[Johnson] Johnson 500 audio shortage

[John Lyles]

Roger
Is it possible that you have got a high pass filter somewhere in the audio chain which is attenuating the low end significantly. This would cause the audio sideband to seem removed from the carrier, in that the higher frequencies would be spread out with less information close in for the bass octaves of audio. This might also explain the low level, as a lot of the 'meat' or punch in audio is found below 1 KHz.Try testing with a scope down at 400 Hz or so too. You would have a coupling capacitor which is very small, not enough for the circuit impedance of the stages it couples.  
73
John 
K5PRO
 
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> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Wed, 2 May 2007 20:45:46 -0400
> From: "Roger Shultz" <nj2r at earthlink.net>
> Subject: [Johnson] Johnson 500 modulation?
> To: <johnson at mailman.qth.net>
> Message-ID: <032701c78d1c$63c53de0$6501a8c0 at dellgx110>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> 	reply-type=original
> 
> I recently did a complete rebuild of the power supply/modulator for a local 
> OM and thought that finally everything was working properly, however it 
> ended with a problem that has me perplexed and I can't seem to resolve.
> 
> This 500 supply was pretty much a rusty basket case but I rebuilt it from 
> the chassis up with the aid of the original assembly manual.
> 
> Everything seemed to work properly RF wise,  on 80 meter CW,  but the 
> previous owner(s) had changed the low level modulator by removing the 
> clipper and had used an external audio driver to the high level modulators. 
> I was able to get the on board driver all working again, still with the 
> clipper bypassed and had lots of drive out of the 6B4. Looking at the 6B4 
> plates with a scope the audio looked reasonably decent with a 1 KHz sine 
> wave input though the phone patch jack. The mod cath meter on the RF unit 
> indicated plenty of modulation but here is the rub.
> 
> When I listen off air and when another station listens, while there is 
> modulation, it is quite weak and perhaps only my imagination but it seems 
> offset from the carrier!
> 
> I am getting plenty of power out on AM (325 watts) and from the modulator 
> cathode indication the modulator tubes sure are working properly but what I 
> don't get is "where is the modulation"!
> 
> It would seem that if modulation cathode reads like it is supposed to read 
> then the 572B's, though of unknown history, would seem to be OK leaving only 
> the modulation transformer as suspect. I then checked the transformer open 
> circuit and find the primary to have about 120 ohms from each side to the 
> center tap and the secondary 200 ohms and nothing is shorted.
> 
> Any thoughts, off the wall or not?
> 
> 73, Roger, NJ2R 
> 
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