Do you mean what you says that phase difference between channels is not 90 degree?
Yes.When the phase difference between channels is not 90 degree them are not locked . The vco ( center frequency : 280Mhz , RFin:140Mhz ) of TDA 8040 is controlled from a demodulator controller and when the phase difference between channels is 90 degree them are locked and the frequency of vco is fixed at 280 Mhz . I see at open loop a correct voltage at vco of receiver difect .
There is one possibility, VCO signal does not drive 90 degree to the mixer du to low Q factor utilization.In this case you may not be obtained sufficient drive level.
What test misure do here ? Misure only level carrier 280 Mhz ?
Another possible problem may be high signal drive ( most possible). A high signal drives input amplifier into nonlinear region and amplifier disturbs phase between I and Q channels.
Are talking of amplifier internal at TDA 8040 ? If yes , output of low pass filter is a input for amplifier of TDA 8040 . If i calculated well the frequency response od filter , it is not flat and when reduce capacitor from 220 pF to 60pF become more flat change signal level and group delay of course.Perhaps an amplifier of the TDA 8040 has characteristics a little diverged from the specific ? I will try to change it again.
I recommend you that to use Bessel type filter for minimum group delay.
I thought at Bessel type filter too but this is not my design and is not prototype because is in production from at least two years.
The problem is only on some receiver , i need to search difect components on bad receiver . I here would like have of the suggestions to do a good diagnosis , search breakdowns .
Otherwise you can be face to face signal delays between lower and upper frequency limit points.
How to do this with instruments ?