Tektronix 1241

[Dave Turner]

I picked up a Tektronix 1241 a few weeks ago un-working and without probes. The idea being to investigate it without any pressure, just for interest.

I plugged it in to see what would happen,having first checked to see that it was set to 250V and not 120V. Not unexpectedly PHUT!. The Plug fuse blew (but not the back panel fuse) with nice sparks approx 3 inches from the back under the PSU,  - so teardown time.

With the exception of 2 boards visually nothing seemed to be amiss.

The exceptions are the PSU board and the Control Processor Board (CPB) It is clear that the battery backup on the CPB had corroded and leaked. It is possible  indeed probable, that this leakage may have affected the PSU board.

I removed the PSU, and on an isolation mat plugged it in again - unsurprisingly the main fuse blew again, and yet again the back panel fuse remained intact. So currently I'm trying to figure out where the fault(s) are on the PSU.

I did notice that there appeared to be diodes (possibly zeners or germanium) soldered on the back of the board as a later update one of which disintegrated on touch. Also one of the main power caps has a sooty deposit on its outside only.

I downloaded the (pdf) service manuals and can identify the parts v the schematics, unfortunately as yet I haven't figured out how to post just the relevant pages  as opposed to the whole manual to poinnt out where.

I have only basic knowledge of how to fault find a switch mode PSU and since the main fuse blows on plug in other than disassembling each component and testing them I'm at a loss.

Any suggestions? - and yes I know it's not a simple question.

cheers

Dave

[grumpydoc]

If it's blowing the plug top fuse violently but not the back panel fuse I'd check:

- that the back panel fuse is the correct rating
- that here isn't a short in the IEC inlet.

Then I'd check the EMI filter components, initial rectifier diodes and the two smoothing caps for shorts - careful as there will be high voltages potentially present on those components even with the mains off (and even though the fuses blew quickly).

Service manual here -> http://www.ko4bb.com/Manuals/Tektronix/Tektronix_-_1241/Tektronix_1241_Service_Manual.pdf if you haven't already found it.

[trackman44]

I would also check the mains power cord for shorts, since the fuse blew at the plug and not the back panel.

Will

[Dave Turner]

Thanks for the pointers.

The power cord is a known good one. Nice point about the back panel fuse, as far as I can determine it's an original, though I'll order some more.

Ok so it's desolder and checking. Going to take some time.