Need help fixing the psu of my Tek410a

[jogri]

I recently got a Tek 410a scope with the bad habit of blowing the fuse every time the switch on the back is turned on. I followed the instructions in the service manual which pointed to the low voltage power supply. I discovered that there was a direct short between the DC outputs of the bridge rectifier and traced them back (starting from the negative pole) to the main switching transistor of the primary side (upper right corner, the heatsink with the warning label). I checked the resistance between Base, Collector and Emitter, they where under 1 ohm each. Desoldered it, same result. So i bought a replacement and installed it. Now the resistance between the poles of the rectifier is approx. 40-50 kOhm, but the resistance (in circuit) of the new transistor between B&E is still 0 Ohms (180/35 kOhm between B&C/C&B). I traced the connections going out from the transistor, but couldn't find the cause of the 0 Ohm problem. Now the question: Am i just missing something blatantly obvious or is there something else broken?

I attached pictures of the board and the schematic i draw from tracing the connections to/from the transistor. The "green thing" i am reffering to is the thing behind the big transformer, it is wrapped in heatshrink(?) and it looks like pin 1&2 are going into a transistor. No idea what the rest does. The mosfet is the one w/o heatsink left from the filter(?) caps. R18 is a functioning 1k Ohm, forgot to put that in the schematic. I marked the ceramic resistors as varistors to differentiate them from normal ones. CR2 and VR4 are probably fried Zehners (VR4 is a 1N5221B, can't read the letters on CR2), they have a forward voltage of 0,4 V (backwards 1,2 V on both). My multimeter shows a resistance of a few Megs, so i guess they aren't the cause of the 0 Ohm problem.

The problem with drawing a schematic is that i have to trace the lines on the pcb and can't tell if it's a valid connection or caused by a short somewhere else when i find 0 Ohm of resistance between two parts. I think i got all the connections, but i didn't test every point against every other bc of the problem i mentioned above.

Would be nice if someone who has the same psu could check the resistance on theirs and tell me if i am just waisting my time.

(No, i will NOT power the damn thing up until i am 99% sure that i fixed all the problems. 3 reasons for that: 1) Don't want to electrocute myself. 2) It's the only pcb i have and there are no replacement pcbs online 3) I have only more replacement transistor, if i kill them i have to wait untill november for new ones to arrive)

(And yes, i know about the existence of the "schematic" from a LeCroy 9384. Problem: The scan quality is extremely crappy and you can't identify a single part)

[BreakingOhmsLaw]

You should get yourself a variable transformer. Worth it's weight in gold for repairing bad switchers.
You can then run the supply at a very low voltage - say, a couple of Volts, just enough to drive the bridge rectifier into conduction - and then diagnose the problem without frying your unit.

It also can keep you from frying yourself.
Beware: The come in two flavours: The ones that are also an Isolation transformer, marked by a sign like this:


...or the deadly type that has PE connected. Or the *really* dodgy onehunglow types where the wiper just taps right into the primary winding. *yikes*

[Cyberdragon]

Or the *really* dodgy onehunglow types where the wiper just taps right into the primary winding. *yikes*

You mean like a standard autotransformer that most people use? They certainly aren't "onehunglow" and dodgey. Have you seen photonicinduction's triple decker 20KW motorized autotransformer?

Variable transformers are not supposed to be isolation transformers. They are seperate things that are sometimes combined. Yes, there are partial (ground tied) and full isolated (load not grounded) types which are more expensive.

[james_s]

I have two of those damn things on the bench from Tek 400 series scopes and so far I haven't been able to get either one working. One of them had a catastrophic failure that damaged the hybrid, that's the green ceramic covered module I think you're referring to. I xrayed it and got to work reverse engineering it to build a replacement but then I got sidetracked by other projects. The other comes up but the voltages are way high, I've replaced the ICs and checked everything I can find and it all seems ok, got frustrated and set it aside. If you do manage to fix yours please do follow up, so far my experience is that they're a weird design that is a real pain. There is a very similar unit in another brand of scope (LeCroy?) I remember finding the service manual which unlike the Tek manual has a schematic for the power supply. It's not identical but it's very similar. Actually the two I have are not identical to each other either.