[SOLVED] UNI-T UTD2052CEL Oscilloscope SMPS repair - Page 1
Forum section: electronics / repair

author: negativ3
author_rank: Regular Contributor
timestamp: July 23, 2016, 04:45:52 am
content: Hi guys, long time lurker here in need of some assistance...Bought this two channel scope about four years ago and it's worked flawlessly for what I need, which is mainly educational low voltage (<50V) stuff.The power supply board has now shit the bed and I'm in two minds to just chuck it and buy a Rigol, but for the sake of a dollar part being busted, I'm reluctant.So, would anyone have access to the power supply schematic for this beast? The PCB has code UTC7.820.039 V1.2 on it... Fuse is fine, zero on all outputs, no signs of magic smoke escaping.Please don't tell me I need a scope to fix my scopeThanksAndy

author: retiredcaps
author_rank: Super Contributor
timestamp: July 23, 2016, 04:56:13 am
content: Quote from: negativ3 on July 23, 2016, 04:45:52 amI fixed a lot of power supplies with just a multimeter. I don't even have a scope. Most of the time, I can't even get a schematic. I did, however, learn a lot just trying a bunch of different things.If your fuse is fine, do you get roughly 1.4 x mains AC voltage (120V AC x 1.4 = 168V DC) on the bridge rectifier (BD1 on pcb)? If yes, check C8, that might be your SMPS startup cap. It should be in the 16V DC range across the capacitor? The SMPS IC, maybe 8 pin IC, should be around there or on the backside of the pcb? If you can find the SMPS IC, you should be able to find a datasheet.Also, check the 5V standby rail to see if you have that present.

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2016, 05:14:03 am
content: Quote from: retiredcaps on July 23, 2016, 04:56:13 am+1 ^^^Once you've identified the SMPS IC and got the datasheet (get a few different versions if you can find them) look for typical application schematics normally listed near the end. These are so often followed very closely by manufacturers and really help if they are the same as your PSU.Other failure points are the rectifying diode from the Aux primary winding that supplies the "running" voltage for the SMPS IC.The dropper resistor/s from HV DC to the IC supply should be checked too. (startup supply)Without a scope to check that's OK while supplying the IC with a LV external PSU (safest method) be sure to supply mains power via a dim bulb tester to limit the current available and prevent any fry ups.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 23, 2016, 05:28:54 am
content: Quote from: tautech on July 23, 2016, 05:14:03 amWhat is scary is that 6 years ago when I started in electronics, I understood none of the above. If it wasn't an obviously bad cap, it went to the "fix later" pile. Now I actually understand it, somewhat. Yes, I'm a slow learner.Learning how to fix a SMPS power supply and its troubleshooting process is one skill that will help you fix a lot of things throughout life. Plus, it is a lot more satisfying replacing one broken component than doing the board swap that everyone including manufacturers do nowadays.

author: negativ3
author_rank: Regular Contributor
timestamp: July 23, 2016, 05:53:24 am
content: Thanks retiredcaps/tautech,retiredcaps - same boat, just dipping the toe into smps...I will take some measurements and attempt a davecad of the circuit.U2 = TOP225YN three terminal off-line PWM switch. Other IC packages are regulators, diodes and opto couplers.Here is the back of the PCB...

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2016, 06:09:22 am
content: Quote from: retiredcaps on July 23, 2016, 05:28:54 amLess than 6 years ago I was at where you are, that's the great thing about EEVblog you need not be an active member just hovering in the background and doing lots of reading can get you a mountain of understanding.Over the last few years we are starting to see more SMPS failing so it's one of the things I've taken an interest in, not in any way to be any expert, just a reasonable understanding of what to look for and check.I've fixed a few but I won't call myself an expert, no way.

