Datron 1041 vintage multimeter

[Zeitkind]

Hi!
On my hobby bench today: A Datron 1041 4-1/2 digits multimeter, some chips date prob. around 1978.

Unit powers up, but display kinda "hangs" at ".9990".
I can select V or Ohms, doesn't matter, display shows ".9990". No click of any relays, but the "+"-symbol changes when I switch from DC to AC. Not dead, but half dead.
I opened the meter, totally unserviced yet, everything looks original. I have the schematics of the big brother model 1051 - which is the very similar 5-1/2 digit one - and the 2 models seem to share some boards. I measured basic stuff like supply voltage (both 5V and 15V seem to be fine) and checked some electrolytic caps (Philips and others, capacity and ESR are all in spec). The EMI filter went up in nasty smoke and my junior got scared - glad I had a cheap chinese spare. It took the EMI filter around 10min to blow up, so I think the meter hasn't been powered up for a very long time.
I wonder if anyone out there ever had this problem with a 1041 or 1051?

- tia Hermann

[Gyro]

Hi Zeitkind, welcome to the forum,

Yes, I can hopefully help you. I have a 1041M (mil version) a 1045 (DC only) and a 1030A (RMS) all of which use the same A/D board.

https://www.eevblog.com/forum/chat/show-your-multimeter!/msg793266/#msg793266

Your 1041 is definitely worth persevering with, it has very decent accuracy for a 4 1/2 digit meter (0.01% +1dig DC), a 10mV range with 1uV resolution and the same true RMS converter as the later 106x and 107x ranges (assuming yours has the AC option fitted). The best thing is that it is completely discrete (no micro, s/w or Cal RAM) so you can maintain it almost indefinitely. Nice Panaplex displays too. You can easily change it to display 10 readings/sec too. So don't bin it!

Discovering and replacing the Mains filter is a good first step , as is checking the three supply rails - the tantalum beads on the outputs of the 15V regulators are also prone to failure. The axial electrolytics seem to last indefinitely so no point in changing them, as you have found. Good that it is displaying something too (+180V, drivers and displays functional).

The 104x and 105x do share most boards (apart from the A/D and display board, there are differences in the quality of ranging resistors etc on the other boards (higher quality on the 105x) but they are interchangeable. Does having the 1051 schematics also mean that you have a 1051?

All the logic ICs are socketed, which will help your debug. The clock oscillator is a simple transistor astable (no pll complication like the 1051), but I suspect that the problem is in the general triggering logic. What colour IC sockets does it have? The translucent poyethene looking ones are less reliable than the black ones.

As the schematics are only available in paper form I will need to get the camera out. In the meantime it would helpful you can post some photos to judge the general condition see if there is anything obvious. Also post which options are fitted.

One other thing, check that all buttons (hold, print inhibit etc) in the centre contol group are not pressed in!

P.S. Does the front panel say 'datron', 'datron Ltd' or 'datron instruments Limited'? This helps to judge the age.

[Zeitkind]

Hi, thx for the answer!
Serial number on the back is 3568.. no idea how many they built, "Datron Electronics Limited"
I did a visual inspection, took out the boards, re-seated most of the TLL stuff. Some look .. gunky, grubby. Just as they have a silicone oil like something on them, prob. gonna clean the boards though they look brand new otherwise. All TI 74L90N have black colored pins.. it's a kind of black paint, weird.
When I disconnect the -15V and/or +15V cables to the other boards, the display shows various numbers, so isn't stuck any more. After dis- and reconnecting that rail, I saw smoke coming of a blue tantalum bead cap (C53 on the AC option board). I desoldered it, it has 0F and 24ohms. Other blue tantalums show in-circuit resistance from <1k up to 5M, prob. I should desolder all below some Meg Ohms and check them. I have no spare ones, so I have to wait for delivery now

The schematics of the 1051 can be downloaded at http://www.ko4bb.com/ soon, Peter from g0rsq.co.uk was so kind to scan and upload them. I don't own a 1051, but I prob. get one in more or less working condition next week. I took some pictures today outside:

[Gyro]

That looks in pretty good condition. 

From the company name and look of the boards, you have a reasonably late one which is good. It looks to be in good original condition, original Wima electrolytics, no obvious signs of anything having been changed. It's good that it has the black IC sockets too, they're pretty reliable.

