Datron 1071 shows Error OL

[dml]

Okaaaay... so I started swapping over some ICs near the section where the NE5534 failed, working outwards. However the unit would go dark on me as soon as I lay it face-up, where the ICs are accessible. I had to do the power-up tests standing it on edge (as seen in the last photo).

This got old pretty fast so I started looking for this intermittent fault instead. It seemed like maybe it was a relay contact having mechanical issues as it would tend to work at one angle, but not at other angles.

Pretty soon after starting this exercise - flipping the thing over and tapping it - the display went dark 100% of the time, at all angles.

At this point I gave the thing a decent bump on the floor and it sprang to life.

For the first time I didn't get an 'Error OL' but instead it immediately showed a fluctuating +/-0.00V reading. Hitting the TEST button yielded only PASS.

Yay! So both the digital and DC sections appear to work - at some basic level at least. But there is a mechanical fault somewhere that I still need to track down. I can maybe do this via the test points.

Not sure if I'll have the same luck with the other two boards but things are looking up

[DC1MC]

Started with the inputs short circuited and all the other boards disconnected, it's now the sockets shit and optos dead or decalibrated. I've wirebrushed all my chips and replaced all the optos.

 Anyway, we see a bit of progress here 

 Cheers,
 DC1MC

[dml]

Started with the inputs short circuited and all the other boards disconnected, it's now the sockets shit and optos dead or decalibrated. I've wirebrushed all my chips and replaced all the optos.

Did you see a change on replacing the optos? I'll try this soon but want to understand the schematic a bit better first for the sections that aren't working yet.

I'm still chasing some problems which didn't narrow down to anything straightforward. Most of the tantalums and electrolytics are replaced except on the display panel and the AC board.

I get an Error 7 from the AC board but the rest is now PASS including Ohms. The Ohms section actually seems to work more or less correctly.

Always Error OL + HOLD on power-up.

On testing, Volts section reports PASS but displays a random fluctuating reading in all ranges when used. Doesn't notice inputs. Not sure what's going on there yet.

So I have working Ohms, a PASS on Volts but with unstable readings and an Error 7 from the AC board. Still some kind of progress though. 

[DC1MC]

Started with the inputs short circuited and all the other boards disconnected, it's now the sockets shit and optos dead or decalibrated. I've wirebrushed all my chips and replaced all the optos.

Did you see a change on replacing the optos? I'll try this soon but want to understand the schematic a bit better first for the sections that aren't working yet.

I'm still chasing some problems which didn't narrow down to anything straightforward. Most of the tantalums and electrolytics are replaced except on the display panel and the AC board.

I get an Error 7 from the AC board but the rest is now PASS including Ohms. The Ohms section actually seems to work more or less correctly.

Always Error OL + HOLD on power-up.

On testing, Volts section reports PASS but displays a random fluctuating reading in all ranges when used. Doesn't notice inputs. Not sure what's going on there yet.

So I have working Ohms, a PASS on Volts but with unstable readings and an Error 7 from the AC board. Still some kind of progress though. 

Yeah, the lack of stability was what determined me to replace all the bloody optos, their current transfer ratio (CTR) decays over the time and this translates in the changing of digital signals delays, because the input signal slew rate is determined by the driving resistor and they were using "clever" techniques  to get some delays between digital signals depending of drive current and the specific characteristic of that individual opto.
Of course this means specially selected and paired optos (this why they are marked with those color dots) and when they decay the CTR is even stronger dependent of temperature and the fun starts.
Basically, if the analogue stuff is correctly set according to the calibration procedure, the ADC is good for 7 1/2 in all the 1061 series as well, the only difference is how it is driven by the digital board and that un-funny unobtanium POS from Ferranto.
One of my little projects is to decompile the software and fully replace the bloody digital part with a modern MCU board, the forrum post is around. It's a fun thing, but I don't see it getting too much traction, the 1061, 1061A and 1071 are becoming rare and not many people are willing to thinker with this stuff when they can buy an HP something, re-cap it and be done with it.
But I find it fascinating.

 Cheers,
 DC1MC

[Kleinstein]

Started with the inputs short circuited and all the other boards disconnected, it's now the sockets shit and optos dead or decalibrated. I've wirebrushed all my chips and replaced all the optos.

Did you see a change on replacing the optos? I'll try this soon but want to understand the schematic a bit better first for the sections that aren't working yet.

I'm still chasing some problems which didn't narrow down to anything straightforward. Most of the tantalums and electrolytics are replaced except on the display panel and the AC board.

I get an Error 7 from the AC board but the rest is now PASS including Ohms. The Ohms section actually seems to work more or less correctly.

Always Error OL + HOLD on power-up.

On testing, Volts section reports PASS but displays a random fluctuating reading in all ranges when used. Doesn't notice inputs. Not sure what's going on there yet.