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2016, 06:18:04 am
content: https://www.power.com/sites/default/files/product-docs/top221-227.pdfIf you look at P 7 It would seem to me that D3, U2 and C5 are critical components for operation, check each.

author: negativ3
author_rank: Regular Contributor
timestamp: July 23, 2016, 07:32:49 am
content: Quote from: tautech on July 23, 2016, 05:14:03 amMains voltage here is 230V AC ish and I am getting 315V DC across the HV filter cap C5 (PCB ref). 0V DC across C8 (PCB ref).Where do I attempt to measure the 5V standby rail?

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2016, 08:21:15 am
content: You'll need to follow the PSU pin outs to the rest of the DSO and attempt to find it.Hopefully the PCB has an overlay on both the PSU and the DSO mainboard.Not all SMPS use a standby to start, in some cases the SMPS is just ticking away ready to supply full load when the mainboard tells it to.(turns on)Strands of the PSU ribbon cable will be paralleled to share the current so looking at the "big" picture will help you understand what's going on there.

author: negativ3
author_rank: Regular Contributor
timestamp: July 23, 2016, 09:57:50 am
content: Quote from: tautech on July 23, 2016, 08:21:15 amOk, with the DSO main board plugged in, all voltages on the ribbon connector are zero or floating a little (sub 0.05V DC). No obvious voltage markers on the main board either, just component legends.My DC voltage reference for all measurements is the -ve side of the HV cap. Not sure if this is good practice...

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2016, 10:29:46 am
content: Quote from: negativ3 on July 23, 2016, 09:57:50 amIf you've confirmed -ve side of the HV cap is common with the secondary 0V then that should be fine, on P7 of the datasheet it seems that topography is used.There's enough info in the datasheet to determine the likely fault and without the PSU in front of me the main suspects I have: Opto, diode, cap and resistors in relation to the Control pin or the TOP224P itself.Not a lot to check with a DMM and if the passives are OK suspect the opto or TOP224P.You should be able to confirm if the opto is OK with your DMM, that just leaves the TOP224P.One more thing.....check the transformer primary is not OC.

author: StillTrying
author_rank: Super Contributor
timestamp: July 23, 2016, 11:55:25 am
content: QuoteJust don't forget that on every -ve half cycle this -ve side of the HV cap / secondary 0V, is at a lethal -320 V from real earth / you.

author: negativ3
author_rank: Regular Contributor
timestamp: July 23, 2016, 12:46:19 pm
content: QuoteOk, passives look fine. I have measured the optos and get odd results as follows:U1, PC817pins 1-2 (+ve on 1, -ve on 2) = 1078pins 1-2 (+ve on 2, -ve on 1) = 620pins 3-4 (+ve on 4, -ve on 3) = OLpins 3-4 (+ve on 3, -ve on 4) = OLU3, PC817pins 1-2 (+ve on 1, -ve on 2) = 1068pins 1-2 (+ve on 2, -ve on 1) = OLpins 3-4 (+ve on 4, -ve on 3) = OLpins 3-4 (+ve on 3, -ve on 4) = 635I have a feeling my results are because the DUT is in CCT, thoughts?

author: StillTrying
author_rank: Super Contributor
timestamp: July 23, 2016, 02:16:57 pm
content: They look OK to me. It's difficult when we don't know how similar your PSU is to page 7.Your U3 looks like the U2 on page 7, could your U1 be just to provide the line-trigger signal?I think you're going to have to reverse engineer at least parts of the schematic.Quote from: negativ3 on July 23, 2016, 05:53:24 amMe too, no wonder the boats sinking.

author: JFJ
author_rank: Regular Contributor
timestamp: July 23, 2016, 04:44:23 pm
content: Quote from: negativ3 on July 23, 2016, 04:45:52 amThe attached schematic may be sufficiently similar to be of use:

