Hi Don,
That's definitely my "do next" item, though the 2N2907 looks like a reasonable substitute for the original BCY70
The following is an extract from an email I received from an Ametek engineer.
==== QUOTE ====
Strange to relate that the 7071 also drifts for a long while after power on, about 15 hours now and the voltage at TP705 is still moving. It too shoots off into the weeds after the lid is removed.
I haven't yet dared to see if my "good" 7081 also does this ...
Yes, I think you might be surprised just how long these units do take to settle after power up. This is why the spec says to leave the unit on for 24hrs before making adjustments and also stresses the point about movement of the lid and says to use a calibration cover. A calibration cover is just a normal lid with holes drilled in it so that adjustments can be made without removing it.
==== END QUOTE ====
((Aside: 36 hours and still moving, but not much now).
I would say that the errant 7081 is somewhat worse than the 7071, but not much, so perhaps this is a feechur after all. If that is the case, then replacing the J511 CRD with an LT3092 would very likely make a *huge* difference to the temperature stability.
BTW do either you or David have experience of the LT3092 to give guidance on what caps to use to quiet the Set resistor and stabilise the whole thing? The data sheet is a little "vague" - just saying a small capacitor across Rset, but giving no clue what "small" might mean - 10pF, 10nF or what? Or is it quiet enough by comparison with a JFET CRD that this won't matter?
Dave
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Don Black
Sent: 08 July 2012 15:52
To: TekScopes@...
Subject: Re: [TekScopes] Re: OT: J511 current regulator diode (FET) temperature coefficient
Perhaps the odd replacement transistor is upsetting the channel balance so the common mode bias change is resulting in a signal output. Try replacing it with the correct transistor now you have it and see if that solves the fault.
When something strange is happening fix the obvious faults first, it may clear up the other problems too.
Don Black.
On 06-Jul-12 7:27 AM, David wrote:
On Thu, 5 Jul 2012 22:03:43 +0100, "David C. Partridge"
<david.partridge@...> wrote:
David,
I found your very helpful reply on the volt-nuts archive (reproduced here):
I doubt the temperature coefficient of D717 matters in this circuit.
Feedback is taken off of the output through TR705 or TR706 (x10
mode) back to the inverting input of IC705. The current source was
used to prevent any significant shift in the operating point of
TR703 for frequency compensation reasons because the switched
cascode has voltage gain which is inside of the operation amplifier feedback loop.
The drift in the cascode bias from D716 is irrelevant for the same
reason.
Having voltage gain inside of the feedback does suggest another
failure mode that could cause drift or noise in the output. Could
IC705 or TR703 be oscillating? Maybe C714 dried out and has low
capacitance.
I have no idea why I never see some posts to that list!
C714 is suspect I agree, and I'll have to order in some 10uF tants. I did find that TR702 had a breakdown problem a while back and I replaced it temporarily with a 2N2907, pending the arrival of some hard to find BCY70 xistors - these are now here, but I've not gotten round to swapping that over.
Oh. If it is a tantalum then it will not have dried out but I suppose
it still could have gone bad. The leakage would need to be pretty
high before causing a problem but I suppose that is possible.
I can definitely confirm that cooling or heating the CRD causes a large shift (millivolts) in the level at TP705. I've found no other component that causes that.
Compare blowing compressed air on various areas like the non-inverting
input of IC705 versus blowing with your breath through a soda straw. I
would suspect humidity and ionic contamination before temperature in
this case.
I need to check my other 7081 and a 7071 (identical) to see if they behave the same.
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