Datron 1281 Repair/Maintenance

[leighcorrigall]

Hello, members.

Many of you are familiar with this 8.5-digital 1281 Datron/Wavetek multimeter manufactured in England, but there does not seem to be a dedicated repair/maintenance thread on the EEVBlog.

I am going to post about my experiences with this unit and I invite others to join in as well.

TiN has a great work-log https://xdevs.com/fix/d1281/ where most of the material is already available for servicing this unit. 

EDIT: rigrunner also has a repair thread for the Datron 1271 https://www.eevblog.com/forum/repair/datron-1271-repair/, which shares many similarities to the 1281. In fact, most of the boards are nearly identical to mine with the exception of the DC board.

The unit I have seems to be manufactured around 1987 to 1996, based on the date codes of the integrated chips. I would say that the unit is from 1991 until I know more. The last calibration was performed on 2017-10-04 by Pylon and expired on 2018-10-04. Calibration seals were unbroken on the calibration lock, and on the screws which secure the top and bottom covers. The battery that sustains the calibration data measures 3.7 V. The previous owner was the Hydro-Québec Research Institute. All options (True RMS AC Converter, 2- and 4-wire Resistance Converter, Current Converter) are included in the unit. The vacuum fluorescent displays are bright and in good condition. The firmware appears to be the latest version (400918-3.12B 10May96, 400918-3.12A 10May96).

The unit was tested (Section 4-30 of the User's Handbook) and inspected (with a Keithley 238 and some precision resistors) prior to purchase. Most of the ranges measured seemed to be decently accurate compared to what I had available for testing at the time. There were some major offsets at the lower ranges as expected for an instrument out of calibration.

Test -> Full Test (select the appropriate ^ arrow below the menu)

OUTPUT: REQUIRES INT. SCRE. CAL. 2105

Test -> Fast Test (select the appropriate ^ arrow below the menu)

Sometimes the fast test is successful. The following errors have occurred:

2.7.3.1 True Zero Checks (possibly because of old electrolytic capacitors)

OUTPUT: "FAIL: DCV 2181 List" - 2181 Noise (standard deviation ≤ 5 μV)

OUTPUT: "FAIL: DCV 2182 List" - 2182 Magnitude (-250 μV < mean 100 mV Zero < +250 μV)

Already, I am very impressed with the documentation available for this instrument.

When I got back to my hotel, I foolishly left the instrument running for several hours to warm up so that I could test it when I got back later that evening. After learning more about the functionality of the instrument in the hotel, I pushed a few buttons and the display information scrambled. No smoke or bad smells, although concerning. The unit was still responsive in that I could somewhat understand what was going on from some of the fragmented information presented and I knew that the unit could switch into various modes. At this point, I turned the instrument off not knowing if it would ever come back. Fortunately, the unit functioned properly the next morning. 

To be continued ...

[mendip_discovery]

Ask to see if they have the old certificates to build a spreadsheet of its history.

[leighcorrigall]

Ask to see if they have the old certificates to build a spreadsheet of its history.

Who is 'they'? I might be able to get calibration information from Pylon. Right now, I am trying to get the unit to operate without errors.

[leighcorrigall]

After considerable effort and help from TheSteve, I have compiled a list of aluminum electrolytic capacitors throughout the instrument. A comparison is made between the Wavetek Service Manual, me, and TheSteve.

Note:

-> TheSteve was able to acquire a more recent Wavetek unit in mint condition with factory seals. This may suggest that the selected capacitors in his unit are more suitable for the instrument. My unit is likely to have been refurbished a few times during service so the capacitor specifications may deviate from the original design.
-> Some of the capacitor identifiers are difficult to read or have been mislabeled in the service manual. I have done my best to identify these capacitors. One way to tell from the bill of materials is to look for CAP AE (capacitor aluminum electrolyte) in the description. I was able to determine the unknown labels this way along with inspection of various reference pictures from my unit and TheSteve's.

The current part shortage made it difficult for me to order selected parts. Some parts have to come from eBay. Most of the items will be delivered sometime in mid-December. Better than nothing, I suppose. The cost was about 120 CAD.

[leighcorrigall]

Since the unit had suffered from an issue related to the digital board, I decided to check the taps. The taps have been identified in the attached images along with labels that correspond with the tap number.

TP503 is the reference for all others:
VRef_2.5V (TP201) = +2.46 V (+1.6 % error)
+11V_UNREG (R307, R517) = +11.65 V (+5.9 % error)
-14V (TP501) = -13.86 V (-1 % error)
+45V (TP502) = +46.8 V (+4 % error)
-14V_2 (TP504) = -14.15 V (+1.07 % error)

There seems to be noise on some of these references, specifically +11V_UNREG and -14V_2. The noise is likely caused by the aged capacitors which have been ordered. Apart from the line regulation, it seems the voltage references are in good shape.

[leighcorrigall]

Members have commented about my unit and its unusual features. Here are a few that I am aware of so far:

--> The vast majority of the integrated circuits have their own sockets. This feature is found throughout the boards and I am lucky to have them.

--> The precision voltage references are mounted upside down. At the moment, I am uncertain as to why this was done, but the feature is original.

--> The current sense option board uses Vishay-Mann LR500BU and VTA series precision wire wound resistors for the current sense. Newer models use Vishay VLR500 resistors. EDIT: The resistors also have a large trace underneath them that suggests a guard.

--> The PL54 is not connected in series with the 1.6 A fuse. See 5.2.4.4 Protection for details. Interesting.

--> The ohms board uses Vishay-Mann resistors for R209, R210 and R213. Two are insulated and possibly hermetically sealed like R204. EDIT: Here is a link to a picture where the resistor is exposed. https://www.eevblog.com/forum/metrology/burster-resistors/msg927671/#msg927671 They appear to be precision wire wound resistors. The 5 MΩ is incredible. Mann Components was apparently bought by Vishay in 1983.

EDIT:

--> The digital board arrangement seems to be different from TiN unit #2 https://xdevs.com/fix/d1281/#dig_pcb_u2.

--> Front terminal release: A rear button retracts the front terminal of the instrument. The terminal block is spring-loaded so that the posts extend outward after a second button press.

(1) Legacy: Flexible PCB with a rigid PCB to secure 3 connectors (my variation). Rear 'terminal release button' that includes a shaft to the front panel assembly.

(2) Revised: A 6 conductor cable with 3 connectors.

--> Older models are capable of over-ranging by double the full-range (FR) even on the latest firmware. This feature is referred to as the full-scale (FS).

[syau]

I got one unit which the power supply cap in the power supply board / cpu board short out (not completely) and dragging the 5V down to 4.x V. Surprisingly, there is no brown out protecting in the cpu side which end up the cal constant corrupted. 

For the replacement cap, I am using the Rubycon capacitor ZLH series whenever possible. Please double check the height of the replacement capacitor on the power supply as the clearance is not great.

If you are looking for the cal key, yes, it can be purchase from RS.

[leighcorrigall]

The Datron 1281 PCBs are elegantly mounted with Nylatch panel fasters, which are still available to order (shop.southco.com, alternatively: www.dbroberts.com). Only the AC and digital boards are screw-mounted to the unit. Due to the age of my unit, the plastics have become brittle and some pieces have broken away.

Specifications:

If you look close enough, the length number (43) is visible on the flange. The G range measurement will confirm this as well as the 'removable panel hole diameter' (6.04 - 6.15 mm).

1/4" Mini Grip Plunger (A = 8.1 mm, B = 7.1 mm)
https://shop.southco.com/en_us/NY-4P-43-4-50 (black Nylon)

1/4" Mini Grip Grommet
Removable Panel Thickness (PCB): 1.6 mm
Removable Panel Hole (PCB): 6.1 mm (~ 1/4")
https://shop.southco.com/en_us/NY-4G-43-20 (black polycarbonate)

I assumed that the plunger and grommet are nylon and polycarbonate, respectively, because I know them to be good insulators.

There are 21 fasteners in total.

I have ordered 84 assemblies to meet the minimum purchase order of 50 USD on dbroberts.com because southco.com will not sell outside the US. Don't you just love the North American Free-Trade Agreement? 

If anyone wants replacements, please let me know.

[leighcorrigall]

...

For the replacement cap, I am using the Rubycon capacitor ZLH series whenever possible. Please double-check the height of the replacement capacitor on the power supply as the clearance is not great.

...

The capacitors that have been specified in my previous post are compatible with the Datron 1281. I noted all the dimensions in my spreadsheet and measured the clearance for the unit.

After considerable effort and help from TheSteve, I have compiled a list of aluminum electrolytic capacitors throughout the instrument. A comparison is made between the Wavetek Service Manual, me, and TheSteve.
...

Of course, it would be great to have high-reliability capacitors. Unfortunately, not everyone has access to electronic components due to the pandemic. The capacitors that I selected are more than adequate for the job but may be difficult to source depending on your location.

[syau]

...

For the replacement cap, I am using the Rubycon capacitor ZLH series whenever possible. Please double-check the height of the replacement capacitor on the power supply as the clearance is not great.

...

The capacitors that have been specified in my previous post are compatible with the Datron 1281. I noted all the dimensions in my spreadsheet and measured the clearance for the unit.

Of course, it would be great to have high-reliability capacitors. Unfortunately, not everyone has access to electronic components due to the pandemic. The capacitors that I selected are more than adequate for the job but may be difficult to source depending on your location.

I source them from Digi-key or Mouser depends on availability and prices.

[leighcorrigall]

I got one unit in which the power supply cap in the power supply board / CPU board short out (not completely) and dragged the 5V down to 4.x V. Surprisingly, there is no brownout protecting in the CPU side which end up the cal constant corrupted. 

...

Good to know. How do you know that the calibration constants are corrupted?

[leighcorrigall]

...

The unit I have seems to be manufactured around 1987 to 1996, based on the date codes of the integrated chips. I would say that the unit is from 1991 until I know more.

...

Given the Renew Batt. dates are 2002 and 2012. I now believe this unit was originally commissioned in 1992.

[syau]

After replacing the capacitors in the power supply, the unit startup with calibration constant (pri & sec) corrupted msg.

[syau]

It turned out that one of the 220uF capacitor give up all its value.

BTW, I found that the C520 in mybdigital board was fitted with 220uF instead of 330uF as per the schematic.

[quarks]

Hello Leigh,

thanks for sharing.

I am a big fan of John Pickering designed gear.
1281 is still one of the best DMMs ever made in my books.
Therefore congratulations to your 1281 buy.

When I replace electrolytic capacitors in my Datron/Wavetek gear, I use Chemi-Con KY-Series with 10000h endurance.
AFAIK the original Chemi-Con KME Series only had 1000h endurance, therefore the KY is a big improvement.

