[AMRadio] Relayed from N4TGC was: GB> VTVM Battery Eliminator

RJ Mattson rjmattson at hvi.net
Fri Jul 21 00:51:17 EDT 2006

Relayed from N4TGC Eric

Don;   It's so simple I could almost recall it from memory, but I
pulled the schematic anyway ... there are a couple-three detriments, which
I'll mention first: the circuit I use only works if the negative of the cell
is grounded (which all Heaths, EICOs and Conars I've ever encountered were);
it puts a slight additional load on the filament winding (tho I've had no
overheating or failures); and the unit has to be re-calibrated, as the
voltage may be off by 0.1v from that of a C cell.

Remove the dead "battery".
 Next, (diagram this before you do it) install a 3-terminal tie
strip that has one grounded terminal, in a convenient spot.  From one
floating terminal, run a wire to the "+" of the cell holder.  Wire two
ordinary silicon diodes in series (one cathode to ground) from there to
ground - this is the regulator.  (I use 200v 1/2a or better, but around 50v,
25ma should work fine.)  From this same 'floating' point, run a 47 ohm, 1/4
watt resistor to the other floating terminal; from there, put a 470u/16v
cap. to ground (observe polarity!)  From the junction of cap. and res., put
the cathode (banded end) of a third diode, whose anode ties to the
un-grounded side of the 6.3vac filament sply.  Fire it off, re-calibrate,
and you're done - 'forever'.

What's so maddening about the original design is that not only does
'everyone' forget and leave the C cell in place, but the idiots at Heath put
it ON TOP, where it could leak and ruin the guts!  Yet, I've found several
that destroyed the holder, but none so far that got down into the other
circuitry.  Am I lucky, or what?!

I've built this circuit seven or eight times so far - only the first one
gave trouble, and then only because I used three diodes in the regulator
string, as the source was indeterminate as to the number needed - I think
they assumed one might be using 'geranium' diodes, which are no longer
common.  (Silicons have a rigid 0.7v junction; germaniums around 0.4, hence
their uncertainty.)

Have fun!    e

-----Original Message-----
From: merz.ds at mellon.com
To: amradio at mailman.qth.net; glowbugs at piobaire.mines.uidaho.edu
Sent: Thu, 20 Jul 2006 12:26 PM
Subject: GB> VTVM Battery Eliminator

Has anyone does this with a circuit that they are happy with? I'd like to do
this to one or more of my VTVMs but I don't want to experiment with it--I 
an optimal circuit that will be reliable. Anybody got one?

73, Don Merz, N3RHT

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