Trevor Wilson wrote:

On 20/03/2014 3:16 PM, wrote:
I found this circuit somewhere, (I can't remember) to replace the 1.50 volt battery in a VTVM. I have an RCA WV98C Senior Voltohmyst VTVM that is in perfect condition but doesn't get used very often. This battery retrofit circuit takes power from the 6.30 volt filament transformer and is built around an LM317. With a trim pot you set it for 1.55 volts and you never have to change a battery again. It also needs to occupy an area no larger than the battery presently does.

As much as I like keeping things original, I'm worried that the battery is going to leak and rot the inside of the unit. I was going to build the circuit but it occurred to me that I should probably be concerned about the current that the LM317 will be dissipating when the meter is used on the low ohms range. Does anyone have a feel for the size that an adequate sized heat sink should be in order to handle this? Thanks, Lenny


**Crazy idea, Lenny:

Move yourself into the 1970s and buy yourself a digital multimeter. Most
of them operate on a 9 Volt battery. Replace the battery every year and
you'll be sweet. Even a US$20.00 meter will give you a vast improvement
in every performance parameter over your ancient POS, which is likely
well outside it's original performance specs.

Like this one:

http://au.mouser.com/ProductDetail/E...qaRLKRQ5wis%3d

One of my meters measures the usual Volts, Amps & Ohms AND light
intensity, sound pressure level, relative humidity and transistor gain.
I recall that it cost me AUS$50.00.

I have a nice collection of analogue meters (I recently acquired a
beautiful AVO 8 Mk7), but I never use them anymore.

Go digital. You will not be sorry.


Yes, he will. A VTVM has a 1 Meg resistor in the probe for DC
measurements, to keep it from affecting the circuit being tested. The
capacitance of a set of meter leads on a DVM will screw up a lot of high
impedance circuits.


Here is one of many simple conversions to replace the battery with a
regulated DC supply. You only need a few mA for the Ohm meter function
in a VTVM. I did this to several VTVMs back in the '70s.

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


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.