HP3458A FLATNESS DAC CONVERGENCE 195 error

[chilternview]

Just started seeing the following error on a HP3458A (old one, 1989 vintage) after an ACAL AC:

"ERR 204 HARDWARE FAILURE - FLATNESS DAC CONVERGENCE 195".

It started a couple of days ago after the meter had been left running for a week; previously it's been fine. Any tips on what to look for?

[PartialDischarge]

Check if is it running in a too warm environment

[chilternview]

Room temp is 24C, TEMP? gives 33.0

[Dr. Frank]

Room temp is 24C, TEMP? gives 33.0

That's not too hot a room temperature, but....

If your instrument is running for at least 2 hours under these conditions, then TEMP? should read at least 12°C higher than R.T., provided the fan filter is clean, the instrument stands free on a table, i.e. nor in a rack, neither any air blocking of fan inlet.

Otherwise, something is not correct with the TEMP? measurement.
Check all power supplies, please.

The error is related to the AC board A2.
It sounds, as if it doesn't happen all the time.
What's the status when the 3458A is just powered up (i.e. still "cool")?

What's the result of the extensive Self Test?

The assumed thermal error might affect the 12bit DAC, U301, or the OpAmp behind.
If the error pops up after warming up, you might specifically cool the ICs, as well U404, U406.


Frank

[chilternview]

Sorry, I should have been more accurate.

Room temp 10cm in front of the 3458A is 20.5C according to a Fluke 87V. The thermal probe reading 24C was between two stacked 3458A's - the lower good one has TEMP?=34.8C. The one with the fault described is on the top with TEMP?=33.7C As far as I can tell both the fan and filter are OK with no airflow blockage.

The error message occurs every time I run a reset or full test or an ACAL AC. As far as I am aware it started yesterday (Tuesday) morning; the unit had been powered down Sat evening and then powered on and left on Monday morning.

It seems to read OK on ACV at least on the 1v/10 ranges and at freqs 10Hz-1MHz. I wanted to keep it powered up for a while as I'm running the drift test, so far  (subject to the power cycle mentioned above, and after only 1 week) it's showing 0.09ppm/week drift compared to the other known good 3458A with 0.03ppm/week.

I'll try some freezer spray on those ICs as soon as its powered down again.

[martinr33]

Good suspect here isthe Elantec comparator, EL2039, I'd start wthe the freeze spray there.

These are super high speed, sensitive parts and somewhat prone to failure. They have not been made for many years, and there are more counterfeit than genuine sources out there.

[MiDi]

Good suspect here isthe Elantec comparator, EL2039, I'd start wthe the freeze spray there.

These are super high speed, sensitive parts and somewhat prone to failure. They have not been made for many years, and there are more counterfeit than genuine sources out there.

TiN seconds that: Flatness DAC convergence: 198

Good luck on finding genuine replacement 

[chilternview]

So I saved the contents of the CALRAM and settings RAMs using WinGPIB to dump to a bin file. Then replaced first the CALRAM after flashing the saved bin file and verified that was OK. Then did same for settings RAMs.

All looked fine although I did see a message about a settings ram checksum error initially. But running a full self test was OK apart from the 195 error. And the meter seemed to be working OK.

Until I tried to query CAL? 72. And found that there was no menu entry CAL?... Half the commands seem to have disappeared.

Question: Is this likely to be due to the settings RAM change? Which of U121/122 is the low address (0x120000-0x127fff) and which the high address (0x128000-0x12ffff)?

[Dr. Frank]

You obviously successfully wrote back the CALRAM content, but yes, you've lost the content of the setup RAM, maybe by confusing Hi and Lo Byte of U121/122.
That's no big deal, I would just manually re program any Keys and settings as before.

For accessing the full menu call: MENU FULL from the key pad.

Frank 
 

[chilternview]

Dr Frank, you're a hero! I was seriously pooping myself I'd done something irreversible...

Are the settings RAMs just that - settings that can be changed by the user and are non-volatile? I'll read the manual on changing the key settings, but in the meantime MENU FULL works just fine.

R.e the flatness error, that's next on the list to check U404, 406 etc. when I can find some freezer spray.

[Dr. Frank]

R.e the flatness error, that's next on the list to check U404, 406 etc. when I can find some freezer spray.

