DSO-X 3024A Power Supply defect

[AndyC_772]

ps. I also stumbled across an older thread containing some useful information - here.

[salvagedcircuitry]

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.

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.

[georges80]

Well, another lineage power supply bit the dust (in my msox2024a). Same old story, blew the transistor (and fuse) and the main cap has gone from 100uF to around 5uF.

Rather than try to repair, I followed the path of others and replaced with a Meanwell. In my case after a bunch of research/comparing, I went with an EPP-300-12 model. Mouser had it for $58. Bought some JST 4 pin females and pins to make the S+/S- and Power-on mate and then repurposed the existing AC plug to fit on 2 of the 3 pins (of the 5 pin connector) on the Meanwell. Cut the 3 GND and 3 +12V wires and crimped each set of 3 to a ring terminal.

It's a tight fit, but will fit and all go back together nicely. The Power-on signal of the EPP-300 talks properly with the scope's front panel button so all functions nicely.

Thanks to contributors of this thread to steer me down the right path to repair my scope. I did power it up with +12V from my powersupply first to verify nothing nasty had happened other than the crackle pop of the original powersupply. All was good so that was a relief and I knew I just need to replace the powersupply to be back in business.

cheers,
george.

[salvagedcircuitry]

You definitely made the right choice! I just don't think it's worth the hassle of repairing and troubleshooting the lineage power supply anymore, as it is really not a good design.

Suitable candidates I've come across so far:
Meanwell RPSG-160-12
Meanwell RPS-300-12
Meanwell RPS-400-12
Meanwell EPP-300-12
Murata PQC250-12
Bel Power ABC201-1012G
...

The original replacement unit I purchased, the Meanwell RPS-160-12, did not include the remote power-on option. I goofed and purchased the wrong version. The unit I meant to purchase was the Meanwell RPSG-160-12. What a difference one letter can make!

The replacement power supply needs to have a remote sense and remote power-on feature as the power switch on the oscilloscope switches the secondary side of the power supply and not the primary side. Without the remote-on feature, the oscilloscope power button does nothing. Plugging the oscilloscope in would be the on/off button, which is not ideal.

All these units seem pretty decent. I'm liking the fact that the Murata unit is higher efficiency and most importantly does not have a perpendicular daughter board, like the meanwells. This should make the murata unit easier to repair, but then again, none of these units should need any attention for a long time.

It doesn't look like the murata has fine adjustment on Vout, so from the datasheet it can't be set to 13v which is the output voltage of the lineage PSU. The meanwells have Vout fine adjustment. The bel power has minimal adjustment. This may not be a deal breaker as these are all 12v class power supplies and the original lineage PSU only had a +-5% voltage adjustment range and poor voltage regulation.

Glad to hear you got the msox2024a back up and running!

[rolkinas]

After reading all this I took apart mine perfectly working DSO-X 2024A to check condition of smps. I use scope not that much and bought it used couple years ago.
Cap measures ok: 107uF.
But those baking resistors gets surprisingly hot. Precious preprogrammed obsolescence.
It seems that psu might survive longer with scope being powered on and fan running. While resistors still heats while unit is powered off. It's not funny having components near 100C in device being in standby...
Just curious if new SMPSs sold as spares by Keysight have such an issue?

[salvagedcircuitry]

After reading all this I took apart mine perfectly working DSO-X 2024A to check condition of smps. I use scope not that much and bought it used couple years ago.
Cap measures ok: 107uF.
But those baking resistors gets surprisingly hot. Precious preprogrammed obsolescence.
It seems that psu might survive longer with scope being powered on and fan running. While resistors still heats while unit is powered off. It's not funny having components near 100C in device being in standby...

This is excellent information. Thanks a bunch for adding the infrared images. Makes me want to pull the OEM PSU out of my other DSOx 

[Madmanguruman]

I'm liking the fact that the Murata unit is higher efficiency and most importantly does not have a perpendicular daughter board, like the meanwells. This should make the murata unit easier to repair, but then again, none of these units should need any attention for a long time.

The Murata PQC250-12 works just fine with three DSOX3014As that I've repaired - the oldest repair has had close to 1 year of constant use with no issues so far.

Like with other units, the wiring harness needs to be modified to make it work but it does offer remote sense as well as remote on/off (possibly with inverted logic, but the button will still work.)

I must also admit that I'm biased - I work at the Murata power supply design center where the PQC250-12 was designed and qualified.

[rernexy]

Finally got around to dismantling my msox3054a after 8 or 9 years. Partly because of a high pitch hissing sound that always seemed to be present, and it's waaaaay out of warranty so now idgaf.

