[Milsurplus] DAV-2 On Off Wiring

[Ray Fantini]

Look at the schematic for the MAB, the evil twin sister of the DAV and you will see the same application of DC voltage to the audio transformer and headset with no apparent DC return path. Perhaps it has something to do with the fact that a 3S4 is a directly heated cathode without a separate cathode from the filament? With all the experts who can speculate endlessly on variations on rarely seen or never produced ARC-5 sets would have thought this would be easy!
Ray Fantini KA3EKH

-----Original Message-----
From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of WA5CAB at cs.com
Sent: Friday, June 18, 2010 11:21 AM
To: milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] DAV-2 On Off Wiring

While having my second cup of coffee, I copied the DAV-2 schematic and 
"rewired" it to eliminate having to trace through four connectors.  You're 
correct that the mic voltage is 6 volts.  Not 1.5 as I think I said earlier.  The 
-4.5 V to ground is also applied to the modulator tube grid.  But the 
series combination of headset and audio transformer secondary is tied to the 
series combination of "A" battery, "C" battery, Mic and Mic Transformer primary 
at only one point.  So although the headset and transformer secondary are 
1.5 volts above ground, there is no mic current flow other than through stray 
capacitance and insulation leakage resistance.  Which when new would have 
both been pretty small (or large).  

So we're back to them using the switch terminal as a tie point and the wire 
from the switch to J401 for dual duty.

In a message dated 6/18/2010 5:30:14 AM Central Daylight Time, 
arc5 at ix.netcom.com writes: 
> I think it's "cheap and easy" sidetone, plus a way 
> to boost the carbon microphone voltage.
> If you follow the circuit out, the mike voltage is the 
> "C" voltage in series with the "A" voltage, giving 
> 6 volts for the carbon mike.  
> Also, the mike voltage gets tapped to the 
> headset, which has a potential difference between
> the two batteries, but no path to ground.
> So you get cheap sidetone.
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
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