Well, I assume that you did not answer the question yet, if this error occurs on a warm instrument only, or if it vanishes if the instrument is cold.

If it occurs also on a cold instrument, this cooling method makes no sense.
So please clarify this effect first.

I also was a bit reluctant to directly point to any of the ELANTEC ICs, like MiDi, as I hope the best for you, that only this DAC is the root cause, which might also be difficult to find.

Frank

[chilternview]

Sorry Dr Frank, so far I've just tried powering off for a short while (10-30mins). On power up, there is no initial error. If a full self-test is run (or an ACAL AC) it shows the error though. I'll try leaving it powered down for longer - e.g. overnight - and see if it passes self test then.

[chilternview]

Well, I assume that you did not answer the question yet, if this error occurs on a warm instrument only, or if it vanishes if the instrument is cold.

Unfortunately with the instrument cold, then powered on this morning (room temp 22.1C), self test failed with the flatness error. So does not appear to be a temperature issue.

Does it make sense checking voltages or waveforms around the flatness DAC? And if so under what conditions?

[Dr. Frank]

Fortunately, I did not experience such a fault on my 3458A yet. So unfortunately for you, I can not give any specific advice on this problem.

You might check whether the DAC receives proper digital input (problem with the ring-shift-registers?), and if it outputs a proper analogue signal, which I assume to propagate across the AC amplifier. I wonder how to analyze the signal before and after the ELANTEC amplifier, maybe apply an AC signal in the correct AC mode and observe what signal comes out of it.

Sorry, I don't want to speculate any further, maybe TiN has some detailed hints on his xdevs pages, or you might contact him directly, how he identified the ELANTEC as the root cause.

Frank

[chilternview]

Thank you Dr Frank, I'll check around U401B/U301/U302 the components around the flatness DAC. I seem to remember seeing someone else had a problem which was traced to bad opamp U401B.

[chilternview]

Some probing voltages on the pins of the flatness ICs on board A2 (marked 03458-69502, ERC4312, 09-9226). All measured with a 34401A with input resistance set to 10Gohm.

U401 (MC34072P)
1: +11.7mV    8: +14.0V
2: +11.7mV    7: +2.0mV
3: +10.1mV    6: +2.0mV
4: -17.7V        5: +1.2mV

The above I believe look OK.

U301 (AD7240JN)
1: +2.0mV    18: -22.0mV
2:  0.0V        17: -0.6mV
3: -1.27V      16: +13.9V
4: -1.27V      15: -1.27V
5: -1.27V      14: +5.0V
6: +5.0V       13: +5.0V
7: +5.0V       12: -1.27V
8: +5.0V       11: +5.0V
9: +5.0V       10: -1.27V

Schematic says pin3 of U301 should be -1.4V but -1.27V is probably OK.

U302 (LT 9150)
1: +11.16V    8: -16.5V
2: +27mV      7: 14.0V
3: -8.3mV      6: +8.5mV
4: -17.7V       5: 11.18V

I'm wondering about these - the opamp output (pin 6) is +8.5mV, the +input (pin 3) is -8.3mV, the -input (pin 2) is +27mV.

[Dr. Frank]

Well, this A302 (wideband amp) probably is defect. Pin 2 should be -8.3mV as well, and pin 6 =  -83mV.
I assume that all voltages are measure relative to AGND.

That ain't no good message either, as this is a special part, maybe HP custom specific, as I don't even find a datasheet.

Let's see what's used on the new A2 board (black edition).
Seems still to be the original one, an LT318A, as denoted in the schematic. Still in production.
The 12bit DAC, U301 = AD7541

Frank

[chilternview]

Well, this A302 (wideband amp) probably is defect.
I assume that all voltages are measure relative to AGND.

Yes measured from TP402 (analog ground).

That ain't no good message either, as this is a special part, maybe HP custom specific, as I don't even find a datasheet.

Let's see what's used on the new A2 board (black edition)

Frank

It's shown as a LT318A on the CLIP schematic, which broadly fits the description.

[chilternview]

Iteresting... I removed the A2 board and took a closer look. It's interesting that the date codes on this board are mostly '92, whereas the other boards are datecode 88. So it may have been replaced early on in the life of this 3458A.

