Interesting point mawyatt. I must admit I didn't think of this before messing with the references. In my setup, there was the K7200 diy PSU (100uF output cap), ~1.5m of wires and a small board with 2 refs that also had a 10uF capacitor and a sliding switch. I tried to recreate the possible surge in LTSpice. Oddly enough the current pulse on the heater pins is bigger with the 10uF cap than without it. The pulse current seems to largely depend on how fast the switch is closed. If the rise time is faster than 1 ms, the chip would be in trouble (transients get up to a few amps).
Thanks for the information about the clones iMo. Those Chinese and Russian ones must be really rare, haven't even found pictures around.
P.S. as a kid I had TESLA bass guitar amplifier. It had just average sound, but was built like tank.. Undestructable ... ![Azn ^-^](https://www.eevblog.com/forum/Smileys/default/azn.gif)
I am very fond of TESLA, had a B100 stereo reel to reel player, it sounded great.
PS: My bet all MAB/MACx99 refs available today at ebay or in various eshops were already carefully "sift through" (mind they are 30+ years out of production), also the markings on their hats might be replaced/forged. All chips are "MAC199" afaik, and the "MAB/MAE" versions were just stamped onto the easily replaceable plastic hat after the binning during the testing..
Who knows... The probability for forgery might be low though, since they are cheap and not that popular. There's still one shop in the Czech Republic selling them for 1,18 EUR / piece, but they sell only locally. The MA* dies are indeed the same. In the factory I think they just sorted for tempco, so there's not a big difference between the MAB and MAC versions regarding noise / other parameters.
[edit]
Found a few meters that used them, the Metra M1T 390 (4.5 digits) and M1T 380 (5.5 / 6.5 digits, quoted as the most advanced meter produced by Metra Blansko). Some pictures are available here:
https://forum.elektrolab.eu/resources/metra-m1t-380.629/. I found the schematic for it and in the part list they wrote LM199 but in reality they used MAB399 (picture 8 in the link above). In any case, I was curious to see if they limited the surge current somehow,
but nope, the heater is simply connected to 15V (generated from a MA7815) and that's it. The zener is supplied from 20V (generated by a MAA723) via a 12K resistor, so 1mA current.
![LM399 based 10 V reference 2053469-0](https://www.eevblog.com/forum/index.php?action=dlattach;topic=15982.0;attach=2053472;image)
[edit2]
On a second look, there is actually a current limiter for the 15V rail (that powers the heater of the reference) in the M1T 380. At first I thought it was a capacitance multiplier, but it actually limits the current to about 375 mA before the input of the MA7815. This may not help the reference too much since I doubt the heater surge is bigger than 150-200 mA on a cold start. I will add a current limit in my circuit as well at the 15V regulator.
![LM399 based 10 V reference 2056394-1](https://www.eevblog.com/forum/index.php?action=dlattach;topic=15982.0;attach=2056397;image)