Be careful of board cleaning and flux generally. There are guarded low leakage areas where you could cause drift. (especially under the black draught shield on the DC board, that's where the factory TC selected and compensated LM312 lives).

Interesting that you now have some activity, it clearly has a few leaky tantalum beads but I wouldn't have expected that to stop the digital section (unless it upset the analogue side of the discrete dual slope A/D converter). Keep a close eye on the -15 and +15V rails. Also note carefully where each connection to the front panel terminal board goes if you remove the AC and Ohms options.

As a start, I've photographed the schematics for the main board which include power supply, A/D and displays. That's the bit that's different from the 1051. It may help get you started.

The schematics of the 1051 can be downloaded at http://www.ko4bb.com/ soon, Peter from g0rsq.co.uk was so kind to scan and upload them.

A warning that the 1051 is rather more temperamental than the 1041, it uses a triple slope A/D conversion rather than dual and its autozero loop is tricky (trying to fix a 1055 board at the moment).

Damn, that means I'm sort of duty bound to start scanning the whole 1041 manual.   It's fairly thick and the schematics are A3 .

I hope the attached shematics help you start to get your bearings anyway.

[Zeitkind]

Thx!!

Without the AC board, the meter shows nothing (blank) on DC, AC or Ohm. I have to wait for the spare parts, and the blue tantals are spread over all boards, most of them show suspicious low resistance - in circuit, but well..
Gonna take a look at the schematics and try to figure out what I should read at the various test point. Some have +4.99V or -4.99V, but on some I see just floating something with my DMM and oscilloscope.

[Gyro]

Ah, actually that is progress.

Blank display (apart from decimal point and mV/V/ohm etc) indicates over-range on these datron meters. The Ohms board intercepts the input Lo connection (the ohms board has a relay on-board).

As the DC volts input is high impedance (>10G) it will float to over-range if nothing is connected (input Lo is open as above). You should find a wire loop on the back of the terminal board that is snipped open (it was snipped at the factory when the Ohms option was added). If you temporarily re-make the link and short the input Hi and Lo terminals you should hopefully get a zero reading.

If you can get to this stage then you can check the operation of DC volts ranges, autoranging functionality etc.


P.S. I know a simple mod to make it display a 'leading 1' rather than blank display for overrange - more like the behavior of most DMMs.

[Zeitkind]

Blank display (apart from decimal point and mV/V/ohm etc) indicates over-range on these datron meters. The Ohms board intercepts the input Lo connection (the ohms board has a relay on-board).

Not sure what you call "Ohms board".  I have a "main" AD-board with the mains transformer, the DC Isolator board next to it and the removed AC board with the Datron RMS-Chip (and the blown tantalum cap) that was below the DC Isolator board. If Ohms need an addon board, I don't have one. 

As the DC volts input is high impedance (>10G) it will float to over-range if nothing is connected (input Lo is open as above). You should find a wire loop on the back of the terminal board that is snipped open (it was snipped at the factory when the Ohms option was added). If you temporarily re-make the link and short the input Hi and Lo terminals you should hopefully get a zero reading.

I can see a loop behind the Lo(black) input plug, but it is still connected and not cut - because the Ohms board is missing.. *damn*
Connecting a DC source - no display, so there likely is a fault on the DC input board or the main board. But I will replace all those blue tantal caps before I dig further.

And - if I get this unit working, I can measure Volts DC and AC, but no Ohms. 

[Gyro]

Sadly, yes your unit does not include the Ohms option. It would have sat next to the AC option (photo of the underside of my 1041M attached, note that the extra cross bracing is for the M version only).

I would concentrate on getting the DC functionality working before plugging in the AC option again. Curious that you had a display before removing it but not now. Is the display completely blank or do you have mV/V indicator and decimalpoint - if not then check the 5V logic supply again.

Also try manual ranging (auto button out) [EDIT: Rem (Remote) button out too!]. Does the decimal point move, mV change to V etc.

[Gyro]

I've scanned and uploaded my 1041 Service Manual to http://www.ko4bb.com/ today. Hopefully it won't take long to go live. 

[Zeitkind]

Thx again! Will take a while to go online, but well...

I measured some more of those blue tantalum caps and - well, many of them should be replaced, they have some k ohms on DC or even below. I think about replacing them with some good electrolytics, but not sure..  .. prob. with a small ceramic disc in parallel?
I have a bunch of smaller tantal caps, but none with 36V .. hmm..

