Stunning 3000A, 2000A, 3000T power supply discovery - MUST READ IF YOU HAVE ONE - Page 1
Forum section: electronics / testgear

author: EE-digger
author_rank: Frequent Contributor
timestamp: January 27, 2024, 12:17:55 am
content: After spending some time on the repair thread for these scope power supplies, I did further research and also prepped a Murata supply which I had on hand, to replace the original Cherokee/GE CCH125 in the scope.So today, I took the factory supply out of my pristine, dare I say virgin, DSOX3104T. The scope is super clean inside and out. Hadn't really looked at the supply much because ... it's a supply.Also, haven't used the scope in weeks but it's plugged in a day at a time. First thing I noticed was that the ac mains cap was hot as hell. (thermo and IR camera in the days to come)Back of the board looks like crap. Not blackened but clearly looked like it was sitting in an oven at high temp.I'll cut this short with more to come later.I've designed SMPS from low level to high level modulars into many medical products but do not profess to be an expert in topologies.So, need some help here. The 420v mains cap is sitting at 397 volts dcThis is on a 120v mains.This is why these scopes are cooking.Might this supply be using doubling up front? You just can't do it with these bleed resistors. They will be running red hot, and right below the cap with thermal coupling via the cap leads. With very little thermal decoupling, these 1W resistors are running at 0.4W, and the thermal rise is increased due to their close proximity.The new Murata is sitting on the bench, most parts cool to touch and the voltage across the cap sitting at ..... drumroll ..... 168vdc. I could hear it saying "take me", "take me", "take me".Ok little Murata, as soon as the connectors arrive.It seems like these scopes are a ticking bomb, almost guaranteed to blow on European and other 230v mains, although I don't know what their design would do there.

author: perdrix
author_rank: Frequent Contributor
timestamp: January 27, 2024, 09:26:15 am
content: It's quite normal to run 120V setting through a doubler (for switchers that convert mains input to DC as the first step). When set for 240V input the doubler isn't used.In the absence of other faults, I would suspect that the cap in question has gone high ESR, and therefore gets seriously toasty.If there are bleed resistors across the cap, you could consider increasing their value - its not uncommon for the value of the bleed resistor to be unnecessarily low.

author: EE-digger
author_rank: Frequent Contributor
timestamp: January 27, 2024, 01:18:39 pm
content: Thanks for that.I'm going to refurb several of these with a higher value bleed resistor and an 8000 hour cap of identical size. I just realized that in the powered off scope, this supply has aluminum chassis on one side which will conduct heat if close enough (not so sure it is) and the bright plated shield on the other side, which may be closing the "oven door" so to speak.Open on the bench, the resistors get up to 100C and the cap to 60+C. I may emulate or just put it back in the scope to test the "oven" effect. But that +397v cuts it too close to the 420v rating for my taste.And as they say, the proof is in the pudding. For a very low hours scope, this supply looks "cooked", just short of board blackening / burning.But for my scopes, this baby is gone

author: TheSteve
author_rank: Unknown
timestamp: January 27, 2024, 06:58:02 pm
content: You've got me curious so I will open my scope this weekend and have a peak. It has been connected to power 100% of the time for many years.

author: EE-digger
author_rank: Frequent Contributor
timestamp: January 27, 2024, 08:44:14 pm
content: Think of it less as a scope and more as a crypt.Can you post a pic of the board bottom?I've installed new cap and resistors. Excellent improvement and thermal images posted below.It's a design flaw. Someone missed the +397 volts sitting across the series Rs.Resistors were at 100C, now at 64C, capacitor pins are about 10C below that (new R value = 68K each, bleed time to 50V = 24s)There are also hot resistors lower in the image but they are only pushing 70C+Capacitor top was pushing 70C, now at 46C (what looks like a hot ring around the capacitor is the emissivity of the shrink wrap tricking the camera)

author: TheSteve
author_rank: Unknown
timestamp: January 27, 2024, 10:55:47 pm
content: Opened up my scope. My cap is sitting at 398 VDC with the scope off - ouchTop side of the cap is 60 CMine has been powered for 6-7 years.Can't say I'm impressed they use a doubler in it - explains the rather poor efficiency this thing has.My cap measures ~82 uF and 1.5 ohms ESR. It is time for replacement. I will be ordering a new cap and higher value bleed resistors.You can see some discoloration of the PCB, but nothing horrid.

