Tektronix TDS1002 w/ Ault SW424RA0000F03

[luizbitencourt]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

I know this is a very old topic, but I need to share!

I came here looking for a solution to this power supply fault; however, I've discovered that C6 caused the fault in my case. This 56 uF bad cap caused the failure.

I have the same oscilloscope model with the same power supply brand but a different part number (PN: SW424KA0000F02). Upon connecting the power cord and switching it on, nothing occurred. After removing the power supply board and measuring the J9 voltage outputs, they're all 0V.

The solution came from changing the C6 to a new one. Now, it works! All the outputs are good!

If someone in the future needs it, it can be a path to the solution!

[CapLeaker]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

glad to hear you got it fixed!

[rnjacobs]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

how annoying that my problems seem to have been due to the lack of strain relief on the power socket.

Well, that wouldn't be the first time.

Indeed! Even after having encountered it on low voltage barrel connectors many times before it just didn't occur to me.

That is an odd number for C6, 82uF?

Just maybe the time constant of 30kΩ and 82uF and D5's zener voltage works out to something?? But that doesn't seem likely.


8 hours of running the power supply on its own (logging the 3.3V and 6V rails with a logic analyzer), and 12 hours of powering the 'scope later ... it seems to be just fine. Success!

Anyway, I'd like to thank all of you for walking me through the debugging process, even if it had nothing to do with bad capacitors

[budm]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

That is an odd number for C6, 82uF?

[CapLeaker]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

"I guess it's in "burn-in" stage now to see if I can coax out a failure again. If I can't, how annoying that my problems seem to have been due to the lack of strain relief on the power socket."

Well, that wouldn't be the first time.

[rnjacobs]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Ok.
Swapped out IEC cable for one that felt "tighter". (Actually did this yesterday)
Reflowed solder on C14 socket, switch, fuse, and thermistor(??—green thing silkscreened TH)

The solder on the C14 socket looked good before hand, but the socket was kinda floppy, and it's definitely tighter now.

Tested, had one more power failure. Removed power supply from mounting posts so that I could get a voltmeter onto the solder side and ... haven't been able to reproduce since.

I guess it's in "burn-in" stage now to see if I can coax out a failure again. If I can't, how annoying that my problems seem to have been due to the lack of strain relief on the power socket.

Especially if it has some stand-by rail which remains active! Which I would say it has - there is likely some constantly-powered switch which turns the main PSU on. My Rigol has it that way…

Not in this supply. Mains—both hot and neutral—is completely isolated from all other components when the switch is off.

So C6 (+ is connected to pin 6 of SMPS IC 1 which should be the VCC pin of IC 1) start up/running cap is only 2.2uF (not 22uF?)? It is good for sure?

I apparently don't have a useful capacitance meter—just an ordinary DMM that goes up to 99uF, and no ESR measurement. C6 is nominally 82uF 35V. R3 and R15 are both 15kΩ 2(?)W.

IC 1 is ?????

KA7552.

D2 is the rectifier diode that rectifies the AC through R10 (24 Ohms) from AUX winding of the transformer to provide running VCC once the SMPS is running.

:enthusiastic learning:

Does anything keep power from being dissipated in the shunt regulator of R3+R15+D5 after it bootstraps?

Q2, Q3, Red cap are the Royer Oscillator that drive the Inverter transformer T2 for the backlights.

Ooh! Thanks for the correct term to look up!

[budm]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

So C6 (+ is connected to pin 6 of SMPS IC 1 which should be the VCC pin of IC 1) start up/running cap is only 2.2uF (not 22uF?)? It is good for sure?
IC 1 is ?????
D2 is the rectifier diode that rectifies the AC through R10 (24 Ohms) from AUX winding of the transformer to provide running VCC once the SMPS is running. D5 is Zener connected to pin 6 and GND for protection.
Q2, Q3, Red cap are the Royer Oscillator that drive the Inverter transformer T2 for the backlights.

