Broken 34401a - where to begin

[jitter]

Indeed, I was aware of that from one of your earlier posts. Perhaps my mentioning the logic voltage levels on the front panel pcb suggested I thought that that pcb was faulty.

My point was that the front panel pcb may force a reset on the main board if there's no communication with the main board within a certain amount of time. It may keep trying to do this, hence why you see the reset pin oscillate. The main pcb might not monitor this communication and reset normally with the front panel pcb disconnected.

I have another (working) meter to compare with. If I disconnect the front panel from the working meter, it still resets the way it is supposed to, so I don't think that the front panel (or the connection to it) is the problem.

Does the reset pin on the faulty main pcb also keep oscillating with the front panel pcb disconnected (it's not clear to me if your measurements were taken with or without the front panel pcb connected)?

[JJalling]

Indeed, I was aware of that from one of your earlier posts. Perhaps my mentioning the logic voltage levels on the front panel pcb suggested I thought that that pcb was faulty.

My point was that the front panel pcb may force a reset on the main board if there's no communication with the main board within a certain amount of time. It may keep trying to do this, hence why you see the reset pin oscillate. The main pcb might not monitor this communication and reset normally with the front panel pcb disconnected.

I have another (working) meter to compare with. If I disconnect the front panel from the working meter, it still resets the way it is supposed to, so I don't think that the front panel (or the connection to it) is the problem.

Does the reset pin on the faulty main pcb also keep oscillating with the front panel pcb disconnected (it's not clear to me if your measurements were taken with or without the front panel pcb connected)?

Ok, I did some measurements on the connector to the front panel (the front panel was disconnected).
FPINT and FPRST are both oscillating at ~4kHz as well.

And yes, the reset pin oscillates no matter if the frontpanel is connected or not.

/Jonas

[jitter]

Okay, it's beginning to look like your hunch that the origin of the reset pin oscillating is to be found at (or rather around) U553 may be correct after all.
At first I couldn't find it as you wrote it was the LT2925, but now I tried LM2925 instead and found the datasheet.

I didn't know a voltage regulator with a reset-out function existed, but this is one. Since voltage regulators are normally very robust, I'd first start looking at the health of the components around U553, especially C556, C557 and C558.
Dried up caps may wreak havoc on the output of U553 and if that's not what it should be, that also generates a reset. It might also explain why it keeps resetting. Perhaps put a probe on the in- and output of U553 and see if anything strange is happening there...

Edit: I read through from the beginning again, and see that you already had a closer look at this reset pin.
What I might do to find out if U553 is the cause or if something else is the cause is desolder / cut the reset pin so that it's electrically isolated from the rest of the circuit and see if the behaviour has changed (on the pin of U553).
I might also desolder U553 and feed in 5 V from an external PSU and connect the reset trace to 5 V through a resistor and see if that changes anything.
What happens if you put another cap of 100 nF across C558?

[JJalling]

Okay, it's beginning to look like your hunch that the origin of the reset pin oscillating is to be found at (or rather around) U553 may be correct after all.
At first I couldn't find it as you wrote it was the LT2925, but now I tried LM2925 instead and found the datasheet.

I didn't know a voltage regulator with a reset-out function existed, but this is one. Since voltage regulators are normally very robust, I'd first start looking at the health of the components around U553, especially C556, C557 and C558.
Dried up caps may wreak havoc on the output of U553 and if that's not what it should be, that also generates a reset. It might also explain why it keeps resetting. Perhaps put a probe on the in- and output of U553 and see if anything strange is happening there...

Edit: I read through from the beginning again, and see that you already had a closer look at this reset pin.
What I might do to find out if U553 is the cause or if something else is the cause is desolder / cut the reset pin so that it's electrically isolated from the rest of the circuit and see if the behaviour has changed.
I might also desolder U553 and feed in 5 V from an external PSU and connect the reset trace to 5 V through a resistor and see if that changes anything.
What happens if you put another cap of 100 nF across C558?

Hi jitter,

I have tried to desolder the reset pin from U553 (actually it is lifted right now, and connected by a wire to the board), but it doesn't affect the reset pin, so I don't think that U553 is the culprit.
I have tried to tie the reset pin high through a resistor, but it didn't work - it still oscillated.

Thank you for your comments and ideas.

/Jonas

[jitter]

I have tried to desolder the reset pin from U553 (actually it is lifted right now, and connected by a wire to the board), but it doesn't affect the reset pin, so I don't think that U553 is the culprit.