author: retiredcaps
author_rank: Super Contributor
timestamp: July 23, 2016, 05:13:38 pm
content: Quote from: negativ3 on July 23, 2016, 07:32:49 am315VDC is good unless there is PFC involved in which case it should be closer to 400VDC.0VDC across C8 may be right, but I haven't looked at the datasheet or attached schematic yet.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 23, 2016, 05:17:56 pm
content: Quote from: negativ3 on July 23, 2016, 09:57:50 amOn the hot or primary side of the SMPS, using the -ve of the HV main fliter capacitor as ground is correct for making DCV readings on the hot side.For the cold or B+ side of the SMPS, you should be using chassis ground or some ground screw on the DSO main board.For fun, I took a working SMPS and did the same as you by putting my ground onto -ve of HV cap and got roughly the same millivolt readings as you on the cold side. Once I switched to chassis ground, I got my expected 5V standby.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 23, 2016, 05:35:55 pm
content: The SMPS that I fix are inside lcd monitors, TVs and ATX PSU. Generally, there is some standby voltage always present as long as the AC plug is plugged into the wall socket. I normally see 5V, but some newer TV sets have 3.3V.The 5V standby is used to power on the main logic board. The main logic board sits and waits for the user to press the on button or listen for the remote control on button.Since I don't own a scope, I'm guessing it works the same, more of less. I'll be back in about 1 hour. I'll take a look at the datasheet and schematic more closely after that. My schematic reading skills are very poor though.

author: boffin
author_rank: Unknown
timestamp: July 23, 2016, 05:51:22 pm
content: Maybe it's me, but that big cap looks a bit suspiciousIs it just a shadow, but there appear to be dark sections on the label and near the PCB (see attached)

author: retiredcaps
author_rank: Super Contributor
timestamp: July 23, 2016, 06:37:49 pm
content: Quote from: JFJ on July 23, 2016, 04:44:23 pmA quick glance on the hot side of the schematic shows it very close to the OP's picture. This schematic should prove to be helpful.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 23, 2016, 06:42:13 pm
content: Quote from: negativ3 on July 23, 2016, 09:57:50 amCan you measure pins 11 and 12 again, but using a ground screw on the main board for your negative probe? It should be 3.3V according to the schematic. All the other pins are unmarked on the schematic. They may have other voltages or be GND.The 3.3V is coming from jellybean 3.3V voltage regulator (U7).

author: Paul Moir
author_rank: Frequent Contributor
timestamp: July 23, 2016, 06:58:08 pm
content: Quote from: retiredcaps on July 23, 2016, 05:35:55 pmThere's no standby supply on that. It's got a real power switch, so there's no need. Also you will normally see a second transformer traversing the isolation gap which provides the standby supply without having to bring up the main transformer & switcher. There are ways around that, but there's no evidence of them being used here. To me this looks like a very simple straight up switcher.The U1 opto is just the line sync input to the oscilloscope, so it's unrelated to the rest of the PSU. So you can ignore it and it's associated parts. In fact, if you have the appropriate desoldering equipment, you could swap them to rule them out. (Though I don't think there's any evidence that it is bad.)To me there are not many parts likely to be faulty. Easy to check the transformer for continuity.C8 & associated just smooths the output from the opto, and you're not getting anything there since the supply isn't lighting up so it wouldn't matter if it was faulty. If you decide the switcher IC is bad, check out the snubber components (diodes & R20 resistor above C8) and make sure they're OK and didn't precipitate the fault. I would expect the supply to be chirping or something if anything on the low side were bad.EDIT: I misunderstood C8 - it is rather critical and also very suspect! Check it's associated resistor too.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 23, 2016, 07:12:02 pm
content: Quote from: Paul Moir on July 23, 2016, 06:58:08 pmAfter staring at the schematic, I started to realise there was no standby supply. My SMPS world consists of repairing consumer electronic equipment that I get for free.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: July 23, 2016, 07:21:51 pm
content: I know what you mean; the only difference with my education is that it started when real power switches were de rigeur. The standby circuits stick out like sore thumbs to me.I don't think I ever had a schematic for any SMPS I repaired. They're all pretty much the same and unless the component is burnt to a crisp you can ID the culprit and replace it. Unless of course it's the transformer, but sometimes those can be fixed too.