C520 is also 220µF in my 1281

Cal key is Lorlin 850

[leighcorrigall]

...

When I replace electrolytic capacitors in my Datron/Wavetek gear, I use Chemi-Con KY-Series with 10000h endurance.
AFAIK the original Chemi-Con KME Series only had 1000h endurance, therefore the KY is a big improvement.

C520 is also 220µF in my 1281

...

Hi, quarks!

I ordered most capacitors with a 10,000-hour rating. There is a world shortage of capacitors and other electronics that is preventing me from getting what I want immediately.

Thank you for your advice about the KY Chemi-Con capacitors. Please let me know if you can spare some.

In later models, it seems that C520 must therefore have a capacitance of 220 µF. My unit has 330 µF. Three of four say 220 µF @ 100 V.

Out of curiosity, what are your {C307,C308,C312,C313} and {C907,C910} on the DC board? Do you have an electrolytic or a tantalum capacitor for C304 on the digital board?

[leighcorrigall]

...

Cal key is Lorlin 850

What single-pole lock switch model number is it? I do not have a key to try the positions.

I am assuming it to be SRL-5-B (parallel lock head with two positions).

[syau]

...

Cal key is Lorlin 850

What single-pole lock switch model number is it? I do not have a key to try the positions.

I am assuming it to be SRL-5-D (tapered lock head with two positions).

Not sure if the attached photo can help.

[Mickle T.]

A few notes about the Datron 1281 found in the archives of British electronics journals when I was preparing material for an article on the undocumented tricks and secrets of the 10x1 and 12x1 series multimeters.

[quarks]

Thank you for your advice about the KY Chemi-Con capacitors. Please let me know if you can spare some.

Out of curiosity, what are your {C307,C308,C312,C313} and {C907,C910}? Do you have an electrolytic or a tantalum capacitor for C304?

let me know what KY you need and I can check my stock

C304 is 47nF 250V MKT
C307, 308, 312, 313 are 100µF 25V
C907, C910 are 33µF 63V

[quarks]

...

Cal key is Lorlin 850

What single-pole lock switch model number is it? I do not have a key to try the positions.

I am assuming it to be SRL-5-D (tapered lock head with two positions).

the Lorlin key you need is labeld 850 (see pic.)
afaik they are all identical and fit all 1271 and 1281

https://uk.rs-online.com/web/p/key-switch-keys/0319792

[leighcorrigall]

After considerable effort and help from TheSteve, I have compiled a list of aluminum electrolytic capacitors throughout the instrument...

I have compiled a list of capacitor specifications for the Datron 1281. According to interviews with EEVBlog members, there appear to be variations in the voltage and capacitance specifications. The variations might be correlated with stock availability during manufacturing, updates to the design, and or maintenance. I have placed '[ ... ]' around magnitudes that have been specified in the service manual that may or may not include reported values. A value without '[ ... ]' indicates a value that is not found in the service manual but has been reported by a member(s).

The maximum diameter and heights for each capacitor are listed as well as the lead spacings for your convenience. Thanks, TheSteve. 

EDIT: I cannot guarantee that all lead spacings will be the same. It is already evident that there are variations in the board layouts. TheSteve has a Wavetek version and I have a Datron version with similar capacitor dimensions and board layouts. Provided enough 1281 owners participate, we will find out in time about the change history.

[leighcorrigall]

The mains input and fuse holders can be replaced with the following parts. Start with the power receptacle and then replace the adjacent fuse holder. The cable lengths are rather short, so be prepared to replace those if you cannot manage to reuse them.

Power receptacle:
1x SCHAFFNER FN9222-3-06 (part number borrowed from, https://xdevs.com/fix/d1281/)

--> Remember to include the terminal guard and thread prior to soldering the blue wire.

--> I suggest insulating the exposed line and common terminals with heat shrink tubes before adding the black cover sheath.

-->The grounding cable for the receptacle does not pass through the wire entry hole, only the line and neutral.

--> A very small gap between the panel hole and the flange.
--> The gap is not significant enough to allow dust to enter.
--> A somewhat taller flanged receptacle might be more suitable.

Keyed Fuse holders:

--> Remember to include all necessary heat shrink tubing and the flange nut prior to soldering.

--> Like the originals, there is limited space between the fuses and the transformer.
--> Bend the live inner fuse terminal to look like a spark plug for the power input fuse, but do the opposite for the current option fuse.

--> Clean the current option fuse cable (two white wires) and handle with gloves near the DC board receptacle.
--> Cable management can be reused. In my case, the adhesives failed and I will replace them all, including Ch. A and B input cables.
--> Strain relief heat shrink tubing is used on the shielded entry gate and somewhere near the corner.
--> Do not blow hot air at the board whenever possible. Instead, position the heat shrink tubes prior to cable installation.

2x BULGIN FX0454 (available in Canada)
--> Reduce panel hole key width from 2 mm to 1.5 mm (L2002 specification) with razer blade as pictured. This is a very simple procedure.
--> It is much easier to remove small amounts of material than regret removing too much.
--> I had no trouble and measured the widths of 1.3 mm and 1.5 mm for the fuse panel hole key.

OR
2x BELLING LEE L2002 (available in USA / Europe, simply install)

[martinr33]

It is rare for an electrolytic cap to be in a circuit where the capacitance value is critical.

Therefore - as a rule - you can upgrade the voltage rating, the life hours - and the capacitance.

I would prioritize:
 - longer working life
 - higher operating voltage
 - higher capacitance value

The higher operating voltage has a material, positive effect on the life of the capacitor. In this unit, the cpacitor height is limited by the lid. Datron put circles of masking tape on the ones in my unit to protect against shorts.

[leighcorrigall]

The Datron 1281 PCBs are elegantly mounted with Nylatch panel fasters, which are still available to order (shop.southco.com, alternatively: www.dbroberts.com). Only the AC and digital boards are screw-mounted to the unit. Due to the age of my unit, the plastics have become brittle and some pieces have broken away.

Specifications:

If you look close enough, the length number (43) is visible on the flange. The G range measurement will confirm this as well as the 'removable panel hole diameter' (6.04 - 6.15 mm).

1/4" Mini Grip Plunger (A = 8.1 mm, B = 7.1 mm)
https://shop.southco.com/en_us/NY-4P-43-4-50 (black Nylon)

1/4" Mini Grip Grommet
Removable Panel Thickness (PCB): 1.6 mm
Removable Panel Hole (PCB): 6.1 mm (~ 1/4")
https://shop.southco.com/en_us/NY-4G-43-20 (black polycarbonate)

I assumed that the plunger and grommet are nylon and polycarbonate, respectively, because I know them to be good insulators.

There are 21 fasteners in total.

...

I replaced 11 damaged Nylatch panel fasteners with the above parts. Apart from the labelling, the parts are virtually identical. The replacement process is much more difficult than anticipated.

--> Warning: inserting the plunger into a grommet is irreversible. Only do this when you are certain about installation.
--> Decapitating the plunger from the neck makes it possible to push the remaining part through the grommet. This step needs to be done to release the grommet from the PCB. You will also want to use pliers to snip the tail so that the grommet fits easily though the PCB.
--> Inserting the grommet into the PCB is difficult because the through-hole tolerances are very tight. This is the most challenging step. I needed to use both hands; two fingers to press down, a thumb or two to prevent the PCB from flexing. Be careful about contacting other board components. I suggest wearing two pairs of gloves so that the sharp through-hole leads do not hurt your fingers or cause board contamination.

After the removal of the DC board, I realized that there are at least two Mini Grip mounts that are broken because of ageing (attached). By the way, the through-hole standoffs fit an 11 mm diameter hole for anyone interested. Does anyone have information on these parts? I plan to order replacements and spares.

[leighcorrigall]

...

After the removal of the DC board, I realized that there are at least two Mini Grip mounts that are broken because of ageing (attached). By the way, the through-hole standoffs fit an 11 mm diameter hole for anyone interested. Does anyone have information on these parts? I plan to order replacements and spares.

The only information I can find is from the Chassis Assembly (see Drawing No. 480737) part list:
Datron part number: 450580-1
Description: plastic standoff

Maybe it is time to fabricate replacements...

EDIT:

-- Attached is a representative CAD model of the part in both STP and PAR formats. An update was made to the original ID (0.250 inch to 0.0252 inch) and also includes a chamfer (45 °) so that the Nylatch fastener assembly can be removed easily.

-- Before installing the standoff onto the board, make sure that the plunger and grommet assembly fits loosely into the standoff hole. Otherwise, there could be alignment issues that will cause damage to the fastener assembly. You have been warned.

-- If you plan on replacing all 21 standoffs like I do, remove all analog boards, THEN remove only the DC standoffs. Removal of a single side at a time will make it easier to identify which of the standoff holes are for the boards. Otherwise, review each board hole position from the illustration (options.png) from post #1.

[leighcorrigall]

...

Datron part number: 450580-1
Description: plastic standoff

Maybe it is time to fabricate replacements.

I have reproduced standoffs for testing purposes. They were quickly made from the attached CAD model. I plan on improving the standoff resolution for the next print.

If anyone is interested in replacement standoffs, please let me know and I can ship them for the base cost. These parts are needed in combination with the Nylatch fasteners that secure many of the PCBs to the frame.

EDIT: The attached images show a comparison between the broken original pieces and the newly fabricated prototypes.

[leighcorrigall]

Calibration data of the Datron 1281 can be accessed and reprogrammed via GPIB.

The attached Python 3.7 scripts save the 'DUMP?' data (confirmed) for backup purposes:

1) The nvram_raw_dump.py script is based on rigrunner's original that was modified by TheSteve to correct for the region order (Python 2.7, https://www.eevblog.com/forum/repair/datron-1271-repair/msg3293128/#msg3293128). This script saves the raw "DUMP?" output from the instrument to a file. The raw file can be converted with the following Linux command line suggested by chekhov:

Code: [Select]

cat <file_name>.raw | sed 's/\(^....\)\| ..//g' | xxd -l 16384 -p -r > <file_name>.bin
2)  The modernized nvram_bin_dump.py script saves the "DUMP?" output from the instrument as a binary.

Both approaches listed above have identical outputs (attached). Binary files match and can be examined with the following command lines:

Code: [Select]

diff -du <file_name_1>.bin <file_name_2>.bin

Code: [Select]

xxd <file_name>.bin
Once your binary file is verified, it can be programmed onto FM1808 as described by the xDevs.com article: https://xdevs.com/fix/d1281/#adapters

If anyone would like to share your bin and raw output files, please do so.

Special thanks to the following people for helping me achieve this outcome:

  rigrunner, TheSteve, ManateeMafia, chekhov, TiN

[leighcorrigall]

Using ibtest, the Datron returns the following line when sending it the data string:

write:

Code: [Select]

EXT_DUE?
read:

Code: [Select]

'"150812  "
'
According to the user's handbook, the description is as follows:

The value returned is the date most recently entered either as a parameter of EXITCAL, or when calibration mode exited from the front panel.

Does anyone recognize this as a date? It might confirm the last calibration for me.

[alm]

In the EU, 15-08-12 would stand for 2012-08-15 (unambiguous ISO 8601 notation).

[dietert1]

And DUE should be "calibration due", not last calibration.

[leighcorrigall]

...

 I plan on improving the standoff resolution for the next print.

...

Attached is the second revision of the Datron PCB standoffs (P/N: 450580) fabricated with a Figure 4 by 3D Systems. I made 84. CAD files have been provided above. The material is called "Pro BLK 10".

[leighcorrigall]

...

2)  The modernized nvram_bin_dump.py script saves the "DUMP?" output from the instrument as a binary.

...

Both the Python 3 nvram_raw_dump.py and nvram_raw_dump.py scripts (provided above) produce identical outputs to the previous nvram-2.out.py Python 2 script:

https://www.eevblog.com/forum/repair/datron-1271-repair/msg3293128/#msg3293128

I verified these scripts by interfacing the Datron with an Agilent E5810A (thank you, pipelie!) and using the vxi11 library instead of the Gpib library which no longer can be installed easily with pip on Python 2. 

Code: [Select]

pi@raspberrypi:~/Downloads/backup $ sudo python2 nvram-2.out.py
Reading nvram from Datron Instruments 1281 to 020206-4.X.nvram
Dumping PRIM to "020206-4.PRIM.nvram"
PRIM done.           
Dumping IPZ to "020206-4.IPZ.nvram"
IPZ done.           
Dumping SECN to "020206-4.SECN.nvram"
SECN done.           
Creating 020206-4.nvram.bin
Finished.
pi@raspberrypi:~/Downloads/backup $ diff -du 020206-4.nvram.bin 2021.12.15.020206-4.nvram.bin
pi@raspberrypi:~/Downloads/backup $
Now that the calibration data backup has been made, I will have no issue handling the Datron digital board. 

Pretty good considering that I had no idea how to use a Raspberry Pi or Python last week. 

Thank you, everyone. 

[syau]

Now that the calibration data backup has been made, I will have no issue handling the Datron digital board. 

Pretty good considering that I had no idea how to use a Raspberry Pi or Python last week. 

Thank you, everyone. 

Feasible to share the dump as I want to restore it on my lost calibration 1281 for some testing.

[leighcorrigall]

Now that the calibration data backup has been made, I will have no issue handling the Datron digital board. 

Pretty good considering that I had no idea how to use a Raspberry Pi or Python last week. 

Thank you, everyone. 

Feasible to share the dump as I want to restore it on my lost calibration 1281 for some testing.

I shared the dumps and scripts already. See post #27:

https://www.eevblog.com/forum/metrology/datron-1281-repairmaintenance/msg3876473/#msg3876473

[syau]

I shared the dumps and scripts already. See post #27:

https://www.eevblog.com/forum/metrology/datron-1281-repairmaintenance/msg3876473/#msg3876473

Missed that attachment, thanks.

[leighcorrigall]

Just my luck. More things to consider...

On removal of the AC board, I found problems with the AC board standoffs. Someone applied too much torque while fastening them to the board and they were damaged or have separated from the metal frame. It will be difficult to fix the press-fit standoffs. If you decide to remove the AC board, wiggle the press-fit standoffs to see if they separate from the board -- they should stay secure.

The male-female threaded insulated standoffs have the following dimensions:

Nylon dowel:
length: 15.9 mm [5/8"]
diameter: 8.1 mm [5/16"]

Brass threads (male and female):
thread type: M3, 0.5 mm pitch
thread length: 6.3 mm [1/4"]

[volvo_nut_v70]

You can replicate them using Delrin rod, it will tap easily and hold a thread like aluminum. A friend of mine who is a machinist made some up for me. Easiest is to drill both ends, tap to 3mm threads and use a M3 screw body for the threaded end.

[volvo_nut_v70]

You do realize that having bought the 1281 (Congratulations) you are now on a slippery slope and starting to slide!  Datron Wavetek calibrator next    Many of us are past redemption now..

This may help you in the future, my present revision of replacement capacitors and other bits for the Datron Wavetek equipment

[leighcorrigall]

After dumping the calibration data to a binary file with the Python scripts provided above, it is possible to completely update the SRAM (static RAM) system with a preprogrammed FRAM (ferroelectric RAM).

Original SRAM assembly:
The calibration data, among other information, is stored on U106 (e.g., Sharp LH5164D-10L). A 3.6 V lithium-ion battery sustains the memory of U106 while powered OFF. My battery is from 'JUN06', which is old. The battery reads 3.7 V. If you prefer this storage approach, then make note of the battery replacement procedure (Section 4.3 of the Service Manual) so that you can maintain your calibration constants.

FRAM assembly:
Fortunately for me, there have been great people working on this challenge and all I had to do was put the pieces together. I selected the following parts for my adapter board:
CD74HC00M96 (TI, IC GATE NAND 4CH 2-INP 14SOIC)
FM1808-70-P (RAMTRON, IC MEM 256kb 32kx8 parallel 28-DIP)
C0603C104K9RAC7867 (KAMET, CAP CER 0.1UF 6.3V X7R 0603)
CRCW060310K0FKEAHP (VISHAY-DALE, RES SMD 10K OHM 1% 1/3W 0603)
310-87-114-41-001101 (PRECI-DIP, CONN SOCKET 14POS 0.1 GOLD PCB)
342-10-114-00-591000 (MILL-MAX, CONN HDR DIP POST 14POS GOLD)

xDevs.com also has a variation: https://xdevs.com/fix/d1281/#adapters

Installation:
1) The binary file for the FRAM is uploaded with an EPROM programmer. My GQ USB Universal Programmer (R 7.29) does not support the FM1808. Instead, I selected the DS1230 as instructed by TheSteve. My binary file was also 'cleaned' so that only the serial number and calibration constants remained. EDIT: Attached are the NVRAM and FRAM binaries so that you may perform a comparison.
2) When you are certain that you have correctly created a backup, cut the battery lead while the instrument is OFF.
3) Remove the U106 chip with proper tooling.
4) Install the FRAM and the adapter into the pre-existing socket.

When the instrument is turned ON again, you should know right away whether or not the operation was successful. No error messages should appear and you should be able to check your stored serial number (Status -> Config -> Ser#).

[leighcorrigall]

So far, I have replaced all of the aluminum capacitors of Datron 1281. The greatest challenge was with the digital board and the DC board because of the rear removal process and the floating capacitors in the pre-amplifier section (C307, C308, C312, and C313), respectively. If you are going to replace the capacitors at the DC board pre-amplifier section, be prepared to replace the components that the capacitor leads are connected to. This will make it much easier to disassemble and then install onto the board.

C312 and C313 are both connected to Q203? (Linear Systems current regulating diode, J500).

C308 is connected to Q307 (Siliconix N-Channel MOSFET Transistor, VN10LM) and R314 (Holsworthy metal foil resistor: 10 kΩ, 1 %, 50 ppm, 0.12 W). I suggest removing M301 for more accessibility.

C307 is connected to Q308 (VN10LM).

Another challenge was that the DC board had damaged traces caused by the previous owners. Not sure who to blame, but luckily the rework was possible. I would like to point out that, in most cases, a square hole represents a positive termination and a round hole represents a negative termination for the aluminum capacitors.

After replacing the capacitors, I attempted to run the internal calibration procedure (with CAL Switch enabled, Cal -> Ext -> Quit -> Trig). Error 2582 and 2602 occurred and point to the Current Assembly board. Apparently, the true zero checks of the 100 µA (error 2602, P091, 0.0114 µA) and 1 mA (error 2582, P089, 0.000114 mA) range are out of tolerance. Both are out by exactly the same magnitude, which makes me suspect that the problems are related.

[leighcorrigall]

Attached is the latest firmware (version 3.12) from 1996 that was loaded on U103 and U104 using AM27010-150DC EPROMS. Enjoy!

[leighcorrigall]

...

When I got back to my hotel, I foolishly left the instrument running for several hours to warm up so that I could test it when I got back later that evening. After learning more about the functionality of the instrument in the hotel, I pushed a few buttons and the display information scrambled. No smoke or bad smells, although concerning. The unit was still responsive in that I could somewhat understand what was going on from some of the fragmented information presented and I knew that the unit could switch into various modes. At this point, I turned the instrument off not knowing if it would ever come back. Fortunately, the unit functioned properly the next morning. 

...

The scrambled screen seems to occur at random. After I replaced the capacitors on the digital assembly, I noticed that the problem worsened. I decided to reseat U110 (20L8), U111 (10L8) and U107 (10L8). This seemed to have corrected the problem. I suspect that the terminal release button in the back knocks the U110 when removing the PCB and may have caused problems with the socketed terminations.

[leighcorrigall]

...

I attempted to run the internal calibration procedure (with CAL Switch enabled, Cal -> Ext -> Quit -> Trig). Error 2582 and 2602 occurred and point to the Current Assembly board. Apparently, the true zero checks of the 100 µA (error 2602, P091, 0.0114 µA) and 1 mA (error 2582, P089, 0.000114 mA) range are out of tolerance. Both are out by exactly the same magnitude, which makes me suspect that the problems are related.

Turns out that the ranges needed to be zeroed. I did this with a calibrated Keithley 238 operating at 0.0000 nA with triaxial test leads connected to the Datron 1281 inputs. Luckily, I did not need specialized calibration equipment to perform the correction. I was convinced for a while that the instrument needed component replacements.

Steps:

1) Set DCI to 100 μA or 1 mA with maximum resolution. Enable calibration with the key switch. Source zero amps with an instrument such as an SMU below the measuring capability of the Datron.
2) Cal -> Ext -> Caltrig (repeat for all effected ranges)
3) Cal -> Self -> Corrections ON -> Trig ... (COMPLETE) 
3) Test -> Full ... (PASSED) 

Note: If the instrument is experiencing issues, the display will read 'SELF CAL. XXX ##:' followed by '?' or '??' during self calibration. When the instrument finally gives up, it will provide a list of associated errors. These are likely errors that can be calibrated out such as in my case.

[TiN]

I don't quite get why need 238 connected to meter. 238 might have offset and that would be adding error to your zero calibration. Just leave posts open and do a zero calibration without anything external. Similar to having just a short for DCV/ACV zero calibration.

[mendip_discovery]

I don't quite get why need 238 connected to meter. 238 might have offset and that would be adding error to your zero calibration. Just leave posts open and do a zero calibration without anything external. Similar to having just a short for DCV/ACV zero calibration.

It is all I would do in the lab to get a zero reading for current ranges.

[leighcorrigall]

I don't quite get why need 238 connected to meter. 238 might have offset and that would be adding error to your zero calibration. Just leave posts open and do a zero calibration without anything external. Similar to having just a short for DCV/ACV zero calibration.

Fair enough. Leaving the input terminals open should improve results if the Datron had the sensitivity. I will leave the inputs open the next time I calibrate the zero.

I sourced 'zero' current on the 1.0000 nA range of the Keithley 238 knowing that the maximum offset for the Keithley is ±450 fA. This offset is much lower than the measuring capabilities of the Datron 1281:

--> ±100 pA sensitivity for the 100 µA range (0.0000 µA)
--> ±1 nA sensitivity for the 1 mA range (0.000000 mA)

[leighcorrigall]

Members have commented about my unit and its unusual features. Here are a few that I am aware of so far ...

The more time I spend working on this Datron 1281, the more I am impressed by its subtle features.

While asking about the difference between Datron's definition of 'full-scale' (FS) and 'full-range' (FR) on xDevs IRC channel, I found out that the earlier Datron branded models are capable of over-ranging (i.e., full-scale) by twice the full-range. Attached are some examples. I tested all ranges with the exception of 1 kV and 1 A, which I assume do not have this over-range feature. 

[TheSteve]

Members have commented about my unit and its unusual features. Here are a few that I am aware of so far ...

The more time I spend working on this Datron 1281, the more I am impressed by its subtle features.

While asking about the difference between Datron's definition of 'full-scale' (FS) and 'full-range' (FR) on xDevs IRC channel, I found out that the earlier Datron branded models are capable of over-ranging (i.e., full-scale) by twice the full-range. Attached are some examples. I tested all ranges with the exception of 1 kV and 1 A, which I assume do not have this over-range feature. 

All versions(Datron/Wavetek) will do twice full scale. The newest firmware(3.12) does seem to limit it to full scale less one count though.

[Mickle T.]

All of Datron/Wavetek 1271/1281 can do 2.35*Full Range, but only in raw mode without any calibration constants applying.

[leighcorrigall]

All of Datron/Wavetek 1271/1281 can do 2.35*Full Range, but only in raw mode without any calibration constants applying.

Hi Mickle,

Can you please explain the raw mode? How does one access it?

Thanks.

[Mickle T.]

An asynchronous access to direct ADC results can be obtained via GPIB. I'll try to write some article for it.
Datron 12x1 ADC is much more complex than described in the service manual. It have an auxiliary measurement cycle for DA induced offset compensation, an additional hardware tricks for charge injection reducing and common current cancellation etc.

[leighcorrigall]

...

When I got back to my hotel, I foolishly left the instrument running for several hours to warm up so that I could test it when I got back later that evening. After learning more about the functionality of the instrument in the hotel, I pushed a few buttons and the display information scrambled. No smoke or bad smells, although concerning. The unit was still responsive in that I could somewhat understand what was going on from some of the fragmented information presented and I knew that the unit could switch into various modes. At this point, I turned the instrument off not knowing if it would ever come back. Fortunately, the unit functioned properly the next morning. 

...

The scrambled screen seems to occur at random. After I replaced the capacitors on the digital assembly, I noticed that the problem worsened. I decided to reseat U110 (20L8), U111 (10L8) and U107 (10L8). This seemed to have corrected the problem. I suspect that the terminal release button in the back knocks the U110 when removing the PCB and may have caused problems with the socketed terminations.

Another possibility that caused the scrambled display problem could be that the original PCB insulator does not sufficiently cover the entire PCB surface (see attached). Now that I have identified a damaged card slot (see attached), it is worth ruling out the possibility of contact shorts. Note that some of the plastics in these ageing units are brittle and should be replaced when possible (see attached).

The double-sided tape adhesives cannot be easily removed. The best way that I know of is to apply an acetone cloth to the surface and leave it there to do the removal work for you (see attached). Make sure not to let anything else plastic come into contact with this strong solvent or you might regret it later!

I measured the internal dimensions of the enclosure to be 13.5 mm x 37.8 mm. The replacement material is an Arc-Resistant GPO3 fibreglass sheet with a thickness of 1/32" (< 0.8 mm). I cut the sheet by running an Exacto blade along a metal ruler a few times until it could be creased. The re-enforced fibreglass sheet fits perfectly into the enclosure and slips under the PBC card slots (see attached). No adhesives are necessary because the fibreglass fits the dimensions exactly and will not move around. It is stiff and very smooth. Do not allow the solder joints to catch while inserting the PCB. I might eventually punch holes for access to the 4 screws that secure the transformers, but what would the point be now that the insulator can easily be removed?

If your original digital board insulator looks as badly implemented as mine, I would suggest updating it as well.

[dmmguy]

Hi,

How much you paid for it (if not s secret :-))?

Thanks

[Micke]

Finally!   
Now I am also a proud member of the 8,5-digit DMM club! 
Yesterday I got my Wavetek 1281, the grey version in very nice condition, options 10, 20 and 30.
Seems to be manufactured circa 2001, high serial number 447xx.
Price was 2700 USD (including 25% tax).
Will change electrolytic caps and change SRAM cal-RAM to F-RAM. Only bad thing so far is that this unit has no IC sockets 
It has F/W v3.14, I have not seen this version anywhere to download, so I attach it here.

[leighcorrigall]

Only bad thing so far is that this unit has no IC sockets 

Only the original Datron has sockets that I am aware of.

[leighcorrigall]

It has F/W v3.14, I have not seen this version anywhere to download, so I attach it here.

Awesome! I wonder what the differences are.

[volvo_nut_v70]

Excellent, thank you so much. I've attached my spreadsheet of capacitors and other parts for the calibrators and DMMs. It has manufacturer part numbers for the replacements.

[leighcorrigall]

Will change electrolytic caps and change SRAM cal-RAM to F-RAM.

Welcome to the club.

You should inspect the electrolytic capacitors before purchasing replacements. There are board variations and alterations to the electrical that might be specific to your instrument. Note the lead spacing, outer diameter, maximum voltage and capacitance. I suggest 10,000 hour rating United Chemi. You can use my spreadsheet (attached), which should be fairly complete.

[Micke]

Thanks Leigh!

Thanks for the list, and also thanks to volvo_nut_v70!
Yes will buy premium brand 105C high ripple current and low ESR, and long life time.
I have noticed the limited height in some places, better watch out!
But they seem to have fixed the bodge jobs of the 100µF/50V caps on the Analog board, layout has been changed 
By the way I have a Volvo V70 (2007 Classic) and I am very happy with it!  Bought it new...

[Micke]

F/W v3.14, Serial 447xx
Mod record = 9

[Micke]

Analog board

[Micke]

Analog board (from other direction)

[Micke]

Digital board

[leighcorrigall]

Digital board

Your instrument looks to be in very good condition and I am glad that my digital board uses the most modern variation. What happened to the serial number?

You should speak with xDevs.com if you need a place to post high-resolution images of equipment. 

[Micke]

Digital board

Your instrument looks to be in very good condition and I am glad that my digital board uses the most modern variation. What happened to the serial number?

You should speak with xDevs.com if you need a place to post high-resolution images of equipment. 

Serial number sticker is fine, it was just me that put a bit of silver tape on top to disguise serial number 
Good idea putting pictures on XDevs.com, will take better pictures and put there!
A difference I have seen also is that I have Vishay VLR500 foil resistors on the Current board, not those green PWW.

[TiN]

Micke, thank you for photos and firmware. You can upload anything to FTP server which is mirrored on my repository. Your 1281 seem to be one of latest before Fluke takeover.
VLR500 also ended up used in 8508A later on, as this photo reveals .

[rigrunner]

It has F/W v3.14, I have not seen this version anywhere to download, so I attach it here.

Awesome! I wonder what the differences are.

I've only had a really quick scan of the rom comparing it to v3.12. It appears as though a few commands are not listed in the v3.14 rom.

TRACK_ON
TRACK_OFF
FILT1KHZ
FILT360HZ
AV32

^^^ Aren't in the main block of commands.

[TheSteve]

Perhaps 3.14 was just a bit of a code "clean up" ?

I was planning to burn some EPROMs later today and install them in my 1281.

[rigrunner]

Perhaps 3.14 was just a bit of a code "clean up" ?

A quick look at the 1271 user manual I have shows AV4, AV16 and AV64 as selectable. The 1281 user manual I have shows AV4, AV16, AV32 and AV64 selectable.
A similar story with the FILT commands too.

It could very well be a simple clean up code version.

[TheSteve]

Not a surprise at all but installing 3.14 in place of 3.12 works just fine and passes self tests with no other changes.

[Micke]

Now I have changed all electrolytic caps in the 1281    Thanks to leighcorrigall and volvo_nut_v70 for BOM lists!

Changed those crappy original binding posts that cracks to Pomona 3770 (Copper Tellurium). Bought them from DigiKey.

Put six metal spacer sleeves M4x6,5mm  (S.A Bourqui Jean 99056-70065 / DRM 4370X6,5) before putting new binding posts. Filed down to get the flat edge.
Luckily I had some spacers in the garage... https://www.elfa.se/en/spacer-sleeve-bourqui-jean-99056-70065-drm-4370x6/p/14801677?track=true&no-cache=true&marketingPopup=false

The result with Pomona 3770 looks much better! Too bad they only are available in two colors...

I also installed a switch (in series with F504 or F505 fuse) on the backside that disables filament voltage to the displays to preserve lifetime. Thanks to Dr. Pyta for this advice!  The switch is of the type you must pull out to be able to operate.

BTW, tried to get the pictures (expandable) inline, but without success...

[leighcorrigall]

...

I also installed a switch (in series with F504 or F505 fuse) on the backside that disables filament voltage to the displays to preserve lifetime. Thanks to Dr. Pyta for this advice! 

...

Nice going, Micke.

Can you please elaborate more on your display mode toggle switch? I like it a lot, but I am wondering how accessible the disassembly would be. Is there a disconnect on the PCB where the switch leads terminate? What is the model of the swtich? I think it compliments the original design well.

I am also somewhat interested in modifying my front terminal. For now, mine work well and have not cracked like yours. Just so that others are aware, there are two types of front binding post assemblies -- the type that Micke has (permanent fixture) and the type that I have (mechanical assembly for storage).

https://www.eevblog.com/forum/metrology/datron-1281-repairmaintenance/msg3842057/#msg3842057

[Micke]

The switch seems to be C&K 7101K2ZQE
https://www.digikey.com/en/products/detail/c-k/7101K2ZQE/253351

I just unsoldered one leg of one fuse and soldered in the cables.

[volvo_nut_v70]

I broke the line at PL4 on the Digital board output connector as shown

[TiN]

3770 Every time I tried to use them they ended up cracked after just few times. Never ever again, rather overpay for LTC posts that are at least last for years.

[Micke]

3770 Every time I tried to use them they ended up cracked after just few times. Never ever again, rather overpay for LTC posts that are at least last for years.

Yes, I have seen that on the forum  I give this a try, it seems Low Thermal 2758 http://www.lowthermal.com/cables-and-connectors.php are at least 3 times more expensive, and I wonder how long the lead time would be to get them to Europe...    But if the Pomona´s break in the future, I will definitely look for alternatives! Right now I thought "It can not be worse than it is right now..." 
Shame on the manufacturer of the Datron binding post, because other than bad plastics choice it is a great binding post, and I guess very low EMF!

[martinr33]

Usual reminder to also check your local Fluke spares for their posts. If you recycle the washers and nuts, they can save you a lot of money. I thing the basic posts are less than $20 this way (just don't expect a deal on blue).
You have to order at leats three parts - the post, the front spacer, and the rear spacer. Washers and nuts are $3 each. Don't buy the steel start washer for $3. 

[volvo_nut_v70]

Now there is a project for an adventurous soul, 3D print replacement plastics since the post and nut metal is okay.

The original style of post is still available from Fluke since they are used on the Datron/Wavetek/Fluke 4955 Calibration fixture for the Datron/Wavetek/Fluke 9500B

I have dug up the Fluke part numbers for the binding posts.

Blue  = 2053775
Brown = 2053766
RED   = 2053753
Black = 2053748
Green = 2053794

The part numbers are good, the pricing not so good, $127 average each

[Micke]

Those binding posts look exactly like those used in Datron/Wavetek 4600 and 4808, where you have to pull the plastic knob to insert a cable. I have a 4600, and they do feel very expensive!  I wish they could have used those instead in 1281! 

[manganin]

The original style of post is still available from Fluke since they are used on the Datron/Wavetek/Fluke 4955 Calibration fixture for the Datron/Wavetek/Fluke 9500B ... The part numbers are good, the pricing not so good, $127 average each

The mysterious "Datron" binding posts are actually Hirschmann PKI 100 in custom color and plating. The standard version costs less than two dollars each.

[leighcorrigall]

Please do not modify one of the best digital multimeters in the world with cheap binding posts.   

[martinr33]

Summary: you can use a dual coil latching relay in a 1271/81. You don't have to run down the single coil version, which appears to have been gone for decades.

p/n S4EB-L2-6V

The 5V version should also work.


The 1281 and 1271 use a Panasonic latching relay that is impossible to find. It is a 6V single coil latching relay. The single coil design requires that you reverse the coil  current direction to switch the relay, which drives circuit complexity.

This relay is unobtainable. However, it turns out that the dual coil version will work just as well. I found a clear case version at our local junk store. Here's a little video

https://photos.app.goo.gl/oHU1koYftAFRfNT49

The reversible drive current in the Datron circuit can effectively flip the dual coil relay.

[rigrunner]

3770 Every time I tried to use them they ended up cracked after just few times. Never ever again, rather overpay for LTC posts that are at least last for years.

I had this with the older 3770 posts. The newer posts are a darker colour and aren't crimped at the end, allowing the thumbscrew to be removed.
Not broke a new one yet.

New on left, old on right.

[leighcorrigall]

Attached is my first draft of the Datron 1281 button.

EDIT: I removed the original file. See below for the next revision(s).

I plan on printing a few as a favour to a fellow member (rigrunner). It will be the responsibility of the purchaser to prepare the raw part with a primer and a paint coat. Is there anyone else that is interested in such parts? 

Please let me know if there are any revisions to be made and I will update the CAD files accordingly.

[leighcorrigall]

Summary: you can use a dual coil latching relay in a 1271/81. You don't have to run down the single-coil version, which appears to have been gone for decades.

p/n S4EB-L2-6V

The 5V version should also work.


The 1281 and 1271 use a Panasonic latching relay that is impossible to find. It is a 6V single coil latching relay. The single-coil design requires that you reverse the coil current direction to switch the relay, which drives circuit complexity.

This relay is unobtainable. However, it turns out that the dual coil version will work just as well. I found a clear case version at our local junk store. Here's a little video

https://photos.app.goo.gl/oHU1koYftAFRfNT49

The reversible drive current in the Datron circuit can effectively flip the dual coil relay.

Essentially, the S4-L-6V and the S4EB-L2-6V are interchangeable. This component is much more accessible than previously thought.

[wutieru]

When I select cal->spec->adc, I got error 2008 & 2050, the cal failed.
Can anyone tell me how to solve it？

[rigrunner]

When I select cal->spec->adc, I got error 2008 & 2050, the cal failed.
Can anyone tell me how to solve it？

What does pathway PXYY show, is the reading stable?

[rigrunner]

The Datron 1281 PCBs are elegantly mounted with Nylatch panel fasters...

If anyone wants replacements, please let me know.

I got some of the fasteners from leighcorrigall. The 3d printed part he created is a perfect fit. You can't really tell the 3d part from the originals. 

Here are some of the poorly stand offs from my 1281 along with one of the printed parts.

[leighcorrigall]

Attached is my first draft of the Datron 1281 button. I plan on printing a few as a favour to a fellow member (rigrunner). It will be the responsibility of the purchaser to prepare the raw part with a primer and a paint coat. Is there anyone else that is interested in such parts? 

Please let me know if there are any revisions to be made and I will update the CAD files accordingly.

The button appears to be successful in terms of its functionality. It holds well and has the correct dimensions (see the attached image showing a printed button polished lightly with 1200 grit). Aesthetically, there is a slight difference in the roundness which I have corrected in the attached file. The roundness has been changed from 1 mm to 1.5 mm.

If members want to order these buttons for the costs of making and shipping them, please let me know. I plan on making a few more of them for rigrunner and it would be good for them to be done in a batch.

[wutieru]

I checked the maintenance manual, Error 2008 is A/D Fail ,Error 2058 corresponds to Pxyw, the data is unstable and Pxyy is stable.

[rigrunner]

I checked the maintenance manual, Error 2008 is A/D Fail ,Error 2058 corresponds to Pxyw, the data is unstable and Pxyy is stable.

All of the +FR + EXT Zero are related tests.

I spent a long time tracking down a 2058 error. I won't give you the long story.
Try changing the integrator capacitor C505. The original in my 1281 was an ASC X363

Both the Dubilier 730P104X9250 and the Nichicon PWS1633100KG work as replacements
I tried a Vishay MKP1839 but several of those failed after warm up, so stay away from those.

[wutieru]

Thanks a lot.

[Kleinstein]

A change in the integration capacitor may need a new adjustment of the trimmers for the correction circuit for the dielectric absorbtion (DA). The DA for different brands or PP capacitors can be somewhat different.

[rigrunner]

A change in the integration capacitor may need a new adjustment of the trimmers for the correction circuit for the dielectric absorbtion (DA). The DA for different brands or PP capacitors can be somewhat different.

A good point.   

Another thing worth mentioning is C505 orientation. The original ASC has outer foil marking, the Nichicon and Dubilier do not.
You will need to test for the outer foil.




 

[wutieru]

I try to replace C505 with Vishay MKP1839410163.

[rigrunner]

I hope you have better luck with the MKP1839 than I did. I tried several of them and they all failed the same way.
After initial power on all self tests pass and ADC calibration is possible. Leave the 1281 to warm up for 40 minutes or so and try the test and ADC calibration again.
Self tests would pass but ADC calibration would fail.

Power off the 1281 and let it fully cool and re run the tests, exactly the same issue.

I replaced the MKP1839 and tried with the Dubilier and the Nichicon and they work perfectly.

[Le_Bassiste]

...
Another thing worth mentioning is C505 orientation. The original ASC has outer foil marking, the Nichicon and Dubilier do not.
You will need to test for the outer foil.
...

i had foil capacitors that had their outer foil markings on the wrong side. it's a good idea to just always check them.

[bfsy]

Self Cal password ?

[rigrunner]

Self Cal password ?

It's user settable.

Move the digital board jumper to the cal password off position and see if that allows access?

This is the off position

[chinapp]

Summary: you can use a dual coil latching relay in a 1271/81. You don't have to run down the single-coil version, which appears to have been gone for decades.

p/n S4EB-L2-6V

The 5V version should also work.


The 1281 and 1271 use a Panasonic latching relay that is impossible to find. It is a 6V single coil latching relay. The single-coil design requires that you reverse the coil current direction to switch the relay, which drives circuit complexity.

This relay is unobtainable. However, it turns out that the dual coil version will work just as well. I found a clear case version at our local junk store. Here's a little video

https://photos.app.goo.gl/oHU1koYftAFRfNT49

The reversible drive current in the Datron circuit can effectively flip the dual coil relay.

Essentially, the S4-L-6V and the S4EB-L2-6V are interchangeable. This component is much more accessible than previously thought.

The  S4EB-L2-6V have two coils, and footprint is different, how to interchange ? plz give more details,thx

[Kleinstein]

With many (but not all) 2 coil latiching relays on can wire the 2 coil in series / parallel to use it like a single coil version. If the current in both coils helps the sereis connection could be used with about the same voltage as the 2 coils in parallel.
As not all (though most) relays work this way it would be a good idea to do an upfront test.

[martinr33]

On the S4EB-L2-6V, either coil is capable of switching the relay.

The second coil sits across two pins that are unused in the single coil version (at the other end of the relay). So the pinouts are compatible when using a dual-coil relay in a single coil footprint.

If you think about it, the design has to be symmetrical for a single coil to work. Adding the second coil makes the drive circuits simpler, but does not change the mechanics of the relay.

Also, you cannot use a single coil version in a dual coil footprint, but that is not really an issue as the dual coil devices are available.

[leighcorrigall]

I have been considering including a switch to interrupt the VFDs on my Datron 1281 without cutting into the existing enclosure. This would avoid disassembly problems of the back panel and maintain the original look while preserving the display.

Options for replacing an existing component:
1) Keylock calibration switch (S2)
2) Mains switch (S3)

Both options would require a three-position switch.

On review of the schematics, the simplest approach for interrupting the power to the VFDs is at the fuses (F504, F505) as recommended by Micke. A modified schematic has been attached.

I think I am going to try and replace the mains power switch (S3) because it is already associated with the power supplies. Does anyone know the model number of the switch or any other details?

[Kleinstein]

Another option for a switch without drilling a hole would be a reed switch and a magnet from the outside.

The main switch is a bit tricky for accidentially interrupting power, when only wanting to turn on/off the display. In addition the isolation between different contacts may be a problem.

For the cal switch the this would increase the risk for accidentally enabeling calibration - not sure how important that protection is.

[TheSteve]

How about repurposing some contacts on the DB15? There are for certain unused pins that go from the DtoA IC socket to the DB15 if you don't have option 70 installed. It would be pretty easy to have an external DB15 male with a shell and toggle switch that just plugs in to enable the VFD.

[rigrunner]

How about repurposing some contacts on the DB15? There are for certain unused pins that go from the DtoA IC socket to the DB15 if you don't have option 70 installed. It would be pretty easy to have an external DB15 male with a shell and toggle switch that just plugs in to enable the VFD.

You would need to double check if the pin you are thinking of using is free. SK8 Pin 10 for example, is marked as unused on the schematic, but on the digital boards I have here it is wired via 100K to TL201.

[rk767]

Well, members, I too have become a Datron 1281 owner. My unit comes in fairy decent physical condition. Initially it had the "MEAS, CORRNS, INVALID 2021" error. After clearing the NVRAM I was able to do a quick and dirty ext cal with a 10V source. Power cycling the unit about 10 times I was not able to reproduce the error. This unit originally was supposed to be in working condition, however, the place where it was was not able to repair the "2021 issue", because of that I got a nice discount  Not sure if something causes the NVRAM to corrupt itself or a mishap when the battery was upgraded. Unit passes full self-test, however, fails 2008 and 2058 during ADC calibration. My assumption would be due to not having went through a proper calibration. More details later, first thing is to replace the capacitors as they all seem original.

[leighcorrigall]

... More details later, first thing is to replace the capacitors as they all seem original.

Congratulations.

I have a partial list of electrolytics (including dimensions) posted above in a spreadsheet.

[Le_Bassiste]

I have been considering including a switch to interrupt the VFDs on my Datron 1281 without cutting into the existing enclosure. This would avoid disassembly problems of the back panel and maintain the original look while preserving the display.

Options for replacing an existing component:
1) Keylock calibration switch (S2)
2) Mains switch (S3)

i'd rather "sense" one of the front push buttons, say, the LOCAL key, and then toggle the VFD supply via some MOSFET driven by a photovoltaic optocoupler and some sort of flip-flop (or, low pin-count PIC8-someting for that matter).

[leighcorrigall]

Self Cal password ?

It's user settable.

Move the digital board jumper to the cal password off position and see if that allows access?

This is the off position

Hi Rigrunner,

Do you happen to know the functionality of the other jumpers on the digital board? This would be beneficial to me.

Regards.

[rigrunner]

TL101 Pulls the CPU Bus ERRor pin high in position B, preventing anything signalling bus error.

TL201 Doesn't have any obvious behaviour. Its state is passed through a 100k resistor to pin 10 SK8
TL202 Again, no obvious behaviour change with this jumper. Pins aren't fitted to the PCB in later revisions.
TL203 Option 70 enable/disable. Position A is disable.
TL204 Callibration menu password required. Position A is disable.
TL205 pulls the CPU DTACK low in position B. Debugging only, CPU will trap and crash pretty quickly in position B.

[leighcorrigall]

I experienced a strange, yet subtle problem, while adjusting ranges on the Datron when I had access to a Fluke 5700A 5720A series calibrator last month. The problem was unknown to me until the adjustment/calibration attempt. The instrument would accept the new adjustment then, when I wasn't looking, it would revert back to its previous value or something similar (REVISED SENTENCE). You can imagine how frustrating this was after spending several hours meticulously adjusting each range and documenting the results to discover that the values were completely different after verification. Sometimes the value would even hold after a reboot! At first, I thought it might have been how I exited the calibration menu where it asks for the next calibration date (options being ENTER or QUIT). My hypothesis was that the digital board had some sort of malfunction in storing calibration constants. Although potentially unrelated, I should have taken more care in diagnosing the scrambled screen issue described in the initial post. I thought that the board might have been shorted because the card slot was damaged and the protective sheet under the digital board was a piece of tape and some card stock. Well, that was only part of the problem.

In a retaliatory manner , I decided to replace all of the socketed components on the digital board to address the problem (what a beautiful design feature of the older Datron models). This is because I was unable to repeat the issue with a simple cause-and-effect process and I figured it wasn't that expensive to replace these components anyways. A Hong Kong-based eBay seller sold me nearly all of the obsolete parts required to do this swap (40 USD total with shipping). The rest were purchased new from DigiKey. Attached is a list of the socketed components that were replaced. I compared my older Datron to TheSteve's Wavetek model from what I could piece together from reference pictures.

Part of the replacement process involved programming 27C010, GAL16V8B, and GAL20V8B chips with the TL866II Plus and XGPro. I bought the TL866II Plus specifically for the GALs because my other universal programmer isn't that universal after all. Attached is a compressed file I used for the EPROMS and GALs organized into the socket designations. SHA256: 835656a4939d1b52c8f5afac1887858536df383ed417720487bb1cf1da4a9730 (checksum)

Another feature of the XGPro software package is that it can test some of the components through Device -> Logic IC Test, which can be rather helpful for isolating the program. I was able to rule out the 74245, 74374, 74165, 7404, 74260, and 74165 chips this way. The ROMs can be simply reprogrammed and then verified that they match a predetermined data dump.

At this point, I have adjusted my Datron to a well-determined Fluke 732A 10 V output and have not noticed a change in value since yesterday. The measurement error range is maybe +/- 0.2 ppm, at most, and is likely caused by thermal drafts. My next plan is to perform a fully automated test that will include a front input short (i.e., zero volts) and a reverse polarity test of a reliable voltage standard using the rear inputs (ChA and ChB) over several weeks at 23 °C. 

[TiN]

It was 5720A, not 5700A :-). I thought it was confirmed that problem saving calibration constants was related to jumper in incorrect position on digital board, no? Also I don't think constant values were randomly changing, but rather reset to stored NVRAM values once unit was reset/enter/exit calibration procedures. Since we did that multiple times trying to "troubleshoot" non-storing values, it may "felt" like random act of ppm pixies..

Also I'd like to use this chance to reiterate that no instrument can be deemed properly operational after even minor repairs until it is actually went thru full "as-received, adjusted, as-returned" calibration and service adjustment procedures. Imagine sending this 8.5 digit meter to Fluke, paying $2000 for calibration, and getting it back with all data lost  .

[leighcorrigall]

It was 5720A, not 5700A :-). I thought it was confirmed that problem saving calibration constants was related to jumper in incorrect position on digital board, no? Also I don't think constant values were randomly changing, but rather reset to stored NVRAM values once unit was reset/enter/exit calibration procedures. Since we did that multiple times trying to "troubleshoot" non-storing values, it may "felt" like random act of ppm pixies..

Also I'd like to use this chance to reiterate that no instrument can be deemed properly operational after even minor repairs until it is actually went thru full "as-received, adjusted, as-returned" calibration and service adjustment procedures. Imagine sending this 8.5 digit meter to Fluke, paying $2000 for calibration, and getting it back with all data lost  .

I forgot what option the 5700A was from memory, so I didn't get too specific.

Option 70 may have been related to my Datron symptoms, but perhaps not. It might have been coincidental that removing U205 and disabling the option jumper improved the situation. The instrument might have been impacted by the ~1400 km road trip to and from NJBH. After more 'long-term' observations, the measurements were still not consistent with Option 70 removed. The entire chip swap has shown the greatest amount of consistency so far. I will need your help next week to set up a long-term performance study to determine if the bugs have been squashed  

By 'random', I meant that the values were not what was expected, given a known input. I agree, the instrument likely reverted back to a previous calibration constant once it detected a fault somewhere. What was troubling was that Datron gave no warning about its decision, which caused a great amount of confusion.

I sent my Advantest in for calibration and found out the hard way that the instrument doesn't meet specifications and I definitely want to avoid that outcome for Datron. Lessons learned.

[TheSteve]

I feel the need to ask which jumper is used to enable option 70?
Did you try switching back to the stock nvram?

[leighcorrigall]

I feel the need to ask which jumper is used to enable option 70?
Did you try switching back to the stock nvram?

Rigrunner has determined the purpose of the jumpers:

...

TL203 Option 70 enable/disable. Position A is disable.

...

The storage has remained FRAM since the update. So far, it does not seem to be the issue because the memory can be overwritten. A more complete picture will come about after I perform a long-term drift study on the 10 V range.

[TheSteve]

It sounds like you've already swapped the main parts but scoping the write enable lines for the nvram may be useful if you continue to have issues.

[leighcorrigall]

...

My hypothesis was that the digital board had some sort of malfunction in storing calibration constants. Although potentially unrelated, I should have taken more care in diagnosing the scrambled screen issue described in the initial post. I thought that the board might have been shorted because the card slot was damaged and the protective sheet under the digital board was a piece of tape and some card stock. Well, that was only part of the problem.

In a retaliatory manner , I decided to replace all of the socketed components on the digital board to address the problem (what a beautiful design feature of the older Datron models). This is because I was unable to repeat the issue with a simple cause-and-effect process and I figured it wasn't that expensive to replace these components anyways.

...

My next plan is to perform a fully automated test that will include a front input short (i.e., zero volts) and a reverse polarity test of a reliable voltage standard using the rear inputs (ChA and ChB) over several weeks at 23 °C. 

Studies
1) 2022 06 11 to 2022 06 22 (11 days) Experiment - Before Digital Board Chip Replacement
- ChA connected to Fluke 732A-2 (+10.0002207 V)
- ChB connected to Fluke 732A-1 (+10.0001320 V)

Note the 6 skew data points on the "stability evaluation" figure.

2) 2022 08 26 to 2022 09 28 (33 days) Experiment - After Digital Board Chip Replacement
- ChA connected to Fluke 732A-1 (+10.0001320 V)
- ChB connected to Fluke 732A-1 (-10.0001320 V)

No skew observations for over a month of data logging.
The spread of the data is greater because there are triple the amount of days in this study.
Datron 1281 temperature coefficient of voltage between 19.5 and 24.2 C °C: α = -0.342 ppm/°C, β = 0.256 ppm/°C^2

Note:
These rough parameters were computed by assuming the Fluke 732A source was independent of temperature.
The temperature sensor used could have an offset up to a few °C relative to other sensors present during the data collection.
I am assuming that these voltage coefficients are associated more with the 13 to 18 °C specifications.

