Agilent 34401a error 606

[bading]

I buyed an used Agilent 34401a.
It had errors  612, 613, 615, 617, 618, 619, 621.
I have replaced U201 and these errors have disappeared.

Now appears Error 606 (This test checks the nominal gain between the integrating ADC and the U500 onchip ADC. The nominal gain is checked to ±10% tolerance)
I measure correctly Vdc , frequency.
I measure incorrect values Vac, R.
Anyone help me?
Thank's
Dario

[bading]

I make a clarification. The error 606 appeared after I accidentally ran tests with a short circuit between HI and LOW terminals bridge.
I blown something?
Thank's
Dario

[Vgkid]

Have you tried running the self test after leaving the meter powered off.

[bading]

Yes, i tried but the error remains.
Also the measures Vdc are wrong.
Thank's
Dario

[HighVoltage]

I make a clarification. The error 606 appeared after I accidentally ran tests with a short circuit between HI and LOW terminals bridge.
I blown something?
Thank's
Dario

You can not blow anything when you short the terminals and run the self test.
That would be bad.

I have repaired many 34401A meters but never had to deal with Error 606
Did you check all inputs and outputs of U500?
That is where I would start.

[bading]

Unfortunately I think that the instrument has been tampered.
The ADC should work.
If I measure 1.5Vdc read 1,1Vdc. If I measure 100Ohm read 60Ohm.
I checked the power supply is seems ok.
I'm arranging the capacitors to do the recap.

I have to fix the part around U301:

It seems that there is no diode:

I took the picture from:
https://xdevs.com/fix/hp34401a/
Thank's

[Vgkid]

If yours is the top image. It needs a good cleaning, and the blue resistor appears cracked.
About the diode, I will see if mine has one(99 datecode).

[bading]

If yours is the top image.

Yes, my is the top.

I will clean and replace the resisistor.
Thank's

[HighVoltage]

I'm arranging the capacitors to do the recap.

The capacitors on the 34401A never go bad or at least I never had a bad one, no matter how old.

Yes, it seems someone has tried to repair your meter very badly.
Check all those components and may replace them and remove all flux after re-soldering with some alcohol.

[bading]

How to remove the ribbon cable of the front panel from the connector on the mother board?
I tried to push as written on the pcb but nothing.

Thank's
Dario

[Vgkid]

No diode in mine.

[HighVoltage]

How to remove the ribbon cable of the front panel from the connector on the mother board?
I tried to push as written on the pcb but nothing.

Thank's
Dario

Push the loose plastic part down all the way and then lift the cable out.

[bading]

I cleaned the pcb and recap.
Now, i have the error 612 and 618.
It works all unless the measure of the resistances.

I have a doubt about the negative voltages -15A and -15B -14,72V and -14.67 respectively.
The negative voltage is -18,31V, zener diode 3,3V, i should have 15.01V.
They are low?

I measured output current from ohm source.
I used external analogue micro amp meter  to measure current from marked node to 34401's LO terminal.
I check current like your said and have:

000.000 Ohm - 1,35mA
0.00000 KOhm - 1,35mA
00.0000 KOhm - 130uA
000.000 KOhm - 13uA
0.00000 MOhm - 6,5uA
00.0000 MOhm - 6,5uA
000.000 MOHm - 6,5uA

The current values are wrong!
Help me!
Dario

[bading]

I did the voltage measures starting from Dr.Frank good multimeter.
See:
https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/

Q201 is dead?
Thnak's
Dario

[Dr. Frank]

Although it seems to be obvious, that Q201 is broken, I'd check if oscillation is visible on output (1)  of the OpAmp.

These OpAmps have failed in all similar cases by silicon breakdown of the inputs, up to now, so this time it could be U201-B, pin5, which causes the trouble, by drawing excess current. I also can't imagine, how Q201 could have failed. So, I would change U201 first.

Anyhow, order both parts, this will definitely repair your instrument.

Frank

PS.: After third thought, Q201 very probably is working ok, as the left current source with Q201-A/R202 is working properly.

It's very probable, that  Q201-B is defective, its excessive current is delivered by the 28.57k reistor in the array, not the FET, but anyhow, the constant current through R202 (40k) is regulated correctly, as can be seen by the involved voltages-so replace U201 only!

PPS: As there is a voltage drop between U201-A, pin 2 (-) = 6.94V and voltage over R202 = 7.12V, the (-) input on pin 2 draws excess current, instead of having low Ibias <200pA.
Therefore, another argument for defect of U201, but not Q201.

[bading]

I replaced Q201 and U201 but the error 612 and 618 remains!
Any idea?
Thank's

[bading]

I suspect problems in the analog switch U101.
Which tests can I do about this ic?
Thank's
Dario

[Dr. Frank]

Hi Dario,

let's continue systematically.. at first, do you now measure correct reference currents ( 1mA, 100µA, 10µA, ..) after the replacement ?

You can measure these currents directly on the Input jacks also, best when you use 4W Ohms.

Especially, are the 500nA correct?

Error 612 and 618 might have the same root cause, as the HV divider is tested in conjunction with the 500nA from the 10M Ohm range, afaik.
So we should investigate  there, first, before focusing on U101.

If the 500nA are ok now, maybe there's mis-calibration, if someone attempted to calibrate with a faulty current source circuitry.

Frank

[bading]

"Keep calm and repair 34401a!"
I reapeated the measures in 4W and with amperometer connected to input terminals but no change!

I have:
000.000 Ohm - 1,35mA
0.00000 KOhm - 1,35mA
00.0000 KOhm - 130uA
000.000 KOhm - 13uA
0.00000 MOhm - 6,5uA
00.0000 MOhm - 6,5uA
000.000 MOHm - 6,5uA

If i repeat the tests sometimes I have 612 and 618 errors  and other times 612, 613 and 618.
Thank's

[Dr. Frank]

You have measured the voltages in the I ref circuit, when being in the 100Ohm to 1MOhm ranges, so R202 is used.
Would you mind measuring these voltages in the 10MOhm range, so R201 is used?

Do the +/- input voltages of both OpAmps still differ?

Frank