author: JFJ
author_rank: Regular Contributor
timestamp: July 23, 2016, 09:04:48 pm
content: Quote from: retiredcaps on July 23, 2016, 06:42:13 pmThe voltages are marked elsewhere on the schematic, so can be traced back. Here's my attempt:Pin 12: +3.3VPin 11: +3.3VPin 10: GNDPin 9: +5VPin 8: GNDPin 7: Line Trigger SignalPin 6: Not ConnectedPin 5: +20VPin 4: GNDPin 3: -11.5VPin 2: GNDPin 1: +8~8.5VTwo pin connector:Pin 2: GNDPin 1: +5VThree pin connector:Pin 3: -5VPin 2: Not connectedPin 1: GND

author: wblock
author_rank: Regular Contributor
timestamp: July 23, 2016, 09:24:26 pm
content: It mightnotbe the Samwha capacitors, but... that would be the first thing I'd suspect. An ESR meter can test some of those in-circuit. If C8 is the startup cap, replacing it alone might get the thing going. But it that does work, replace all of the eletrolytics from C8 to the right, because that would mean the rest are probably near failure, too.

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 01:32:59 am
content: Quote from: wblock on July 23, 2016, 09:24:26 pmThanks all,So... is replacing C8 the next logical step to take? I do not have an ESR meter

author: tautech
author_rank: Super Contributor
timestamp: July 24, 2016, 01:36:29 am
content: Quote from: negativ3 on July 24, 2016, 01:32:59 amYep, sometimes that's just all it takes.But you do have a capacitance mode in your DMM don't you, if it measures more than 10% low, replace it for sure.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 24, 2016, 01:43:40 am
content: Quote from: negativ3 on July 24, 2016, 01:32:59 amIf ESR is high, you can try warming up just C8 with a hair dryer or adding some fresh solder to the bottom of the legs with your soldering iron.Heating C8 up slightly will lower its ESR maybe enough to start the power supply?

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 03:30:11 am
content: QuoteQuoteI tried heating the pins on C8, no difference. My DMM does not have cap mode.I removed C8 and measured across it in ohms mode and it shows charging and discharging (+3.25 meg connected correctly and -3.25 meg connected reverse).C8 is a 47uF 50V. Closest I have is 22uF or 100uF 25V which are waaay off.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 24, 2016, 03:59:16 am
content: Quote from: negativ3 on July 24, 2016, 03:30:11 amEither 22uF or 100uF should be "good enough" for testing purposes. I have done the same replacement many times and it works. That is, if C8 is at fault.The voltage across that cap should only around 5.7VDC if I'm reading the datasheet properly.

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 04:07:04 am
content: Quote from: retiredcaps on July 24, 2016, 03:59:16 amThanks all for the incredible assistance with this.Ok, I have replaced the cap with 100uF type and it is the same, all outputs zero V. 0V across C8 with reference to case/earth point on the PCB.

author: StillTrying
author_rank: Super Contributor
timestamp: July 24, 2016, 04:09:03 am
content: Quote from: retiredcaps on July 24, 2016, 03:59:16 amI was just reading that! The control pin charges the cap to 5.7 - 7V, so 100uF 25V is close enough to check.It's looking like the TOP225YN then.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 24, 2016, 04:41:35 am
content: Quote from: negativ3 on July 24, 2016, 04:07:04 amC8 is on the hot side. You should be using the -ve leg of the main filter capacitor as ground or you can just measure the voltage of C8 across the legs of the capacitor.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 24, 2016, 04:47:08 am
content: @OP, does R21 measure as 9.8 ohms? In a post, you said all passives check out okay, but I just want to verify.There is one more simple check you can do . See if TOP225YN is shorted. With power off, see if S-D, S-C and C-D are shorted. You can make this measurement in circuit. Any reading less than 100 ohms is suspicious.TOP225YN is available at mouser.com for $4.03 USD. If you are not in the USA, TOP225YN can be found on ebay and aliexpress.com

