Electronics > Repair
HP 34401A Error 606 on Self Test (Solved?)
_rushin_or_draggin_:
Hey all,
I ran a self test on my 34401A and it has the error 606. This error says that it "Cannot calibrate rundown gain: This test checks the nominal gain between the integrating ADC and the U500 onchip ADC. This error is reported if the procedure can not run to completion due to a hardware failure. I saw (I believe on this forum actually for a 34970A) that someone replaced U420 and U402 to fix this issue. I am not entirely sure how to test these opamps without taking them out of the circuit so I went around different components in the ADC schematic page. I noticed that +5VB going into U411 is actually around 9.7V and seems kind of unsteady. What I'm confused about is where the +5VB is coming from as I am struggling to find anything in the schematics that show it. Is it actually supposed to be somewhere around 5V or is this 5V referenced to somewhere else? I checked the 5V rail and it is rock solid at 5V.
Thanks,
Josh
Kleinstein:
The 5 VB should be very stable. It is a voltage generated via U401B (pin7) from the main 7 V reference, not the normal 5 V supply. It is also related to the 5 V that goes to U500 for it's internal ADC. The actual voltage can be a little off, like 4.9 V.
U401B is wired as a simple buffer, so one could check the input and output of that relatively easy.
A dropping voltage could also be cause from excessive load.
_rushin_or_draggin_:
I’ll have to check that. I’m just confused why I’d be getting almost double the voltage on +5VB. Also, do you know the difference between +5VB and +5BV? I see multiple voltages like this on the schematic.
Kleinstein:
There seem to be several parts of the 5 V supply, for more analog and more digital part.
Having really 9.7 V for 5VB is bad and very means a bad U401. Or a serious problem with the -15 V.
The high voltage may have damaged U411 and worst case also U500, but there is a series resistor that could have saved the day. The 74HC4053 is usually relatively robust, but if one orders a replacement for U401, one could as well get a HC4053. In my tests with a similar ADC the HC4053 from Ti was slightly better than the chips from NXP and ST, though for the 34401 the difference likely won't matter.
_rushin_or_draggin_:
I see on the theory of operation part for the ADC that the FLASH input is clamped to either 0 or 5 VDC as a protection, plus the series resistor you mention. The funny thing is, the meter (atleast voltage, frequency, and resistance) measures spot on. I haven’t checked current yet. It’s just when I do the self calibration it gives me the error 606. I’ll check the voltage across R442 to see if it’s feeding a good 5V into pin 5 of U401-B. If it isn’t I’ll have to backtrack to the U403 voltage reference. I’ll be sure to check the -15 volt line that you mentioned. I don’t think the filter cap or regulator were bad when I checked, but then again I didn’t look too closely. I’ll update you when I get to check it later tonight.
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