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: [EMAIL PROTECTED] To: amradio@mailman.qth.net; [EMAIL PROTECTED] 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 want an optimal circuit that will be reliable. Anybody got one? 73, Don Merz, N3RHT