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 05:46:23 am
content: R21 = 9.8 ohms.S-C = 39.8 ohms.S-D = 3 Meg ohms and rising.C-D = OL.From this the TOP225YN looks favourite?Sent from my iPhone using Tapatalk

author: retiredcaps
author_rank: Super Contributor
timestamp: July 24, 2016, 06:08:56 am
content: Quote from: negativ3 on July 24, 2016, 05:46:23 am39.8 ohms doesn't look right. You might want to desolder the IC and see if you get the same readings out of circuit.I'm not 100% familiar with this off-line PWM switch and a 20 page datasheet at midnight isn't my idea of bedtime reading. In 30 minutes, I'm off to bed.Having said the above, if it still reads 39.8 ohms out of circuit, I would say it is bad. As you might have noticed, others have more experience and knowledge with this type of power supply.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 24, 2016, 06:17:53 am
content: One other tip before bed.If/when you order a new IC, measure it before installing it. All three measurements should be open circuit.

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 06:37:33 am
content: TOP225YN removed and C-S reading 40 ohms. I will read the datasheet again. Thanks retiredcaps.Sent from my iPhone using Tapatalk

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 06:55:14 am
content: Are these sleeves used for isolation or something else? Tempted to remove them and test again...

author: Shock
author_rank: Super Contributor
timestamp: July 24, 2016, 07:01:10 am
content: Quote from: negativ3 on July 24, 2016, 06:55:14 amThey are ferrite beads.

author: tautech
author_rank: Super Contributor
timestamp: July 24, 2016, 07:20:51 am
content: Quote from: negativ3 on July 24, 2016, 06:55:14 amLeave them be for testing, they're there to manage EMI.

author: Shock
author_rank: Super Contributor
timestamp: July 24, 2016, 07:26:29 am
content: Just something I observed from your posts, when testing the low voltage side did you confirm if there is good continuity between the test point you are using as ground and the pin out grounds mentioned by JFJ? Because those are good ground reference points (for the low voltage side) you can clearly see they are common on the PCB.Next have you tested for voltages both with the output cables connected and disconnected? I'm not sure if I saw this clearly mentioned anywhere. Often when diagnosing power issues people tend to jump pretty quickly to the power supply itself but I don't recall reading you have confirmed the output voltages one by one, or tested across the secondary caps for voltage, or when the smps is removed from the mains and discharged checking the continuity across those outputs to see if any short is present.

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 08:45:39 am
content: Quote from: Shock on July 24, 2016, 07:26:29 amThanks Shock,Ok, I have verified the HV cap -ve and chassis earth arenotconnected to secondary GND.I have measured all outputs (cable connected and disconnected) WRT DC GND and theyallshow 0.000V DC output.With the PSU discharged, between DC GND and outputs, I measure the following resistance(s):3.3V = 0.732K5V = >1MLine Trigger = 0.97K20V = 3.81K-11.5V = 1.62K~8V = >1M

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 11:20:19 am
content: The transformer has been measured with the following:Primary:1-3 = 1.2 ohms4-5 = 0.4 ohms3-4 = Open cctSecondary:6-7 = 0.2 ohms6-8 = 0.3 ohms6-9 = 0.3 ohms6-10 = 0.5 ohmsOpen cct between primary and secondary.

author: JFJ
author_rank: Regular Contributor
timestamp: July 24, 2016, 12:53:26 pm
content: Quote from: negativ3 on July 24, 2016, 08:45:39 amThe secondary GND should be connected to chassis earth via the main PCB (through the 12-way ribbon cable).Incidentally, I notice that you only give single values for each of your resistance measurements. When measuring resistance in-circuit, it's best to do it in both directions (by reversing the meter leads). This can help to reduce some of the misleading resistance readings that can occur in the presence of semiconductor junctions.A long time ago, while testing faulty PCB, I checked the resistance of an 18K resistor that I suspected might have failed open circuit. The reading on my analog multi-meter was exactly 18kohm. I then wasted several hours performing other tests, before I noticed that there was a small diode (obscured by larger components) connected in parallel with the resistor. The forward biased diode just happened to read 18kohm on my meter (reverse biased it read infinity).

