Author Topic: Datron 1081 Teardown and repair (Read 15422 times)

dacman · « **Reply #25 on:** April 07, 2019, 03:11:37 am »

Error 6 is usually caused by high resistance in the input path (or current path) which can be due to a relay contact or the 2-Wire/4-Wire switch.

MaxFrister · « **Reply #26 on:** April 07, 2019, 03:41:33 pm »

Because the 1081 has a sealed case, the interior is remarkably clean and free of dust. Despite that. there is some schmutz on the pcbs. I’ve been careful not to contaminate them further by wearing surgical gloves while working on it. (Wearing gloves and using forceps to repair the power supply, I felt like a real doctor.)

Anyway, I’m looking for advice on post-repair cleaning (or not cleaning) of sensitive pcbs.

The other issue is that the soldermask has become brittle so the cleaning should be as physically gentle as possible. The service manual mentions using IPA but also warns that even that can contaminate the boards.

splin · « **Reply #27 on:** April 07, 2019, 05:57:55 pm »

Is there any chance of getting a copy of the 1081 EPROM images? I have a 1081 that I want to install a GPIB board from a donor 1061 but the GPIB firmware is different for each model and has to match the rest. Just the GPIB ROM would be a good start as I could dissamble it and adjust any addresses referencing the main ROMS but it would be a lot easier with a matching set.

MaxFrister · « **Reply #28 on:** April 07, 2019, 09:40:51 pm »

I will eventually read the eproms for backup, and I can let you have copies at that time. In the meantime, I have memory dump of 2 other 1081s from another member that I can share with you. Just pm me an email address.

doktor pyta · « **Reply #29 on:** April 08, 2019, 07:02:37 am »

http://rfscientific.eu/memory-dump-directory

splin · « **Reply #30 on:** April 08, 2019, 01:17:50 pm »

Quote from: doktor pyta on April 08, 2019, 07:02:37 am

http://rfscientific.eu/memory-dump-directory

Thanks - I found those ROM images before but neither contains the GPIB ROM.

MaxFrister · « **Reply #31 on:** April 08, 2019, 01:52:12 pm »

I gave the meter exterior some attention. As usual, it was filthy. The display got brighter after cleaning both sides of the lens.

The top and bottom covers were scratched down to white fiberglass. I decided to repaint them. There are worse color matches than the paint shown in the photo.

maginnovision · « **Reply #32 on:** April 24, 2019, 07:00:30 pm »

Have any pictures all painted? Curious how it turned out. (I just bought a 1082 with some case issues)

MaxFrister · « **Reply #33 on:** April 25, 2019, 01:55:48 am »

Here's a pic of the machine temporarily put together.

I'm still working on the tant replacement and trying to track down a problem that only occurs when cold.

MaxFrister · « **Reply #34 on:** May 12, 2019, 05:31:07 pm »

The “last” problem with the 1081 was interesting. When cold, for a 1K resistor, it would read ~10M for the first 10 or 12 minutes. Then it would read ~700K for a few minutes, and then start to read correctly. I eventually traced it to a bad Fairchild F40014 inverter on the analog board that was part of the auto-ranging circuit.

While waiting for the obsolete IC to arrive, I finished replacing all the tants. For the digital board, I used an isolation transformer and lifted ground for the soldering and desoldering irons (I don’t own a battery operated one). I was able to complete the replacement without loosing the calibration constants. Hint for the next person: C33 is .68uF, not 68uF. As a 555 timing cap, the wrong one has the expected result.

For cleaning the analog board, I used MG flux cleaner and a soft brush. I then blotted the residue from the board with a kim wipe. Normally I would rinse with distilled H2O but I didn’t want to spread flux residue further. This seemed to work well and did not damage the fragile solder mask.

The next step is to see if I can improve the calibration without a full recal.

maginnovision · « **Reply #35 on:** May 12, 2019, 06:37:42 pm »

Glad to see you've got it all figured out. Hopefully the replacement inverter works well. Luckily the Cal ranges can all be done independently. Do you have a decent calibrator? I was very lucky with mine and the DC/AC calibration seem right on. DC matches my 4000 and both of my ltz1000 references. AC matches my fluke 5200 and Prema 5017sc. I do need to clean mine up still.

MaxFrister · « **Reply #36 on:** May 18, 2019, 08:03:28 pm »

Quote from: maginnovision on May 12, 2019, 06:37:42 pm

...Do you have a decent calibrator? ...

