Electronics > Repair
DSO-X 3024A Power Supply defect
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AndyC_772:
ps. I also stumbled across an older thread containing some useful information - here.
salvagedcircuitry:

--- Quote from: AndyC_772 on August 27, 2020, 10:04:29 am ---I'm a little hesitant to make a resistor modification like that because I'm not sure I can see a sufficiently robust way to mount a TH part off the PCB.

The SMT resistors may get very hot, but they are at least light weight and unlikely to detach and become dangerous if the scope takes a knock. I'm concerned that the added weight of a TH part could cause it to detach the already weakened pads from the PCB, and in the worst case this means a loose end at 400V flapping about.

As an aside, I was faced with a very similar problem here. After I posted that thread, the equipment in question did actually fail again, and it was the resistor chain exactly as I'd predicted.

I've removed the resistors from my scope PSU just to check the condition of the board and solder joints. The board is blackened and the solder mask is cracked, but the pads are still firmly attached, and following a good clean with IPA, I'd put it in the "I've seen worse" category. I'll order some replacement resistors anyway, no sense refitting old ones.

Given that the failure is entirely predictable, I've also ordered a replacement main capacitor (this one) for my other scope, by way of preventative maintenance. If I can spend half an hour swapping a single component, rather than waiting for a potentially catastrophic failure that will inevitably happen at the most inconvenient moment, that has to be time well spent, IMHO.

One thought I did have was to fill the space between the resistors and the internal aluminium chassis of the scope with a thermal gap filler pad material like this stuff. This would conduct some of the heat out of the board and into the chassis, which in turn would keep the temperature of the capacitor down and help it last a bit longer.

--- End quote ---

Yeah. I'm not under the impression that this was a decent quality power supply. The PCB material around the resistors was quite charred on my unit, which may indicate a low TG fiberglass board was used. I have not experienced many boards where neighboring soldermask cracks and flakes off after a desoldering procedure.
You have a very valid point, the weight of a THT resistor may be too much for whatever is left of the compromised pads that endured such elevated temperatures. That thermal pad looks like a promising solution. I'm guessing the pad would have to be around 6mm thick to sink the heat to the aluminum shell, but I think there are some noticeable diminishing returns with increased thermal pad z-height.

I'm glad you were able to get the scope back up and running!
AndyC_772:
Today the last few parts arrived to complete the PSU repair. I reassembled the unit and connected it to a 5R dummy load, to give it an opportunity to prove its reliability before being connected to my expensive scope.

It's as well that I ran the test. It lasted about 10 minutes before there was a bang and the fuse in my isolation transformer blew.

The damage was interesting - Q8 showed evidence of arcing from the tab to the heat sink, and it looked as though there may have been a tiny nick in the kapton tape which caused it to fail as an insulator.

I fitted my one spare Q8, and replaced the kapton tape under it with a proper thermal pad. This time it managed only 10 seconds before there was another bang and a visible puff of smoke.

I don't doubt it could be fixed again, but if there's a Mean Well that'll fit, then that has to be the better option now.
AndyC_772:
Today I installed a Mean Well RPS-160-12 into my 3054A, which had graciously agreed to donate its original power supply to keep the 3104A alive.

The MW has a different connector and pin-out, which requires a new wiring harness. It also requires a little filing of the mounting holes to make it fit, as they don't quite line up with the standoffs on the scope chassis. (Take care doing this, as a couple of them do make an important earth connection - and test for connectivity afterwards!)

There's an adjuster on the MW which can be used to set the output voltage. Noting that the silk screen on the scope's own dc-dc board refers to +13V from the PSU, I adjusted its output up to +12.9V.

Result? The scope turns on just fine, but the front panel power switch no longer does anything. The MW doesn't have the remote power-on feature that the original supply does, and there's nowhere to connect the REMOTE wire from the scope. It looks as though the RPS-400-12 does have remote power on/off, so with hindsight that could have been a better choice even though it's a more costly unit.
HighVoltage:
Nice that you got it partially working.

It might be a good idea to try the RPS-400-12 with remote control and see if you get it to work.

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