[SOLVED] UNI-T UTD2052CEL Oscilloscope SMPS repair

[negativ3]

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 scope  

Thanks
Andy

[retiredcaps]

Please don't tell me I need a scope to fix my scope  

I 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.

[tautech]

Please don't tell me I need a scope to fix my scope  

I 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 on the bridge rectifier?  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 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.

+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.

[retiredcaps]

The dropper resistor/s from HV DC to the IC supply should be checked too. (startup supply)

What 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.

[negativ3]

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...

[tautech]

The dropper resistor/s from HV DC to the IC supply should be checked too. (startup supply)

What 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.

Less 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.

[tautech]

https://www.power.com/sites/default/files/product-docs/top221-227.pdf

If you look at P 7 It would seem to me that D3, U2 and C5 are critical components for operation, check each.

[negativ3]

Please don't tell me I need a scope to fix my scope  

I 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 on the bridge rectifier?  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 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.

+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.

Mains 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?

[tautech]

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.

[negativ3]

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.

Ok, 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...

[tautech]

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.

Ok, 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...

If 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.

[StillTrying]

My DC voltage reference for all measurements is the -ve side of the HV cap. Not sure if this is good practice...

If 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.

Just 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.

[negativ3]

If 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.

Ok, passives look fine. I have measured the optos and get odd results as follows:

U1, PC817
pins 1-2 (+ve on 1, -ve on 2) = 1078
pins 1-2 (+ve on 2, -ve on 1) = 620
pins 3-4 (+ve on 4, -ve on 3) = OL
pins 3-4 (+ve on 3, -ve on 4) = OL

U3, PC817
pins 1-2 (+ve on 1, -ve on 2) = 1068
pins 1-2 (+ve on 2, -ve on 1) = OL
pins 3-4 (+ve on 4, -ve on 3) = OL
pins 3-4 (+ve on 3, -ve on 4) = 635

I have a feeling my results are because the DUT is in CCT, thoughts?

[StillTrying]

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. 

retiredcaps - same boat, just dipping the toe into smps...

Me too, no wonder the boats sinking.

[JFJ]

... would anyone have access to the power supply schematic for this beast?

The attached schematic may be sufficiently similar to be of use:

[retiredcaps]

Mains 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).

315VDC 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.

[retiredcaps]

Ok, 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...

On 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.

[retiredcaps]

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.

[boffin]

Maybe it's me, but that big cap looks a bit suspicious

Is it just a shadow, but there appear to be dark sections on the label and near the PCB (see attached)

[retiredcaps]

The attached schematic may be sufficiently similar to be of use:

A 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.

[retiredcaps]

Ok, 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...

Can 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).

[Paul Moir]

Standby Supply

There'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.

[retiredcaps]

There's no standby supply on that.  It's got a real power switch, so there's no need

After 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.

[Paul Moir]

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.

[JFJ]

All the other pins are unmarked on the schematic.

The voltages are marked elsewhere on the schematic, so can be traced back. Here's my attempt:

Pin 12: +3.3V
Pin 11: +3.3V
Pin 10: GND
Pin 9:  +5V
Pin 8:  GND
Pin 7:  Line Trigger Signal
Pin 6:  Not Connected
Pin 5:  +20V
Pin 4:  GND
Pin 3:  -11.5V
Pin 2:  GND
Pin 1:  +8~8.5V

Two pin connector:
Pin 2:  GND
Pin 1:  +5V

Three pin connector:
Pin 3:  -5V
Pin 2:  Not connected
Pin 1:  GND