[Behemot]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

If there is some load on the supply I think the cap charge will be discharged quickly. Especially if it has some stand-by rail which remains active! Which I would say it has - there is likely some constantly-powered switch which turns the main PSU on. My Rigol has it that way…

[Longbow]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

As I was saying...if a supply has a complex secondary failure, even one that shuts down the output voltages, voltage on the main supply filter will not just vanish. In fact, if the supply quits, the 160 volts should actually rise a little because there is no load. A typical place for trouble is the AC switch solder pads because the switch is being constantly pushed on and off...or right where the 160v connects to the switching transformer. Sometimes you cannot see the intermittent connection on the larger pads, but you can spot it with a meter. Admit it, its fixed!

One sneaky thing that can happen is that one rectifier in a bridge can fail. The unit will work to some extent but the supply can't get up to full output under load. But again, these diodes should be located next to the 160 volt filter.

[CapLeaker]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Only have 3DMMs, but:
3.3V line: 3.353, decays rapidly on failure
-4V line: -3.81V, falls to 1V on failure and then decays slowly
(from memory, not retested) 35V line: 31V, decays very slowly on failure
main input filter cap: 161.2V, falls almost instantly to 25V on failure, then slowly falls if supply stays in failure state

Secondaries all come up about 2 seconds after the main input goes back to 160V.

This tentatively looks like the same behavior if I just use the power switch ... which implies I should first investigate the input filters? So I tried removing LF2 (the second/lower one) and bypassing it. No change. Getting to LF1 is a good deal harder, I'll try that tomorrow.

(Ok, that's the 2.2nF capacitor on the KA7552's "CS" pin). It looked pretty good to me, but lacking any reason not to, I tried reflowing the solder, tested, removed the solder and put down new solder, tested again. No obvious change.

O.k. sometimes solder joints look bad on the picture.
Anyhow I guess there is something to chew on, because the main filter capacitor goes low intermittent. That should not be. So easy game from here on. Make the PSU fail again, check the input of the bridge rectifier (AC) and see what happens there. 120VAC x 1.414 = 169.68V DC. If the AC disappears, follow it back to the switch and compare input to output of that. Then it turns out to be a broken switch or cord. lol.

As Longbow already said, there is very few components on the AC circuit of the primary.

[Longbow]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

161.2V, falls almost instantly to 25V on failure, then slowly falls if supply stays in failure state

This one observation tells me that you have a problem in the primary circuit of the supply. Crackling noises with immediate loss of 160v. have to mean intermittent connections right near the AC input.

Since there are very few parts between the plug and the 160v. filter cap, I would look carefully at the common mode choke, main rectifier bridge and even the AC switch - with attention to the traces leading up to same.

Using a rubber or plastic hose as a stethoscope can serve as a way to isolate where the crackling is coming from. Let us know.

[rnjacobs]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

I'd like to know the voltages on all secondary rails and the main filter cap when this thing clonks out.

Only have 3DMMs, but:
3.3V line: 3.353, decays rapidly on failure
-4V line: -3.81V, falls to 1V on failure and then decays slowly
(from memory, not retested) 35V line: 31V, decays very slowly on failure
main input filter cap: 161.2V, falls almost instantly to 25V on failure, then slowly falls if supply stays in failure state

Secondaries all come up about 2 seconds after the main input goes back to 160V.

This tentatively looks like the same behavior if I just use the power switch ... which implies I should first investigate the input filters? So I tried removing LF2 (the second/lower one) and bypassing it. No change. Getting to LF1 is a good deal harder, I'll try that tomorrow.

While you are at it check this what I think is a bad solder joint.

(Ok, that's the 2.2nF capacitor on the KA7552's "CS" pin). It looked pretty good to me, but lacking any reason not to, I tried reflowing the solder, tested, removed the solder and put down new solder, tested again. No obvious change.

[CapLeaker]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

+ 1
You also can use heat (hair drier) or cold (can of air upside down), an old plastic toothbrush or something like that to press (or stress) on things, while the PSU is turned on.

I'd like to know the voltages on all secondary rails and the main filter cap when this thing clonks out.

While you are at it check this what I think is a bad solder joint.

[stj]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

resolder everything heavy or on a heatsink, and remove all 4 transformers.
then inspect where the wires are soldered to the pins.

