HP 34401A hacks and upgrades

[Ecogeek]

Yes I did similar, soldering some copper wings on the LRs to at least attempt to lower the peak temperatures and speed up the distribution of heat by way of a greater surface area.  My mains voltage is typically 240V but ranges from brief periods of 230V to 250V during the day with all our solar PV systems lifting the mains voltage. Most houses where I live have solar PVs. The other thing I did to paint the internal aluminium shield matt black, which goes from a very good IR reflector to a good absorber and radiator to speed up the thermal transfers to hopefully reach some equilibrium quicker.
The Linear Regulars are dumping a lot of heat, the designers of the 34401 made sure the DMM was under regulation down to 200V or below (for 230V mains) or so. 
The thermal environment of the 34401 has both external ambient and internal heating to contend with.
The ideal world would stabilise both the mains voltage to just above the LRs regulation point around say 210VAC and then the external ambient temperature.
I did plot the temperature of the internal shield, it takes hours to stabilise, a log curve, you need at least two hours before it flattens off then with both the mains voltage variations (variable LR heat dissipation) and ambient temperature fluctuations.
I did a quick check with all the covers off and used my soldering heat gun set to 100C i.e. a small point source of hot air. While watching the external Voltage plot from the DVM, applied the heat source briefly to the various components.  The good news most of the instrument makes only small differences, but the three Vref  0.01uF X7R capacitors are extremely sensitive and cause significant changes to the measurement voltage.
Replacing only three 0.01uF capacitors from X7R  to COG around the Vref circuit would eliminate one source of TC instability affecting the warm-up period in particular. One could ask why such unstable TC components were used in the Vref circuit in the first place apart from the cost 10+ years ago.

[iMo]

So, the potential changes around the Vref..
The 78/7915 are the placeholders for a low drop regulators (18.3->15V), there are just the 3V2 dropper zeners there now (and the same situation at the ADC powering too).

[Andreas]

Hello,

caution. 7915 (and usually LDOs) need more than 100 nF as output capacitor.

And do not forget the reverse bias diodes (anti parallel to output) to prevent latch up during start up.
(especially when putting a large capacitor across the heater).

with best regards

Andreas

[babysitter]

So, the linear regulators love some lower mains voltage, sometimes?

A trick for old radios comes to mind:
Put a 24V mains transformer in a box and connect it so that the 24VAC are substracted from the mains voltage before feeding the unit.
cooler device without opening.

BR
Hendrik

[iMo]

One hack I tried has been with the RC filter after the HP399.
Tried with 10k/47k/100k and 3.3uF wima foil.
Cut the trace and carefully soldered the RC in (WARNING: any soldering around the opamp's pins WILL move the calibration by some ppms!).
I did not see any "significant" change at 10V, however. It would need more elaboration..
See below ADEV with 10k/3u3..

I saw a significant change when I replaced the HP399 with my lowest noise LM399 - some 20-30% based on a quick stddev evaluation. It would require a complete readjustment of the meter, however. Moreover, my LM399 does shot from time to time (like 2-4x per day) making around 5uV (@10V range) shift in one direction.

Another thing to eleborate is to add a small capacitor at the input of the ADC in the MCU.
There is 3K16+1k resistor in front of it, so perhaps 100-300pF against chip's ground might show something..
Also decoupling of the MCU might help.
The ADC's comparator is in the ASIC, so decoupling of the chip there could be worth of try as well.

And the replacement with a 1399 will be the best move.. It means to add the RC snubber (do not do it at the 399 pcb pads directly as that would increase the noise due to thermal flows), and replacement of the zener's cathode resistor.
Needs complete readjustment.

[iMo]

..and a preregulator like in the Keithley 2001 might help too..

V2 is the voltage generated by an LED->optocell, the LED driven by electronics on the secondary side..
Keeping "stable" 220V AC at the primary.

[Kleinstein]

The µC internal ADC has to be quite fast as the feedback waveform does not stop. Adding a capacitor there would add a delay and settling time.

The comparator performance is not relevant. The comparator is only used to control the feedback and it's noise has essentially no effect unless excessive to cause saturation of the integrator. The "comparator" could even be something like a schmidt trigger input and not a real comparator.

The main point with the ADC is that the quantization noise is limiting and there is nothing (except a complete redesign, like a different µC) that can be done about this part. So all improvements for the shorter time scale are small.

The preregulator like in the K2001 could help a little with the thermals - lot of effort with a limited potential.

[iMo]

The µC internal ADC has to be quite fast as the feedback waveform does not stop. Adding a capacitor there would add a delay and settling time.

Indeed, but worth of seeing the impact.. We do not know how is the exact timing of the ADC's sampling..

[Kleinstein]

We don't know the exact timing, but there is not much room to play. The switching is with some 1 µs and 1.6 µs and the sampling time of the ADC is some 1 µs - no barely enoug time and of the long time slot is used any delay effects how much of the last feedback step is effective.
One could test the impact, but i have little hope for an improvement and it can have a negative effect on the linearity.

[iMo]

80C196KB has got a separate Vref input for the internal 10bit ADC, so we may try to filter there..
ADC timing - see below (we run it at 12MHz).. For higher than 6MHz clock the prescaler should be on..

[Kleinstein]

The ADC needs to run rather fast and there is now good way to change anything there. So likely no room for the prescaler. I had just looks at a shorted DS from Intel, that shows 1 µs as min sampling time. Even that is not very fast for the given signal that is modulated to some 370 kHz and thus not stable for the 1 µs. Already the quantization noise is limiting, extra noise from the ADC would be on top. How noisy the ADC is with the relatively high clock and very likely no prescaler is another question.

