PLL Reference Oscillator Range Test

[Chris Kantarjiev K6DBG]

I just bought K2 s/n 3641. It's in good shape, but suffers from the
dreaded 'low audio'. Sidetone volume is loud enough - I wouldn't
like to operate at 255, but it wouldn't drive me from the room,
either (might damage my hearing with headphones).

I decided to go through the alignment instructions, both to see if
I could find a problem and to familiarize myself with the rig.

I found that the AGC voltage threshold setting was a little low,
so I corrected that (page 46 in my rev E manual) - about 3.36v, I
changed to 3.8v.

The displayed voltage is 13.2v for input of 13.8v; I expect 0.2v
drop across the diode (indeed, I measure 13.6v there). I guess this
is within 5% of 13.6, but it's a little disappointing :-)

The PLL Ref Oscillator Range Test went OK, but my range came out to
be 13.38 KHz, outside the stated 9.8-13 KHz range. Looking through the
reflector archives, it appears that an upper range of 15 KHz is more
suitable, post-temperature compensation circuit (which I believe
this has - the s/n is late enough, past 3446).

Just want to confirm that - should I bother to diddle the turns on T5,
as Don W3FPR suggested?

73 de chris K6DBG

P.S. I was a little surprised when the VFO Linearization display jumped
from the 40m band that I had selected according to the directions,
to the 12MHz range ... it seemed to work OK, but ...

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[Don Wilhelm-3]

Chris,

I did not suggest changing the spacing on T5 to change the PLL range!!! If I
said that, it must have been a typo, and I retract it.

Changing anything on T5 will not change the PLL range.  If you want to
tighten up the range a bit, you can change the black (shielded) inductor L31
on the bottom of the RF Board.  If your inductor is 12 uH, it can be changed
to 10 uH - if it is already 10 uH, you would start by changing to a
different crystal, but the 13.38 kHz range is not far off, and likely will
be just fine.

Changing the turns spacing on T5 will only modify the voltage ranges read at
the left end of R30 - this sets the VCO operating range, but does not change
the PLL.

You may want to lower that AGC Threshold down a bit from the 3.8 volts to
see if that improves your low audio.  Higher voltages produce more AGC
action and thus hold the receiver gain down.  After optimizing a K2, I find
the AGC Threshold voltage often is between 3.65 vlts and 3.8 volts.

Toggle the AGC on and off by pressing the AGC and PRE/ATT buttons together
(with no antenna attached to the K2) and see if there is any change in the
receiver noise level - a change in noise level indicates that the AGC
Threshold voltage is too high.  The exact voltage it should be depends a lot
on the 8 volt regulator output - 3.8 volts will be correct for regulators
with at least 8 volts output, but if it on the low side of 8 volts, the AGC
Threshold must be set lower than 3.8 volts for optimum receiver performance.

Also check the orientation and turns ratio of T7 - the green winding is
connected to pads 1 and 2 while the red winding connects to pads 3 and 4 -
this is different than the other transformers in the K2, and builders
sometimes get it backwards.

If all that is OK, you might just have higher expectations than the design
allows - the K2 has ample volume to fill a normal room with the AF Gain near
the 9 or 10o'clock position, but it does not have 'ear-splitting' reserve
volume as found on many other transceivers.  The K2 was designed for low
receive current draw, and a higher power audio amplifier would have defeated
that goal.  If you need 'ear-splitting' audio, I recommend an external audio
amplifier - many folks have reported good results with the powered Motorola
speakers (check Ebay).

BTW, yes the CAL PLL function demands to be on 40 meters, but it indicates
the actual range of PLL/VCO frequencies while running and that starts near
12100 and decreases from there - what you saw was quite normal.

73,
Don W3FPR

> -----Original Message-----
>
> I just bought K2 s/n 3641. It's in good shape, but suffers from the
> dreaded 'low audio'. Sidetone volume is loud enough - I wouldn't
> like to operate at 255, but it wouldn't drive me from the room,
> either (might damage my hearing with headphones).
>
> I decided to go through the alignment instructions, both to see if
> I could find a problem and to familiarize myself with the rig.
>
> I found that the AGC voltage threshold setting was a little low,
> so I corrected that (page 46 in my rev E manual) - about 3.36v, I
> changed to 3.8v.
>
> The displayed voltage is 13.2v for input of 13.8v; I expect 0.2v
> drop across the diode (indeed, I measure 13.6v there). I guess this
> is within 5% of 13.6, but it's a little disappointing :-)
>
> The PLL Ref Oscillator Range Test went OK, but my range came out to
> be 13.38 KHz, outside the stated 9.8-13 KHz range. Looking through the
> reflector archives, it appears that an upper range of 15 KHz is more
> suitable, post-temperature compensation circuit (which I believe
> this has - the s/n is late enough, past 3446).
>
> Just want to confirm that - should I bother to diddle the turns on T5,
> as Don W3FPR suggested?
>
> 73 de chris K6DBG
>
> P.S. I was a little surprised when the VFO Linearization display jumped
> from the 40m band that I had selected according to the directions,
> to the 12MHz range ... it seemed to work OK, but ...
>
>

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