Tektronix T912 Oscilloscope w\ broken trigger.... Help? (teardown pics inside)

[lunarfox]

This is my first post here, please be gentle! As such, I also thought I should introduce myself with some nice internals of a Tektronix T912 scope (unfortunately broken) seeing as nothing came up on the search. It was given to me by a friend 'cause I wanted something a bit smaller and more featured to replace my Type 503 scope; the storage function was also a huge bonus for me 'cause I sometimes want to look at really low frequency waveforms. It's really quite interesting watching a low speed trace "writing" the wave form to the storage screen. Anyone know exactly how that works? I'll make a video of it when I put it back together; sadly I did not think to take any pictures before taking the case off, but there's plenty if you just search google.

Before I get onto the pictures though, a bit of a dilemma. Everything seems to work well (though some of the pots needed a bit of cleaning) except the trigger which will work fine for 10-15 minutes (until it starts getting warm) then completely quit. I've scanned the easily removable boards (see pictures) for cold solder joints or obviously dead components, but everything looks like it's in excellent shape! Anyone have any suggestions?

I've tried to find a service manual, and keep finding myself pointed to this page which requires you to log in to download, but doesn't actually do anything when you register! If someone can get me that service manual, I'll host it indefinitely on my DropBox!

I've never trouble shooted a scope before, nor have I taken one apart (aside from the side panels on my 503 to tweak and clean a few things), and the way this thing is put together is actually quite nice! Almost everything is modular, and the main boards come out with just a couple screws each. It doesn't feel cheap though! It also has little rubber feet on the back of it so it can be stood on end when you're removing the case and tweaking it, excellent! Everything is easy access which is honestly not something I expected seeing how cramped it looked inside. Oh, and before anyone says anything, this may be my first time inside a solid state scope, but I know perfectly well about the high voltages inside!

Thanks in advance to anyone willing to help; even if I can't fix it I hope everyone enjoys the pictures! I'd be happy to take more if anyone's interested!

Anyways, on to the pictures! I'm gonna post a handful in IMG code, but I'm hosting all the high resolution pictures here: Tektronix T912 guts


Front:


Sides:


Bottom with and without input board removed, shows trace and trigger board (right):



Also, one more little question, what is this coil of wire for? It's connecting to and from the same board (the input board specifically), has two really thin conductors, and is heavily shielded:

[maelli]

hello lunarfox
you can find the manual and lots of info on the tektronix "hoover" scopes here:

http://www.logwell.com/tech/oscilloscopes/Tek_T922R.html

the line you mentioned is the delay line.

I do have the T935A, faster, dual timebase, but no storage.
After some troubles with the HV supply (had to roll my own HV multiplier)
 it works perfectly.

[lunarfox]

Thank you very much maelli! I've already found a lot of valuable information! I'm gonna work through the manual's troubleshooting for the trigger circuit and check VR4367 as suggested by the service notes on that website. But that's for tomorrow, I'll report back with the results!

[cracklingaudio]

Sorry for this tardy reply.
I have the same unit T912 and may have the same problem. I'm frying R458(150ohms).
What is the HV multiplier and how would I "roll my own" HV multiplier?
20-30 seconds  after turning the unit on R458 gets really hot and if I don't turn it off it will fry completely.
Thanks ahead of time.
Crackling

[gkmaia]

Have any of you still have the 912 scope and may want to sell a few parts of it?