Electronics > Repair
Another TDS420 with broken PSU
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nikodem:
Hi!

I've recently got new project-osciloscope - a non-working TDS420. It seems to have a dead PSU, but without a schematic and any obvious blown parts I don't know where to start.

I don't have a lab bench PSU powerfull enough to run it (5,1 V 15 A  :wtf:), but after testing it with a tad lower voltage on that rail the CPU seems to be alive-ish.

What would be the best course of action here? PSU rebuild or change to a replacement? Ot maybe first trying to test the scope with some lab PSUs to see if it is alive at all?


Regards
u666sa:
PSU like this?

Fix it.

Poroit:
G'day Nikodem,

Refer the link below.

https://www.eevblog.com/forum/repair/tektronix-tds420tds460-smps-green-hybrid/

There is also a large amount of videos on the web relating to repairs/faults with this model that should be able to guide you.


MathWizard:
So what does the PSU do when turned on ??, do you ever see any lights or hear anything ? If I was worried about the PSU, but still wanted to power it up a bit, I'd use a current limiting series lightbulb, and/or an auto-transformer. With the lightbulb, it may or may not beable to startup fully, but then you should see it resetting/cycling as it try's to pull too much current. And then of course with a scope you can just watch the rails on startup.

Me with a lot of time, I would remove the PSU section, and check as much as I could unpowered, and probably remove and check some caps. See what all chips are controlling things, and probably make a schematic if I didn't have one. And then I'd power up the PSU and try some dummy load resistor tests of the different rails.

Then I might try to only power the 5V or +/-12V rails, and to see if none of the logic circuits explode. Then I'd try powering the whole scope. (if I was already worried about the PSU or shorted rails)
nikodem:
Thank you all for the response. There are two issues that I have here:

1) The PSU
It's dead, totally. But with no obvious signs of any malfunction - no blown ICs, leaking caps, no shorts, nothig. I don't know where to start or IF to start, because if the "SMPS green hybrid" is shot, then this is basically just a fancy paperweight. Without the schematic it's very hard to tell. On the primary side - all the caps, filters, rectifier bridge - it's all ok. The rectified line voltage is present as expected, but nothing is happening. I still haven't got time to poke with a scope on the secondary side (i'm too afraid of blowing my working scopes by poking on the primary). The PSU should autostart, according to the manual.

2) What of the scope itself is dead?
I want to power the scope from bench PSUs, I've applied all the required voltages to pins of the cable connecting the PSU and rest of the scope. Got some LEDs light up on the CPU board and the fan starts, but that's it. No CRT, no nothing. I think i'm missing somethng, any ideas?
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