Heathkit v7a vtvm battery
 

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?

Heathkit v7a vtvm battery
 

On Nov 29, 8:12*am, Bryce wrote:
My v7a vtvm (built by a guy who looks like me but a lot younger)
uses a 1.5v battery

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

There aren't satisfactory zeners with voltages under 5V, so
a bandgap reference is more usual. Rectifier, capacitor,
and an LM317 regulator (which uses a bandgap reference and
handles 1.5V output) will do just fine.

Heathkit v7a vtvm battery
 

The accuracy of a Zener is not good enough for your application. What
you need is a very low noise, high stability, low source resistance
precision power supply to replace the 1.5 V Battery. I would use a 1.5
V battery, and change it once every year.


Jerry G.



On Nov 29, 11:12*am, 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.

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

Heathkit v7a vtvm battery
 

On Sat, 29 Nov 2008 11:12:45 -0500, 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.

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


There is an absoute reason for never ever using ziner references in ohms
measurement.

The voltage must be stable and totally free of ripple or other electronic
noise. There s no ziner driven source that can do this cheaply.

Jist install a quality battery they last the normal shelf life of the battery
and are cheap. Nix on using ziners.

Gnack

Heathkit v7a vtvm battery
 

"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.

Heathkit v7a vtvm battery
 

On Sat, 29 Nov 2008 16:15:50 -0500, "DaveM"
put finger to keyboard and composed:

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

AISI, that will give you an output of only 0.6V, ie two diode drops
minus the Vbe junction voltage. Replacing one of the 1N4001 diodes
with a red LED should get you closer to the mark.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Heathkit v7a vtvm battery
 

Thanks to all for the guidance and amazing ASCII art!
This is high on my list of low-budget winter projects
now that leaf-raking season endeth.

I should admit that it had been a VERY long time since
I had even seen my first Heathkit. I opened it expecting
to see serious damage from battery acid. Happily, the
"Radio Shack New Formula Steel Clad, Custom MFD in Japan"
was intact (although totally dead). It had been in there
for at least ten years; maybe twice that. My vtvm serves
my needs but my old friend deserves some attention.

Bryce

Heathkit v7a vtvm battery
 

"Franc Zabkar" wrote in message
...
On Sat, 29 Nov 2008 16:15:50 -0500, "DaveM"
put finger to keyboard and composed:

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

AISI, that will give you an output of only 0.6V, ie two diode drops
minus the Vbe junction voltage. Replacing one of the 1N4001 diodes
with a red LED should get you closer to the mark.

- Franc Zabkar

Thanks for catching that error, Frank. I was going to copy & paste two more
diodes but the phone rang and I forgot. I hit the send button while the phone
was hung out of my ear. Never try to multitask while one of the tasks is
talking on the phone.. never works!!!

Using an LED will work too... thanks for the suggestion.
--
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.

Heathkit v7a vtvm battery
 

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.

--
*The problem with the world is that everyone is a few drinks behind *

Dave Plowman London SW
To e-mail, change noise into sound.

Heathkit v7a vtvm battery
 

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

Heathkit v7a vtvm battery
 

On Sat, 29 Nov 2008 11:12:45 -0500, Bryce
put finger to keyboard and composed:

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 was curious, so I had a look at the manual:
http://bama.edebris.com/manuals/heath/v7a

Here is the circuit:
http://bama.edebris.com/download/heath/v7a/v7aschem.gif

It seems that you first set the FSD by selecting your particular ohms
range (with no resistor connected) and tweaking the OHMS ADJUST pot.
The meter should then reflect the OC voltage of the 1.5V battery. If
you then measure a resistor whose value is equal to the internal range
setting resistance, the meter reading should be exactly mid scale.
However, the lowest range setting resistor is 9.1 ohms (instead of
10R) which suggests to me that the designer may have allowed 0.9 ohms
for the ESR of a typical battery of that time. If you were to now use
a precision 1.5V supply with an ESR of zero ohms, then your scale
would read 10 ohms for a 9.1 ohm resistor.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Heathkit v7a vtvm battery
 

Franc Zabkar wrote:

On Sat, 29 Nov 2008 11:12:45 -0500, Bryce
put finger to keyboard and composed:

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 was curious, so I had a look at the manual:
http://bama.edebris.com/manuals/heath/v7a

Here is the circuit:
http://bama.edebris.com/download/heath/v7a/v7aschem.gif

It seems that you first set the FSD by selecting your particular ohms
range (with no resistor connected) and tweaking the OHMS ADJUST pot.
The meter should then reflect the OC voltage of the 1.5V battery. If
you then measure a resistor whose value is equal to the internal range
setting resistance, the meter reading should be exactly mid scale.
However, the lowest range setting resistor is 9.1 ohms (instead of
10R) which suggests to me that the designer may have allowed 0.9 ohms
for the ESR of a typical battery of that time. If you were to now use
a precision 1.5V supply with an ESR of zero ohms, then your scale
would read 10 ohms for a 9.1 ohm resistor.

- Franc Zabkar

Half-scale is 10 ohms and I agree with your analysis. I have always
thought of an aging battery as having a larger than usual internal
resistance; i.e., open circuit voltage may still be about nominal
but the output voltages collapses under load. Doesn't that mean
this sort of ohmmeter is inherently inaccurate without a "nominal"
battery regardless of the ohms adjust setting? Not a big error
for an analog readout though.

Bryce

Heathkit v7a vtvm battery
 

On Mon, 01 Dec 2008 16:01:58 -0500, Bryce
put finger to keyboard and composed:

Franc Zabkar wrote:

On Sat, 29 Nov 2008 11:12:45 -0500, Bryce
put finger to keyboard and composed:

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 was curious, so I had a look at the manual:
http://bama.edebris.com/manuals/heath/v7a

Here is the circuit:
http://bama.edebris.com/download/heath/v7a/v7aschem.gif

It seems that you first set the FSD by selecting your particular ohms
range (with no resistor connected) and tweaking the OHMS ADJUST pot.
The meter should then reflect the OC voltage of the 1.5V battery. If
you then measure a resistor whose value is equal to the internal range
setting resistance, the meter reading should be exactly mid scale.
However, the lowest range setting resistor is 9.1 ohms (instead of
10R) which suggests to me that the designer may have allowed 0.9 ohms
for the ESR of a typical battery of that time. If you were to now use
a precision 1.5V supply with an ESR of zero ohms, then your scale
would read 10 ohms for a 9.1 ohm resistor.

- Franc Zabkar

Half-scale is 10 ohms and I agree with your analysis. I have always
thought of an aging battery as having a larger than usual internal
resistance; i.e., open circuit voltage may still be about nominal
but the output voltages collapses under load. Doesn't that mean
this sort of ohmmeter is inherently inaccurate without a "nominal"
battery regardless of the ohms adjust setting? Not a big error
for an analog readout though.

Bryce

I've never thought about multimeters much until your post, but it does
seem to me that anything that relies on an unregulated battery supply
cannot possibly lay claims to any sort of precision.

In your case I would try a precision voltage reference for the 1.5V
supply and a 10R 1% resistor in the lowest ohms range.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.