Yet another DATRON 1071 repair/resore, advice sought

[DC1MC]

Hello dear voltnuts, especially DATRON series 10X1 series owners, while waiting for people to decide getting the connectors for my Solartron 1061, I thought that it will feel alone and got a DATRON 1071 for a reasonable price (for Germany ), sold as: "Working, but sold as defect because but I've had enough replacing the tantalums, everytime I replace one, another one pops and errors appears...". If you search fleabay in German, you'll get plenty of them. Also, "it starts with an error, but if you let it run long enough it will eventually come..."

So the instrument comes (nicely packed, I have to admit) and of course the truth is out there  but not here, the instrument starts in hold mode, and produces a cornucopia of errors, from 0L to 7, I think I've seen them all. After reading whatever I could find (and it was a lot) I've disconnected all the boards on the bottom ( DCI, R and ACV ) in turn, all 3 of them were producing errors, finally, just with the DCV ( the upper side ADC board, near the digital board) the device started normally and allowed a zero to be made. So all boards on the bottom are screwed, one way or the other.

The replaced tantalums, BTW, were exactly ONE, on the ACV board, that I found it fully massacred, all the test points, cut and the device was delivered with the connector from it unplugged (also the retaining plastic bracket lost). Also some precision capacitor was touched with the soldering iron and whoever cretin tried to do repairs blocked a trimmer by touching it with the soldering iron. And this is some kind of old shop with a reputation 

I fucking HATE the equipment sellers that are trying to do these clumsy miserable repairs and running (semi)permanently some nice device, one moron there even peeled off the label of the "True RMS" specialized chip on the AC board that is really unobtanium, even if it actually has just 5 NPN bipolars, why would one do this, is beyond my human comprehension  . I pray that he didn't destroy it, but chances are that is gone, he seem to have cut the original tantalum and clumsy soldered some ordinary elco on the pins, frakking some other components in the process, it was probably with a soldering iron used for plumbing coper pipes.

The main DCV itself is not fully OK, if left with the inputs floating, it will slowly increase the voltage until some large values and marching for the dreaded Error 0L  .


Naja, I can send it back or I can try to repair it myself, even if my blood pressure will increase everytime I'm looking into these "Pfuscherei", so I have a couple of questions for the more experimented people:


 - Tantalum capacitors, is it worth replacing all of them on all the boards, even if they are looking OK, what would be some acceptable brands ?

 - All the ELCOS will go, the gang of 6 from the ADC board seem to be a main candidate, and the tired looking ones form the power supply as well, is it worth increasing a bit the values ?

 - I've seen in another thread some troubles regarding the rectifiers bridge, is it worth replacing them, they are in rather clumsy to reach position, but if it's recommeded, I'll do it ?

 - There is a lot of corrosion  on metal parts, ground contacts and such, I believe this must be cleaned to at lest have a faint hope of a low noise and sustainable calibration ?
 
 - Speaking of corrosion, the input terminals are connected with screws and nuts and they are really corroded and actually look like ordinary posts, if I'm to replace them, what do you recommend ?

 - The calibration key, does anybody knows where to get one, I don't want to do obvious kludge jobs ?

- Any advice and gotchas are extremely appreciated as it's my first restoration of a (relatively sick) precision instrument.

 Best regards,
 DC1MC



 

[Kleinstein]

If the tantalum caps are from a bad series, like those yellow drop shaped ones it might be better to replace them all. The nasty thing about some of those old tantalums is that they not only fail short but in some cases also "burn" quite vigorous and do quite some damage.

With a high impedance (e.g. > 10 G) input it is quite normal to see it slowly drifting all they way to the limits. If the drift is slow, this is a good sign, showing a low input bias. One way to check the input bias is to add a small cap (e.g. 1-10 nF) and look for the drift speed.

I would not have too much hope on lower noise from cleaner ground contacts. The 1061 does not look like very low noise anyway, but the calibration should be stable.