author: negativ3
author_rank: Regular Contributor
timestamp: July 24, 2016, 02:18:57 pm
content: Quote from: JFJ on July 24, 2016, 12:53:26 pmHi JFJ,I meant to state the resistances quoted were the lowest readings with probes in both directions during test.You are of course correct, the DC ground goes to chassis via the DSO main board.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 24, 2016, 07:56:22 pm
content: Quote from: negativ3 on July 24, 2016, 02:18:57 pm1. If IC is shorted both ways (40 ohms) out of circuit, then I think it is safe to say it is bad.2. JFJ brings up a good point about a multimeter's voltage possibly turning on junctions. There was a thread about the this athttps://www.eevblog.com/forum/testgear/multimeter-with-low-continuity-test-voltage/?alljjoonathan asks about this inhttps://www.eevblog.com/forum/testgear/need-suggestion-about-dmm-kyoritsu-1012-vs-hioki-4252-vs-fluke-17b/?all3. If you are going to order the IC from mouser, you might want to think about changing out the Samwha caps to something like Panasonic, Rubycon, Nichicon or United Chemi-con.4. Measure the resistance of the empty IC pads on the pcb to make sure they are open circuit.5. Please report back if a new IC fixed your problem or not.

author: negativ3
author_rank: Regular Contributor
timestamp: July 25, 2016, 05:22:59 pm
content: New IC and caps ordered. Will take about a week for delivery. I will for sure update when I get to the next step.Thanks all for your fantastic support.

author: negativ3
author_rank: Regular Contributor
timestamp: July 29, 2016, 08:53:51 am
content: New caps and IC delivered. Old caps removed and replaced with new.Now working 100% and on an extended test.Thanks again all who helped me out with this.

author: tautech
author_rank: Super Contributor
timestamp: July 29, 2016, 09:11:10 am
content: Nice one.We all learn a little bit more each time we have a win.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 30, 2016, 02:10:31 am
content: The good thing about this power supply is that it is relatively all laid out pretty flat and easy to repair. As I mentioned, I mainly work on consumer SMPS like an ATX power supply. I hate how everything is crammed in their on stand-up daugther boards and everything is impossible to measure safely on the hot side unless you solder jumper wires.How do you begin to troubleshoot this?http://images.anandtech.com/galleries/3560/CRM1000_13_575px.JPGhttp://www.kitguru.net/wp-content/uploads/2015/06/AS7V08471.jpg

author: negativ3
author_rank: Regular Contributor
timestamp: July 30, 2016, 03:01:23 am
content: One last point to consider... when reassembling ANY equipment, make sure the correct screws go back in the correct positions.I would remove the ATX PSU fully from the metal frame, then begin...

author: SeanB
author_rank: Super Contributor
timestamp: July 31, 2016, 08:08:12 am
content: Quote from: retiredcaps on July 30, 2016, 02:10:31 amJust like the Dell power supply I have that blew up. A replacement power supply was more than the book value of this Optiplex ( 2 years old), and was in any case a "refurbished" power supply. 250G hard drive is going in as an upgrade of the replacement "spare" computer, as the existing 80G drive ( Yes, I know, it filled up with Windows updates, till it had zero free space, but still actually boots and runs Outlook, which is the only thing used. I need more space to actually log in with a domain admin to prune it down, but currently just use a live Opensuse disk to mount and prune it down) is full.Something inside blew up and let the magic smoke out, but as this is shoehorned into a tiny box, and seemingly is assembled by elves and hotsnot, it is really hard to troubleshoot. Plus is the detonation did not kill the HDD, but the motherboard uses that wonderful Dell tiny and non standard connector, so I might just use a regular PSU and frankenstein the connector, though it looks like it will be scrap metal "real soon", and I will use the CPU, laptop DVDRW drive and ram elsewhere.