[iMo]

Does it sample at 370kHz modulation? I can hardly imagine it can do something with that data at 12MHz clock and intel 16bit architecture (many clocks per instr.).
Moreover the ADC conversion time will be something like 23us at those 12MHz.. (say max 50kHz sampling rate)..
My bet would be it does sample the residual voltage at the beginning and after say 1000 runups or something like that..

[Kleinstein]

There is no way to stop the run-up modulation. It has to contineously run to avoid saturation. IFAIK it only samples the voltage 2 times, once for the start and once for the end of the effective integration. So only 2 µC internal ADC conversions per conversion, but still with the 375 kHz of modulation going on all the time. The effective integration time is defined by the time when the µC internal ADC reads the integrator.

Already the µC internal ADC can not follow the 375 kHz - so at most the µC could sample the voltage for every 4th cycle and average a little. This could give a little improvement, but I doubt that they use this.

[iMo]

The ADC "conversion time" cannot be 2us. Noooo way. It is 10x time longer at least..
ADC's "S/H acquisition" time could be 2us when overclocked.
I highly doubt the 80C196 at 12MHz with its ADC can follow 370kHz modulation and do something with the data (or the above DS table is wrong)..

[Kleinstein]

In the DS that I have they claim down to 1 µs aquisation time, though already with a desiclaimer : "1. These values are expected for most parts at 25°C but are not tested or guaranteed." They would want a pretty short aquisition time, even though the relevant part is mainly from the end of this time anyway. So even if they have more only the last 500 ns or so would really matter if the signal changes.

The 375 kHz feedback cycle is handled by the ASIC and the control is via the "comparator" input there and not from the µC. The µC only has to somehow synchronize the ADC conversion with that cycle and do the 2 conversions for start and stop and read the counters inside the ASIC.

The somewhat tricky part not described that well in the patent is when to exactly sample the integrator and how to handle the feedback phase during the sampling. Chances are this would include 2 feedback steps that are only partially effective. Chances are they have to do some calibration measurement to see how to split the 2 steps to before an after. They may avoid the split steps when sampling just before the actual variable time, even though this includes the reference switching.

[iMo]

They define in DS the minimal "conversion time" is 91 "state times".
The state time is 2 clocks (based on the DS) that is say 166ns at 12MHz clock.
The internal clock of the MCU == state time (166ns period with 12MHz external clock).
That is 15us for the conversion time when overclocked (no prescaler).
Add the "acquisition time" which is minimal 8 state times when overclocked (DS) that is 1.3us.
So you cannot get an ADC value into the MCU's ADC register earlier than after 16.3us overclocked.
Then you have to do something with the ADC value, you would need a couple of us to place it somewhere, no math, no decisions, imho..
The average instruction exec time with primitives is aprox 6 state times (aprox 1us) with simple math instr. perhaps 14 or more state times (User's Guide p 20-21).

[Korchahin]

Got to my HP 34401. I was lucky and the multimeter is in good condition. It looks like it was not used much. The screen is in excellent condition.

Question about electrolytes: The capacitors have typical measurements and it looks like it is in good condition (for example, i add foto other new narmal and Low-ESR capacitor). Do they need to be replaced? I am considering quality Nichicon UHW1H102 capacitors (High Reliability 10kHours, Low Esr)

[iMo]

Mine is from 1999 and I replaced the all big elytes with those black-gold ones. The old one were all ok, so there was no need to replace them, actually. Also the tantalum one seem to work ok. The issue with tantalum caps is when they got short they will smoke the zeners (usually short them). The zeners create +/-15V off the +/-18V (a primitive series dropper) on many places and when a zener is replaced the adjusting might be needed.

[Korchahin]

I want to do a thorough preventative maintenance and upgrade all reasonable weak points, and install the ADR1399 and send it in for recalibration after minimal aging.

[iMo]

Then people recommend to replace all tantalum bricks with same capacitance bricks but higher max voltage (like 25V or 35V - doublecheck) than the original one, afaik.

PS: with 1399 you have to change the zener's cathode resistor (such you get 3.5mA) and you have to add the RC snubber. Also I would not solder anything directly at the 1399's socket pins (from the bottom side) as it visibly increases noise (perhaps because the higher thermal flows) then.

[Andreas]

I want to do a thorough preventative maintenance and upgrade all reasonable weak points, and install the ADR1399 and send it in for recalibration after minimal aging.

Do you really think that replacing a manufacturer selected (from many, to meet the specs) and pre-aged LM399 with a arbitrary ADR1399 with unknown drift and history is a upgrade?
Before doing this I would pre-age a bunch of ADR1399 and then select them for minimal drift, 1/f noise and T.C.

with best regards

Andreas

[Hydron]

I don't know why you would want to replace an electrolytic that tests good unless it's extremely old, in a position where failure would cause damage, or there's another reason to be suspicious (e.g. another of the same type is bad, or it's a known problem part - are these known to go bad often in a 34401A?).
I've got plenty of stuff from the 90s and earlier built with quality caps that work as good as new (at least where they aren't abused or in particularly stressful use).

[Andreas]

are these known to go bad often in a 34401A?).

I am only aware of some Keithley production lots which used leaking electrolytes (from 2002-2004)
https://www.eevblog.com/forum/testgear/keithley-2000-and-the-leaking-caps/msg458613/#msg458613

with best regards

Andreas

[Hydron]

Thankfully my Keithley gear is mostly from the brown era (and by and large has not needed any caps replaced, other than the early 80s K181 which had been abused by a prior owner). If I had a K200x I'd certainly be swapping caps, I've seen enough horror repairs documented on those!

[Dr. Frank]

My 34401A is one of the first instruments, I bought it in 1990.
Up to now, no need at all to replace the capacitors, they do not leak, and ESR / cap value are perfect for their age.
Please use an ESR meter, before uselessly changing them.

I even had not to replace U203 in the Ohms current source yet.
Frank