[lowimpedance]

Looks like a rather interesting 'old timer', Cant seem to find any pics on the web  hmmm.
Thanks for the internal pics chaps. Nicely built, always nice to see inside these oldies and that they are not forgotten in particular.

[lowimpedance]

@gyro
 Just checked the linked thread, nice vintage pile there  .
Just had a quick look on that auction site but nothing came up. I assume they must be rather rare beasties then ?. As opposed to the common as mud 106x 108x types  .

[Gyro]

Thx again! Will take a while to go online, but well...

I measured some more of those blue tantalum caps and - well, many of them should be replaced, they have some k ohms on DC or even below. I think about replacing them with some good electrolytics, but not sure..  .. prob. with a small ceramic disc in parallel?
I have a bunch of smaller tantal caps, but none with 36V .. hmm..

You're welcome. If you wish, PM me an email address and I'll send it direct (it's only about 3.25MB), your choice though.

I noticed earlier that you mentioned testing the Tantalum capacitors "in circuit".  You won't get a proper leakage reading that way - there is too much other circuitry across the supply rail. Unsoldering them to test will probably kill them anyway so not really an alternative. I would replace the one that has smoked (obviously) but would be tempted to leave the others unless they run warm. The only Tantalums I have seen fail (over several meters) are the ones nearest to the voltage regulators, however your failure on the AC board is an exception.

Yes I would use higher voltage rated caps for any replacements, I think 25V is the next step up from 16V, there is plenty of space anyway. Modern low esr electrolytics should be fine too.



@lowimpedance: Yes, they're nicely built and still very useable meters (well I use mine day to day anyway), 10G input impedance and 0.01% on 4 1/2 digits is pretty much what you read on the display (+/- 1 digit), with a 10mV range they can be more accurate at 10's uV levels than longer scale meters. I'll do a basic teardown of mine (1041, 1045 (DCV), 1030A (RMS AC) if there's interest.

[Zeitkind]

You're welcome. If you wish, PM me an email address and I'll send it direct (it's only about 3.25MB), your choice though.

Shouldn't be a problem, but I can wait, no need to hurry.

I noticed earlier that you mentioned testing the Tantalum capacitors "in circuit".

That's quite easy: If I see >1M?, it should be fine. If I see anything below, it might be ok or might be bad - depending on the circuit. So I desoldered one pin of only those that showed a significant low resistance and tested again.

however your failure on the AC board is an exception.

The one that failed is directly connected to the 15V rail and is rated.. 16V. So well, if the rest is as tight as this one, I prob. replace them all. I remember voltage*3 for tantal, so.. 35V? Duh..

Modern low esr electrolytics should be fine too.

On the power PCB I don't see a problem, but I'm yet to understand the AC and DC input circuits. If there is any HF action going on, I prob. should add a 1nF ceramic disc?

[Gyro]

I've just noticed that the manuals are now available for download...

http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=06_Misc_Test_Equipment/Datron

In my own defense I should point out that I was working from a photocopy and don't have access to an A3 scanner (1041 Service manual). I did make sure that there is plenty of overlap on the schematic pages though.

The 1051 manual has much more detail than mine (another photocopy), so I might stand a better chance of getting my 1055 A/D board working again.

[Gyro]

@gyro
 Just checked the linked thread, nice vintage pile there  .
Just had a quick look on that auction site but nothing came up. I assume they must be rather rare beasties then ?. As opposed to the common as mud 106x 108x types  .

Yes, unfortunately they are getting a lot less common. They do come up on ebay occasionally for little money (compared to the 106x plus). There was a flurry of 1041Ms in the UK a while back, obviously some establishment clearing out. Worth keeping an eye out for though.

[Zeitkind]

Some progess...

I replaced all tantal caps. Some with decent eletrolytics, some with tantals with much higher rating (16V -> 35V oder 40V). Found another roasted one (C26), it is totally black colored (was as blue as the rest before) and had a little hole.. You can see it on the main board just above the two 1000µF electrolytics, right beneath a good blue one. Also a 16V rated that is connected to the 15V rail.
The unit powers up and I can measure DC voltage - but only up to 19.999mV. As soon as it gets to and above 20mV the displays is blank.  For example +14.68mV is shown as +1.4667V - see picture. Have not tried AC yet, simply because I have nothing that can give me max. 20mV AC..
I can trim to 0.0000 with shorted leads with the potentiometer on the DC board. All range switches have no effect at all, neither Auto nor the voltage ranges do anything. DC/AC switch seems to work and I can see some floating AC noise on the DC supply I used.

