Datron 1081 Teardown and repair

[MaxFrister]

I recently acquired a Datron 1081.  The Datron 1081 is a 7½ digit meter and is similar to the Datron 1071 but higher specs. 

Basic 24 hour specs:
                   1071                                  1081
10VDC        +/-2ppm +/- 4 digits         +/-2ppm +/- 1 digit
10VAC        +/-0.04% +/- 40 digits      +/-0.01% +/- 0.005%
10k            +/-5ppm +/- 2 digits         +/-3ppm +/- 1 digit

The 1071 was designed about 1975, the 1081 about 1983.   The 1081 achieves its resolution using a network of 4 selected temperature compensated zener diodes.

My unit was advertised as broken and no display.  Unfortunately, for once, the advertisement was right.  Preliminary checks showed the wrong fuse installed.  After installing the right fuse, the -175V rail was missing.  Further checks showed the transformer damaged.  The previous owner apparently used the transformer as a fuse.  There was also serious corrosion on one of the voltage regulators (a common problem, perhaps related to the wrong flux).

I’m waiting for caps and other parts to arrive to do some basic repairs to the power supply before I continue testing.  In the meantime, I thought I’d post a few pics.

Basic machine



The magic, 4 zeners


Corrosion on TO3 package wires



p.s.,
As I mentioned, I’m probably in the market for a new transformer.  If you happen to have a parts machine and you’re willing to part with it send me a pm (I believe that the 1061,1062, 1065, 1071, 1072, 1081 and 1082 all use the same transformers).

[maginnovision]

I've got 4 datrons and none of them have issues with the 5V reg or wiring, seems like I got lucky. After a bit of cleaning it'll look nice, really sad about the transformer though. I saw that in another unit and passed on it for that alone.

[Johnny10]

Are you in US?

[branadic]

Missing display? I'm working on a replacement solution.
https://www.eevblog.com/forum/repair/datron-4000a-getting-at-test-points/msg2295153/#msg2295153

-branadic-

[DC1MC]

That's not looking like a damaged display (except the broken ones with compromised vacuum, all Datron displays turn on a bit if powered correctly), it looks like a display without power !!!

After you're done with restoring the powers supply fully and getting rid of the 169% dead elcos you may have a nice surprise  .

 Cheers,
 DC1MC

[maginnovision]

That's not looking like a damaged display (except the broken ones with compromised vacuum, all Datron displays turn on a bit if powered correctly), it looks like a display without power !!!

After you're done with restoring the powers supply fully and getting rid of the 169% dead elcos you may have a nice surprise  .

 Cheers,
 DC1MC

Or a reason to sell to me! 

[feedback.loop]

How about rewinding the transformer? It is doable.

[maginnovision]

If it really is the transformer and not the bridge rectifiers I guess you could rewind it but I'd probably just buy another unit for the transformer and keep the rest for parts(relays, adc, display, buttons, etc...)

[MaxFrister]

I spent a long time last night replacing the corroded wires to the LM309K regulator and cleaning up the acid damage.  The design of the power supply does not leave much access for rework.

Thanks for all the good suggestions regarding the transformer.  I may have a lead on a replacement.  Anyway, that problem is in the future.  The current workflow is:

• Wait impatiently for the caps to arrive in the mail
• Install replacement electrolytics (none of the tants are obviously shorted)
• Try to see if basic machine works via GPIB
• Try to see if display is working using an external 200V source

[maginnovision]

Actually once you disconnected all the leads and pull the back away it's pretty clear.

[MaxFrister]

In your picture you can almost see the LM309k leads.  There might be a little more to it....

There are, of course, other ways to approach it.  In my case, replacing the leads for the LM309k required:

• Disconnect and remove the back panel
• Disconnect and remove GPIB board
• Disconnect and remove the Digital board
• Desolder the LM309k leads and transistor leads from the back pcb
• Unscrew the heatsink
• Drill-out the LM309K mounting screw!
• Clean and make new leads
• Attach regulator and transistor to heatsink
• Attach heatsink while trying to allign 6 leads with hidden holes in the back pcb
• Resolder and reassemble

If I had to do it again I would cut the plastic fastener on the transistor and leave the transistor soldered in place, but that will make reattaching it to the heatsink a challenge.

[maginnovision]

Taking the back panel off it took me about 10 minutes to R&R the 309 including replacing the plastic fastener that broke. Add 5 minutes to remove the back panel, gpib board, digital board. Not too bad really. I think they look a lot worse to work on than they are.

Once you get it all working it should be pretty nice.

[DC1MC]

I spent a long time last night replacing the corroded wires to the LM309K regulator and cleaning up the acid damage.  The design of the power supply does not leave much access for rework.

Thanks for all the good suggestions regarding the transformer.  I may have a lead on a replacement.  Anyway, that problem is in the future.  The current workflow is:

• Wait impatiently for the caps to arrive in the mail
• Install replacement electrolytics (none of the tants are obviously shorted)
• Try to see if basic machine works via GPIB
• Try to see if display is working using an external 200V source

The tants are really  tools, it will even look good in a LCR meter and then suddenly, under working voltage, will go directly full-short, no warning signs, nothing 

[MaxFrister]

I have lots of progress to report:

• I replaced the electrolytic caps in the power supply
• The power supply is working except for -175V
• I was able to connect and test the machine via GPIB
• The analog board is mostly working with an occasional Error 5 when cold
• The resistance board fails with Error 6
• The AC board has not yet been tested
• The cal constants are still there

Most importantly, the display worked when driven from an external power supply.  Thanks to a forum member, I also have a new transformer on the way.

I've attached a few more pics of the analog board showing the board over-all, another view of the zener reference, and some precision resistors.

[DC1MC]

Yes, I knew that was not a dead display  , congrats for almost 100% revival !!!