author: 2N3055
author_rank: Super Contributor
timestamp: January 27, 2024, 11:15:47 pm
content: Would somebody, please, be so kind and say what are resistor case size and what capacitor size is?What is resistor value now? What are you going to replace them with?I would like to order them up front, before opening the scope...Thank you..

author: TheSteve
author_rank: Unknown
timestamp: January 27, 2024, 11:26:23 pm
content: Original Capacitor is 420 volts @ 100 uFI will be replacing it with a 450 volt @ 100 uFDiameter is 25mmlead spacing 10mmOriginal height is pretty short at 23mm. I will be replacing it with a taller capacitor as there is plenty of room below the metal shield.I was thinking of maybe a ESCOS-TDK B43644B5107M000I'm open to other suggestions, but can really only consider parts from Digikey.Original resistors are 22K, size 2512I will replace with 68-100K each.Perhaps Bournes 75K CRM2512-FX-7502ELF

author: 2N3055
author_rank: Super Contributor
timestamp: January 28, 2024, 12:26:59 am
content: Thanks a lot, much appreciated!

author: EE-digger
author_rank: Frequent Contributor
timestamp: January 28, 2024, 01:37:14 am
content: B43644B5107M000 is the same cap I used. It's 25 x 25 and is fully below the module height. Digikey has stock.Removal is a royal PITA. I first sliced the silicone beads around the cap with a thin blade.After vacuum sucker and solder wick couldn't finish the job, I ended up using some Chipquick low temp alloy, then heated and pressed each lead downward with a stainless steel pin, alternating leads until it dropped.

author: TheSteve
author_rank: Unknown
timestamp: January 31, 2024, 04:19:14 am
content: Rework done.Installed the ESCOS-TDK B43644B5107M000 capacitor and Bournes 75K CRM2512-FX-7502ELF resistors.PCB was a little toasted under the original 22K resistors, the glue that held the parts on was very crispy and burnt.Good for another 10 years I hope.

author: EE-digger
author_rank: Frequent Contributor
timestamp: January 31, 2024, 04:34:07 am
content: Looks good and the 75k resistors will keep it comfortable for a long time. My new Murata is hissing at me so the cleaned up original may go back into the scope.

author: tautech
author_rank: Super Contributor
timestamp: January 31, 2024, 08:26:17 am
content: Seems an unusual SMPS design needing bleed resistors on the HV DC cap.Normal modern SMPS design has the controller remain working until the HV DC cap gets discharged to low and safe levels.

author: TheSteve
author_rank: Unknown
timestamp: January 31, 2024, 04:15:01 pm
content: Quote from: tautech on January 31, 2024, 08:26:17 amThe datasheet tells the tale - it has very poor efficiency(but good power factor). I guess the design is minimum 15 + years old but even then it is pretty weak.

author: EE-digger
author_rank: Frequent Contributor
timestamp: January 31, 2024, 04:47:50 pm
content: I have a second Murata coming in at noon. If it is quiet, it's going in and the cleaned up original goes on the shelf until I need to use my service agreement. Then the old supply goes back in, etc.And Steve, you can't say these supplies haven't been burned in

author: SeanB
author_rank: Super Contributor
timestamp: January 31, 2024, 06:37:57 pm
content: One capacitor only says this has active PFC, so very likely the active PFC is running all the time. those 22k resistors are likely a start up supply for the main controller, which then will use the main transformer to generate it's own supply, which will also drive the boost converter doing active PFC.would also sufggest checking for dry joints on the transformer, and that join, likely a heatsink, above those new resistors does look rather suspect with a ring around it.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 31, 2024, 07:21:44 pm
content: Thanks for pointing this out.Time to open all my scopes, I guess.I had a similar problem on a 7000 series Agilent scope.