Unfortunately, or maybe fortunately, I’m only at the entrance of the metrology rat hole so I don’t have anything that I can use to calibrate the Datron 1081. All I can hope to achieve is to adjust it to correspond to low-grade and uncalibrated standards.

Status update:

1. After leaving the machine unpowered for several days, and the weather turning colder, it booted up in an odd state. Both DC and ohms were unrealistic (negative ohms?). After several minutes of this I turned it off and back on and it was fine. I have not been able to replicate this so far.

2. Because all the ranges are off in the same direction by roughly the same amount I thought I would attempt the “Standard” adjustment that adjusts for drift of the voltage reference. I bought what I had read was the correct calibration key, and it was not. I then tried shorting the key contact and I still can’t get it to shift into “cal” mode. I’ll have to start troubleshooting that one.

3. I’d like to run some long-term logging runs and see how it behaves with respect to drift. However, I’m leery of wearing out the display. On many meters, you can turn the display off via GPIB. I don’t see a way to do that on the 1081. I’m thinking of adding a physical switch to shut down the +175V power supply. There is room at the back of the machine for such a switch.

This is turning into a bit of a slog. Not a Tin-style epic slog, but a slog nevertheless.

maginnovision · « **Reply #37 on:** May 21, 2019, 02:21:57 am »

For cal mode turn all the GPIB switches to on, if it doesn't go into cal mode then.

I believe a switch for long term logging may be good, alternatively just pull the connector. If you hook up to GPIB it really shouldn't matter if you need to go in/out of it when you want the display back.

MaxFrister · « **Reply #38 on:** May 22, 2019, 12:46:23 am »

Quote from: maginnovision on May 21, 2019, 02:21:57 am

For cal mode turn all the GPIB switches to on...

Nice one! I never saw that in the manual. I spent a while tracing the "cal key" signal through the circuit (sort of fun, 74ls logic and all), but it seemed like it was fine. I was starting to wonder if it was a timing or software problem. Apparently it was a software problem with the user. Thanks for that!

e61_phil · « **Reply #39 on:** May 22, 2019, 08:28:56 am »

Quote from: MaxFrister on May 22, 2019, 12:46:23 am

Quote from: maginnovision on May 21, 2019, 02:21:57 am
For cal mode turn all the GPIB switches to on...

Nice one! I never saw that in the manual. I spent a while tracing the "cal key" signal through the circuit (sort of fun, 74ls logic and all), but it seemed like it was fine. I was starting to wonder if it was a timing or software problem. Apparently it was a software problem with the user. Thanks for that!

That tooks also a lot of my time a couple of years ago with my 1071

I thought I reported it here, but I didn't..

MaxFrister · « **Reply #40 on:** May 23, 2019, 03:29:40 am »

Here is the meter in its new home near it's cousin, a nixie version of the HP 5340A:

With GPIB disabled, the machine boots into a slightly different mode (not in hold), and allows manual calibration. I was able to change the "memory" settings and performed the "standard" adjustment. It helped get the dc voltage closer but still not very accurate. It did not appear to help ohms which reads consistently -0.3%.

It is a bit much to expect that a machine with unknown provenance, repairs to the power supply and re-cap'ing all boards would be in calibration.

Work plan includes:

1. Read the 1071 manual and try to figure out how to use the various filter, averaging, and math options.
2. Use it for a few weeks and make sure it is stable.
3. Perform what adjustments that I can using my limited equipment.

Unfortunately, my prom programmer (TL866) apparently does not support the eprom chips so I can't make backups of them.

maginnovision · « **Reply #41 on:** May 23, 2019, 03:59:11 am »

This is the most useful stuff.

MaxFrister · « **Reply #42 on:** June 13, 2019, 10:48:31 pm »

The 1081 has continued to operate nominally and I started setting up for some stability logging. I don’t have access to the 1081 user’s manual so I have to guess what the GPIB commands are based upon the similarity to the 1071. I found a couple of interesting things:

While I have not found a way to turn off the display for logging runs, issuing the K keyboard command has the side effect of blanking the display until the next trigger. The result is a brief flash upon read and the “rem” remote annuciator always lit. Almost good enough.

