[AMRadio] My Viking 2 is done


Larry Szendrei ne1s at neandertech.com
Sat Mar 1 18:09:26 EST 2008


Ellen Rugowski wrote:
> The Viking 2 is basically done - the $75 non-runner I picked up in Nov. 2007 is up and running.  Wahoo!!
>
>   
Congrats, Ellen! Hope to hear you on the air soon!

I just got done fixing up a junker Viking II for a future AMer (except 
he doesn't know it yet!). I got into the mods a bit more heavily than I 
wanted to, but it seemed to need it to get decent audio out of it.

In a nutshell, I:
(1) replaced the HV bleeder/voltage divider resistor with dropping 
resistor and a pair of OA2s in series to stabilize the 807 modulator 
screen voltage. The resistor was missing anyway, so I needed to do 
something, and this gives a nice stable voltage source for the 807 
screen grids.

(2) stabilized the modulator grid bias with 3x10V 1W Zeners in series

(3) took the DC current off the primary of the audio driver transformer 
and changed the audio driver from the triode-connected 6AU6 to a 
triode-connected 6AQ5, with the accompanying necessary circuit changes

(4) added inverse feedback from the top of the mod secondary to the grid 
of the 6AQ5 audio driver

(5) added a 6C4 stage under-chassis with ~20db gain between the 6AU6 1st 
audio and the 6AQ5 audio driver, because after (3) and (4) above, I no 
longer had enough gain for the typical hi-Z microphone

(6) changed circuit values to ones more conducive to fuller-range audio

And I managed to do all the above without drilling a single new hole in 
the chassis or panel.

It makes a lot more, and a lot better, audio than stock now, but, like I 
said, I got a lot deeper into it than I really wanted to.

73,
-Larry/NE1S



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