Electronics > Metrology

Buyers beware, misleading 34470A reference modules (clones) on eBay.

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TiN:
All know that I have soft spot for ultra-stable DC Voltage references. When I saw some interesting looking module labelled as Semi-finished 34470A Ultra Precision Voltage Reference Board Without LTZ1000ACH. Note that it does not say "Keysight" anywhere except the PCB, and rightfully so. It's not a Keysight product, but a clone. Perhaps sellers main target audience are hobbyists who interested in upgrading and modifying their 34460A/34461A/34465A 6½-digit DMMs for better temperature stability and lower noise? I was thinking "price looks not too bad, maybe should get one or two to test next year?"

Below screenshot from Dave video, featuring real deal Keysight 34470A teardown and showing voltage reference module construction in great detail.


We can see that PCB quality and assembly is impeccable in 34470A with OSP-plated pads and precisely applied solderpaste. While reference circuit design is not very good in 34470A (compared to full potential that LTZ1000A chip is capable of in good hands), it is very good reference totally adequate for 7½-digit DMM. It is essentially replicate somewhat flawed reference circuit design from older legendary HP 3458A, repeating same high oven temperature problem that hurts long-term DCV stability of these instruments. This was deliberate decision to meet +50 °C operating temperature requirements.

Now is photo from the eBay listing for "34470A reference":


By obvious markers it is indeed very clearly different to original design made by Keysight, and does not look same quality even remotely. But it's not the main issue for me, after all soldering quality is fixable. What matters - it even looked pretty close with same looking 15k \$\Omega\$ and 1k \$\Omega\$ R4 and R5 resistors used for LTZ1000A super-zener temperature setpoint (marked with red square and arrow). Ratio of these two resistors is one of the key performance components that define stability of the LTZ1000A 7V output.

Anyhow, I thought it would be interesting to do some measurements and test these little modules in terms of temperature coeffient stability, noise and long-term drift. Seller accepted $30 USD best offer and two modules were on the way. Too good to be true?


Few weeks later boards arrived, but to my great disappointment PCBs I received are NOT what was shown on the photo in eBay listing



Board assembly quality and overall PCB quality is rather poor, but that does not bother me much. What is a much bigger problem - wrong resistors R4 and R5. Both boards that I received have standard SMD1206 1% resistors and not the special better resistors that are SHOWN ON EBAY listing photo. They may look the same to unexperienced eye, but pay attention on the digits font. Only one manufacturer of SMT resistors using special rounded font shown on both EBAY photo from seller listing and also on real genuine Keysight 34470A from EEVBlog's teardown video.

So this is false advertisement and eBay seller is misleading buyers as he mislead me here.



Also second issue - since I bought two modules, they both have this problems with R4 and R5 resistors, but second module also missing capacitors C1 and C2! Why?
I have messaged seller to either modify listing to reflect correct information, or to refund the order. Will see what happens next.

Some more photos. Heater drive transistor is good old 2N3904 in TO-92 package, just like real 34470A ref. No issue here.



And yes, I know that proper LTZ1000A board components just in resistors would cost 5 times the price of this whole module. Have more than 7 years of experience with various LTZ1000-references and have lot of different designs. It was especially interesting to see how bad is "bad" 34470A reference PCB design is versus expensive typical LTZ1000A with VPG foil resistors and fancy PCBs.



At least 34470A module is smallest there is for production-grade module. It is half the size of old 3458A A9 LTZ1000A module and more than 10 times smaller than my latest quad LTZ1000A ultra-low noise xDevs.com QVR module.



Tests and performance numbers are to be done next year.

UPDATE.
Seller promptly responded and will organize return shipping and replacement to two boards with "proper correct" resistors. I was promised a photo of the new boards before shipping. So hopefully this will be resolved to everybody's satisfaction.  :-+

floobydust:
It's like a science experiment to see how many people stabbed each other in the back. I'm told that once you get your product made in china, the first thing they do is rip off and distribute all drawings and IP.

I saw a documentary going after methlamine and fentanyl producers in china and the video showed knock-off shops in Shenzen, an entire building full of people hand-painting classic art works.

But here it looks like a port from Mentor to Altium, the silk font reveals that look at the "6". I would wonder if the People's Op-amp LM358 snuck in there.
Check the solder bridge on J1 pin 1-3 at R96.

Dr. Frank:
Hi TiN,

I also have seen this PCB, and from the ebay photos it was quite evident that the components were assembled manually.. leadfree soldering by hand is not easy.  :-DD
So chances were high, that the resistors were also not low T.C. ones.

Anyhow I also ordered one, and received it last Monday, see top and bottom shots.

My plan is to fully reverse engineer the PCB, to find out how it's connected to the 34465A/470A, in conjunction with the existing LM399H drive circuit.
There was already a partly deciphered circuit on the forum, but a few tracks were still missing, i.e. these on bottom where a sticker was placed, and a few below the connector and the LT1013.. What I've measures so far, these latter lines make sense. (Kelvin connection  ;D)

Even if this PCB is a clone, they will probably have copied the circuit correctly, and I have the possibility to compare the bottom side against an original KS board.
The quality of the PCB  also does not matter, as Keysight has made the worst design ever, anyhow.. too thin lines, slots, and so on.

TiN, where did you find differences in the layout, already?

My further intention is to replace all 5 precision resistors with small TH metal bulk resistors from AE, the FLCY, which can be squeezed on the existing pads.. and also setting the oven temperature to reasonable 60..65°C (12.5k/1k). T.C. compensation will also be done.

Then I will assemble it into my 34465A so that I can put in the existing LM399H back, if required.     

Let's see, if everything works directly, or if this reference would need a SW enabling for the 34470A.. different analogue interface, different ACAL functionality maybe.

Frank

floobydust:
I drew this schematic of the board a long time ago, hoping to copy it and make million$... well actually to look at its design.
I could not see underneath the 8-pin header for what pin 2 is about, and if there is a V- rail. I see they can monitor heater output.
Please let me know of any errors/updates and I will update this post.

edit: 2020-12-25  Updated schematic to rev2 based on post #7.

TiN:
Seller replaced photos in his listing, now showing only same crappy film resistors your and mine boards have. Seems like they dont have proper resistors to sell boards with. Was expected something along these lines to happen ($30 shipped, can't expect $45 resistors on the PCB) but still hope dies last.

I might keep one board as received and replace resistors on second to fancy VPG VSMP1206 15k, 1k and 100 ohm to test. Most likely real 34470A module performance would be somewhere in between. That's low priority project for next year/

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