[Gyro]

Well that looks like significant progress. 

It's encouraging that you have mV readings (even if it shows up on the wrong range) and the ability to zero the meter, that means that the critical DC gain block is working and probably undamaged.

You've probably realised by now but your problem lies somewhere in the front portion of the DC Isolator board - the small area between the push buttons and the  six optoisolators which drive the gain switching fets and relay. The first thing to check is the ribbon cable between the two boards - it could be something as simple as a bent pin preventing the 5V DC supply reaching the range logic.

It should be a fairly simple task to tie down the fault with reference to the DC Isolator board schematic. Note that the logic in the range selection area is 74C series, not TTL (HC would probably substitute ok as long as there are no timing issues). Make sure the remote button is out as it will otherwise disable the logic (check the signal from the switch in case of bad contacts). Concentrate on manual ranging, once that works you'll probably find that autorange starts working too. The range logic also controls decimal point position on the display and mV/V annunciators (via that same ribbon cable), - another useful debug indication (as it is proving to be already).

[eblc1388]

As per Gyro's advice, please check that the logic levels on these ICs output pins shown below do change when you manually selects different range settings or DVM functions. I have had a working 74C157 gone bad for no apparant reason.

[Zeitkind]

You've probably realised by now but your problem lies somewhere in the front portion of the DC Isolator board - the small area between the push buttons and the  six optoisolators which drive the gain switching fets and relay. The first thing to check is the ribbon cable between the two boards - it could be something as simple as a bent pin preventing the 5V DC supply reaching the range logic.

Yeah, already poked around in this area. Cable seems fine, all resistors show the correct values - besides two, gonna check them later. Transistor and diodes checked, reseated all ICs and checked the voltages - 4.6-4.8V, a bit low. When I disconnect the DC input board, the voltage on the main board goes up to 4.9V. The 7805 seems OK though a bit rotten. Nothing gets hot, no obvious damaged parts.

It should be a fairly simple task to tie down the fault with reference to the DC Isolator board schematic. Note that the logic in the range selection area is 74C series, not TTL (HC would probably substitute ok as long as there are no timing issues). Make sure the remote button is out as it will otherwise disable the logic (check the signal from the switch in case of bad contacts). Concentrate on manual ranging, once that works you'll probably find that autorange starts working too. The range logic also controls decimal point position on the display and mV/V annunciators (via that same ribbon cable), - another useful debug indication (as it is proving to be already).

The last 3 wires (J4 14-16) transfer the data, next step will be hooking up my scope and check if I can see any signals and check the levels.
74C NOS are hard to get, prob. have some 74HCT.. hmm..

As per Gyro's advice, please check that the logic levels on these ICs output pins shown below do change when you manually selects different range settings or DVM functions. I have had a working 74C157 gone bad for no apparant reason.

Some kids help me repairing, so we will try to draw a decent table.. they need to learn something.. *g*

[Zeitkind]

Funny meter. Today it only measures in the 1V-Rang , bang on with my Sanwa. But not above 1.9999 - as yesterday within the 10mV range, range switches still off duty. Seems a little bit random on what range the meter starts - and stays.

Switching area around M1 und M2 seem to be OK - on the input side. At least we see H and L when switching, we see Auto on/off etc.
We can see the corresponding H and L on the input of the 74C193, but not on the output.
M2/74C193:
Pin 1,10,11(Auto),15 - H or L when switching
2,6 - 0.9V
3 - 0.1V
4,5,12,16 - all H
7,8,9,14 - all ground/low

The output is 0.9V on pin 2 and 6 and 0.1V on pin 3 no matter what is on the input. Oh well... getting complicated.. 

[Kleinstein]

The connection to pin 11 could be a problem -  so not loading new values to the C193.

The 74HC193 should be a suitable replacement. HCT might work too, but the input levels are slightly different.

With a rather week output of the 74C193 the 0.9 V level might be still a valid low with no real problem.

[Zeitkind]

Pin 11 is either high or low if Auto button is pushed or not, so it works as it should I guess. But pin 2 and 6 always stay at ~0.9V and pin3 is always low at >0.1V.
If I short pin 1 and 2 = pull output 2 high, a relais klicks and the display switches from 1.9032V to 0.0019V - prob. 1kV range or 100V range.

