Teardown: Valhalla 2707A

[Jorn]

Sometimes you have to take a chance. Such as the other day where a Valhalla 2707A programmable precision dc voltage standard was for sale on eBay. Apparently a model similar to the 2701C but no information is to find on the entire Internet about the 2707A. No specifications, no YouTube videos, no nothing at all. Should I bid $500 or not? 

You have probably already guessed the outcome. I won (with no competing bids) the Valhalla 2707A which is the subject of this mini teardown. 500$ plus another 300$ tax and shipping. Did I overlook something? Suggestions whether this is a bargain or not is welcome.

After nearly two weeks the unit arrived from the US and frankly it's stupid not to turn it on before you take it apart. So I did. The unit turned on without funny sounds and odors and a short test revealed that the unit was completely operational.

This is the unit. Besides the model number it looks exactly like the 2701C which is still being sold.

All voltage ranges work, and compared with my Keysight 34470A the 2707A agrees within 21 ppm. Excellent.  However, no "120 mA current source" was installed. What a pity. 
Now turn it off and take it apart.

[Jorn]

Now teardown time.



The picture shows the main board and it appears to be in excellent condition. Judging by the date codes on the components, it's from around 1999 or a few years later. The unit is clean and in good condition. No questionable repairs.

After the initial euphoria of the unit being in complete and in decent condition the search for the LM199/399 reference began. Where the heck is it?  Checking Master Shahriar's tear down of the 2701C points me in the right direction.

(https://www.eevblog.com/forum/testgear/teardown-repair-calibration-of-a-valhalla-2701c-programmable-precision-source/msg840669/#msg840669)




But - no reference is installed! This can't be true. The unit outputs voltage close to specification!  Zooming into the PCB where the reference is supposing to be, reveals wire leads that is soldered from the bottom of the PCB. What’s going on?

Turning the unit upside down provides the answer.



An aluminum box probably enclosing a voltage reference mounted beneath the main board. Very exciting. Kind of a Chinese box.  Details on the following pictures.








Let´s crack it open as Dave would have said. 

But you will have to wait until the next post.

[Jorn]

Ready?  Aluminum box opened here



Isolation removed on the next picture:



Now let's remove the three screws and flip the PCB around:



What a voltage reference. Two identical LTZ1000 (non A versions) on one PCB, loaded with Vishay precision resistors and an Analog Devices AD708B dual precision amplifier.  All thermally isolated from ambient with polystyrene (or similar) plates. Note the date codes on the LTZ1000.

PCB is marked Alphanetics Inc. An engineering company still around. If anybody know this company, please let me know.

Now it's reverse engineering time. But you will have to wait until the next post.

[AF6LJ]

This is too cool.
so someone fill me in; why two LTZ100s?

[Jorn]

Here follows a crude schematic of the reference.



Two identical references whose outputs are averaged to reduce noise. Each LTZ1000 reference section is built almost exactly as prescribed in the LTZ1000 data sheet. Both LTZ1000’s are properly raised a millimeter above the PCB. Nice attention to details!

70K resistors are made up of 50K in series with 20K, both 0.01% Vishay resistors. The "120 ohm" resistor is two 0.01% 250 Ohm Vishay resistors in parallel resulting in 125 ohm which is the only deviation from the LTZ1000 reference circuit in the datasheet, I have found.

The important temperature set point resistors 13K/1K resistor pair is interestingly an ordinary 13K/1% resistor that is marked with a small number as if it was selected. The 1K resistor is a Vishay Mill specified 50 ppm resistor marked RV60C1001B.  Question is if this pair is TCR matched as it should be. Otherwise there is room for improvement.
Lowering the operation temperature could also be considered.

The presence of a low noise high stability reference in this unit indicates that the rest of the hardware actually can benefit from not only a single LTZ1000 reference but two with reduced noise.   This comes as a surprise as the noise specs for the LM399 based 2701C isn’t all that great as I recall.

But this is of cause wideband noise that tends to be averaged during DMM calibration. The dual LTZ1000 reference will probably greatly reduce noise below 10 Hz compared to a single LM399 which is of great importance when used as a DMM calibrator or voltage transfer standard.

With respect to stability it’s interesting if the PWM based voltage generation now becomes the weak link. The crystal might be an ordinary 30 ppm type.  Absolute precision is unimportant but most ordinary fork type crystals have temperature coefficients of about -0.05 ppm/C. Aging is often better that 3ppm/year. However, Valhalla could have selected a crystal with much better specs in the model.  Would an OCXO be a worthy upgrade? Maybe.

[Jorn]

For those of you who already own a 2701C it might be worth a reference upgrade. 

The main board of the 2707A is labled 2701:



which suggest that the two units are almost identical.

