As you can see from the diagram, IC2 is a GP214D
dual PLL chip. Only the left hand side is used in these micro transmitters.Commercial transmitters have a long time constant, many take several seconds to lock.
This technique ensures that the PLL completely ignores signals in the audio region, even right deep into the low end region.
This prevents bass roll-off caused when the error voltage tries to correct and therefore 'wipe out' bass content.
Of greater advantage is that it can help prevent the nasty whine which is typical of the 668A.
This mod is for the ChaoYu 668A MK2 which is best described as being the unit which has the built-in LED Flashlight.
As you can see from the diagram, IC2 is a GP214D dual PLL chip. Only the left hand side is used in these micro transmitters.
The tuning voltage is produced internally by the GP241D and appears at CP1 (pin3). C1, C2, C3, R3, R4 act as a network to iron out any ripples which may be produced by the charge pump as well as any other high frequency synth noise.
Increasing the time constant will allow the bass response to roll off lower as the loop will have less potential to 'wipe out' any lower frequency audio component.
Locate capacitor C2 which is connected to pin 3 of the PLL chip. Change it for a 10µF tantalum observing polarity. Note, there's not much clearance with the back of the battery compartment.
Increase the value of C3 as far as you can go without incurring excessive lock times.
As yet incomplete and in places erroneous schematic from what I've pieced together.
Requires Eagle by CadSoft
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