[Gyro]

It sounds as if it might be a faulty 74C193 (M2) then.  ...Although it would also be worth checking the outputs again with M7 (3 input NAND) removed in case it's inputs have failed (or its VCC missing) and loading down M2's outputs. Also check with M4 removed (2 input NAND).

Sorry to sound like a stuck record, but two things...

Firstly do leave Auto alone for now, in manual mode, then M2 is just operating in parallel input mode (pin 11, parallel load, low). rather than doing any counting - the outputs should just follow the inputs.

Secondly double check that the Remote signal to M1 pin 1 and M8 pin 1 (via D24). This selects between push button inputs and the remote control inputs (which would come in via J4, the unpopulated dip socket next to the ribbon if the option was fitted). There is a danger that you may be getting unpredictable data into M2 if the remote select signal is bad.

As I say, in manual mode, the outputs of M2 should be following it's inputs, as static signals. If this isn't happening then that's where you need to concentrate with a meter, continuity, shorts etc.

[Zeitkind]

It sounds as if it might be a faulty 74C193 (M2) then.  ...Although it would also be worth checking the outputs again with M7 (3 input NAND) removed in case it's inputs have failed (or its VCC missing) and loading down M2's outputs. Also check with M4 removed (2 input NAND).

I already poked around that area behind M2, checked the opto couplers and transistors and also removed both NAND gates (M4 & M7) that are connected to the output of M2. No change, the 3 outputs stay at those levels. Will get replacement HC chips in a few days. Those 74-series chips are really funny - you always have a big pile of them - but never ever the one you need.. ^^

Firstly do leave Auto alone for now, in manual mode, then M2 is just operating in parallel input mode (pin 11, parallel load, low). rather than doing any counting - the outputs should just follow the inputs.

We only tested if Auto, DC, AC etc. switches actually do anything. i.e. pull some input high or low and checked the results with the schematics.

Secondly double check that the Remote signal to M1 pin 1 and M8 pin 1 (via D24). This selects between push button inputs and the remote control inputs (which would come in via J4, the unpopulated dip socket next to the ribbon if the option was fitted). There is a danger that you may be getting unpredictable data into M2 if the remote select signal is bad.

Well, the outputs of M1 and M8 change state when pressing the range switches, so M2 gets various inputs - but does not reflect the signals on the output side, they stay low or undefined permanently. So I don't think a wrong remote signal is involved.
I don't see any signals with higher frequencies around that range select section anyway, it's fairly just a bunch of static (logic) switches (at least in maual mode).

As I say, in manual mode, the outputs of M2 should be following it's inputs, as static signals. If this isn't happening then that's where you need to concentrate with a meter, continuity, shorts etc.

Checked all that could be involved (or better: I let it get checked by the kids  , that's how we noticed that shorting pin 1&2 on M2 activates the relay..  ) This repair is a kind of extracurricular private activity project for some kids interested in electronics). We now wait for the spare parts and prob. build a little circuit in which we can check the 74C193 - or build some more cheap chinese DIY kits awhile.
The good thing on those older stuff is, that you can actually 1. repair them and 2. get the schematics and 3. at least have an idea how they work. Repairing an eg. iPhone or a Playstation is more or less.. just changing parts.

[Gyro]

An excellent learning opportunity for the kids  . A nice diversion from the single chip world of Arduinos etc. Having all the ICs socketed is a real bonus.

Hopefully the arrival of the new HC193 will bring a smile to their faces and yours! 

The end result will be worth the effort, a nice high impedance meter with lovely 4 1/2 digit display, excellent accuracy, stable 1uV resolution and responsive 10 rdgs/sec (see the timing diagrams at the end of the user manual). "Vintage" in all the right ways!

[Zeitkind]

An excellent learning opportunity for the kids  . A nice diversion from the single chip world of Arduinos etc.

Well, doing "old school" electronics is harder than when I was young. We used to grab old broken stuff from the garbage, took them apart and build other stuff out of the parts we found. My first stereo amp was totally made of parts I salvaged from old TVs and radios. Of course it was horrible. But the speakers I had were even worse..
Nowadays - when you open a device you prob. find a switching something for power and a second PCB with a single chip solution.

Hopefully the arrival of the new HC193 will bring a smile to their faces and yours! 

