Low PLL Output

[Chuck-51]

Greeting to the List,

This is my first post.  I'm building K2 sn5939 and have hit a snag at alignment and test part II.  It actually went through all of the tests and alignment OK, but when I hooked an antenna up to try the receiver I notices some unusual symptoms.

To cut to the chase, what I have found is the PLL reference oscillator output low.  It measures .3v at pin 1 of U4.  The expected value is .8v-1.8v .  The frequency counter picks up the output and the output frequency varies over the proper range.

I have checked yet again for bridges, cold solder joints and all component values.  All seems correct.

The DC source voltage measures 1.27v with an expected value of .8v .  This tells me Q19 is not conducting as it should.  Before I declare Q19 lightly toasted, does anyone have any ideas or suggestions?

Thanks.

Chuck K4QS



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[Chuck-51]

I should have proof read this first.  That is the source voltage on Q19 that is off.


Chuck

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[Don Wilhelm-3]

Chuck,

You are on the right track, but I don't believe you can conclude that Q19
itself is at fault.  The higher voltage you find at the source is caused by
Q19 conducting more than it normally does, and that indicates the gate bias
is running too high - this JFET (like many other oscillators) uses self
biasing which depends on the oscillation itself to properly control the
bias..

First thing to check is R21, followed by D18 and the PLL crystal and L31.
Lastly check capacitors C64 and C65 and varactors D16 and D17.
Check all these components for correct values and good soldering.  Check the
diode for proper orienation (refer to the parts placement diagram).

If you have doubts about what a good solder connection should look like,
take a look at the soldering information on the Builder's Resource page at
the Elecraft website www.elecraft.com .

73,
Don W3FPR

> -----Original Message-----
>
> To cut to the chase, what I have found is the PLL reference
> oscillator output low.  It measures .3v at pin 1 of U4.  The
> expected value is .8v-1.8v .  The frequency counter picks up the
> output and the output frequency varies over the proper range.
>
> I have checked yet again for bridges, cold solder joints and all
> component values.  All seems correct.
>
> The DC source voltage measures 1.27v with an expected value of
> .8v .  This tells me Q19 is not conducting as it should.  Before
> I declare Q19 lightly toasted, does anyone have any ideas or suggestions?
>
>

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