So I have working Ohms, a PASS on Volts but with unstable readings and an Error 7 from the AC board. Still some kind of progress though. 

If Ohms work correct, but voltage ready odd (e.g. like an open input), this points towards trouble with a relay in the Ohms section.

[dml]

Yeah, the lack of stability was what determined me to replace all the bloody optos, their current transfer ratio (CTR) decays over the time and this translates in the changing of digital signals delays, because the input signal slew rate is determined by the driving resistor and they were using "clever" techniques  to get some delays between digital signals depending of drive current and the specific characteristic of that individual opto.

So it sounds like it's not just a case of replacing them with newer or faster ones - but matched to the original devices, which have decayed by now and the selection criteria isn't written down anywhere

When I was looking at replacement optos I was wondering if a different device e.g. MCT61/62 with higher CTR would cover the selection criteria - but sounds like it would just screw up timing even more.

I have two of these 1062 units - so if I could rig something to read out the CTR differences on each IC. That might show something about the selection criteria for each slot (or dot colour). Two samples isn't really enough to do it with any confidence but combining 2 units with the white/green/red dots (assuming all white dots mean roughly the same CTR) would maybe give enough info to get a clearer idea.

Of course this means specially selected and paired optos (this why they are marked with those color dots) and when they decay the CTR is even stronger dependent of temperature and the fun starts.

If it were just one or two optos it might be better to use a different device and adjust the resistors to get the right behaviour. But there are lots of optos in there - many dual devices.

Also, hand-selecting the 'right' values these days would probably be difficult - many components don't vary much within a production batch. I was hand-sifting some power transistors recently for a repair and they were all effectively identical.

Basically, if the analogue stuff is correctly set according to the calibration procedure, the ADC is good for 7 1/2 in all the 1061 series as well, the only difference is how it is driven by the digital board and that un-funny unobtanium POS from Ferranto.

Which one is that?

One of my little projects is to decompile the software and fully replace the bloody digital part with a modern MCU board, the forrum post is around. It's a fun thing, but I don't see it getting too much traction, the 1061, 1061A and 1071 are becoming rare and not many people are willing to thinker with this stuff when they can buy an HP something, re-cap it and be done with it.
But I find it fascinating.

I picked these up because I wanted something with good precision, not too expensive (and they weren't!) and with a lit display which I can read from a few feet away in crappy lighting conditions. So that means LEDs or a chunky VFD. No LCD stuff. I think these orange displays look much better and clearer than any of them. I was lucky to get two with low hours - one has some fogging behind the glass in places but is evenly bright. The other looks factory new.

I went for two because I understood the faults could be severe and swaps would improve chances of getting one to work. I think it should be possible to get both working now - getting quite close to it. Unfortunately the issues seem to be similar in both so 'swaps' haven't been as revealing as I was hoping.

Also I enjoy messing with 70s-80s thru-hole tech Parts which I can actually see & maintain without a microscope and don't lose in the carpet at a 50% success rate. So I don't mind if it takes me a while to bring them back to life.

[dml]

If Ohms work correct, but voltage ready odd (e.g. like an open input), this points towards trouble with a relay in the Ohms section.

I'm still trying to figure this one out - not had as much time on it recently as I hoped - but your suggestion sounds about right. Either a relay or a board connection fault somewhere.

At least one issue with internal test failures was resolved by re-flowing the board interconnection pins around the frame. I had noticed various squeaking sound effects on lifting and moving the board, either PCBs rubbing on each other or pins rubbing in joints, with any light twisting force. After doing this on one of the units, those sounds are gone and one of the internal tests began to pass for the first time.

So the ohms and volts tests pass on that unit now, but AC test fails. And volts still shows no correlation between the input, the range and displayed reading. Ohms readings look correct (although I tested it only briefly so far with a few values in a couple of ranges). All volts ranges show some fluctuating, different value. I'm wondering if half of this is due to someone trying to store calibration in the volts ranges with faults present between the boards and I'm seeing large offsets due to that...

Both units I have boot up with Error OL always - this goes away in ohms mode so long as a resistance is present across the inputs. I'll need to spend some time studying the overload detection circuits to understand whats going on here.

Anyway I'll investigate the relays next and see if that helps with volts mode.

[Kleinstein]

With the ohms option installed, the voltage input first goes to the ohms board and only from there to the terminals. So the fault is likely in the Ohms board, as the ohms reading (2 wire ?) seems to work.

A calibration error (e.g. lost memory) would result in a more or less constant reading. An open input (e.g. failing relay) would result in a highly drifting voltage reading, especially after switching from ohms to volts.

[dml]

With the ohms option installed, the voltage input first goes to the ohms board and only from there to the terminals. So the fault is likely in the Ohms board, as the ohms reading (2 wire ?) seems to work.