Kids need a feeling of success when doing such stuff, so I really hope we get it fixed...

[Zeitkind]

Step by step..

The replacement 74HC193 arrived and we changed the chip today. Improvements: Range switches work - kind of.
The output of 74HC193 is now as expected and the range switches cover 001 to 111 (ABC). But still the meter doesn't really work:

10mV range: 1,23mV is shown as .1230 V  ;wrong unit and wrong decimal point
100mV range: 12,34mV is shown as .0123V, sometimes as mV ; wrong unit and wrong decimal point
1V range: seems fine, 1.234V is shown as 1,2340V
10V range: 1,2V is shown as .0120V ;wrong decimal point
100V range: same as 10V ;wrong decimal point and prob. wrong range
1kV range: same as 1V-range! ; weird
In Auto mode, the ranges are shown as in manual mode, so Auto increases and decreases the ranges somehow, but in 10V it's going nuts turning the relay on and off.
http://www.mein-zeugs.de/pub/VID_Datron_1_288p.mp4
mp4/H.264 file, Firefox doesn't like them much, use Opera, Chrome, IE or Safari.

So I now have a working 1V-range and the other ranges show correct values but with wrong decimal point. Waiting for new NAND gates (M4 & M7), to lazy to test them somehow on a breadboard. We tested the old 74C193 on a breadboard (with some LED's and switches) and all 3 outputs are indeed dead, so prob. another chip down the output line is bad too.

[Gyro]

Doh!

Still, it's progress.

The Decimal point decoding is done by M6 (74C42), followed by the M5 inverter package and switching transistors. I suspect M6, but worth adding both to your shopping list. M6's outputs feed back into the autorange gating which could also explain the 'hunting' behavior. There are so few ICs in that section of the board that I'd probably look at getting replacements for all of them at this stage.

It's encouraging that the actual measurement function seems to be working though and apparently in spec.


P.S. Curious that it seems to be multiple ICs on that board - they are the only 74C series ICs in the unit, everything else is TTL. It sounds like there may be some dodgy aging mechanism with 74C.   I've re-used lots of older 4000 series CMOS ICs and never had a failure with those.

[eblc1388]

The integrity of IC M6(74C42) and M5(14069) can be easily verified on a breadboard.

74C42 is a BCD to Decimal decoder and M5 is an HEX inverter. Should be a fun and good exercise for your boys in understanding logic function.

[Zeitkind]

Was thinking about that. A little fun project over the next months is a LED lamp (or better a LED-wall..) with a bunch of 74HC595 and a sound input to control the "LED snake" crawling forward. No idea yet, but I prob. add some more IC's like 4011 and others to that lamp to get some "effects" or several ways to control the "LED snake". We will try to build one lamp for each, but that is not decided yet.
Chips like the 74595 are great for such projects and are easy to understand. Logic stuff like NAND and XOR.. well.. we'll see.. ^^

[Zeitkind]

Finally. We replaced the two 74C00, the 74C10 and the 74C42 today with new (NOS I guess..) HC series. Everything is working now , DC is in spec (9.999V and 2.501V on 10,000V and 2.500V) and AC is working too. Can't say if AC is in spec, I don't have any reference for AC here, but it's fine with my other meters, so I think it's also in spec. 
Not sure if both NAND gates are bad, I just distrust all of them and replaced all IC's on the DC board with new HC-versions. I'm a little bit disappointed that there is no Ohms-boards in this meter, I will look for one, but they are for sure hard to find.. Perhaps I will find a broken meter with a working Ohms-boards..
Many thanks to all!!

Repair history: Replaced AC mains filter, replaced all tantal caps with new tantals (with higher voltage rating) or electrolytics, replaced all 74Cxx on the DC board.

[Gyro]

Hey, congratulations to you and the kids. 

I think you're right, those 74C parts are only good for the bin! Useful to know that it does all work properly with HC.

You have a nice meter there, easy to maintain, accurate, and no worries about EEPROMS, backup batteries etc. Yes, it's a shame you don't have the Ohms board, especially as it does 4 wire guarded measurement.  There's little chance of finding one on its own, but you might want to look for a broken parts donor. You can also use one from a 1055 1051, it is electrically identical, just using closer tolerance parts. You could maybe try to build one from the schematic I suppose, but your time would probably be better spent making an accurate external current source instead.

