Help me fix my Tektronix 485 'scope!!

I have a Tektronix 485 oscilloscope that needs a new power supply assembly. AFAIK, that’s the only problem with the unit. Does anyone know where I can get this part cheaply? I’d repair the board, but the critical parts all have Tektronix proprietary numbers, so I have no idea where to begin. It looks as though someone attempted a repair, but the soldering is so poor, I can only assume they totally screwed it up. Help!

You can get this unit on Ebay
Tektronix 485 350MHz Oscilloscope

Depending on price it might be cheaper to part out the old unit and use it’s PS.

Do you have a schematic?

I’ve never seen a power supply board for a scope, but I can’t fathom why most of the parts can’t be replaced with generic components. Is it a linear or switching supply? What’s wrong with it? Did you lose a rail? Is it blowing a fuse? Too much noise or ripple?

It’s a switching power supply. The main problem is it isn’t switching, and I suspect the one or both of the two large chopper transistors on the board. However, as I said they are marked with Tektronix proprietary numbers and cannot be cross-referenced. A schematic would be helpful, unfortunately I have been unsucessful in locating one, and the service manual for this scope is Quite Expensive.

I think you’re on the right track; when a switcher dies it can usually be traced to a blown transistor.

Can you tell if it’s a FET or bipolar? If it’s a new scope I would suspect a FET. At any rate, it can be easily determined by the biasing arrangement. What’s the number on the transistor? Perhaps I could help. Worse comes to worse, you can just try replacing it with (what we believe would be) a close match.

This scope was originally manufactured in the 70’s. This assembly is marked “Copyright 1070 Tektronix”. the two transistors in question are marked 151-0368-00. They have the Motorola “M” logo on them, but Motorola is of no help either. I’d really prefer to replace the whole assembly, since as I said, someone attempted a previous repair and really butchered the job.

That should read “1970”. I’d be impressed it had been made in 1070! :eek: