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.