Conclusion
The Datron 1281 does not produce skew data anymore after replacing the digital board chips. I have also not witnessed a scrambled screen anymore. I suspect the PALs were going bad and possibly some other chips. Note that the FRAM modification presented above was left alone and is not suspected to be the problem. I also removed Option 70 and placed a wire jumper like other Wavetek models.

My only concern now with the Datron 1281 is that it might have a relatively large temperature dependence compared to the specifications. If there are members with quantitative results to compare with, I would appreciate it if they provided feedback.

The next task will be to completely replace all tantalum capacitors on each board.

Thanks!

[Micke]

I did an experiment a while ago to investigate the TC of my Wavetek 1281 vs a borrowed Keysight 34470A.

Conditions:
1) Measuring 10VDC from a LTZ1000 placed in temperature chamber set to 34C (only able to heat) in a separate room from the DMM´s, shielded RG316 cable from LTZ1000 to DMM´s.
2) Both DMM in the same room
3) The temperature logged is the room with the DMM´s
4) Started a 2000W heating fan to heat up the room with DMM´s.
5) 1281 in 8.5 digit mode, WinGPIB samples temperature and 1281 every 25s. Data from 34470A saved to USB memory, tried to match its trigger time to match 1281. Did not succeed to let WinGPIB to log both instruments.

My setup and measurements might have flaws, but my results were TC of +0,2ppm/C for 1281 and -0,6ppm/C for 34470A.
The 1281 is more long time stable, did a 139h test, the 1281 tracking ambient temperature perfectly, while 34470A drifting slightly upwards during time.

[leighcorrigall]

While stuck at home sick, I took the time to examine the performance of my Datron 1281. Members such as RigRunner and MartinR33 have suspected that my Datron troubles are not over and I want to find the root cause of this mystery problem.

If you compare the  2022 08 26 - Datron 1281 closed case example.png (380.65 kB. 2970x1980 - viewed 101 times.) figure with  2022 10 25 - Datron 1281 open case study.png (579.25 kB. 2979x1989 - viewed 89 times.), there doesn't seem like a whole lot of difference between when the Datron 1281 top cover has been removed or when it is closed. This to me is suspicious.

I then decided to compare the lesser Advantest R6581T with the Datron 1281 using a Fluke 732A-1 (+10 V) as a common voltage reference  2022 10 27 - Datron 1281 and Advantest R6581T performance comparison.png (590.94 kB. 3116x1989 - viewed 77 times.). Clearly, the Advantest R6581T has won the stability award which tells me that there are still some serious problems with my Datron! 

The plan moving forward:
1) Measure the -7.2 V references at TP405 (M408) and TP406 (M409) with TP102 ('MECCA') as common. This can be accomplished with the Advantest R6581T now that I know it is reasonably stable and temperature insensitive.
2) Measure the buffered voltage references at TP403 and TP404 with TP102 as common.
3) Measure the -35 V and +35 V supply voltages as recommended by RigRunner. He suspects that the designer balanced these supplies with R904 and R906 to enhance the performance of the instrument. The resistors are both labelled "FSV" which may be an abbreviation for 'factory selected value'. The 35 V supplies influence many critical areas of the DCV board. None of the other inside guard regulators have been trimmed in this way. My DCV board shows the most amount of abuse in this area from a past owner. Perhaps this could be an area to work on first.

All future plans will start measuring after the instrument has reached a steady-state condition with the cover off and after the cover has been reinstalled. See  2022 10 26 - Datron 1281 open to closed case reversal error.png (565.82 kB. 3121x1989 - viewed 71 times.) for an example of when I placed the top cover on the Datron 1281 while continuously measuring a Fluke 732A. These conditions may provide insight into how voltage measurements behave as a function of temperature. I wish I could do more than one measurement at a time with the Advantest R581T. This may take a while.

[alm]

I wish I could do more than one measurement at a time with the Advantest R581T. This may take a while.

I realize this is yet another project to build and verify, but one way around that would be to build a low-thermal-EMF scanner like this to allow a single DMM to measure multiple signals. I would imagine it's really useful to see relations between the voltages. An alternative, if you have lower resolution meters that can log, could be if you have a stable voltage source of nearly the same voltage, like a divided 10V from 732A, and have a lower-resolution meter measure the difference, kind of like a differential voltmeter. I've used this trick in the past to use 6.5 and 5.5 digit meters to monitor power rail stability.

[Kleinstein]

Having quite some drift in the trun over error is suspicious. This could be a linearity problem, but also an offset problem. Different from the R6581 meter the 1281 uses a kind of chopper stabilized amplifier and am ADC that is made to have relatively low drift even without an auto zero mode.
Ideally the chopper part has no drift, but at the µV level and below, there is a chance for some problems.
So I think a check of the offset drift / stability would be a good idea. The 100 mV range would show more sensitivity to amplifer offset drift and less from the ADC part. So it could help to localize the offset drift.

For the longer time tests with the 10 V on the drift one chould also check on how much of the drift is corrected with a ACAL call.

I don't think the +-35 V supply should have very much influence it mainly for the supply to less critical amplifiers. A point were is may have an effect is with some JFET switching, but don't think gate leakage there would be that critical. The much more critical supply would be the +-15 V supply (it is used e.g. for quite some CMOS switches and the supply can effect charge injection and on resistance).
There are also 3 sets of resitors that effect the gain factor for the 10 V range:
1) the gain setting resitors, that should be acting as a divider by 2 for the 10(20) V range. I somewhat doubt they would automatically switch between an extra range with 10 V max and the 20 V max range, as this would cause discontinuity problems near 10 V.
2) The 7 to 10 V gain for the ADC reference. (not very much effect, but still possible error source).
3) The resitors at the ADC input

A temperature effect that can increase from a partial defect are leakage currents. A part could have developed higher than normal leakage current and such leakage is often quite temperature sensitve. However leakage in most cases would give more like an offset and not an effect on the gain.

An indirect effect can be the fan speed / air flow.

The tests so far still run with a relatively small variation in temperature. A larger temperature swing could show a clearer effect.

[leighcorrigall]

...

The plan moving forward:
1) Measure the -7.2 V references at TP405 (M408) and TP406 (M409) with TP102 TP101 ('MECCA') as common. This can be accomplished with the Advantest R6581T now that I know it is reasonably stable and temperature insensitive.

...

 2022 10 27 - Advantest R6581T stability according to Fluke 732A-1 (+10 V).png (761.76 kB. 2979x1989 - viewed 84 times.) is an example of the Advantest R6581T [AZERO, 100 NPLC] measuring a Fluke 732A-1 (+10 V) for 33 hours taken from the previous post https://www.eevblog.com/forum/metrology/datron-1281-repairmaintenance/msg4489450/#msg4489450. During that time, the ambient temperature decreased from 24.2 to 22.1 °C (ΔT = -2.1 °C). The measurement error changed from 0.13 to -0.38 ppm (Δε = -0.51 ppm). Perhaps the R6581T has a temperature dependency of 0.25 ppm/°C. It would also be beneficial to get a sense of the measurement dependency while the temperature increases as well. Another indication of performance would be measuring the Advantest while the FRONT is shorted. If only I had a low thermal scanner or a secondary Datron. 

Using  2022 10 27 - Advantest R6581T stability according to Fluke 732A-1 (+10 V).png (761.76 kB. 2979x1989 - viewed 84 times.) as a control, it is now possible to roughly evaluate the Datron 1281 (DCV, FRNT: short) voltage references (M408 at TP405 and M409 at TP406):

•  2022 10 28 - Datron 1281 M408 stability according to Advantest R6581T.png (680.86 kB. 2979x1980 - viewed 91 times.) is an example of the Advantest [AZERO, 100 NPLC] measuring TP405 for 12 hours. During this time, the ambient temperature increased from 21.8 to 22.6 °C (ΔT = +0.8 °C). The measurement error increased from 0.5 to 0.9 ppm (Δε = +0.4 ppm). Perhaps the combined temperature dependency of the Datron (M408 at TP405) and the Advantest is approximately +0.5 ppm/°C.  2022 10 28 - Datron 1281 M408 stability according to Advantest R6581T.png (680.86 kB. 2979x1980 - viewed 91 times.) and  2022 10 28 - Datron 1281 FRNT short.png (590.37 kB. 2979x1989 - viewed 79 times.) were measured simultaneously.
•  2022 10 30 - Datron 1281 M409 stability according to Advantest R6581T.png (792 kB. 2979x1989 - viewed 73 times.) is an example of the Advantest [AZERO, 100 NPLC] measuring TP406 for 30 hours. During this time, the ambient temperature increased from 22.5 to 23 °C (ΔT = +0.5 °C). The measurement error increased from -0.13 to 0.25 ppm (Δε = +0.38 ppm). Perhaps the combined temperature dependency of the Datron (M409 at TP406) and the Advantest is approximately +0.75 ppm/°C.  2022 10 30 - Datron 1281 M409 stability according to Advantest R6581T.png (792 kB. 2979x1989 - viewed 73 times.) and  2022 10 30 - Datron 1281 FRNT short.png (607.03 kB. 2979x1989 - viewed 74 times.) were measured simultaneously.
  

Comments:

• I believe that most of the temperature dependency must be attributed to the Advantest. The time required for the measurement to drift as a function of temperature is small according to the figures, which suggests that the Advantest is responsible. Another way of interpreting this hypothesis is to think about the Datron voltage references as being surrounded by a blanket of hot air without circulation. The Advantest uses a fan and has plenty of air vents.
• The M408 is connected to the reference inverter circuit, which would annihilate most of the temperature-dependent error when compared to the M409 buffered reference source. See sheet 4 of the D.C. Assembly Schematic. At this stage, I do not suspect the Datron voltage references to be responsible for the significant temperature dependencies demonstrated in the previous post.
• https://www.eevblog.com/forum/metrology/datron-1281-repairmaintenance/msg4489450/#msg4489450 shows that the Datron deviated by +3 ppm when a temperature change of -1.5 °C occurred over 29 hours (i.e., 2 ppm/°C) while measuring the Fluke 732A-1 (+10 V). I would expect that if the Datron voltage references were the immediate problem, the Advantest measurements would have detected something greater.

