"Bryce" wrote in message
...
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.

Is there some reason that a zener powered from rectified filament
voltage won't work here?


I performed similar surgery on an old Knight kit VTVM years ago, and it worked
great. Actually, I used a diode/transistor circuit to make a simple 1.5V
regulator. No more dead batteries at the exact moment you don't have any spares
and all the stores are closed. Just make sure that the ohmmeter circuit doesn't
need to be isolated from the heater circuit. If the heater(s) isn't tied into
the tube's DC circuit somehow, it should be fine.

If you want to go the Zener route, just make sure that you do the math and get a
Zener that can handle the current (assuming that you can find a 1.5V Zener).
Put a new battery into the unit and measure the current from it when on the
lowest ohms range, with leads open and shorted. Use the highest current that
you measured to calculate the needed Zener current and wattage. Remember that
the Zener will have to absorb the full current when no current is being drawn by
the ohmmeter circuit if it's not switched in with the meter's function switch.

Here's a quickie regulator that should give you something close to 1.5VDC.

View in a fixed-width font such as Courier.



+---+ 1N4001 TIP29
| | +-----++ C E 1.5VDC
| +---+ |+---+-----+-----\ /---------+-------o
| | +-----++ | | \ / |
6.3VAC| |+ +++ --+-- |
| | -+- | | B | |
| | 50uF -+- | | +-----+ |
| +---+ | | |1k | | |+
| | | -+- +++ | | -+-
+---+ | Gnd | | | -+-
-+- +--------+ | | 10uF
Gnd | |+ |
| -+- |
+++ -+- |
| | | 10uF |
1N4001 | | | |
+-+ | -+-
+++ | Gnd
| |
+-+ |
| | -+-
1N4001 | | Gnd
+-+
+++
|
-+-
Gnd

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster
it goes.