Jukka,
You are on the right path. When the voltage is high, there is too much
capacity in the circuit and the VCO/PLL is trying to reduce it - but it
runs out of steam at about 7.5 volts and cannot change the capacity any
more.
C75 in your K2 needs less capacitance. You can try several other 470 pf
capacitors (NPO disc ceramics or C0G mono capacitors) to see if you can
discover one that is on the low side of tolerance rather than the high
side (like yours), or you can replace C75 with a 390 to 450 pf capacitor
and add padder capacitors to bring the total capacitance up so the 160
meter VCO voltages are in the 1.5 to 7.0 volt range over the entire
tuning range.
73,
Don W3FPR
Jukka Tarvainen wrote:
Hi!
I got suggestion from Rick, KC0OV, to check VCO control voltages.
1800 kHz 3,5 V
1960 kHz 7,5 V
1970- kHz 8 V
Other bands are well inside 1,5-7,5 V.
I search archives and found case where it has been below 1,5 V and that was
cured with additional capacitance in C75. I test with some other big size
capacitors around 450 pF and found that VCO voltage can be lowered. I need
to buy several small caps to make suitable replacement for original 470 pF.
We will see if this helps.
73
Jukka OH4MFA
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