Repaired a Datron 4708 - but need an LTZ ref - anyone have a spare? - Page 1
Forum section: electronics / general

author: TheSteve
author_rank: Unknown
timestamp: May 02, 2019, 05:23:10 am
content: I recently purchased a broken D4708 with all options. It arrived in good condition but I sadly discovered someone had swapped the LTZ ref for the 4000 diode ref.I have since repaired the 4708 and it is working well but I'd really like the proper LTZ1000 ref - does anyone have one they would consider selling?I only need the ref module itself, the divider board is the proper 4708 model.For reference here are the parts that my D4708 required changing:All 10 uF 25 volt tantalums - several where shorted on the + and - 15 volts rails on several boards. They must all be changed. Replace with 10 uF 35 volt(minimum).D29 (ZN458) on the power amplifier board - this is a precision 2.45 volt zener used as part of the + and - 15 volt supply rail check, it was shorted, discontinued but available on ebay.C26 on the power amplifier board - 100 uF 40 volt electrolytic, it had leaked, thankfully it wasn't acidic, no PCB damage.Q1 on the in guard supply - this is a TIP141 transistor operating as a pre-regulator for the + 15 volt rail, it was shorted C to E.M1 on the in guard supply - this is a 7815 regulator that had been tortured when Q1 shorted.That was it to get it running. While I had it on the bench I also replaced nearly every electrolytic in the power supplies and all of the 22uF and 33uF 25 volt tantalums. They all test fine but are 30 years old. Hopefully it is good for another 30 years.As you can see it is badly in need of a calibration as the swapped ref isn't even close to correct. For now I'll see how the diode ref behaves but am sad about the missing LTZ, paypal is ready, anyone?

author: hwj-d
author_rank: Frequent Contributor
timestamp: May 02, 2019, 06:58:34 am
content: Why not buy one from Digikey? Cellularmitosis made a board, but don´t know, it´s running properly.https://oshpark.com/shared_projects/jP5xkfJyCan't you use one of his boards there?

author: TheSteve
author_rank: Unknown
timestamp: May 02, 2019, 07:21:14 am
content: The ref board from the Datron has an LTZ1000 at its core but it boosts the voltage to 21 or so volts. The 3458A ref isn't a drop in replacement.And yes, we have the Datron LTZ1000 ref schematic but could never build one exactly the same as they used custom ordered Vishay resistors.It is still something I may consider with different resistors but at this point I'd rather purchase a known good one with known specifications.

author: maginnovision
author_rank: Super Contributor
timestamp: May 02, 2019, 07:21:36 am
content: Is it not stable? It may just need a calibration. Is every output off?

author: TheSteve
author_rank: Unknown
timestamp: May 02, 2019, 07:27:14 am
content: Quote from: maginnovision on May 02, 2019, 07:21:36 amIt does appear stable, and does just need a calibration. To meet proper 4708 specifications though it would need the LTZ1000 based reference.However the 8 diode based reference is actually very impressive for what it is.I will perform a calibration once I have run it a little longer.

author: maginnovision
author_rank: Super Contributor
timestamp: May 02, 2019, 07:50:46 am
content: You could check it and see how it does in the mean time. My new to me datron 1082 shows <1ppm error(typically zero so far) on my LTZ refs compared to an in cal 3458 once it's warmed up. It only uses a 4 diode reference. Of course it doesn't try to display the extra digit of the 3458. Did you get a decent price on the 4708?

author: Johnny10
author_rank: Frequent Contributor
timestamp: May 02, 2019, 01:21:47 pm
content: How did they switch out the Reference.Same voltage/location?I know there are similarities between the two units 4000 and 4708.So does the 4708 use same reference PC board?

author: TheSteve
author_rank: Unknown
timestamp: May 02, 2019, 04:39:57 pm
content: The reference board plug into the divider board. They can be easily swapped.Attached is a picture of my diode ref and a picture of the LTZ ref (courtesy of user lowimpedance).

author: branadic
author_rank: Super Contributor
timestamp: May 02, 2019, 04:56:01 pm
content: Are you sure the reference boards has been swapped? Maybe some units were sold with the 4000er reference boards, since they already had them? Wouldn't surprise me if they used what they had in stock for the first models, before they went for LTZ reference boards.You could reproduce the board if you were able to get the Vishay resistor networks. If the reference boards were interchangeable, can a Datron 4000 be upgraded for lower reference noise?-branadic-

author: TheSteve
author_rank: Unknown
timestamp: May 02, 2019, 05:03:22 pm
content: I am sure you could put the lower noise ref in a 4000 but I don't think you'd end up with full 4708 specifications as the divider board is different. My 4708 is from 1990, Datron had the LTZ ref well before that. I was able to get some money back from the seller because of the swapped reference. I believe they spoke to the source they acquired it from and confirmed it had been swapped.btw I have no doubt the diode ref will perform very well, especially being more then 30 years old.I will log its data to see exactly how it performs.It is hard not to want another toasty LTZ ref in the lab though.

