Another TDS420 with broken PSU - Page 1
Forum section: electronics / repair

author: nikodem
author_rank: Regular Contributor
timestamp: November 01, 2024, 03:16:39 pm
content: 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), 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

author: u666sa
author_rank: Frequent Contributor
timestamp: November 01, 2024, 10:09:32 pm
content: PSU like this?Fix it.

author: Poroit
author_rank: Frequent Contributor
timestamp: November 02, 2024, 06:09:06 am
content: 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.

author: MathWizard
author_rank: Super Contributor
timestamp: November 03, 2024, 08:18:12 pm
content: 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)

author: nikodem
author_rank: Regular Contributor
timestamp: November 09, 2024, 01:54:04 am
content: Thank you all for the response. There are two issues that I have here:1) The PSUIt'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 PSUshouldautostart, 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?

author: Smoky
author_rank: Frequent Contributor
timestamp: November 09, 2024, 04:15:00 am
content: I can tell you with almost absolute certainty, that the leaking electrolytic capacitors in that scope have done extensive damage on almost every board in that machine. For instance, I had opened one scope and removed the front panel board and found almost every trace completely black especially around the rotary switches because the silicone membrane holds the fluid in place. Some of the leaking capacitor's fluid seeped under the plastic connectors too and went to town devouring the copper traces and every pass through hole that you can't see until you remove the connector. IMO, most of the non "A" version 420/460 scopes are beyond repair if the leaking fluid hasn't been addressed by now.https://www.eevblog.com/forum/repair/captdons-tds420-oscilloscope-repair/I suggest that you take that scope totally apart and clean everything thoroughly and then start doing continuity tests on every trace around every electrolytic capacitor.

author: nikodem
author_rank: Regular Contributor
timestamp: December 19, 2024, 04:45:29 pm
content: Despite what some of you wrote, I've decided that checking, if the scope is alive at all is my priority. So after inspecting the scope towards any leaking caps (none found, actually the PCBs etc are in really good condition) I've powered it using external lab PSUs... and it booted! It goes throygh self test (see photo attached to the post) with some fails, yes, but I guess this might be due to configuration switches on the CPU board etc... I didn't run it for too long, as it draws a lot of current from the bench PSUs and it seems the cables I've got for them, are not of stellar qualityso I didn't have time to do more thorough testing.So the main problem is the PSU. I have no good idea, how to approach this repair. Is there any schematic or service manual with more details than "exchange the module for new one"? Or at least some documentation regarding the infamous "green hybrid" etc.

author: Smoky
author_rank: Frequent Contributor
timestamp: December 19, 2024, 06:54:00 pm
content: I would contact Shakalnokturn and see if he has any remaining PCBs for sale that would replace the original green hybrid module in the power supply.He reverse-engineered it and created a drop-in replacementRead the bottom of this thread:https://www.eevblog.com/forum/repair/looking-for-the-new-tektronix-smps-green-hybrid-100004-pc-boards/msg3650590/#msg3650590I'd check for signs of any burns on the power supply board first and then start with a total electrolytic capacitor replacement, including the opto-isolator.A few incredible minds have put forth hundreds of hours fixing this exact power supply and there are things that must be done as a minimum to bring it back to life....or you can wait for used power supply to pop-up on Ebay for over $200 hoping that the green hybrid is good and go through the same steps above to revive it, or in most cases, to save it!

author: nikodem
author_rank: Regular Contributor
timestamp: December 21, 2024, 12:49:22 am
content: Quote from: Smoky on December 19, 2024, 06:54:00 pmThere is a special place in hell for designers, using such proprietary, not documented at all components.The PSUs PCB does not have any obvious signs - burns, leaks etc. The capacitors in the whole system are in suprisingly good quality. Of course, at some point I will recap it, at least to some degree, but for now I have to see, if it will have any sense. I'm trying to keep this thing as cost-effective, as possible, and recaping a dead scope is not really that.I will contact Shakalnokturn, I think he will be able to help also with initial debugging, in case it it not the hybrid. On the other hand I've seen someone selling some third party PSUs, compatible with this scope, although I don't know how reliable they were, as it looked like bundled AliExpress modules on a piece of plastic or something.