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Old 22nd Aug 2022, 12:22 am   #8
unitelex
Hexode
 
Join Date: May 2005
Location: Greater Manchester, UK.
Posts: 492
Default Re: PHILIPS B5X44A VHF Tuner head wrong LO frequency

Thanks Peter,
This is quite interesting, I could read the first installment however I need to complete my registration with Radiomuseum to see the rest, I have requested to register...

Problem solved:
OK so to update my progress, I re-drew the schematic to make it more understandable from my perspective. That helped.
I could see from my measurements that the local oscillator tank circuit was not being effective, I checked continuity of the inductors and checked the value of the capacitors with my capacitance meter, all seemed OK. Then I began to suspect C407 coupling the 2nd triode anode to the oscillator tank circuit. The schematic has this as 8.2pF but the one in my set is marked with colours for 6.2pF and measures 6.4pf on my meter. Strange.. Since this capacitor couples the triode to the tank circuit and the IF transformer if this capacitor is faulty then it could explain two of my observed symptoms i) the output IF transformer resonating too high frequency and ii) no LO oscillation.
It measured OK on the meter but given the above and the value discrepancy I decided to swap it out. I did not have a 8.2pF HV ceramic but I had a 10pF silva mica. I put that in the circuit in place of the suspect 6.2pF and lo and behold I got VHF reception!
I have to say that C413 (105pF) is still padded out with additional 50pF and it needs this to remain stable.

Out of interest I also tried it with a Russian ECC85 near equivalent and that worked also.

So I have a tubular ceramic that tests OK on the capacitance meter but fails in circuit...
or was it just not quite enough capacitance to couple the tank circuit?

I will now continue to work on the crackling RH channel and the stereo decoder which also has a noise RH output

Incidentally I also found some very useful information about the VHF tuner in the Radio&TV servicing book (J.P. Hawker) Volume II 1955-1956, the preface section "Developments in Radio Receivers" explains a lot about the emerging VHF tuners and discriminators and discusses some of the techniques used. It is quite easy reading and gives a good background.

Regards
Chris

Last edited by unitelex; 22nd Aug 2022 at 12:24 am. Reason: correct typo
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