[Central-Electronics] The Case of the 100V Audio Filter

[JAMES D COMSTOCK]

 Last time I had a similar problem with my 100v I gave up and completely rebuilt the filter with all new components. It worked for a while.
 Eventually I ended up running a tiny steel brush through the socket connectors and cleaned, tightened and re-soldered all the connections. The problem has not returned. If it does I will replace the socket.   Jim K0jjf



> On 01/05/2021 10:47 AM Robert Nickels <ranickels at gmail.com> wrote:
> 
>  
> I can toss out a mystery for the group to ponder:
> 
> I've been trying to find the reason why the output power of my 100V is 
> low.  Sometimes it would start out OK at 100 watts but then dropped to 
> 60 watts or less a half hour or so later.
> 
> I've narrowed the problem to the Audio Filter module but it's puzzling 
> because I can't find anything wrong with it.    If I bypass it by 
> jumpering  pin 2-5 so the phasing system is driven directly from the 
> output of the Audio Limiter module, I can get full 100 watt output with 
> 2.25 volts peak-to-peak out of the limiter.    The output of the filter 
> is about 1/2 that level and the waveform becomes distorted as audio 
> level is increased.   If I adjust the audio input to provide that same 
> 2.25 volts out of the limiter with the filter in place the transmitter 
> power output is only 30 watts.   Even if I increase the audio level 
> greatly the output never gets to 100 watts.
> 
> This makes it seem that the Audio Filter is defective, but I've run the 
> module on the bench for hours, sweeping from 100 to 5000 Hz and it works 
> perfectly.    The voltages in the transmitter are correct (6.5VAC fils 
> and 315 VDC) but instead of the nice sine waves and consistent output 
> within the filter bandpass I see on the bench, the output becomes 
> distorted and triangle wave shaped as the level is increased, and will 
> not drive the transmitter to full output even then.
> 
> The next stage in the 100V is the phase inverter which presents high 
> impedances so it can't be a loading effect, besides the limiter will 
> drive this stage just fine.
> 
> I'd like to have the audio filter inline but am getting nowhere in 
> figuring out why it works on the bench but not in the transmitter and am 
> about ready to bypass it and call it good. Wonder if anyone has run into 
> this mystery before or has any suggestions?
> 
> 73, Bob W9RAN
> 
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