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
> On Feb 28, 2019, at 12:25 PM, Mike Bryce <prosolar@sssnet.com> wrote:
>
> I’ll have to dig through the piles of manuals.
>
> mike
>
> “The trouble with quotes on the Internet is that you never know if they are
> genuine”
>
> —Abraham Lincoln
>
>> On Feb 28, 2019, at 12:50 PM, MadScientist <dukeshifi@comcast.net> wrote:
>>
>> You wouldn’t happen to have a schematic showing the later version?
>>
>> Gary
>>
>>> On Feb 28, 2019, at 11:48 AM, Mike Bryce <prosolar@sssnet.com> wrote:
>>>
>>> Gary…. yes they did. I too had a scout on my bench and it had FET drivers.
>>> If I remember correctly, there were FETs in the PA as well. At the time, I
>>> called and talked to Gary at Ten Tec and he told me they made the change
>>> due to manufactures discontinued parts. The pcbs were slightly changed as
>>> well
>>>
>>>
>
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