[Johnson] J500 external audio input

[John Lyles]

On my DX100 I brought a separate balanced audio circuit into the unit, so that I could use a broadcast audio processor, having 150 or 600 ohm balanced audio lines. To the cathode circuit of the second audio tube, 12AU7 in the 100, I added a 150 ohm resistor, unbypassed, and the remaining cathode R is above this in series, to the cathode, with a bypass cap. I reduced the value of it, so that the total R is about what the original tube used, and what the databooks from the 1950s recommend for this tube in class A audio service with a particular plate resistor. I connected an "ouncer" UTC line/line audio transfomer, with the secondary connected through a series 'lytic capacitor, across the 150 ohms (to keep DC cathode current off the transformer). Now I have a totally isolated audio input which I can apply line level audio (+10 dBm) into the radio, without modifying the old microphone input. It had fine quality audio response and distortion. The same idea can be applied to your J500. 
73
John
K5PRO


> Message: 1
> Date: Wed, 9 May 2007 18:19:30 -0400
> From: "Roger Shultz" <nj2r at earthlink.net>
> Subject: [Johnson] Johnson 500 WA1HLR mod question?
> To: <johnson at mailman.qth.net>
> Message-ID: <083601c79288$1d6b5750$6501a8c0 at dellgx110>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> 	reply-type=original
> 
> I've completed the WA1HLR audio stage mods on the local OM's Johnson 500 but 
> have one question about the procedure which is unclear.
> 
> It states to "reroute the fone patch line to a different part of the audio 
> circuit" without saying what "different part".
> 
> There is an attached figure 2 to the procedure that shows a method to add an 
> adjustable input for outboard audio processing that uses the phone patch 
> jack and old clipping control but I don't know if this is what he was 
> referring to as the "reroute".
> 
> Can I leave the phone patch input connected to the cathode of the second 
> stage of the 12AX7 V109B as it was originally? Should I disconnect it 
> altogether thus eliminating the ability  to inject a test tone? Where is the 
> place that it "should be rerouted" if all of the above is incorrect?