Hi,
as I already told my Valhalla 2703 is a bit unstable. Therefore, I opened it up and measured power supply ripple and so on. Everthing was fine. As a next step I planed to measure the reference voltage stability.
The schematic shows a LM399 as reference. However, I can't find one in my Valhalla 2703. IC112 (LM399 in the schematics) has an TO-92 package. The circuit around IC112 looks very similar to the schematics.
Has anyone looked into a Valhalla 2703 and found a LM399?
Best
Philipp
There was a variant of LM399 called LM3999 that was packed in TO-92 case. It had negative heater pin connected together with zener circuit negative inside case, hence 3 pins...
Apart from that it was LM399 .. I would presume it wasn't quite as good as LM399 proper, plastic case, shared zener and heater current on negative pin etc. But still quite OK. Probably cheaper and could be used in same schematic as LM399..
Also no housing around it as you have found. Valhalla obviously found this package a good choice in the 2703.
Would interesting to see how stable the reference actually is in the 2703.
Thanks for all your replies.
The TO-92 is labeld 3999
The schematics shows a 4-Pin device and it is labeld LM399H. Perhaps, a different version of the PCB.
I will take some measurements now, but I doubt it is the reference.
I've attached a measurement with 10V, 1kHz output. Measured by an Agilent 34401A.
The curve looks like there is a lot of disturbance, like really bad popcorn noise, maybe a bad connection somewhere.
I would not replace the LM3999 with a "new" one - probably the higher quality LM399 is the better choice, even if this means adding an extra wire. I would expect to find suitable points to solder 4 Pin version as well - so more like an PCB with optional either LM3999 for a low cost version or a LM399 for the real thing.
I think the only reason for the LM3999 was for NS to be able to offer a heated zener reference in a cheap plastic package. The shared ground current between the heater and the zener is a problem due to lead resistance, solder joint, and PCB routing. It's also non hermetic.
As Kleinstein says, fitting an LM399 and running a separate heater ground wire back to a non current sensitive 0V point would be easy. The only requirement is that the heater ground must not go above the zener ground by more than a fraction of a diode drop (the parasitic substrate diode on the LM399).
As long as the two zener pads are maintained at equal temperature, there shouldn't be any additional thermocouple effects from adding a thin heater ground wire.