Xitron 2000M (Teardown)

[little.tesla]

Hello,

I've got recently a more recent version of this cute little box and I wanted to share some impressions. I had to fix this unit (still waiting for some parts) and I haven't done any measurements so far. But I don't think I can do any usefull measurements, since my best meter is only a Keysight 34401A (last cal 3y ago).

Xitron 2000MN from 2015

Analog board:
One difference is the stacked SN74HC595N. (The socket was added by me since one IC of the stack was faulty in my unit)

[little.tesla]

Xitron 2000MN from 2015

Power Supply board:
This board was redesigned, but some bodges are visible. I've measured the off current from the battery to be 84.3uA, which is not too bad.
The charger is realized around an LTC4010.
TPS5420 is used as a step down converter (I think for the CPU)
The custom power supply to the analog board was replaced with a COTS from RECOM.
To isolate the communication from the CPU board to the analog board it uses (2x) ADUM2400 and (1x) ADUM1200. On my unit one of the ADUM2400 was broken.






CPU board:

[Anders Petersson]

Tell me where did you get the schemes? I can't find any documents

I asked Xitron for a copy of the service manual (via email). The service manual had very close to correct schematics. I had reverse-engineered the power supply board before I thought to ask for schematics. 

Can you share the service manual, please?

I bought a unit based on this thread and it is thoroughly not working -- V output is random and drifts and A output was first close to 20 mA and later on 150 mA. It displays "Outside limits" when starting. Also the battery is dead.
Thanks

[eurofox]

Just score one on ebay and is supposed to be fully working 

[little.tesla]

Actually this devices are well repairable since they use only standard components. The overall construction is not service friendly, as mentioned earlier but it's possible to fix them.
My unit was initially broken as well and I was aware of that. Trying to fix equipment (and sometimes successfully fixing it) is a very good exercise to gain more knowledge about electronics.

Maybe for others:
On my unit one of the isolators was bad and initially none of the relays were switching (output was stuck at some voltage; relays in some undefined state). After fixing that only some of the relays were switching and I discovered that one of the shift registers on top of another (this is the case/difference with the newer units) was broken as well. After this replacement and two burnt resistors the unit was operational again. My luck is that I did have a second operational unit for comparison.

Regarding the service manual: I did contact Xitron as well and initially they responded very fast and answered some of my questions. Than suddenly after I politely asked if they could share a service manual I did not receive any further answer.

Overall I like this unit even it's not really a 6.5 Digit calibrator. But for many application it's more than enough.

[Anders Petersson]

I bought a unit based on this thread and it is thoroughly not working -- V output is random and drifts and A output was first close to 20 mA and later on 150 mA.

An odd discovery; in a corner of the analog board, several bits of cut through-hole legs were stuck in flux residue, including across guard traces. I removed them with tweezers and cleaned with isopropanol. Instead of the previous random working of the relays, the unit is now consistently not using any relays and the output is a steady 3.3 V.  I got the service manual now so we'll see.

I'm surprised of both all the flux, including on the guard traces, and how the device could ever function with metal bits stuck there.

Just score one on ebay and is supposed to be fully working 

Yeah so was my unit. 

[little.tesla]

The PCBs on my unit were clean without any flux residues. Only thing I discovered is that somebody attempted a repair and one of the burnt resistors was desoldered on one side and pointing up in the air.

Regarding software versions:
I've got a MEJ with a software version F.03 from 2014 and a MN with software version F.05 from 2015. I was explained that the difference is mainly locking of ranges. The MEJ and MN are basically the same.  Any version beyond F.05 has more locking of the ranges with the latest revision F.07 that one can lock any range.

[Anders Petersson]

After my previous fix, my unit is mostly working!
One of the relays sometimes fails to disconnect the 'reset' pole when going from 'reset' to 'set'. Hence, both poles will be connected simultaneously and the output falls towards zero. Does anyone know a pin-compatible replacement for the now obsolete relay TN2-L2-5V?

Other problems with my unit:
- Some loose connection for the LED backlight
- The battery is dead (tips what to replace it with?)
- Touching the output low terminal causes the output voltage to change from the set value of 10 V to 23 V! I don't understand what can cause such a thing.

[little.tesla]

After my previous fix, my unit is mostly working!
One of the relays sometimes fails to disconnect the 'reset' pole when going from 'reset' to 'set'. Hence, both poles will be connected simultaneously and the output falls towards zero. Does anyone know a pin-compatible replacement for the now obsolete relay TN2-L2-5V?

Are you sure it's really the relay and not the drive signal? On my unit one of the shift registers was bad and therefore one relay was not switching properly. I somehow doubt that a relay is the culprit Have a look with a scope on the control signals. You should see a nice low and high (5V) pulse, if it's not like that, than it's one of the shift registers. Does your unit have stacked shift registers like on mine?