I would not increase the capacitance of the filter caps at the supply (maybe going to the next common value if odd sizes (like 350 µF) were used). Higher capacitance puts more load in the transformer and diodes.

[DC1MC]

If the tantalum caps are from a bad series, like those yellow drop shaped ones it might be better to replace them all. The nasty thing about some of those old tantalums is that they not only fail short but in some cases also "burn" quite vigorous and do quite some damage.

With a high impedance (e.g. > 10 G) input it is quite normal to see it slowly drifting all they way to the limits. If the drift is slow, this is a good sign, showing a low input bias. One way to check the input bias is to add a small cap (e.g. 1-10 nF) and look for the drift speed.

I would not have too much hope on lower noise from cleaner ground contacts. The 1061 does not look like very low noise anyway, but the calibration should be stable.

I would not increase the capacitance of the filter caps at the supply (maybe going to the next common value if odd sizes (like 350 µF) were used). Higher capacitance puts more load in the transformer and diodes.

Just in case, we're talking here about a DATROM 1071, maybe is a bit better than the 1061, the 1061 is a Solartron tat waits for enough people to decide for connectors.

 Cheers,
 DC1MC

P.S> A comatose/zombie tantalum, exhibit some measurable properties before burning, or is a nice behaved capacitor until it shorts and bursts in flames ?

Also, what brand for replacement ?

[Kleinstein]

The tantalum caps can behave nice until they brake down. However it usually takes some triggering event, like turn on after a long time off, over-voltage or very high current peaks to trigger a fault. For the caps to really ignite it likely takes a powerful (high current) supply - most caps just fail short without fireworks. So I would most worry abut the 5 V in the computer part. The analog board is more like low power and would hardly deliver more than 2 A.
Modern, major brand caps should be OK, as burning caps are not that popular anymore.

There are Datron Models 1061 and 1071, that are rather similar. Both seem to use essentially the same dual slope type converter with averaging for higher resolution. So the noise is not expected to be super low. Still there might be faults causing higher than normal noise, but that is not very likely.

The coarse adjustment of the voltage reference is done by cutting links - so some cur wires that might look like test-points might be normal.

[DC1MC]

Thanks @Kleinstein for the informative answers, I hope you'll come again when the calibration will start  , now my Datron 1071 is a bunch of PCBs spread around and a chassis.
I have managed to get rid of most of the stickers, except two, these are  's stickers, because they seem to resist to everything I've thrown of them from water and soap to turpentine, acetone, IPO, label remover based on citric acid and rust removal, I may need to attack them with a metal grinder  .

The first candidate for rejuventation was the power supply, here the two  1000uF/40V lost some like 40% of capacity and have a huge ESR, the 10uF/350V was long gone and dry, something 0.5uF and 2Kohm ESR !!, the tantalums were behaving strangely, sometimes showing huge capacity, some times close to 0 at repeated measurements  !?!?!, kind of strange zombies and the huge surprise, the 4700uF/16V showed 6000uF but sadly 1,2ohm ESR, I've got a 4700uF/25V Kemet that is measuring 4550uF with 0.01ohm ESR that will take its place.

For whoever may look into this topic and have a bit of experience with restoring the capacitors on these old instruments:

 1) I have a Tantalums BOM like N x10uF/16V, M x10uF/25V, T x10uF/35V, can I consolidate all of them to 10uF/35V, I understand that for the manufacturer it may have had some marginal cost reductions, but does it affect if I buy just the highest voltage at the same capacity, there is ample space around, any pro or contra opinions ? Those guy are decouplings, nothing special.

 2) Can I replace all those bloody tantalums with hi quality, low ESR modern aluminium caps ?


Finally a BIG question, that is bothering me for a long time: where the hell can I get some of this glue used to glue stuff on metal/glass/plastic on modern phones and devices, that never gets completely hard, become fluid while heated  and will help me glue back the front plate that was fixed with dual sided Scotch/3M tape, that is now either very expensive at that size or unobtanium ?

 Thanks and cheers,
 DC1MC