As a side note, measurements made by the Advantest at both TP405 and TP406 changed according to the measurement made by the Datron (e.,g. cycling each channel [FRNT: short, ChA: +10 V, ChB: -10 V] causes slightly different results (~= 1 ppm) measured by the Advantest).  2022 10 28 - Datron 1281 M408 stability performace while cycling channels.png (1476.52 kB. 2979x1989 - viewed 82 times.) is an example. I suspect this effect is normal.

EDIT: Note that the voltage range is set to 10 V for all studies, including the FRNT short.

[Kleinstein]

I agree that the temperature effect shown in the last plots seems to be mainly from the R6581T. There are considerably more resistors inside the DMM than for the relative simple 7 V to 10 V step in the Datron meter. The capacitive inverter should be pretty stable and not cause much error.

There is some offset effect at the DA1281 from temperature changes (looks a bit like the dT/dt as the main factor), but this is relatively small (<1 µV range), though still not very good. Part if this could already be the short / cables / terminals.

The effect of the measurement performed by the Datron meter on it internal measured reference could be an effect of the layout / placement of the ground test point. If this would be a true effect on the references it would be bad for the linearity.

[leighcorrigall]

...

There is some offset effect at the DA1281 from temperature changes (looks a bit like the dT/dt as the main factor), but this is relatively small (<1 µV range), though still not very good. Part of this could already be the short / cables / terminals.

...

The Datron 1281 FRNT short is simply a copper wire compressed on the binding posts in the following order I+, HI, LO, and I- (i.e., an upward C-shape).

As for Ch A and B, they are made of shielded 22 AWG silver-plated copper-stranded wire with PTFE insulation in twisted pairs. The connector pins are gold platted and soldered with regular Pb-Sn wire. They are similar to the xDevs article https://xdevs.com/fix/d1281/#diyrear. The spades are pure copper crimped, not soldered.

Attached is a series of figures that have been taken from https://www.eevblog.com/forum/metrology/datron-1281-repairmaintenance/msg4448029/#msg4448029 that show the Datron cycling through all three channels. Note that all measurements have a 25-moving average filter applied for legibility. There is a strong correlation between temperature, reversal error, and short offset.

[leighcorrigall]

I had a change of heart and decided to go after the linear regulators L7815CV (M901) and L7915CV (M902) again. They seemed too hot, even when the thermal paste was replaced and when the instrument was left to stabilize with the cover off. These components are still available and cheap so it was easy. I decided to include thermal pads between the heat sink and the PCB to dissipate the heat into the board more efficiently.

Under the recommendation of RigRunner, I disabled the option boards (see highlighted areas in blue of the attached DCV board layout). The purpose is to isolate the Datron from the rest of the analog boards and to reduce heat caused by the ±15 V regulators. The boards can be disabled by changing the jumpers and disconnecting the ribbon cables as follows:

AC converter board (Option 10): LK704 and PL105
Ω converter board (Option 20): LK702 and PL109
I converter board (Option 30): LK703 and PL107

Afterwards, I calibrated the DCV mode (-FS, zero, +FS) for each range (100mV to 1000 V) with the Fluke 5440B. The Datron seems to have a recurring issue with the ADC internal calibration (errors: 2008 and 2054/2056/2058). These errors seem to be persistent now. I went as far as clearing the NVRAM in an attempt to remove these ADC errors. Fortunately, RigRunner is rescuing me again. He suggests that C505 could be responsible. See sheet 5 of the DC assembly schematics that shows the 100nF capacitor acting as a bridge from the integrator section to the null detector with R545 (brown red black yellow brown - 1.2M 1%), R546 (brown grey red green brown - 18.2M 1%), and R547 (brown black black yellow brown - 1M 1%). A previous post already exists on this matter: https://www.eevblog.com/forum/metrology/datron-1281-repairmaintenance/msg4123972/#msg4123972

The DCV calibration constants have been stored and seem fairly stable. I quickly tested the reversal error and they are negligible even after a slight temperature change. I will know more once Datron 1281 passes the ADC, INT SRC CAL, and the FULL TEST LOOP. Then the real test will begin as a stability test over a few days. Eventually, I will get back to measuring the buffered voltage references with the Advantest while the Datron measures the Fluke 732A-1 (+10 V) reference.

[leighcorrigall]

...

--> Front terminal release: A rear button retracts the front terminal of the instrument. The terminal block is spring-loaded so that the posts extend outward after a second button press.

...

Members have asked me to inspect the front posts of my Datron 1281 unit.

There are some interesting features to point out:
- Guard and Ω Guard posts are gold plated
- I+, I-, Hi, and Lo are of a copper alloy (perhaps tellurium-copper, C145)
- The connections are made with a flexible PCB with gold contacts
- The post-assembly can be retracted for safe handling as discussed before

EDIT: After cleaning, the difference in post coatings could actually be that the guards show fewer signs of wear. Now it seems clear that bare copper was deliberate.

[Kleinstein]

Retracting the therminal block may be for more than safe handling: it also gives some shielding against air flow and thus temperature fluctuations - however at a possibly different mean temperature.

It makes sense to have gold on the guard terminals: those are not very critical in respect to thermal EMF, so not problem with gold there and more normal gold coated brass can be used. It depends on how the switching for the resistance function is, if the I+- terminals are also used for small voltages (e.g. for 4 wire ohm).

[leighcorrigall]

...

The Datron seems to have a recurring issue with the ADC internal calibration (errors: 2008 and 2054/2056/2058). These errors seem to be persistent now. I went as far as clearing the NVRAM in an attempt to remove these ADC errors. Fortunately, RigRunner is rescuing me again. He suggests that C505 could be responsible. See sheet 5 of the DC assembly schematics that shows the 100nF capacitor acting as a bridge from the integrator section to the null detector with R545 (brown red black yellow brown - 1.2M 1%), R546 (brown grey red green brown - 18.2M 1%), and R547 (brown black black yellow brown - 1M 1%). A previous post already exists on this matter: https://www.eevblog.com/forum/metrology/datron-1281-repairmaintenance/msg4123972/#msg4123972

...

After a frustrating experience with a UPS delivery, I got the replacement parts for the ADC. C505 caused the ADC CAL errors (2008 and 2058) as indicated by RigRunner in previous posts. Apparently, the dielectric characteristics of the 100 nF capacitor are very important for ADC performance. The polarity (orientation) is also important as indicated by the layout schematic.

I tested the following capacitors:
1) Nichicon PWS1633100KG FAILED ADC CAL in both orientations
2) Cornell Dubilier 930C2P1K-F FAILED ADC CAL in both orientations
3) Cornell Dubilier 730P104X9250 PASSED ADC CAL in the orientation pictured below

These results are not to say that the 730P104X9250 will always work or is the only candidate. Unfortunately, this capacitor has the lowest quality construction out of all the candidates tested. Sometimes it is about functionality I suppose. Since I purchased two additional PWS1633100KG capacitors, I may try them out later after I have resolved the thermal dependency problem with the DC voltage measurements.

In the process of waiting for a UPS order that was delivered damaged, I refurbished the following ADC components:
- rebuilt the entire read-rate board and underneath resistors
- updated R545, R546, and R547 assembly to prevent debris and vibration (likely overkill)
- replaced M503 through M505 and surrounding resistors
- replaced M507 support capacitors
- replaced M506, M508, and M511
- cleaned the pins of M509 with grit paper to ensure proper contact

[Kleinstein]

If you have found a working capacitor, there is no real need to check if another type also works.
The point where C505 and R545-R547 meet is the integrator input and thus a slightly critical node. The isolation sleve at the resistors is kind of parallel to the capacitors and could also have an effect.

Ideally, after changing C505 the trimmer R544 may want a new adjustment. The part around R544 is some circuit to compensate dielectric absorption (DA) effects of C505 to avoid INL errors.
The other capacitors may just need a different setting of C505. Not sure if there are good instruction on how to adjust R544 - expect this to be a bit tricky. Even if not intending to adjust R544 this procedure could give a hint if the setting is about right or maybe still off quite a bit.
I would expect the largest DA related nonlinearity for small votlages, like +-50 mV for the 10 V input range.  So it may be a good idea to test the linearity in this range. With the rather small voltages this looks feasable even without very special instruments.

I would be careful with replacing the CMOS switches. These can effect the ADC performance and new chips may very well use a slightly difference process and new may not be better in all aspects. As the switches are at the reference side I would not expect that much effect, but why take a risk ?

[shenxiaoming]

My 1281 AC board
CLA5532(M412) on is missing, it seems to be customized! Is it possible to DIY one?
?

[Kleinstein]

The chip should be some custom logic. So in principle it could work to replace it with a CPLD of some kind. I don't expect a super complicated content, though it looks mainly like a digital FLL / PLL.
It would still be quite some effort and CPLDs with 5 V outputs are rare by now.  Besides the FLL there seem to be a few extra ouput for things like error, maybe frequency range.
A frist point would be understanding the circuit part (much of the page in the plan). There could be the alternative to replace the complete FLL part with something more modern.

[chinapp]

If you have found a working capacitor, there is no real need to check if another type also works.
The point where C505 and R545-R547 meet is the integrator input and thus a slightly critical node. The isolation sleve at the resistors is kind of parallel to the capacitors and could also have an effect.

Ideally, after changing C505 the trimmer R544 may want a new adjustment. The part around R544 is some circuit to compensate dielectric absorption (DA) effects of C505 to avoid INL errors.
The other capacitors may just need a different setting of C505. Not sure if there are good instruction on how to adjust R544 - expect this to be a bit tricky. Even if not intending to adjust R544 this procedure could give a hint if the setting is about right or maybe still off quite a bit.
I would expect the largest DA related nonlinearity for small votlages, like +-50 mV for the 10 V input range.  So it may be a good idea to test the linearity in this range. With the rather small voltages this looks feasable even without very special instruments.

I would be careful with replacing the CMOS switches. These can effect the ADC performance and new chips may very well use a slightly difference process and new may not be better in all aspects. As the switches are at the reference side I would not expect that much effect, but why take a risk ?

yes,anyone can find the factory repair manual of the 1281 ?maybe fluke has the detail files

[Arhigos]

My 1281 AC board
CLA5532(M412) on is missing, it seems to be customized! Is it possible to DIY one?
?

I think so! I have old 8508 dmm and it has datron AC board fitted with a special fpga-adapter instead of your chip.

[leighcorrigall]

My 1281 AC board
CLA5532(M412) on is missing, it seems to be customized! Is it possible to DIY one?
?

I think so! I have old 8508 dmm and it has datron AC board fitted with a special fpga-adapter instead of your chip.

Thanks for sharing!

It would be great if you could provide more details so that others may benefit 🙂