author: TheSteve
author_rank: Unknown
timestamp: January 31, 2024, 07:38:47 pm
content: Quote from: SeanB on January 31, 2024, 06:37:57 pmYes it does have active PFC and as you go higher on the resistor values the minimum start voltages rises.

author: tautech
author_rank: Super Contributor
timestamp: January 31, 2024, 07:50:54 pm
content: Quote from: TheSteve on January 31, 2024, 07:38:47 pmSo they're not HV DC cap bleed resistors then ?And instead just the ordinary dropper resistors found in most any SMPS to allow it to charge the controller Vcc cap to exceed the startup threshold ?

author: SeanB
author_rank: Super Contributor
timestamp: January 31, 2024, 08:13:18 pm
content: The resistors go to a trace not ground, likely with a small capacitor on the other side of the board. Higher value just means longer time to bootstrap the PSU, as most of the controller IC's will have undervolt lockout that holds them off till the capacitor charges up to a high enough voltage.you can see the trace leading off the bottom to the daughter board, and the capacitor, and as well on the other side of the daughter board the SOT diode that feeds it in operation, with another diode isolating the feedback and power pin so the controller can sense the transformer state for feedback.

author: tautech
author_rank: Super Contributor
timestamp: January 31, 2024, 08:18:21 pm
content: Quote from: SeanB on January 31, 2024, 08:13:18 pmThese are another common failure point in SMPS when this cap goes high ESR.

author: tautech
author_rank: Super Contributor
timestamp: January 31, 2024, 08:26:46 pm
content: Quote from: SeanB on January 31, 2024, 08:13:18 pmDo you know the SMPS controller pt# Sean ?

author: SeanB
author_rank: Super Contributor
timestamp: January 31, 2024, 08:40:31 pm
content: No, it can be any number of them, you find them almost to be generic, hundreds of part numbers, all doing roughly the same work. Same for the PFC, which will also be on this board.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 02, 2024, 06:34:26 pm
content: Four 1206 resistors fit pretty well in the original 2512 footprint

author: Someone
author_rank: Super Contributor
timestamp: February 07, 2024, 10:37:49 am
content: Quote from: SeanB on January 31, 2024, 08:13:18 pmPut 100k parts in place of the 22k, works fine!

author: EE-digger
author_rank: Frequent Contributor
timestamp: February 08, 2024, 01:43:29 am
content: Reply #53 in this thread may be of interest to you:https://www.eevblog.com/forum/repair/dso-x-3024a-power-supply-defect/50/At 400k+ ohms, the startup voltage was 135 volts. Perhaps not an issue for you but would be on 120v mains.

author: EE-digger
author_rank: Frequent Contributor
timestamp: February 10, 2024, 03:00:18 pm
content: Another happy camper with a Murata PQC250-12 installed.It took a few resistor moves to invert the remote on/off signal and it took a bit more doing to get rid of very annoying acoustic noise whether on or off.Spoke with Murata and one of the posters and no one has had this problem. I had it on the two supplies I tried.It comes down mostly to one capacitor (ceramic, 10uF) that sounds like a dentist drill if you gently touch the top with tweezers. My solution was to replace it with a Murata metal mounted cap of same value. You can also get interposer caps. I first made my own interposer before the Muratas arrived by simply mounting the cap piggyback on a 1 Mohm 1206 resistor. I have to say that this was quite good.If anyone want the on/off inversion info, I'll post it.

author: Helix70
author_rank: Unknown
timestamp: July 18, 2024, 02:00:03 am
content: Quote from: EE-digger on February 10, 2024, 03:00:18 pmYes please, my MSOX3014T (early model) power supply just died (GE CCH0125F1-Z03A) and am researching the best replacement options. Having the correct switch position is important to me as it is a work scope and I don't want the risk assessment!

author: djsb
author_rank: Super Contributor
timestamp: July 19, 2024, 07:34:14 pm
content: I have a Keysight MSO-X3054A. Is this scope affected by this and what should i be looking to replace in the power supply and with what? Thanks.