[Anders Petersson]

I did measure that both poles of this relay had low resistance at the same time coinciding with the output dropping out, a behavior the other relays don't have. But you might be right. I will check the control signal too.
Yes the shift registers are stacked -- this is another thing I find odd about the device. There are a lot of hand-soldered fixes.

[Kleinstein]

The stacked shift registers are likely used to drive relays. Even with 2 x HC595 in parallel this is still very much boarder-line.  An AC595 may have more drive capability, but is also more sensitive to clock ringing.

Those TN2 relays are really tiny.  If there is space, one may be able to bend the pins on panasonic Tx series inwards to fit the footprint.

[Kean]

There is an eBay seller in the UK offering the TN2-L2-5V relays, but they are pulls from old equipment...
https://www.ebay.com/itm/114487561523

If you can have a little extra space, the TQ2-L2-5V also has the same basic pin out just with wider spaced rows of pins. As they are low profile you could potentially mount them sidewyas with extension wires if it is a tight fit. Mouser, Farnell, etc have stock of those.

[jchw4]

Another happy owner of this device here. I got mine 2000MN (likely 2012-built) with completely discharged battery.
To the point that none of my meters could detect any voltage on it. I removed the battery, charged it and it seems to be in good condition.

Which led me to charging circuit checks. I used my HP 66311B to simulate the battery (the battery is 6x3300mAh Ni-Mh battery pack).

Scenario 1: Calibrator is switched off. Plugging in power adapter.

1. If the battery voltage at the time of adapter connection is <2V, it will not charge at all.

2. If the battery voltage at the time of adapter connection is >=2V but <= 5.1V battery will be charged by 367mA CC up to 11.5V. At 11.5V charging will end.

3. If the battery voltage at the time of adapter connection is >= 5.1V battery will be charged by 1.8 A CC up to 11.5V . At 11.5V charging will end.

I tried playing "negative dV/dt" to simulate end of charge condition but did not succeed. It will continue charging at the selected rate up to 11.5V.
I did not test 10Hr time charging limit, maybe it works.


Scenario 2: Calibrator is on on battery.

1. If battery voltage was less than 7.1V when calibrator was switched on, front panel green LED will start blinking.
(I am not completely sure about the voltage because the start up current is 1.5A and I did not have proper connection here. So it could be somewhat less than that.)

2. When battery voltage was good at the start but then battery discharged, green LED will start blinking at 6.6V.

2.1. Calibrator will switch off when battery reaches 5.9V

3. When battery voltage is less than 6.5V calibrator will not turn on.


Scenario 3: Calibrator is on charging the battery.  Unplug adapter.

1. If battery voltage was <7.35V when adapter was unplugged, green LED will start blinking.


I was surprised with 1.8A charging current. To double check, I inserted in charged battery (at 8V) and plugged in the adaptor.
And calibrator did start charging at 1.8V  . So it was not a glich with my test setup.

Could somebody double check my measurements? Is it by design? (I mean 1.8A charging.)

[Anders Petersson]

I think something is broken for you. See attachment.
Let us know what you find.

[Anders Petersson]

I realize now that the description I posted applies to the lead acid battery. I guess the charger for the Ni-MH option may behave differently.

I'm trying to revive my own battery pack (6 cells). Does anyone know what current and voltage levels to use? Should I charge and actively discharge a couple of times?
So far I tried to charge to 7.4 V slowly. After this, the pack falls back to 6.28 V under no load, perhaps lower if I wait more than 30 minutes.

[Kleinstein]

NiMH cells should be a bit over  1.2 V per cell with no heavy load (more directly after charge). Getting 6.3 V suggest that this are only 5 cells that are alive and 1 cell dead.

To revive NiMH or NiCd cells that were fully dicharged it sometimes helps to charge with a high current pulse (e.g. 10 C or even more) for a short time and only later charge more normal (e.g. 1 C). Dead cells may have a kind of internal short that can be opened from short pulse of high current.
Than charge normally till full and maybe do another discharge / charge cycle.
Normal charge for NiMH is something like 0.5 to 1 C with a controlled turn off from dU/dt and/or temperature. Much slower charge makes it hard to detect the end.

Old NiMH cells tend to develop a high rate of self discharge and this can lead to battery packs to develop imbalance, especially if no used regularly.

[fenugrec]

I haven't seen any firmware dumps yet - here's one from my unit, a 2000MEJ from 2012, ROM version F.03.

[EDIT] - the firmware is custom to each unit, and contains a reference to the serial number, because

The active cold junction compensation module can only be repaired/replaced by the factory. Calibration data from this module is stored in PROM

Also just uploaded to KO4BB, it might not be visible yet there.

[fenugrec]

For the 0.5 people who may be interested : I made a device file for HKJ's "TestController" software : https://github.com/fenugrec/xitron_tools/tree/master/hkj_testcontroller

It's just basic support right now. Didn't implement a number of features (Measure, Calculated, active/passive CJC, ...)  and only tested with RS232 interface.