[time-nuts] Subject: Subject: Re: HP5065A C-field current voltage mod

[Poul-Henning Kamp]

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In message <1530394039.1439027.1441348869239.JavaMail.yahoo at mail.yahoo.com>, Pe
rry Sandeen writes:

>OK, but why?  The temperature would be stable so the resistorsand
>reference diode wouldn't drift.

No, the temperature is not stable, not even close.

Given that the TL431 is quite good at measuring temperature, that
makes it unsuitable in this case.

I have identified one source of the temperature fluctuations:  The
series pass transistor of the +20V rail is located right beneath
the A15 PCB, so any variation in mains (or EXT DC) voltage causes
the A15 temperature to change accordingly.

More details here:

	http://phk.freebsd.dk/hacks/HP5065A/20150903_psu/


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