PHILIPS B5X44A VHF Tuner head wrong LO frequency
Working on a Philips B5X44A with no VHF FM reception.
Tested the IF with a modulated 10.7MHz and that is working fine, right through to the demodulator, and the demodulator is set up correctly.
The VHF tuner however (ECC85 based) is not operating properly.
The first stage (RF amp) seems to be OK, correct voltages and the small 90...100MHz signal can be seen on the scope.
The mixer oscillator is not oscillating at 80..90 MHz though, it is generating a large >5V P/P sine wave at 13MHz. It is generating this independently of the preceding RF amp as the 13MHz persists even if the RF amp grid is grounded.
It is also oscillating independent of the other parts of the set, for example oscillation continues with the tuner output coax completely disconnected.
So it seems the LO Mixer Oscillator triode is oscillating close to the IF frequency via some local feedback path.
The mixer oscillator anode voltage is ~98V as read on a Fluke meter rather than the 150 shown on the service sheet however another good working set of a similar type (also Philips also read 98V on the same Fluke meter at this position)
Adjusting the main tuning capacitor does not change the 13MHz however adjustment of the IF output transformer core (S409/S410) does move the frequency up and down by 2 MHz, the lowest achievable being >11MHz
The amplitude is highest at the point S409-R403-C413
Have checked R's and C's and Swapped out the ECC85 for a NOS but no improvement.
Have put additional decoupling (ceramic cap with short leads) on the incoming HT with no improvement observed.
Grounding of the VHF tuner seems solid to the chassis
I have cleaned the valve base. There was a fair amount of oil present which seems to have come in via the tuner slugs. I have cleaned off as much of this as possible
The oscillation does stop if C413 (110pF)is padded out with additional capacitance, adding 47pF is enough however with this padder there is no correct LO frequency (80MHz) present, but I can see some of the incoming 90...100MHz once the 13MHZ is quiet
I have not adjusted the main LO frequency setting trimmers. There is a possibility that these have been upset before I received the set.
I am loathed to adjust these until other causes have been eliminated.
Do the experts here recognise this instability issue? and could it be caused by incorrect setting of the main LO trimmer capacitor and inductor?
Thanks,
Chris
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