The 10mV range is very nice for showing the kids thermocouple effects in the uV region and for use with low drop current shunts. Note that different meters can take different times to settle to 0uV after power up with the input shorted (the spec requires 2 hours warm-up to meet full performance).

There are a couple of quick mods that can make it much nicer to use.

1. The horrible blank display on overload:
This is easy to change. On the A/D board, connect a diode (ought to be schottky, but 1n4148 works fine) between M15 pin 8 (cathode) and M9 pin 6 (anode), no track cuts needed. The output of inverter M9 is open-collector so the mod doesn't cause contention. This will make the display show '+/- leading 1' on overload (the same behaviour as most DMMs).  An easy mod to do on the bottom side of the board.


2. 10 Reading per second display (this also makes autoranging performance very fast):
As shown on the timing diagram at the end of the instruction manual (Fig 5.4, Closed loop timing) it is possible for the meter to run closed-loop at its full speed rather than being internally triggered at 2 1/2 reading/sec. To do this:
/HOLD needs to be held low (to disable internal triggering)
/DELAY needs to be held low (this sets the conversion rate to match the Filter setting)
/PRINT COMMAND needs to be connected to SIGNAL INTEGRATE (to cause immediate start of a new conversion as soon as the previous one is finished).

All of these signals are available on the bottom side of the unused edge connector J4, where digital output option boards would fit. You can implement this on an offcut of 0.1" stripboard and carefully plugged in. Please double check these, but /HOLD is on 6L (Lower), /DELAY on 2L, /COMMAND on 5L, and SIG on 3L. GND is on 7L.

This mod can be a little temperamental on the 1041 (works every time on the 1045), it can sometimes freeze on startup (needing power cycling) or if the Hold button gets pushed. I suspect that there is a marginal timing in the logic somewhere, but if it doesn't work for you then you can simply unplug the stripboard (or investigate further, the presence and value of C49 seems partly responsible).

J4 is a very useful connector. It gives you access to all output data in parallel BCD form, together with range, polarity, status etc. It would be very easy to put a proto board in there (needs double sided access to the connector fingers) and with some logic and an Arduino or PIC, convert the data to serial or USB serial for data logging experements. If you also connect a header to J4 on the DC Isolator board (coincidence) you could also remote control the ranges, AC/DC, Filter etc. I keep meaning to do this myself.

I hope this helps. 

[eblc1388]

J4 is a very useful connector. It gives you access to all output data in parallel BCD form, together with range, polarity, status etc.

Yes, but I don't seem to be able to locate a pinout mapping for the connector J4 in the service manual.  Therefore I decided to roll my own, as in image attached.

Hope someone will find it useful.

[Gyro]

You're right, the only ways to decode the pinout of J4 (A/D board one) are to either pull out the sometimes cryptic signal connections from the schematic or work back from the 50 pin Amphenol description. I have a scrappy pencil marked-up o/p board schematic, but I've grabbed a copy of your nice neat one, Thanks.

BTW. I was playing around with the 10 rdgs/sec mod again yesterday, I suddenly realised that tying the /FP override signal (J4 L8) low disables the buttons in the 'Control' block and prevents accidental pressing of the Manual button (manual trigger not manual ranging) from locking it up. I still can't get it 100% reliable from power-up though (which it is on my 1045). Something fails to trigger the first conversion I think.

If an output board option is fitted, the 10 rdgs/sec option can all be implemented on a 50pin Amphenol plug of course and easily fitted/removed as required.


One other mod that might possibly be of interest... 'Autorange-up' happens at 100% overrange (19999 rdg.) but 'autorange-down' doesn't occur until 10% or range (1000 rdg.). For a 4 1/2 digit meter this is quite a loss of useful resolution. This behaviour can be changed by cutting the track between M31 pin 11 (down latch clk) and M20 pins 1+12, and then adding a wire to connect M31 pin 11 to M20 pin 9 instead. The track is accessible top-side next to M31, or alternatively pin 11 can be lifted from the socket.

With this mod there is no longer any hysteresis between ranges, so it may hunt if the input is hovering between 19.999 and 20.00 for instance but I haven't found this to be a problem in practice. On the rare occasion that it is, it can be overcome by going to manual ranging anyway.

[eblc1388]

'Autorange-up' happens at 100% overrange (19999 rdg.) but 'autorange-down' doesn't occur until 10% or range (1000 rdg.). For a 4 1/2 digit meter this is quite a loss of useful resolution.

