Tektronix TDS1002 w/ Ault SW424RA0000F03

[rnjacobs]

What: Oscilloscope (Tektronix TDS1002) power supply made by Ault

Symptom: Power supply randomly browns out and reboot. Temporarily fixed for a year or two by unplugging the CCFL from the power supply. (Turned it on again today and now it's browning out even so)

Power supply: Ault branded switching power supply that emits (according to the silkscreen) -4V, 3.3V, 5V, and a floating 35V (for LCD contrast).

All but one capacitor labeled "Lelon" ... which I see is in the "bad manufacturers" list. None obviously damaged, but I lack a capacitance meter that goes above 20uF.

Between the symptoms and the capacitor manufacturer, is there any reason I shouldn't just recap this? Does it seem likely that this is enough to fix things? Is there a correct order? Any capacitors that I could get away with not replacing?

Thanks, all!

[Behemot]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Well you mostly need ESR meter combined with capacitance meter rather than just a capacitance meter by itself.

Generaly, the input caps tend to be OK, especially when there is no PFC as in here.

Seems like everything else is low-ESR, use those.

Are you sure the lamp is OK? Quite often at least in desktop displays the lamp soldering is poor and the wires tend to burn and heavily oxidate over time, to the state when the conection is severed. Sometimes it can arc and close the circuit but that will deteriorate as well. Usually the inverter is separate and when it detects problem, it turns just itself off, here it may have single driver for the whole PSU resulting in shutting down the whole thing.

[CapLeaker]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Stick the board back in there, heat all the caps up with an hair dryer so they are nice and warm and fire the ole O'scope up. See what happens.
Change out C28, it looks a little off compared to the others. I bet that is for the 35V rail, the one you have trouble with.
Change C6, start up cap.
I can't see what C29 does, but if it is connected to the opto PC1 (feedback to PWM), change it out too.

[Longbow]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Symptom: Power supply randomly browns out and reboots. Temporarily fixed for a year or two by unplugging the CCFL from the power supply.

How can something be fixed if you have to disconnect stuff to get the power supply running?

You might want to start over with a better problem description. What tests did you do to determine that the power supply "browns out"?. What does that mean anyway? In any case, it is the micro controller/system control section that controls "rebooting". This function is normally referred to as POR, or power on reset, and happens when you plug the unit in. Exactly which voltages are you looking at here?

[Longbow]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Stick the board back in there, heat all the caps up with an hair dryer so they are nice and warm and fire the ole O'scope up. See what happens. Change out C28, it looks a little off compared to the others.

The power supply board looks like it is in primo condition from the photo. If we are dealing with a no-test-equipment situation and unlimited time, then fine, replace some capacitors. How difficult would it be to check the regulated voltages with your MM and look for filtering problems on the secondaries with a scope? Chances are you might find a single part on the supply that is causing trouble, or just as likely you may find that the supply is not your problem at all. In the end, you will find the problem quickly and you will actually know which part(s) have failed.

Let everyone in on the fun. Let's see the power supply schematics.

[sam_sam_sam]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

I have a few de-soldering stations that turns on heating element ON before the switching power supply settled down and become stabilize

The micro controller does not have enough of a pause before it turns on heating element ON so it keep rebooting it's self

The new version of the same model has fix this problem it wait two seconds before it turns on the heating element

In the older version it has a transformer type power supply so with out the delay was no problem

The power supply is 18 volts@12 amps

[rnjacobs]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Are you sure the lamp is OK? Quite often at least in desktop displays the lamp soldering is poor and the wires tend to burn and heavily oxidate over time, to the state when the conection is severed. Sometimes it can arc and close the circuit but that will deteriorate as well. Usually the inverter is separate and when it detects problem, it turns just itself off, here it may have single driver for the whole PSU resulting in shutting down the whole thing.

Total use on the 'scope is really quite low, maybe a thousand hours total, and with none of the standard CCFL aging behaviors. But the only test equipment I have, other than this 'scope itself, is a logic analyzer and three voltmeters (that are only rated up to 600V)

I can't see what C29 does, but if it is connected to the opto PC1 (feedback to PWM), change it out too.

C28 and C29 are filter caps for the 35V LCD contrast supply.

PC1 is supplied from a pair of traces the winds all the way up the left (and low voltage) side of the board: IC2, R32, R29, R28.

How can something be fixed if you have to disconnect stuff to get the power supply running?

Well, ok, more exact description of symptoms: With backlight plugged in, 'scope would randomly reboot after a minute to ten. With backlight unplugged, 'scope would work fine for unlimited time ... at least until yesterday.

If I plug in just the power supply and CCFL, and NOT the logic board, it still power cycles automatically.

Exactly which voltages are you looking at here?

This is the only 'scope I have, so at best I can test things on a DMM:
Not plugged into logic board, but CCFL plugged into (and so power supply is cycling)
+35VR(2) to +35V(1): 30.5VDC
+5V(4) to COM(3): 6.8VDC (goes to linear regulator on logic board)
+3.3V(6) to COM(5): 3.3VDC
+3.3V(7) to COM(8): 3.3VDC
-4V(9) to COM(10): -3.78VDC
LT(11) to COM(10): 1.78VDC + 2.5VAC ("LT = line trigger" ?)

(The logic board only has tantalum and tiny SMT ceramic capacitors)

Let everyone in on the fun. Let's see the power supply schematics.

Best I can do is provide a picture of the reverse of the single-sided board...

[Longbow]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

Total use on the 'scope is really quite low, maybe a thousand hours total, and with none of the standard CCFL aging behaviors.

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

C28 and C29 are filter caps for the 35V LCD contrast supply.

so? Do they appear to be faulty? electrolyte leaking onto the pcb? Does the backlighting contrast change as the unit warms up?

With backlight plugged in, 'scope would randomly reboot after a minute to ten. With backlight unplugged, 'scope would work fine for unlimited time ... at least until yesterday.

Great! That is an accurate problem description minus all the other distracting stuff.

If I plug in just the power supply and CCFL, and NOT the logic board, it still power cycles automatically.

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

+35VR(2) to +35V(1): 30.5VDC

You are right, this one voltage would make me look carefully at this part of the supply. The line may also have switching noise on it, making it appear to be unstable so go ahead and replace those 2 filter caps on the 35 volt line. Also check the rectifier. See if the output stabilizes around 35 vdc after the replacements are installed. The higher volt lines like this one are typical problem areas because they can respond to line surges faster than the supply can correct for them. Wise to follow the 35 volt output to the CCFL and look for other filter caps that are downstream. Agree. Agree.

+5V(4) to COM(3): 6.8VDC (goes to linear regulator on logic board)

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

LT(11) to COM(10): 1.78VDC + 2.5VAC ("LT = line trigger" ?)

This output probably supplies a sync pulse to the scope AC line triggering function and should not be an issue here.

(The logic board only has tantalum and tiny SMT ceramic capacitors)

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.

Best I can do is provide a picture of the reverse of the single-sided board...

The board looks fine on both sides in the photos.

[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?

[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.

[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

didnt tek used to have a lifetime warranty on transformers?

[CapLeaker]

Re: Tektronix TDS1002 w/ Ault SW424RA0000F03

arcing will most definitely shut down the PSU.

[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?

[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.

[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.

[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.

[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.

[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

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.