author: branadic
author_rank: Super Contributor
timestamp: May 02, 2019, 05:13:12 pm
content: Same reference board is used in Datron 4808...Source:https://xdevs.com/fix/w4808/-branadic-

author: TheSteve
author_rank: Unknown
timestamp: May 02, 2019, 05:25:14 pm
content: Yes, the 4708 and 4808 are 95% the same internally. Basically it just got a facelift.

author: branadic
author_rank: Super Contributor
timestamp: May 02, 2019, 05:25:38 pm
content: QuoteWell, I'm not sure that they really swapped the reference. Found an ebay auction selling a Datron 4708 Reference Divider 40652 And Reference Assembly 400452https://www.ebay.com/itm/Datron-4708-Reference-Divider-40652-And-Reference-Assembly-400452-Tested-/263965491332-branadic-

author: TheSteve
author_rank: Unknown
timestamp: May 02, 2019, 05:28:38 pm
content: I believe that is really from a 4700, not the 4708. Datron has a lot of different options and not a ton of documentation at this point.We do have the service guide from the 4708 and it clearly shows only a single reference - the LTZ1000 based one. Also the specifications of the 4708 pretty much require an LTZ1000 to be true.

author: guenthert
author_rank: Frequent Contributor
timestamp: May 04, 2019, 05:40:39 pm
content: Quote from: TheSteve on May 02, 2019, 05:28:38 pmCould you elaborate on that?The 1N829 (?) based reference of the Datron 4000 uses selected parts and gets through great length to minimizes temperature drift. With the comparatively high current of ~7.5mA their noise should be low and at this age, their drift should be very low too. I wonder, whether a LTZ1000 truly performs better or is just easier (and hence cheaper) to use, as it doesn't involve the time consuming aging and selection process.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 04, 2019, 05:48:39 pm
content: With several 1N829 there is also the option to use a series connection to get to the 21 V or at least close. A single LTZ1000 based reference would need scaling and would thus be less stable than the pure LTZ. It could be more limited by the resistors.Well aged and selected 1N829 can be quite good as well.

author: Johnny10
author_rank: Frequent Contributor
timestamp: May 04, 2019, 05:57:16 pm
content: TheSteveI am always jealous of your equipment!

author: TheSteve
author_rank: Unknown
timestamp: May 04, 2019, 11:32:52 pm
content: Made myself a calibration key last night with some help from Samogon and Nikonoid.Samogon used a flatbed scanner to scan a 4708 key. I printed it at 100% size, then trimmed it out and placed it over the blank. I used a black marker to color where material needed to be removed. 5 minutes with some small files and it worked perfect first shot.The lock is from Yale. The exact keycode should be DC800 if you can get one cut by number. The blank is a type Y11, also called a type "80" at HomeDepot. The lock is also used in a filing cabinets and small lock boxes etc.I have attached the pdf Samogon provided for anyone else who wants to spend a few minutes with a file.

author: maginnovision
author_rank: Super Contributor
timestamp: May 05, 2019, 01:04:03 am
content: I've just been replacing all my Datron locks.

author: TheSteve
author_rank: Unknown
timestamp: May 05, 2019, 02:09:56 am
content: Quote from: maginnovision on May 05, 2019, 01:04:03 amThat's no funI want mine all original, and making the key was awesome.

author: branadic
author_rank: Super Contributor
timestamp: May 05, 2019, 08:02:50 am
content: My Datron 4000A came with four keys DC751, so if someone is in need to have a key, just write me.-branadic-

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: May 05, 2019, 03:17:28 pm
content: Quote from: TheSteve on May 02, 2019, 07:21:14 amThe custom P/N should be available from Vishay Precision Group. Just ask for a quote. It will take some time, and it will cost you some money (a LOT of money-- I'm guessing between $100 and $200), but they will send you just one!AFAIK, John Pickering was not happy with VPG not being able to match resistors (inside the custom network) as well as they claimed. You may be better off with one of the designs on this forum-- it will be a "mechanical hack" to put it in there, but I think it will perform quite well, IMHO.An alternative would be to make a (mechanically) "drop-in" replacement out of modern components (FRSM resistors from VPG for the "critical" resistors, etc.), a ceramic packaged LT1013 (now available from TI), etc.