Dave Plowman (News) wrote:

In article ,
Bryce wrote:
My v7a vtvm (built by a guy who looks like me but a lot younger)
uses a 1.5v battery for resistance measurements. I'm tempted
to replace it with a zener reference. Looks easy: the battery
is grounded on neg side, so is one side of the 6.3vac.

I guess we didn't have zeners fifty years ago. Dunno why Heath
didn't use several 1n34's in series (I think we DID have those!)
- thermal stability I suppose.

Must be more to it than that - the voltage from a dry cell varies by a
large amount over its life, hence the set zero facility.

I found the Heathkit assembly manual, which includes scrawled notes
by the builder to the distant future and a schematic.

Resistance measurements were made by measuring the voltage at the
probe which is fed through a precision resistor string from the battery.
The lowest range resistor is 9.1 ohms, so the battery must supply as much
as 165 ma. Heath does not sense battery terminal voltage directly, so
internal resistance of the source must be much smaller than 9 ohms to
maintain calibration. Looks like a good candidate for voltage regulator.

My archives also yielded a 1957 Radio Amateur's Handbook. I probably
oughta clean the basement. It lists a page of germanium crystal diodes,
a couple of were rated at 300 ma. Also a page of (mostly PNP) transistors.
So the stuff needed to leave out the battery existed in 1957.

I'm going to call Heathkit and ask them why they didn't do that.

Bryce