[#3300] Wavetek 7000, the hidden gemstone.

[chekhov]

Today little accident likely revealed the whole story of failure of this particular unit.
Attempt to connect chassis to 10V LO output lead to immediate 'reboot' of the unit  .
Rather quickly the culprit was found, it was a battery pack:

One of bolts that keep this battery pack shorts single cell to chassis, depending on how it was screwed and addition al unit movements.
Thus, initially burned 1 ohm resistor was likely the consequence of doing the same what I did, but for a longer period of time. Could you imagine, this little mechanical issue lead to whole this story  .

[TheSteve]

Wow, a pretty terrible assembly job of that battery pack, nice job finding the fault.

[branadic]

Just thinking loud...

When I've build the reference presented here, I was first measuring the initial and residual t.c. with the oven set to the z.t.c. point of the zener and found it to be at +0.175 ppm/K. The reference was buffered only.
I've then compensated this t.c. and ended up at -0.0105 ppm/K after this step.
Afterwards I've installed the boost to 10 V and found the t.c. to be -0.195 ppm/K, that was then trimmed within the boost stage.

So question is, could the residual positive t.c. of the reference have compensated for the negative t.c. of the boost stage? Seems like I need more investigation to answer that question, so I've already removed the compensation on that board.

-branadic-

[Kleinstein]

It is well possible that they only cared about the TC of the final result and choose the parts for the boost stage to compensate for the small TC of the zener part.  The compensation could be intentional (select the parts before soldering) or just luck (some units work well, other may get a rework or selected trim parts).

[dietert1]

As far as i remember we have seen schematics that included a nice measurement of zener temperature from the base-emitter voltage of LTZ1000 Q1 and then a resistor array used with jumpers to set TC in a binary fashion, with very fine LSB. No luck involved!
By the way my LTFLUs include a very similar temperature measurement circuit, but i roughly compensated TC and then used the residual nonlinear TC to adjust the TEC oven for the zero TC temperature. In the LTFLU case the 10 V boost is included in the same oven.

Regards, Dieter