What does the reset pin on U553 do when you cut that wire so that the pin is floating from the board? Does it still oscillate?
Leave that pin disconnected from the board and measure what the signal looks like on the pad/trace where the pin used to be (XPONRST in the schematics).
They should not be the same when they're separated.

I have tried to tie the reset pin high through a resistor, but it didn't work - it still oscillated.

Trying to pull the reset line high through a resistor may not work with the reset pin of U553 still in place. Try it again with that pin disconnected.

The way I see it, the reset of U553 is only an output (going to reset-inputs on U500 and U501), and certain situations described in the datasheet (input transients, low input voltage, etc.) that result in the voltage output going out of regulation can trigger a new reset.

If pin 5 of U553 still oscillates when completely lifted off the board, then it must be U553 that's causing the reset to oscillate.
But the root cause may not be U553 itself. But that's the next step.

Thank you for your comments and ideas.

You're welcome.

[JJalling]

What does the reset pin on U553 do when you cut that wire so that the pin is floating from the board? Does it still oscillate?
Leave that pin disconnected from the board and measure what the signal looks like on the pad/trace where the pin used to be (XPONRST in the schematics).
They should not be the same when they're separated.

When the reset pin on U553 is disconnected, it stays low - hmmm... The XPONRST pin is still oscillating.

Trying to pull the reset line high through a resistor may not work with the reset pin of U553 still in place. Try it again with that pin disconnected.

I have connected XPONRST to +5V(pin 2 of U553) through 1K, but it still oscillates.

The way I see it, the reset of U553 is only an output (going to reset-inputs on U500 and U501), and certain situations described in the datasheet (input transients, low input voltage, etc.) that result in the voltage output going out of regulation can trigger a new reset.

Yes, thats how I understand it. But now it seems like it never goes high, so maybe U553 is broken after all - I will try order a replacement.
But the reset line is still oscillating - should I try pulling it up with a lower resistor value?

If pin 5 of U553 still oscillates when completely lifted off the board, then it must be U553 that's causing the reset to oscillate.
But the root cause may not be U553 itself. But that's the next step.

[jitter]

Now that I know that the reset pin of U553 stays low when disconnected, I'm not convinced that the oscillation is caused by that pin. Your first scope image already seemed to indicate that the reset circuit of U553 was working correctly because the time it takes to change state from low to high after power on is what it should be with the value of C558 that was used (~ 250 ms).

Another thing that seems to indicate that something else may be wrong is the fact that the reset pin of U553 stays low when disconnected. If you look at the block diagram of the LM2925, you'll see that there's a line drawn over the text RESET, meaning it's inverted. In other words: low = active. For some reason, U553 seems to detect a fault. The attachment shows situations in which the reset pin will go active.

Before ordering a new LM2925, I would try the experiment with an external 5 V PSU and XPONRST tied high. Also measure the current drawn, perhaps a bad tantalum is on the verge and already drawing excess current.
Should that not solve the problem, it sure looks like something else is the root cause and not U553 itself.

I don't like XPONRST oscillating on its own, it looks like it might be shorted to another trace.

[JJalling]

Before ordering a new LM2925, I would try the experiment with an external 5 V PSU and XPONRST tied high. Also measure the current drawn, perhaps a bad tantalum is on the verge and already drawing excess current.
Should that not solve the problem, it sure looks like something else is the root cause and not U553 itself.

I don't like XPONRST oscillating on its own, it looks like it might be shorted to another trace.

I now have removed the jumper JM553, and connected an external 5V supply. The meter draws around 130mA from the 5V rail, but the XPONRST still oscillates.

[jitter]

I'm afraid that does not look good, it rules out U553 as the cause and it may mean that U500 or U501 is defective.

At this stage I strongly suspect something might be very wrong with U500 or U501, one of the reset inputs seems to act as an output... perhaps internally shorted.
Might be worth checking with an ohmmeter on XLPORST and adjacent traces if there's a suspiciously low resistance between them. But I'm running out of suggestions what else to check.

Ah well, you can still use it for spare parts...

[JJalling]

I'm afraid that does not look good, it rules out U553 as the cause and it may mean that U500 or U501 is defective.

At this stage I strongly suspect something might be very wrong with U500 or U501, one of the reset inputs seems to act as an output... perhaps internally shorted.
Might be worth checking with an ohmmeter on XLPORST and adjacent traces if there's a suspiciously low resistance between them. But I'm running out of suggestions what else to check.

If I measure the resistance between +5V and gnd, I get 6K5 ohm on the broken meter, and 9K5 ohm on the working meter - capacitors?

Ah well, you can still use it for spare parts...