[rnjacobs]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Less convenient thing: Removing L5 (the ferrite choke that connects the CCFL inverter to its -4V supply) doesn't fix anything.

More convenient thing: I can make the arcing sound (and power supply shut down) appear by just rotating the power supply in space. (While mounted to the oscilloscope: I'm doing my best to avoid any flexing) However, I still can't get to happen in a reliable enough manner to e.g. localize the sound using a microphone.

Any suggestions for other ways to find it?

[CapLeaker]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

arcing will most definitely shut down the PSU.

[stj]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

didnt tek used to have a lifetime warranty on transformers?

[rnjacobs]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

irrelevant for your troubleshooting purposes. You don't clarify if the backlighting is working???

Glows the standard CCFL white (i.e. not old CCFL pink). Visual inspection of the wire shows no visible defects. Inspection under the right-angle protectors at the ends of the tube show an intact wire. Continuity test shows nothing funny from each end of the tube to the connector.

so? Do they appear to be faulty? electrolyte leaking onto the pcb?
[...]
Any of which can fail just as rapidly as any electrolytic...especially the tantalums. Usually a bad tantalum will become a short circuit, turn brown and/or explode. Visual inspection is the first step.

The board looks fine on both sides in the photos.

There's no visible damage on any component anywhere. No bulging capacitors, no leaking electrolyte, no burned wires, no visible heat damage on the FR4 or hot melt, no broken epoxy packages.

Does the [LCD] contrast change as the unit warms up?

Nope, seems stable. The 'scope's service manual ("tek_tds_1000_2000_series_svc.pdf") says that the voltage on those pins should be 30 to 40Vdc, and is subsequently regulated on the logic board to the desired 22-28VDC for LCD contrast. Also that it should only supply 15mA.

be sure that the on-board regulator is actually producing +5 volts.

Multimeter says it is.

It would be great to see the psu schematic at this point.

Is that a request to draw one? To contact Ault (now SL Power) and see if they're willing to sell/give me the schematic?

The non-passives on the board seem to be:
IC1 - KA7552
IC2 - TL431
Q1 - ??20337 / ?ORF / 3N90 (power MOSFET of some sort? label isn't fully readable)
Q2 and Q3 - 2SC3205
Q4 - MCR72
PC1 and PC2 - PC123
PC1 and IC2 form a feedback path controlling 3.3V.
PC2 generates LT, and is not part of the feedback path.
Two dozen various diodes

The cycling would most likely be a power supply failure if that is correct, but one would think of current foldback problems rather than a real uPC "reset" situation. What happens if you unplug everything including the CCFL board? Does it still cycle?

Cycle frequency seems to be a function of load. Without any load at all, it sometimes never cycles. With more load, it seems to cycle sooner. When the logic board was plugged in, power cycles sometimes would happen in response to pressing buttons on the front panel.

What you really want to do is use all the available meters you have and hang them on the psu output voltage points. Observe which output voltage(s) are caving when you get a failure.

Certainly, in the past, the backlight and 3.3V rail would collapse and come back at the same time, with a couple second delay.

Right now, they all collapse at the same time, and they all come back up at the same time... but the amount of time spent off seems to behaving more like a thermal issue.

Wise to follow the 35 volt output to the CCFL and look for other filter caps that are downstream.

The CCFL power supply appears to derive from the -4V supply. The ≈35V supply is definitely only for LCD contrast, and fully isolated on the power supply (albeit tied on the logic board)

Error log never said anything, just incremented the power-up count.

—
Actually ... now I hear something arcing, sometimes, when the power supply fails. Can't see anything even in the dark, nor smell ozone. Uuuuugh. I guess I could try desoldering the CCFL backlight inverter transformer and hope it's not the main transformer.

[stj]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

check or replace the caps and stop going on about how it's hardly used.

bungs harden & perish, electrolytes crystalize.
electrolytic caps dont last forever regardless of if you use them or not.
specially small ones that often have a lifespan in single digits.

[Longbow]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Here is the manual. No schematics, but it should be of some help getting this problem fixed.
This unit has an error log. What do those error messages say?