Glowing Tek RM17 updateWorked some more on it.
Last night I looked into AC/DC input coupling switch to see why it was stuck / inoperable.
Turns out it was very much like Papa Smurf had recently on his Tek acquisition. The shaft was kinda "welded" to the body of the switch.
However luckily, just like him I was able to revive it, so it was not yet welded solid..
Using pliers with long handles, and with a lot force, I was able to turn the shaft, which was good news.
![](https://media.monmagasingeneral.com/media/catalog/product/cache/1/image/1200x/9df78eab33525d08d6e5fb8d27136e95/2/E/2E756F1W2W-pince-multiprise-standard-facom.jpg)
So I put the scope on its arse to have gravity help me a bit, and applied a bit of penetrating oil and worked the switch a few times. Help only marginally... so I soaked it with penetrating oil again and went to bed, letting it soak over night. Must have worked. This morning there was a noticeable improvement, so much so that I could now use the knob to turn the shaft, no need for pliers anymore, great. However the knob with so much less lever action, was of course extremely stiff, unusable. I could turn it using my fingers, but it was very hard/painful. So I put some machine oil this time, and worked the switch a 100 times for 20 minutes. Eventually managed to make it 100% smooth again, phew !
![Phew :phew:](https://www.eevblog.com/forum/Smileys/default/phew.gif)
Works as good as new now, super happy. However the skin on my fingers came off, now it hurts and will take a few days to heal.
![Angry >:(](https://www.eevblog.com/forum/Smileys/default/angry.gif)
But well, at least I achieved something, so I don't mind the pain too much.
Then after that to finish it off, I replaced the knob because it was damaged. A small chunk of it was missing at the top on the edge, right by the white dot. Not much of the dot is left.
That's when you appreciate having lots of Tek scope donors you can just pinch whatever part you need from !
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/cheesy.gif)
Then a drop of machine oil in the detent mechanism of the attenuator and time base switch.. did wonders, they turn much more smoothly now.
Some contact cleaner in the attenuator switches too, to see if that would solve my problems.
So now I was ready to test drive the scope again to see if that made any improvement to my two problems :
On ranges 20 / 2 / 0.2 and 20mV / DIV settings, it fails to display the square wave. Instead, it displays sharp negative and positive pulses. That is, the square wave is like going through an RC "differentiator" circuit. a parasitic one of course ! Must be a simple fix I guess... broken connection, bad contact somewhere...
Since it affects only all the 20/2 etc ranges, the problem must be localized to a decade selector, hence should be able to narrow it down quite a bit in the schematic, I would think.
Zero improvement there, contact cleaner didn't help. So there must be a solid failure in the x2 attenuator RC network (see below).
Need to locate it, hoping it's accessible enough for me to inspect it up close.
- On the 3 most sensitive ranges, 50 / 20 / 10mV, which are all AC only, the amplitude of the trace is way too small ! Like 2 or 3 DIV instead of 10, something like that. Also, the trace incessantly moves up and down, like a roller coaster. So me think the AC coupling cap is dead, and passing only part of the signal.
With AC/DC switch now working as good as new, I was able to test the AC coupling cap. It works perfectly. So that's not causing my problems. It must therefore lie in the circuitry of these 3 AC only ranges. However, I am confused because looking at the schematic... I don't see anything circuitry specific to those 3 AC only ranges
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif)
So I don't see what the problem could be
I had another play with the DC balance trimmer. Looks like it's very finicky. Very touchy and can't keep its setting very well... and there is a huge asymmetry in it's adjustment range. Can make the trace only one division upward, but downward it can make the trace go off the screen way off, at the bottom.
So much so that I though well maybe again a tired 6AU6 tube in the vertical amp, a classic problem. But the amp looks fine to me : with the input grounded, and the vertical control centered, the trace is centered as well. And it I turn the knob end to end, trace can easily go off screen either side.
So the DC balance asymmetry is specific to that control... need to look that up in the schematic.
Cooling fan now turns 100% freely, very happy with that. There is a bit of axial play in the rotor, one millimeter or so, that makes the baldes hit the air filter if you raise the front of the scope a bit with a spool of solder as you do... but well, a rack mount scope is not meant to anything but level so... I can't really complain. Maybe I can take the motor apart and add a washer to take up the slack, as I gather people do that kind of thing on old TE...
That's all for now. If you have brilliant ideas to help me solve my 2 problems, don't be shy, you will have my immense gratitude, if that's any motivation.
Time to go do some more productive work now, on my garage construction. It can't be all electronics 24/7, life is more diverse than that...