Electronics > Repair
DSO-X 3024A Power Supply defect
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AndyC_772:
Just to add another data point - this morning my MSO-X3104A died  :-BROKE

It was sitting on the bench, powered up but not actually connected to anything, when the display suddenly went blank without warning. I removed and reinserted the power cable, there was a loud bang, and the GFCI for the house tripped.

The main reservoir cap in the PSU has vented, and was very hot indeed. The current mode controller has blown the top off its package, and the gate resistor for the MOSFET Q1 is toast too.

Ordinarly I wouldn't even consider repairing an off-the-shelf PSU module, but this one appears to be a custom part that's not readily available, even though it looks like a standard item.

Has anyone replaced one of these with something else recently? Is there anything particularly special about the Lineage module that makes it hard to substitute for another unit?
HighVoltage:
I have always repaired the Keysight power supplies, because usually it is very simple.

What is different on the MSO-X3104A?
You have pictures?


AndyC_772:
I don't think the MSO-X3104A has a different power supply. I've asked Keysight to quote for a replacement part, and there's always the option of fitting an alternative with a suitable wiring adapter, but since I have some spare time this weekend I figured it was probably worth seeing if the damage looks repairable.

I had a few different dead parts than others have seen; MOSFET Q1 has a series resistor which is 5.11 Ohms 0805, and R7 is 110mOhm 5% (probably 2W).

In my PSU both were open circuit; the gate resistor was blown to bits, and the larger one I suspect was damaged during extraction of C11. They're stuck together with white goop which made removing them to access the current mode controller (also blown to bits!) tricky.

The MOSFET Q1 had a big diagonal crack all the way across the package body, the main cap had vented, and the board was dark and discoloured around the 22k resistors on the underside. Part of me is curious to know what happened in what order. Another part of me is cursing a wasted day and just wants it back working again.

I was able to determine the values of the dead resistors by measuring them in a working PSU, which I borrowed out of my MSO-X3054A. Since I rely on my scope to make a living, having a spare on hand was never optional, and today my decision to keep two that are very similar has paid off. I've put the PSU from the spare scope into my 3104A which is now working fine.

Even the working PSU in the 3054A was very hot when I removed it, having been running for several hours since the 3104A died. I'm really not surprised they fail.

My MOV and NTC seemed OK, there's high impedance from L to N and about 7 Ohms across the NTC and chokes.

I should warn anyone else looking to work on one of these supplies, you'll need a powerful soldering iron and a good range of tips. The board is constructed with no thermal relief on any of the through-hole pads, and the copper draws heat out like no other board I ever remember working on. Desoldering the heat sinks and large components without damaging the board was exceptionally difficult even with my JBC, and you'll need to be both gentle and patient. The main cap C7 was especially reluctant to come out, and it's fortunate that the little copper pad on the top side of the board, negative terminal, wasn't actually connected to anything.

I wasn't able to find an exact replacement for the 110mOhm resistor, so I've ordered 100m + 10m to fit in series. All the other parts were available from stock at either RS or Farnell, including some parts which looked OK but which I had to remove to gain access to others. Given the cost of new components vs my time, I rarely re-fit used ones to anything.

Fingers crossed nothing else has failed, and when it's all reassembled I get a working PSU again. I'll be running it for some time out of the box on a decent load before I trust it with my scope.
salvagedcircuitry:

--- Quote from: AndyC_772 on August 22, 2020, 05:08:48 pm ---I don't think the MSO-X3104A has a different power supply. I've asked Keysight to quote for a replacement part, and there's always the option of fitting an alternative with a suitable wiring adapter, but since I have some spare time this weekend I figured it was probably worth seeing if the damage looks repairable.

I should warn anyone else looking to work on one of these supplies, you'll need a powerful soldering iron and a good range of tips. The board is constructed with no thermal relief on any of the through-hole pads, and the copper draws heat out like no other board I ever remember working on. Desoldering the heat sinks and large components without damaging the board was exceptionally difficult even with my JBC, and you'll need to be both gentle and patient. The main cap C7 was especially reluctant to come out, and it's fortunate that the little copper pad on the top side of the board, negative terminal, wasn't actually connected to anything.

I wasn't able to find an exact replacement for the 110mOhm resistor, so I've ordered 100m + 10m to fit in series. All the other parts were available from stock at either RS or Farnell, including some parts which looked OK but which I had to remove to gain access to others. Given the cost of new components vs my time, I rarely re-fit used ones to anything.

Fingers crossed nothing else has failed, and when it's all reassembled I get a working PSU again. I'll be running it for some time out of the box on a decent load before I trust it with my scope.

--- End quote ---

My PSU repair was rather painful, but successful. Overall, I'm not too fond of the repairability of the Lineage PSU design. It definitely took a lot of patience, flux and heat to get the aluminum heatsink off and not destroy the staking pads. I think I ended up having to super-glue a heatsink pad back on at the end of the repair process. I opted to leave the daughter PCB soldered in and just desoldered the failed soic-8 current mode controller on my unit. Since the lineage design does not instill confidence, I went with a meanwell RPSG-160-12 as a replacement power supply to have on hand in case the failure occurs again. Unfortunately, I goofed and ended up with the RPS-160-12 which does not have the remote-on feature which this oscilloscope needs as the power button is designed to switch the secondary side.
https://www.arrow.com/en/products/rpsg-160-12/mean-well-enterprises

If I could do the repair all over again, I would desolder the (4) series 22k 1w SMT resistors below the main filter cap and replace them with one ~88k 4w or 5w through hole resistor and just leave the resistor spaced off the main pcb. This way the resistors can't directly heat up the back side of the filter cap. There is enough space between the bottom face of PSU PCB and the aluminum shell of the agilent oscilloscope to accommodate the resistor. Additionally I would add electrical tape on the back side of the oscilloscope casing where the resistor may come close. On closer look, It may be a set of 2 power resistors, I see a trace between the 3rd and 4th resistor on the back side.

Something like this:
AndyC_772:
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.
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