[SOLVED] Tek 2221A suddenly no power

[Fractal]

Hey there, so my scope used to work fine till last night;

today I press the power button and nothing happens; no fan spin, no light, no screen, no sound.

I've checked the power cord, the fuse, all good.

Opened up, seems like AC comes in past the power switch, but I can't figure it out past that 

Any help?

Thanks!

[Fractal]

Ok update, there was actually a faint whining noise repeating every sec or so; probed around and found a shorted diode, CR907; removed it already, will go find a suitable replacement at my local electronics shop tomorrow

[Fractal]

Okay replacing the diode didn't work out; started probing around for shorts in fets, found out Q946 and Q947 had short on 2 pins, removed them with the heatsink, but they now test find out of circuit

Show up the short remains on the board, I'm getting it between TP950 and TP940 (42.8/43V?)

This seems to narrow down the issue to the preregulator area, but now to locate it...

I can't rely on my IR cam either as the unit only pulls 100mA pulses every second

Any help appreciated

Cheers!

__Yves

[Fractal]

Well after facing crickets for the past couple days and since I have no idea what I'm doing I figured I'd just go the voltage injection way lol

I sent 1A through the 43V rail with my bench PSU and immediately observed Q935 gobbling all the current on the IR cam.

Removed it, reinstalled the inverter transistors and the diode, and sent power again.

And what do you know, it powered on beautifully.

I guess I'll just have to find a replacement triac; says MCR72 on it but seems long discontinued...will see to find a similar alternative.

Service manual BOM says Thyristor, SCR:8A,200V,SENS GATE,TO-220W/LE

Any suggestions welcome I guess

[m k]

It's just a crowbar, all similars are suitable.

Maybe you should replace VR935 zener also.

[Fractal]

Hey there friends!

Just received the spare triac, a matching spec MCR72-8G from ON

Installed it, put my goggles on, pressed the power switch aaaand nothing happened.

Turns out I forgot to plug back Q9070 lol

Turned it off, plugged it back aaaaand... It lives!!!1111!

No further damage to any components up or down the line, before or after the triac replacement. Very odd.

But, super pleased that I can put it back to use, thank you for the tips!

Cheers,

__Yves