The 2707A dual reference board has 5 connections. +15V, GND, -15V, REF OUT and REF GND as expected. 4 plugs directly to the LM399 pins on the PCB. The fifth is the -15V supply taken from the nearest op-amp.

At least in theory it should be a simple upgrade. I'd guess you can desolder the existing LM199/399 and plug in your LTZ1000 reference as shown in this teardown. Of cause you will do it on your own risk.

Building a LTZ1000 reference is very easy just read this thread https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/.

Before installing your own reference start checking that the LM399 heater voltage is 15 volts. Then remove the Zener current limiting resistor to the LM399 and you are probably good to go. The picture of the 2707A mainboard confirms that this resistor is omitted in the 2707A unit. 

Hope you have enjoyed this teardown.  I'm sorry for the low resolution on the pictures. All my savings goes to 15+ years old equipment on eBay rather than a new digital camera

Now it’s lab time. Stability measurements will take place during the next couple of weeks.

-Jorn

[AF6LJ]

Thank you this has been enjoyable and educational 

[TiN]

Thanks for  teardown. Not often we see dual reference in standards .
No PCB cutouts, voodoo-routing and so on, as expected since whole thing is in own chamber and likely equalized thermally.
Is there heater on the box?

[FlyingHacker]

Nice teardown. Great score!

Thanks for sharing.

[Vgkid]

Thanks for the teardown. That is a rather unusual modification that was performed, I guess they needed the extra stability.

[zlymex]

Thanks very much for the teardown.

The important temperature set point resistors 13K/1K resistor pair is interestingly an ordinary 13K/1% resistor that is marked with a small number as if it was selected. The 1K resistor is a Vishay Mill specified 50 ppm resistor marked RV60C1001B.  Question is if this pair is TCR matched as it should be. Otherwise there is room for improvement.

That's looks very wrong to me. It is not only about the TCR match, but more importantly about the aging. The exact copy of the schematics from the datasheet indicate that they didn't give any thought to the circuit.

[Jorn]

Thanks for  teardown. Not often we see dual reference in standards .
No PCB cutouts, voodoo-routing and so on, as expected since whole thing is in own chamber and likely equalized thermally.
Is there heater on the box?

No heater in the box except for the 2xLTZ1000
Current consumption, after stabilization is 68mA for the positive rail and 4mA for the negative rail. This equals 1.08W.
-Jorn

[PA4TIM]

Nice pictures,
I just repaired a 2701 with 120mA option. The option is described in the 2701 service manual (including the schematic) and it are just a few normal extra parts on the empty spots on the pcb behind the range knobs.

I'm just finishend so the calibrator is still here, I will try to make some pictures today as a compare. (I also made a copy of the firmware) A bit strange I am pretty sure I made at least 2 pictures but I can not find them on my camera. I always do for safety, if some parts fry them self I can look at the pictures for the partnumber or how it was mounted, which wires go where etc. Better safe then sorry. I probably saved them in the wrong file or trew them away by accident.

ps, all the IC branded caps were bad. (most had very high DC leakage, but in yours they already replaced most of them)

[Mickle T.]

"2730" mark on the PCB - Valhalla 2730GS M/F Calibrator.

[Jorn]

Nice pictures,
I just repaired a 2701 with 120mA option. The option is described in the 2701 service manual (including the schematic) and it are just a few normal extra parts on the empty spots on the pcb behind the range knobs.

I'm just finishend so the calibrator is still here, I will try to make some pictures today as a compare. (I also made a copy of the firmware) A bit strange I am pretty sure I made at least 2 pictures but I can not find them on my camera. I always do for safety, if some parts fry them self I can look at the pictures for the partnumber or how it was mounted, which wires go where etc. Better safe then sorry. I probably saved them in the wrong file or trew them away by accident.

ps, all the IC branded caps were bad. (most had very high DC leakage, but in yours they already replaced most of them)

The big question is if the firmware is identical for the versions with or without 120mA source. And are the 2701C and 2707A firmware identical? The firmware from the unit in this teardown is marked 2707 on top of the eprom.  I need to get a cheap programmer to read/backup the 2707A firmware. Then we might exchange firmware files to do further analysis of the differences.

Btw. if You happens to have the 2701C service manual on pdf and would like to share it,  I would be very happy to get my hands on it. Searched the internet for it but with no luck.

-Jorn

[Jorn]

"2730" mark on the PCB - Valhalla 2730GS M/F Calibrator.

That's interesting. Safe to conclude that the same reference is used in both units.

However not much information is to be found on the internet about the 2730GS AC/DC calibrator.  

Do you know anything about the performance of the 2730GS Mickle?

-Jorn

[PA4TIM]

http://schneiderelectronicsrepair.nl/?p=408 some pictures of the 2701C's guts

[Mickle T.]

VS 2730GS spec.

[quarks]

interesting, I never saw a 2707A before.
Very nice