Electronics > Repair

Tektronix 2235 Oscilloscope Channel Repair Help

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EpicIntelGamer:
Hello everyone!

I'm a amateur electronics hobbyist, I've watched a lot of and enjoy the EEVblog videos on YouTube, I've been learning a bit here and there about electronics for about a year now, here and there, and that's on top of what I know about electrical stuff. Thus I am fully aware of the mains potential and also HV for the CRT inside these and understand how to go about my poking around safely.

Back in my early days of learning about electronics I had accidentally fed the second channel of my Tektronix 2235 oscilloscope 800v, much more than the 400v limit. Ever since then when I try to use the second channel, whether its in AC, DC, or grounded, I get no trace. It's not a real big deal for me to fix this channel because I rarely use the scope and 1 is usually good enough, but if its easy enough to fix, I figure why not.

I decided to crack it open today and look for anything obvious, unfortunately nothing stood out in those mountains of complex pcb's.

I am wondering if there is anything in specific that I should check out because as I said, I saw nothing obvious and the service manual I downloaded is also no help.

I can provide pictures (low resolution) if needed.

Thank you in advance.

EpicIntelGamer:
Anybody?

I'd like to get this fixed guys.

mtdoc:
In case you haven't seen it yet, THIS might help.

Good luck.

EpicIntelGamer:

--- Quote from: mtdoc on July 30, 2013, 02:39:01 am ---In case you haven't seen it yet, THIS might help.

Good luck.

--- End quote ---

No I've not seen that yet.

I took a look at it and began the process for troubleshooting the vertical section and the strangest thing happened.

I went to change the trace thing to show channel two and that was when I noticed, it actually showed a trace for channel two. I was intrigued and I connected a battery with some wire to the input and I'm very delighted to say that it does indeed seem to be working now!

I think I know why too. When I had the scope opened up earlier today I was examination the rotary switches used. They are very different to most others and definitely a world different than the rotary switches in the tektronix 2225 tear down video, these are pieces of thin metal that are parallel to the vertical PCB and they are pushed down at certain positions by the rotation of the rod connected to the switch because of bumps on the rod, the only way I can think to explain this is like a camshaft hitting the pushrods, instead of pushrods, its contacts.

Since I found these contacts interesting I sat for a while and just played with the knobs as Dave does. I think that perhaps when the 800volts was applied a year ago that the ticking noise I heard when the cap (the sorce of the 800v, its a long story) discharge might've welded some of the contacts down with some very tiny force and me playing with the switches freed them up.

Either way both channels are now working! The only problems remaining on this oscilloscope are my 1) lac of actual probes, 2) missing clear plastic indicator with numbers on it that should be around the knob of the second channel voltage selector (its not a huge problem, I just have to look at channel 1's knob to see where the second knob is at as a reference), and 3) I do not know what state this oscilloscope's calibration is in.

Any suggestions on how I can calibrate this? The only relevant things I can think of to calibrate it is the scope itself and a 555 chip I have, I dont have a function generator or anything.

mtdoc:
Glad it's working! :clap:

Reasonable probes can be had for cheap ($10 or so) on ebay, etc.

Depending on what you're going to use it for it may not need calibration.

Have you checked the probe compensation output?  It's probably a 1kHz 0.5V square wave. If that output looks ok you're probably good to go for most basic electronics.

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