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Trevor Wilson

On 21/03/2014 2:04 PM, wrote:
On Thursday, March 20, 2014 12:16:03 AM UTC-4,
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

I do own several digital voltmeters and they do have their place. You
can't beat a digital for making precise measurements. But there is a
bit of a sentimental value here too. I built this meter from a kit
when I was a kid in 1963. Since then I've repaired a lot of TV sets
and other things with it over the years, and although I don't use it
much anymore it's nice to still be able to when the need arises. So
just where the hell do you come off insulting me?

**Who insulted you and how?

I understand sentimental attachments. I still have my first multimeter.
My dad gave it to me. A Sanwa. I never use it, of course. Same deal with
my AVO 8. I bought it because I always wanted one, certainly not because
I needed it. I have some other, quite exotic and rare meters too. I
still prefer my bench meter and my Fluke.

I'm just curious now. Did you ever try to peak or null a circuit with
a digital?

**Yes. It's a doddle. Both my regular digital meters have a fast bar
graph display, along with the regular digital one. Perfect for those
times when you need to see what is happening quickly.

It's a pain in the ass. How about looking for a change in
logic state?

**Same as above.

I prefer seeing an immediate change rather than watching
a decay. Why don't you try going on the grid of a tube with a low
impedance meter and see where that gets you? Mine has an 11 meg ohm
impedance. It won't load a vacuum tube circuit. So much for my piece
of **** meter, huh?

**My Fluke 85 has an input impedance of 10 MOhm. The bench meter has in
input impedance of 20 MOhm. Plenty high enough for the vast majority of
applications. I have a 100 MOHm, shop built probe somewhere around the
place, for the odd occasion when I require very high impedance/ high
Voltage measurements.

But I have a better idea Trevor. Why don't you take your prize
digital you've raved about and stick it in your ass? That way you'll
never lose it, and it will always be available. Lenny

**I started using digital meters back in 1975 (Fluke). I didn't much
care for them. It took me many years to wean myself off an analogue
meter. Once weaned off them, I've never looked back. Analogue meters are
cool, but for day to day use, nothing beats a digital meter.

--
Trevor Wilson www.rageaudio.com.au