But lo and behold look what I've found on the power supply, the blackening around the 22K resistors under the big arse 420V cap.
The little black chunk to the left is what flaked away when I gently prodded between the leftmost 2 22K resistors, it's exposed a track.
Also you can see that on the underside of the shield it is starting to discolour/blacken in line with either the resistors or that mosfet (don't know).

Maybe time to throw the parts-cannon at it for pre-emptive maintenance.

[Jwalling]

In years past, HP/Agilent would issue service notes on things like this.

[rolkinas]

Hi, don't forget to check capacitance of 420V cap. It probably is semi-dry.
I am thinking about mounting something like power resistor on psu shield-heatsink in place of smd. something like ROX5SJ91K from Digikey.
Leaving these baking 22K where they are now is asking for trouble.

[rernexy]

Decided to desolder the 22K resistors.
They were hiding more blackening/carbonising/burning of the PCB. Pretty sure that wasn't my desoldering.

[rernexy]

Different view of the pads (continuing previous post).
Shining a light from the opposite side, after removing the 420V cap, you can see cracks more easily on the inside of the resistor solder pads.
There is a faint crack at the upper right resistor, bottom pad.

[rernexy]

Parts I ordered arrived today. Bought 2 different models of the resistors and capacitor.

Measured the ESR of the cap from the supply and the new ones using an "ESR Meter Mk II" as seen in the photo.
1.4R for the old cap, 0.25R for the new ones.

Measured the capacitance of all of them with various multimeters, all read within 87uF to 95uF.
Then measured capacitance (not ESR) with the LCR45 at different frequencies, theres a few photos of that and a table below.

-	Old	New	New (high ripple current)
1kHz	46uF	87uF	84uF
15kHz	1.08uF	cycled negative to ~110uF	cycled negative to ~110uF
200kHz	282nF	6uF but cycled slowly between 5 and 8 uF	5.9uF but cycled slowly between 5 and 8 uF

Here are specific parts I bought:

100uF cap:
LGW2W101MELA25 450V high ripple current https://www.nichicon.co.jp/english/products/pdfs/e-gw.pdf
LGUW6101MELA same as on the supply https://www.nichicon.co.jp/english/products/pdfs/e-gu.pdf

22K resistors:
CRGP2512F22K  https://www.te.com/global-en/product-4-2176331-1.html
352122KFT  https://www.te.com/global-en/product-8-2176070-1.html

[rernexy]

Continuing from previous post, here are scope captures of 200kHz 1Vpp square wave fed into the old cap and the direct equivalent new cap (LGUW6101MELA). Just like what @w2aew demonstrates at https://youtu.be/115erzCCxgE

capture 1: old cap, scope DC coupled
capture 2: old cap, scope AC coupled
capture 3: new cap (LGUW6101MELA), scope DC coupled
capture 4: new cap (LGUW6101MELA), scope AC coupled

[Leo Bodnar]

My MSO-X 3024A PSU blew up as well - just now. 
Nice consistency Agilent, or Shitsight or whatever you are called now.
Leo

[rernexy]

My MSO-X 3024A PSU blew up as well - just now. 

Do you have any photos of what specifically blew up?
How old was your 3024A?

[Leo Bodnar]

Looks like synchronous rectification MOSFET.
It's 7-8 years old - obsolete by iPhone standards.
Leo

Do you have any photos of what specifically blew up?
How old was your 3024A?

[rernexy]

Is the PCB blackened/burnt under the 22K resistors on the bottom of the PCB aswell (like others in this thread)?

[Leo Bodnar]

It has discolouration and solder resist cracks.
Aren't these HV safety bleed resistors?  They can probably be increased in value in such case - but fixing it is polishing a turd.
It was probably designed and tested in 120V mains zone.
Leo

Is the PCB blackened/burnt under the 22K resistors on the bottom of the PCB aswell (like others in this thread)?

[2N3055]

Looks like synchronous rectification MOSFET.
It's 7-8 years old - obsolete by iPhone standards.
Leo

Do you have any photos of what specifically blew up?
How old was your 3024A?

That must have been quite noticeable bang...
Is that a blob of ejected melted transistor metal on that mounting clamp ?

Stay safe,

Sinisa

[Leo Bodnar]

Yes, it's a lava bomb ejected from inside the FET.  The explosion was very loud indeed.
Leo

That must have been quite noticeable bang...
Is that a blob of ejected melted transistor metal on that mounting clamp ?

[gman76]

Can you provide specifics on the harness re-wiring?  Is it just a matter of a couple wires?  Will it be obvious once I have the supply in hand?  I plan to use either the PQC250-12 or EPP-300-12.  Haven't purchased yet.