Pulse Pendulum Projects

[TinselKoala]

Oh no, we've got to get that classic Tek 422 working again if it's at all possible! Fortunately this scope seems to be easy to work on and the photos I'm seeing show that all the transistors are in sockets.
http://w140.com/tekwiki/wiki/422#Specifications

(My traces are quite a bit different from yours but that might be because my scope is faster; in your first shot on the top trace you seem to be seeing, almost, the train of oscillatory pulses-within-pulse that I am getting.)

Is there a "Beam find" button hidden away somewhere? I guess not...   

You may find this link helpful, manuals are linked from there.
http://w140.com/tekwiki/wiki/422#Specifications

Step Zero: confirm that there is a problem:
Disconnect the probes from the scope. Then set all the Red knobs fully clockwise into their "Cal" detents. (Or to the "cal" midrange position if no Cal detents present). Set the display controls on the left of the CRT like you had them in these photos. Look on Page 13 of the manual here:
http://w140.com/tek_422.pdf
and set the controls as specified in Step 1 of the "first time operation" section.

Now power on.... what happens? Do you see a trace at all?

If not...
1st thing to try is to clean all the control contacts with a good brand of electro contact cleaner. I prefer LPS brand but you might not be able to find it where you are. Get a can, power off the scope and follow the directions on the can, spray every switch contact and inside every rotary potentiometer and work them (gently but firmly) through their full ranges several times. Remember where the "gain" and "step atten balance" pots are positioned and set them back to there once you've sprayed and worked them thru their full ranges. Gently pull out the transistors and the 8056 Nuvistor tube (in the earlier models)  one at a time and reseat them carefully. Don't mess with the CRT connections at this point. The Nuvistor, if present but nonfunctional, can be replaced with a common MPF102 FET, according to the link.

Repeat Step Zero, do you see a trace?

Do you have a function generator that you can use to provide a signal for further testing?

[PhiChaser]

@TK
I had the ground clip on both probes attached to the negative leg between the capacitors so I think it was just showing the smaller (1000uF) cap? Positive leads were between the resistors and between the transistors. I should have shown how I had the probes hooked up, sorry.
Beam find button LOL!!! No, that would be a nice feature though. Gotta wiggle knobs to find them on this one.
I tried the setup procedure; the trace is gone.
I haven't really tried to take it apart yet, the power supply (backside) seemed really clean inside when I took it off. There were some socketed transistors (?) in that part...
The power light comes on so the power supply appears to be still working. It connects to the rest of the unit through an amphenol (sp?) type of connector where the two parts come together.
I guess I will have to find some contact cleaner. One of the channel knobs occasionally gave the trace 'twitches' when it was being adjusted (and sometimes set the trace too high on the field) so maybe taking it apart and cleaning all the contacts will fix that (and whatever else might be the problem).
I will take pictures (and notes if I think I need them) when I take it apart.
There can't be that many socketed parts to test right?
It seems a daunting task but I'm game...
Thanks for the help TK, I will keep you (and whoever is reading this thread) updated.
Looks like I have some reading to do, thanks for the links!

Peace,
PC

[TinselKoala]

Bummer.
The "beam find" button is a nice feature that is found on many oscilloscopes, it tests the HV supply by taking the horiz and vert amps out of the circuit so you just get a spot  more or less centered on the screen, as long as the scope is capable of making a beam at all.

But you have no traces on either channel... so it is not likely to be a blown input FET/Nuvistor or the tunnel diodes. It could be that the "unblanking" circuit is bad or out of adjustment... or that there is no HV.

I had a problem with one of my scopes that turned out to be a DC offset swung all the way over, so the trace could not be moved onto the screen, this turned out to be a blown input FET and the associated diode. But since both your traces are affected this is probably not your problem.

Here's where I would get out the DMM and start checking power supply voltages as indicated at the test points on the schematics in the manual. The fact that the power light comes on doesn't necessarily mean the power supply is good or on the correct voltages ... it just tests the line cord, switch and bulb!

Peer into the neck of the CRT to see if the filament is glowing. If the CRT is bad, it's probably not worth fixing, you can get another working 422 for the cost and trouble of replacing the CRT. Sometimes the filament is hard to see even if it's good, so don't give up at first glance even if you can't see it.

Page 44 of the manual shows a troubleshooting flowchart that will help you to narrow down the problem. I'd just check the power supply voltages first no matter what, the procedures for this start in the Calibration section on page 81, section 6-6 of the manual linked above. Checking the HV supply is important, be careful here, you may need to find a suitable HV meter or make a voltage divider to do this part.

Once you know that the power supplies are within tolerance you can start checking the voltages at the test points indicated on the schematics. Look close and you'll see voltages indicated at various places on the schematics, and there are photos of the boards so you can identify where things are at. This will help you to find a bad part in one of the subsections.

Suspect the "blanking" circuit first, you may be able to fix the thing just by tweaking the blanking center adjust pot, see the manual in the Calibration section.

This scope does not look like it would be that hard to repair, and the information you need is probably all there in the manual.

[PhiChaser]

It looks like the HV supply is gone. I get about 6+ volts instead of 1385+/-69.5volts at the back of the CRT socket.
I figured my meter would at least 'peg' to the 1. (overload) if there was HV there but there wasn't...
Looks like you were right about the power light TK.
Now how much is it worth my trouble to hunt down the ass end of a 422?
I definitely need more coffee for this type of thing...

Peace,
PC

[Kator01]

Hello PC,

you may find this here interesting. This guy is serious and has a lot of experience. He has only posted
a few experiments but the quality is excellent

https://www.youtube.com/watch?v=Tm1LmkmYkjQ&list=UUVZmmMOhzhWEUamhGLPSTFQ

Regards

Kator01