You've lost me here about the 'loss of resolution'. When the meter autorange-down at 10%, isn't the signal now displays at 100% of the new range?

Does your proposed mod triggers the autorange-down operation at 20% instead? 

[Gyro]

Yes, it triggers down at 20% (rather than the original 10%). It makes more difference than you might think - in practice if you are measuring in the 10-20V range (eg. a 10V reference) you tend to get an extra digit on the display compared to the original autorange behavior (I think because the autorange normally counts down through the ranges). It makes a difference for me anyway. 

[eblc1388]

With this mod there is no longer any hysteresis between ranges, so it may hunt if the input is hovering between 19.999 and 20.00 for instance but I haven't found this to be a problem in practice.

Would you consider the following mod which would only trigger the autorange-down operation if the meter reading is below 1800 (18% of current range), instead of your 20%?

M31-11 pin is kept low, which allow auto range down to occur. If the count reaches 1800 or more, then the auto range down operation will be disabled. After the range changes down, the meter will display 17999 or less. This would add 2% hysteresis to the range change operation.

 

[Gyro]

Perfect! I searched around for a spare gate to try to implement something of the sort but didn't think of a discrete solution. Yes that looks like the optimum solution for maximising the number of digits displayed whilst allowing some hysteresis between (auto)ranges. Thanks. 

[Gyro]

@Zeitkind:

Looking back at your pictures, I noticed that your DC isolator board uses a temperature compensated and selected LM312 (under the black thermal shield) as used on the 1045, rather that the LM394/LF355 combination used on later 1041s.

There is little or no difference in performance between the two implementations but the LM312 variant was much harder to manufacture. It required multiple temperature cycles to determine the 'select-on-test' (FSV) resistor values. The zero-drift compensation procedure is over a page long and includes a couple of recursive loops! Very time consuming. 

Hopefully you will never have to touch this section but I have attached a photo of the equivalent 1045 schematic page for reference.

[Zeitkind]

1. The horrible blank display on overload:
 [...]

2. 10 Reading per second display (this also makes autoranging performance very fast):
[...]

These hacks sound interesting. Don't have much time atm, but will likely try it.
Accessing the data in binary form or controling the meter - hmm, not sure. I can attach eg. my Sanwa DMM to a PC and it's quite accurate, so well.. The meter will have its place in further electronics tinkering with kids, though we normally do not need 4,5 digits.. ^^ But who knows. One brought his dad's dead Metrawatt DMM, that was killed by an electric fence, another kid has a dead Sony amplifier, but we will first repair an old Sega Gamegear etc. etc.. Enough work.. 

[Zeitkind]

Hopefully you will never have to touch this section but I have attached a photo of the equivalent 1045 schematic page for reference.

When checking at the beginning, I already noticed that some parts may be hard to find if broken. So yes, I'm quite happy, that the meter is still in spec and the problem was "only" in the ranging TTL section. A complete recalibration ... *yikes*
But well, tbh, it's nice to have a working vintage 4,5 digit meter, but new modern DMM are fine for me too. I like to repair old stuff built of descrete parts, simply because you can indeed repair it and not just change boards. But if something is broken beyond reasonable efforts? I have eg. a working Tek 453A (but rarely use it now). If it will break some day - it will be dead. To complicated for me, to complicated to take apart, high voltage (8kV) I don't really like to work on etc. And you can get a used CRO for much less the spare parts will cost..

[Gyro]

1. The horrible blank display on overload:
 [...]

2. 10 Reading per second display (this also makes autoranging performance very fast):
[...]

These hacks sound interesting. Don't have much time atm, but will likely try it.
Accessing the data in binary form or controling the meter - hmm, not sure. I can attach eg. my Sanwa DMM to a PC and it's quite accurate, so well.. The meter will have its place in further electronics tinkering with kids, though we normally do not need 4,5 digits.. ^^ But who knows. One brought his dad's dead Metrawatt DMM, that was killed by an electric fence, another kid has a dead Sony amplifier, but we will first repair an old Sega Gamegear etc. etc.. Enough work.. 

In usage terms, the strengths of the Datron (apart from its beautiful display  ) are its 1uV resolution and >10Gohm input resistance. These can be used to advantage for educational experements, high impedance circuits, tracing shorts by voltage drop method etc. Most of the time a DMM is fine but there are times when you'll be really glad that you have it.