Yep

/Jonas

[jitter]

Don't know. Caps with leakage would probably result in much lower resistance...

[teksturi]

I would still try to cut xlporst trace between U500 and U501. Then you may at least know where the fault is coming from. Try to cut as close U500 (I think it is the reason). If oscillation is coming from trace (very very lucky shot) you can then use jump wire.

I found this thread where symptoms are similar to yours. https://www.eevblog.com/forum/testgear/hpagilent-34401a-dmm-startup-problem/ But theres no help 

[JJalling]

I would still try to cut xlporst trace between U500 and U501. Then you may at least know where the fault is coming from. Try to cut as close U500 (I think it is the reason). If oscillation is coming from trace (very very lucky shot) you can then use jump wire.

I found this thread where symptoms are similar to yours. https://www.eevblog.com/forum/testgear/hpagilent-34401a-dmm-startup-problem/ But theres no help 

I just tried to cut the trace near U500. The pin at U500 oscillates and the other side of the cut, is low.

/Jonas

[teksturi]

I still think that it might be something to do with mcu program. It is very unlikely (I think needs another opinion) that there is something broke inside mcu because oscillation happens also some other pins. Reset pin can also act as open-drain output (page 6) and there is pull-up resistor in reset pin (page 9). There can be some situation where watchdog gig in. Example LSENSE sees only 50Hz or 60Hz because in diode bridge one diode might be faulty. Then there is programming error and there will be forever while.

You should think why MCU is doing reset and what states should be in input when system is going up.

mcu datasheet
http://media.digikey.com/pdf/Data%20Sheets/Intel%20PDFs/8xC196KB_KB16.pdf

[JJalling]

I still think that it might be something to do with mcu program. It is very unlikely (I think needs another opinion) that there is something broke inside mcu because oscillation happens also some other pins. Reset pin can also act as open-drain output (page 6) and there is pull-up resistor in reset pin (page 9). There can be some situation where watchdog gig in. Example LSENSE sees only 50Hz or 60Hz because in diode bridge one diode might be faulty. Then there is programming error and there will be forever while.

You should think why MCU is doing reset and what states should be in input when system is going up.

mcu datasheet
http://media.digikey.com/pdf/Data%20Sheets/Intel%20PDFs/8xC196KB_KB16.pdf

Excellent info. I will try to do some digging in the datasheet. I will get back here, when I get further.
Thanks!

/Jonas

[JJalling]

I'm giving up on this - is there anything in the meter worth selling, or should I just dump it in the garbage?

BR Jonas

[HighVoltage]

I'm giving up on this - is there anything in the meter worth selling, or should I just dump it in the garbage?

BR Jonas

Sorry to hear.
The Display is sometimes selling well, if it is nice and bright.
The rest is probably not much worth in this stage.

[JJalling]

I would still try to cut xlporst trace between U500 and U501. Then you may at least know where the fault is coming from. Try to cut as close U500 (I think it is the reason). If oscillation is coming from trace (very very lucky shot) you can then use jump wire.

I found this thread where symptoms are similar to yours. https://www.eevblog.com/forum/testgear/hpagilent-34401a-dmm-startup-problem/ But theres no help 

I just tried to cut the trace near U500. The pin at U500 oscillates and the other side of the cut, is low.

/Jonas

I just found this meter in a box in the workshop - does anyone have a last suggestion before I throw it out?
Reading through the previous comments and my own notes, it seems like the resetpin on U500 is oscillating when not connected to anything. Is U500 fried?

Thanks,
BR Jonas

[teksturi]

It is probably faulty U500. At least do not trash it. Sell it to someone who need it. You can sell it and tell that U500 is faulty and you do not want to repair it.

[teksturi]

FPINT and FPRST are both oscillating at ~4kHz as well.

Do you mean that U500 reset pin is oscillating same frequency? This is kinda strange to me.

Have you confirm that XLPORST work correctly after you cut U500 out. Have you check crystal of U500?

[JJalling]

FPINT and FPRST are both oscillating at ~4kHz as well.

Do you mean that U500 reset pin is oscillating same frequency? This is kinda strange to me.

Have you confirm that XLPORST work correctly after you cut U500 out. Have you check crystal of U500?

I just measured again - the signal on the \RESET is 5.7kHz - so does FPINT and FPRST.

The XLPORST just stays low, so something is probably wrong with the regulator (U553) anyway - I thought it was working, so I'll try to order a replacement for that one.
For a test, should I try to pull the RESET high through a resistor?

Thanks

BR Jonas

[teksturi]

Yes, you can try. Good find that U553 is broken.