Tektronix TDS1002 w/ Ault SW424RA0000F03

[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.

[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...

[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

[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.

[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?

[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.

[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.

[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!