the switch) and all of the contacts. It didn't help. I can rotate the switch to all four voltage positions using pliers so the switch isn't physical blocked but it is VERY hard to turn.Any suggestions?

author: 0culus
author_rank: Super Contributor
timestamp: September 27, 2020, 06:10:48 pm
content: There may be corrosion present even if you lubricated. I'd use some deoxit on it.

author: Stray Electron
author_rank: Super Contributor
timestamp: September 27, 2020, 07:00:46 pm
content: No signs of corrosion at all. It looks very clean inside. I used Deoxit on the switch contacts and rotors but from the small amount of testing that I did there doesn't seem to be a problem with dirty contacts. The only problem that I found so far is that I can't get it out of the 15 volt range because the stiff switch. The knob slips on the shaft rather than turning the switch.

author: 0culus
author_rank: Super Contributor
timestamp: September 27, 2020, 07:07:05 pm
content: Quote from: Stray Electron on September 27, 2020, 07:00:46 pmEven if it looks clean otherwise, there could still be hidden corrosion. You may have to take the switch deck out and inspect it. If it's otherwise clean, something is clearly not right if it's that stiff.

author: Larryc001
author_rank: Regular Contributor
timestamp: September 28, 2020, 12:03:35 am
content: Hi. This is a familiar problem I have faced on lots of Tektronix equipment . Everything appears clean and normal but the switch or pot won’t turn. The problem I have found is actually in the bushing where the shafts pass through the front panel. I have tried disassembly of the switch but have been unable to get it to free up. I am afraid the only thing I have found is to replace the entire switch. You could try the TekScopes.io group for more help. Good luck.https://groups.io/g/TekScopes/messages

author: srb1954
author_rank: Super Contributor
timestamp: September 28, 2020, 07:49:50 am
content: Sounds like that the grease inside the switch bushing has dried up. I have seen this problem quite frequently with other Tek equipment.Stand the 576 on its end so that the switch shaft is pointing upwards. Squirt a little bit of WD40 around the shaft where it enters the bushing and leave to soak in. Rotate the switch if possible to loosen the dried up grease and allow the WD40 to wash it away. Repeat the procedure over several days until the switch moves freely.Since WD40 is not useful as a long-term lubricant you will want to re-lubricate the switch bushing with some light sewing machine oil or similar.