Btw, for the resistance test make sure that the device is switched to 2-wire resistance measurement and nothing is connected on the measuring terminals.

Good luck with the new trafo and keep us informed.

 Cheers,
 DC1MC

[maginnovision]

Only a few more issues to figure out then, pretty good. I actually just bought two more 1062 for the option 12 AC boards but if you've got a transformer coming now I guess I don't need to offer one. Can't wait to see it all better.

[MaxFrister]

The new transformer arrived  .

[DC1MC]

This looks like a perfect display  .

 Cheers,
 DC1MC

[maginnovision]

Not too bad. Pass self test now?

[MaxFrister]

Alas it still fails the resistance selftest. The ac board is still out of the machine and untested.  Tantalums are on order for the ohms board.

I went through the manual last night and prepared an order for the remaining tantalums.  I don't normally do preventative recapping but there seem to be a lot of reports of popping tants in Datron machines.   Unfortunately, replacement will be tedious; there are 7.5-digits of tants (actually ~75).   

I'm curious about the failure mechanism.  At least on the 1081, most of the tants are conservatively rated (35V on 15V).  They did use 6.3V on 5V rails and on the ohms board at least a couple rated 16V for 13V.  I wonder if excessive ripple on the failing electrolytics is damaging them...  Then again the original brand "Union carbide" does not install confidence.

For those considering defering replacement, it appears that "high voltage" leaded tants are becoming rare.  The 68uF/35v has already gone extinct at Mouser, Digikey, and Allied.

[maginnovision]

I replaced all tantalums, 250/400V film caps and electrolytics on all of my datrons(5 * 1062, 1 * 1072, 1 * 4000A) and don't regret it. Only the 1 and .1 are 35 V rated, the higher the capacitance the lower the voltage. I replaced the tantalums with all 35V. I also replaced the bridge rectifiers with 2A rated parts. The film caps I used direct replacements. Cost was about 110$ per meter, the 4000 was about $400. It really doesn't seem like they were appropriately derating as AVX would now recommend so I basically followed their advice.

Once you do that hopefully it's all good. If not you could attempt calibration of whatever boards are failing, at least the hardware Cal even if not fixing the software constants.

[Kleinstein]

Alas it still fails the resistance selftest. The ac board is still out of the machine and untested.  Tantalums are on order for the ohms board.

I went through the manual last night and prepared an order for the remaining tantalums.  I don't normally do preventative recapping but there seem to be a lot of reports of popping tants in Datron machines.   Unfortunately, replacement will be tedious; there are 7.5-digits of tants (actually ~75).   

I'm curious about the failure mechanism.  At least on the 1081, most of the tants are conservatively rated (35V on 15V).  They did use 6.3V on 5V rails and on the ohms board at least a couple rated 16V for 13V.  I wonder if excessive ripple on the failing electrolytics is damaging them...  Then again the original brand "Union carbide" does not install confidence.

For those considering defering replacement, it appears that "high voltage" leaded tants are becoming rare.  The 68uF/35v has already gone extinct at Mouser, Digikey, and Allied.

The failure mechanism with tantalum caps is usually small crack in the dielectric layer that under good conditions would repair with not much degradation. The power on after longer storage can be more critical.  However if too much it can lead to local short and possible a thermal runaway up to fireworks if the power source is too powerful.  So the good failure mode is a short, the bad one is a small fire that might burn through the board.  In many cases one could use a modern low ESR Al based electrolytic cap as well - these were not available in the early 80s.

[Johnny10]

I have replaced all the tantalums on both my Datron 4000 units and 1062's I also use a variac set to 115 Volts.

Have not had any problems since.
Kleinstein is right about units that sit unused being more susceptible to burned out tantalums on turn-on.

Ah! the smell of burning tantalums in the morning.

I know using the variac shouldn't make a difference but it is my experience that certain voltages are excessive at 125V Mains power.

[MaxFrister]

The ohms board is working.  I wish I could claim brilliant trouble-shooting but the reality is that after a couple of hours of proding, probing, cleaning, warming and throwing switches it just started to work.

I installed the ac board and, after sufficient warmup, it too passes self-test.

I'm waiting on two orders of tantalum caps and will install them once they arrive.

It appears the calibration is bad.  This may be as simple as as needing a software adjustment for the reference aging or a serious problem as the equipment required for a full cal is extensive.  I'll assess following the cap replacement.

The reference voltage is slightly out of spec according the the 1071 service manual, but there is no spec that I can find in the 1081 manual.

One disturbing finding is that the service manual has a different version of the digital board.

[maginnovision]

The ohms board is working.  I wish I could claim brilliant trouble-shooting but the reality is that after a couple of hours of proding, probing, cleaning, warming and throwing switches it just started to work.

I installed the ac board and, after sufficient warmup, it too passes self-test.

I'm waiting on two orders of tantalum caps and will install them once they arrive.

It appears the calibration is bad.  This may be as simple as as needing a software adjustment for the reference aging or a serious problem as the equipment required for a full cal is extensive.  I'll assess following the cap replacement.

The reference voltage is slightly out of spec according the the 1071 service manual, but there is no spec that I can find in the 1081 manual.

One disturbing finding is that the service manual has a different version of the digital board.

When you do the tantalums take care not to use an earthed iron on the digital board unless you want to calibrate. Of course when/if you do the battery make sure the unit is on. Whatever the reason it's good that it works.

For the calibration it's not necessarily all that difficult. It's virtually the same as 106x and 107x except you can't coarse adjust the reference you can only manage linearity from the looks of it. The real issue is without a very good calibrator there isn't much you can do with the 1081 that makes sense. The ohms is easy to adjust and the AC is not easy to adjust, I bought a 5200A and am having custom transformers made to manage that bit.