The manual seems to imply that turning on the low-pass filter should result in 7.5 digits but the only way I’ve found the actually get it to report 7.5 digits is to place it in high-res (averaging) mode. By the way, that mode is apparently “A1” on the 1081.

Since I don’t actually want it averaging for the logging runs, I’m going to experiment with block averaging and see if I can use a block size of 1 and still get 7.5 digits.

The graphs are from 3 short test runs. Don’t read much/anything into the data. Both meters and references were starting cold and the reference has not been aged.

The first run shows the Datron in low-res. The second and third and the Datron in high-res. It is interesting to note that the shape and even the noise are similar in these.

I’ve added some temperature logging and I’ll setup for a longer logging run.

maginnovision · « **Reply #43 on:** June 13, 2019, 11:29:42 pm »

A rolling average of 16 samples should be the default and seems to give good results for me so I would just use Filter and HiRes without changing anything. Once you enter a number it's a block average, or continuous for 0. I haven't had time to get back to mine to setup logging what are you using for that? I was going to just write my own program but I've had very little time lately.

splin · « **Reply #44 on:** June 14, 2019, 01:46:39 am »

The 1081 low res results show quantization steps of 10ppm which doesn't make much sense for a 6 1/2 digit meter.

[EDIT] What were the 34401A settings - AZ, NPLC?

MaxFrister · « **Reply #45 on:** June 14, 2019, 01:28:48 pm »

The data is collected by the worlds dumbest python script. I've used variations in the past. I can share it with you after I shake out a few bugs.

The "A1" mode may represent "continuous" averaging. More GPIB experimentation is necessary.

Indeed, I'm scaling the 10V range Y axis by 10E5 instead of 1E5.

As for the 34401A settings, it is using the default autozero and NPLC. A better question is what settings should I use?

After an hour or so of logging, I'm getting failures that I think are related to the calibration constants. It's a bit hard to tell since I'm turning the display off and the GPIB read only reports "!". Looks like I have more troubleshooting in my future.

MaxFrister · « **Reply #46 on:** June 15, 2019, 02:30:50 am »

This meter has periodically shown odd behavior related to the digital section. Yesterday it would run logging for 20-60 minutes and then stop with an error. With GPIB removed and the display restored it showed “fail” sporadically and the then continuously. The fail message indicates corrupt calibration constants.

With the calibration constants clearly lost, I took the opportunity to pull and clean all of the ics and sockets on the digital board. I then gave it a quick adjustment (“calibration”) and started the logging back up. So far, the logging has run for more than 8 hours without errors.

I’m hesitant to say that I fixed the digital problems. Every time I think I’m finished with this meter some new problem crops up. My plan is to continue testing and if everything looks good, restore the adjustment (“calibration”).

The graph shows stability relative to an HP 34401A measuring a LM399-based reference. The green dots are temperature. I changed the Datron logging to “A2” which I believe means hi-res with 16 sample averaging.

maginnovision · « **Reply #47 on:** June 15, 2019, 02:37:15 am »

You never replaced the battery?

MaxFrister · « **Reply #48 on:** June 15, 2019, 03:11:28 pm »

The battery has been replaced and the failures first showed up while under power when the battery should be mostly irrelevant.

I've also seen bad initial program loads. I'm guessing that the bus is (was?) noisy. Some the ics had visible tarnished pins. I'm hoping that cleaning fixed it.

I continued the logging run overnight and was just about the declare victory until I looked at the graph. That is one ugly graph.

maginnovision · « **Reply #49 on:** June 15, 2019, 05:52:23 pm »

Did you go through and check the DC board and power supplies using the internal adjustments procedures? That's pretty odd that you lost cal while running. After I repaired the analog board on my 1072, the day I got it, I had an issue with the display showing garbage about 60% of the time when starting up. After doing something similar with the IC's(moving them in sockets to help contacts) it got a little better. I don't think I stopped seeing the issue until after I had gone through and replaced the battery, the caps, and did all of the internal adjustments(I think specifically the reset was mis-adjusted on it). I really hope you can get all of this sorted.

Now that I have even more extra parts(but good) let me know if you find you need anything you couldn't find elsewhere. I've got 2 full spare units now(I took the display from one for my 1082, but even that display is fine it just wasn't really nice). When you were looking for a transformer I'd only had 1 spare and couldn't stand to lose it but I'm pretty well set for spares now.


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Author Topic: Datron 1081 Teardown and repair (Read 15422 times)

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