This was my understanding but it is very helpful to have it confirmed, thanks.

A calibration error (e.g. lost memory) would result in a more or less constant reading. An open input (e.g. failing relay) would result in a highly drifting voltage reading, especially after switching from ohms to volts.

Repeated power-on-off cycles caused the drifting voltage problem to disappear. Both units now read 0.00V with the inputs shorted. So a relay contact is probably involved. I will consider removing them and cleaning the contacts.


Both units read zero volts on AUTO range with inputs shorted. The first unit with all suspicious components replaced (except for the AC board) settles to 0.0000mV dead-on and remains there.

The second unit with only essential repairs (PSU + 6 electrolytics on analogue board) drifts around 0.00xxmV but doesn't settle well. Within 5 minutes the range is 0.0030mV +/-0.0020. After 30 minutes its more like 0.0030mV +/-0.0010.


The first unit has a fault in the AC circuit, showing 'Error 7'. It always boots with 'Error OL'. I have mostly ignore it until now, focusing on DC Volts first.

The second unit passes all internal tests but usually boots with a blank display, which sometimes recovers and switches into DC volts, or recovers with 'Error OL'. Other times it remains blank until power-cycled again. This could just be a power bypass issue one one of the digital boards or excessive ripple on one of the rails. The PSU does still need some attention as at least one of the caps is known to be dry.

Overall, looking good. Will see if I can get the remaining problems with the AC board and the erratic bootups resolved.

Figuring out the 'Error OL' problem might come last.

[Kleinstein]

One should be careful about cleaning the contacts. Some relays might have gold contacts with just a very thin layer that is easily damaged. Sometimes just some use is enough cleaning.

[dml]

One should be careful about cleaning the contacts. Some relays might have gold contacts with just a very thin layer that is easily damaged. Sometimes just some use is enough cleaning.

Then I'll stick with 'some use' for now and see if the problem recurs. I'll treat cleaning as a last resort. Maybe with alcohol on a lens cloth between the contacts.

[dml]

So the latest on this is both units now report PASS for DC,Ohms,AC/RMS.

I didn't do anything particularly relevant - was tracing differences around the AC board, found some small differences in one circuit near test points 7 & 9 - but nothing that led to a real problem. By the time I was finished with that, the 'Error 7' had disappeared. It hasn't returned.

So it may have been caused by another relay contact and fixed by a lot of power cycling and turning the unit over.

It has become clear that some of the self-test errors in these units can come from relay contacts or maybe solder joints if the thing has been sitting on a shelf for years, or had a rough journey with a courier. I would expect that from time to time but both of these units had at least two cases each of problems magically going away after some use.

Both units still boot up with Error OL, which doesn't return while using it or running TEST, even if set back to the default range/mode manually. The Error OL can be cleared by simply pressing HOLD.

No progress on understanding that one yet but will be looking into it now.

[dml]

A quick update on the 'Error 7' - it returned after being powered off for a day. It cleared after being powered on for 15 mins or so.

Looks like it is temperature related and affecting this AC board only (swapping the boards also clears the error when still cold). The measurement error in all modes is much higher when the unit is cold but settles to very low error after 30 minutes. Since the error seems to be in the AC board, it's probably not being caused by optocouplers. In any case I have seen the 'Error 7' from this unit both before and after these were replaced.

I hadn't noticed the temperature influence before because it had not been left running long enough for the error to clear until other repairs had been done.


I'd be interested to know if it's typical for these units to fail TEST early in the warmup period - or if TEST should always pass, from cold.

There are still some other startup stability issues, sometimes causing the display to remain dark until the UI buttons are used, as if the software is already running but the display wasn't successfully enabled during startup. Will see if I can find out more about this one.

[dml]

In the process of an experiment described here:

https://www.eevblog.com/forum/repair/datron-1062-dmm/msg1812974/#msg1812974

...I stumbled into the apparent cause of the 'Error OL', without doing any thinking. The real cause was found somewhere between the seat and the steering wheel.

Disconnecting the IEEE board from the MPU board allows the unit to boot without the error. Reconnecting the IEEE board causes the error to return.

Bearing in mind the fact both DMMs show this error - meaning both IEEE boards are behaving the same way - I figured this must be something really basic.

I looked up the function of the DIP switches hanging off the back of the unit, linked to the IEEE board. These appear to configure the operating mode for a DMM as a standalone bench meter vs an automated systems unit. My dip switches were configured in systems mode, with a valid address. I changed this to 'talker only' and 'no address' (all switches = 1) and rebooted the unit. 'Error OL' has gone away.

Hopefully this helps someone else who picked up a Datron 10xx recently, carried out the obvious repairs and still saw this confusing error... RTFM!