Dear all,
It is with much regret that I have to inform you that the PSU is dead. RIP, poor SMPS...
I was (scope) probing on the PWM chip, near the mains filter cap, and I guess the probe must have slipped and shorted the mains in some way shape or form but... I do'nt remember exactly the chain of events as I am still under shock !
. All I remember was that all of sudden the board caught fire violently, a fireworks followed, all I could think of was run away in case the huge filter cap exploded and might injure me.
Luckily it didn't, and after a few seconds the fireworks ended, phew...
See pictures. PWM chip is cracked, and a couple components are burnt to complete destruction. From past pictures I would say they used to be a resistor and a little signal/glass diode. The big filter cap did catch fire but it's perfectly fine. I pulled it off for closer inspection, only the outer plastic foil started to melt, but the can/body of the cap itself is completely undamaged. Cap still measures just fine. There just wasn't enough energy in this fire to damage such a big guy.
I do happen to have a spare PWM chip of this reference, as luck would have it, but I have no way of knowing the specs and value and the two components that got destroyed. Plus, even if I could fix this, I would still, at best, get back to my original problem which I failed to diagnose.
Last night I did manage to probe around the thing with the scope without blowing anything, but all I could see were waveforms that defied common sense, so I gave up. this supply is just too complex to be tackled without schematics... too many ICs, too many discrete transistors, two many diodes...
So I give up for now. This scope has eaten enough of my time and money and is not worth it. sometimes you have to draw a line somewhere and call it a day...
But for the record, and in case I do come back to this repair (much) later, when I am more experienced and knowledgeable about SMPSes, or if the schematics somehow surface one day (highly unlikely I know...) here are some screen shot from last night.
The CRT/Fan supply is properly rectified but ripple is horrendous, > 500mVpp
interesting things is that ripple is pretty much as horrible on ALL the other rails ! Just slightly less horrible... maybe 300mV.. but still waaaay out of spec I would think, whatever the specs might be ! To be honest I find it quite miraculous that the scope ever managed to work at all with so much ripple everywhere.
What I find even more troubling is the fact that this ripple has a very odd frequency : 15Hz or so ! Not 50, not 100... but 15Hz. Where the hell would that come from...
Also, when I tried to check the basics and look at supply voltages of the PWM chip and and nearby LM393 comparator.... instead of a clean DC voltage, I found some weird waveform ! The comparator Vcc pin had huge 7Vpp triangular waveform, again at 17Hz, sitting on a 10Vdc level. Surely this can't be normal...
As for the PWM chip, even more weird. The Vcc pin was seeing a PWM like waveform ! Yes, I triple-check chip orientation and pinout in the datasheet... I did probe the proper pin for all I know.
So really, without a schematic, I give up on this thing. Still, I will reassemble the scope, which works fine otherwise. I might find a cheap donor to help me fix this scope, or more likely this scope will be the donor to fix another TDS 300 scope featuring better specs...
So this scope served his purpose well : I learned some things and got some more hands on experience on SMPSes , without schematics. Valuable to me.
Also learned that there is no shame in giving up/admitting defeat... sometimes you have to be reasonable.
Now, let's clear the bench and get going with the next project... I have got an old Schlumberger/Enertec universal counter to fix, hopefully with better luck than this SMPS !
Going to open a new thread about this little guy soon, for those who might be interested.
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