A closer look shows the EL2039CN has a datecode of 99 - and look at the back of the board, it's been replaced with the flux left as soldered!

I'll still swap U302 for a new LT318A, which should arrive today, but maybe that Elantec opamp is worth checking too.

[Dr. Frank]

All these Elantec ICs are the weak spot of the 3458A, over long time and / or long usage.
Therefore, you're lucky that it failed already and had been replaced.. I would leave it alone, as it probably is not the root cause in this case.

Would be really great if U302 was the only culprit.

Btw.: Have you already updated firmware (to 8 or 9) and refreshed the nvSRAMs?

Frank 

[chilternview]

All these Elantec ICs are the weak spot of the 3458A, over long time and / or long usage.
Therefore, you're lucky that it failed already and had been replaced.. I would leave it alone, as it probably is not the root cause in this case.

Would be really great if U302 was the only culprit.

I hope so!

Btw.: Have you already updated firmware (to 8 or 9) and refreshed the nvSRAMs?

Yes the firmware was updated to rev 8 and new(ish) DS1220Y-150 and DS1230Y-150 in. The calram contents were copied OK.

[chilternview]

Well some LT318A arrived in the post this morning and so U302 duly replaced. Alas it did not make any difference, self test gave the flatness dac error. So I guess the next to try replacing are U301 and U401?

[Dr. Frank]

Hello,
you had measured the voltages of Input + and Input - on this U302... which showed an offset. Have you repeated this measurement, and did that improve?
Do you see any oscillation on this OpAmp?
To replace U301 would be another action, although it makes sense only, if that offset is still present.
Why U401? Have you seen any strange voltages on these?

TiN made similar measurements on U404, and found a big Offset as well, so that would be the next test to do.

Let me find the link: https://xdevs.com/fix/hp3458a_u4/

He applied a DC signal in the ACDC mode, obviously, and measured across the signal path in the AC/DC amplifier, including U404

Frank

[chilternview]

Hello,
you had measured the voltages of Input + and Input - on this U302... which showed an offset. Have you repeated this measurement, and did that improve?

No, with the replacement U302 the voltage on pin 2 (-ve input) is +110mV, pin 3 (+ve input) is -0.96mV, pin 6 (output) is -8.5mV.
I'm still unclear why the + and - inputs are different, it seems odd to me. The voltage of the junction of R303/R304 is +0.04mV which is consistent with the output, but I don't see why the inverting input has quite an offset.

Do you see any oscillation on this OpAmp?

No, no oscillation.

To replace U301 would be another action, although it makes sense only, if that offset is still present.
Why U401? Have you seen any strange voltages on these?

It was a thought as U401B drives the reference input of U301, the DAC. Although the voltages on it seem fine.

TiN made similar measurements on U404, and found a big Offset as well, so that would be the next test to do.

I checked U404, the +ve and -ve inputs both read +6.970V which is as expected. This is the EL2039 that appears to have been replaced in the past.

[Dr. Frank]

No, with the replacement U302 the voltage on pin 2 (-ve input) is +110mV, pin 3 (+ve input) is -0.96mV, pin 6 (output) is -8.5mV.
I'm still unclear why the + and - inputs are different, it seems odd to me. The voltage of the junction of R303/R304 is +0.04mV which is consistent with the output, but I don't see why the inverting input has quite an offset.

No, there might be something wrong with one of the 3 resistors, R302, 303, 304.
If pin2 is more positive than pin3, the output should be - V_supply.
If you measure with your DMM (10M input) on pin2, and assumed, that the feedback path via R304 is open, then you will measure about zero Volt, or 10M times the bias current of the OpAmp, +/- 6nA * 10MOhm ~ +/- 60mV, which is about the 27mV you're measuring. These 40mV between R302 and R303 should be exactly 1/10 of V_out, therefore this does not fit as well.
I suspect that R304 is broken, or has an open solder joint, or maybe any of the other resistors.
Please measure continuity (i.e. the resistances) of all 3 resistors, by probing between the affected pins of the OpAmp, ground and the common junction of all 3 resistors.

That would be a very uncommon error, but I also once encountered defective Thin Film resistors in our applications.
The protection film on top of the resistors had pinholes, and humidity plus voltage across the resistor oxidized the NiCr thin film layer until the resistor was open.

You have an old unit, so who knows.

Frank