Whoa!
That’s a great catch. I’d never would have suspected the Jones filter to cause
a transmit issue. Are you producing full RF output now?
Only once did I find a shorted .01 capacitor and that was in a Drake W4
wattmeter. I found one of the old micas (the rectangular shaped ones with the
dots) that was dead short in an old tube audio amplifier.
Working with these old radios reminds me of my ex-wife’s cooking—Every bite was
different!
mike, wb8vge
“The trouble with quotes on the Internet is that you never know if they are
genuine”
—Abraham Lincoln
> On Feb 28, 2019, at 2:27 PM, MadScientist <dukeshifi@comcast.net> wrote:
>
> Thanks!
>
> I now find that, no matter how long I work on stuff like this, I never stop
> learning new things! I guess that’s part of the reason I do it.
>
> This Scout came to me with “no transmit”. It did receive.
>
> I made measurements on the driver stage, based on the schematic, and found
> the “base" voltages to be quite far off (greater than 0.7 volts base to
> emitter) so I assumed that the devices, which at the time I thought were
> bipolar transistors, according to the schematic so I assumed that they were
> bad, possibly from an oscillation problem, so I ordered replacements using
> the numbers on the devices that were in the radio. I had no reason to
> question whether they were bipolar or FET’s since the schematic was quite
> clear on this matter.
>
> Then, in an effort to test the calibration of my signal generator and
> attenuator for checking sensitivity of an Omni 6 I am repairing, I found that
> the Scout had very weak reception, only showing about S 9 at 0 dBm! You could
> still hear signals but touching the center of the unterminated coax connector
> only produced a barely noticeable hiss, not the screaming noise to which I am
> accustomed in my noisy basement shop.
>
> I concluded that the receiver also had problems. After a long and frustrating
> evening of following the signal from the antenna terminal to the IF board,
> through the filters etc., I found that the Jones filter was attenuating
> signals by about 40 dB!
>
> Thanks to a reference on one of the ham sites, I found a link to the patent
> for this filter. That gave a detailed schematic and technical description of
> the filter so I decided to try to fix it. I terminated the ends with 50 ohms
> and applied my spectrum analyzer’s sweep generator to the input and followed
> signal through the filter with the analyzer’s input.
>
> I didn’t have to go very far. Much to my surprise, the 0.01 uF ceramic
> capacitor coming from the input terminal to the first crystal was dropping 40
> dB from one end to the other.
>
> Now I have never in my life seen an open ceramic capacitor, especially in low
> level circuits like this. I have seen them explode in TV high voltage
> supplies and in electron microscope supplies but never in small signal
> applications.
>
> I replaced the cap and got exactly the passband shown in the patent at all
> control voltages, with only a few dB insertion loss.
>
> Then after I reinstalled the filter into the radio I checked for TX drive at
> the input to the predriver transistor. I found that I had drive to burn! So
> the Jones filter was the culprit all along!
>
> Put this one into the archive…
>
> Gary
>
> W0DVN
>
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