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Old 15th Aug 2022, 2:38 pm   #1
unitelex
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Default 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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Old 15th Aug 2022, 4:59 pm   #2
frsimen
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Default Re: PHILIPS B5X44A VHF Tuner head wrong LO frequency

C413 is part of a neutralising circuit. Murphy used to use something similar and had a note in their service manual that the value is critical. Too low and you get unwanted oscillation, too high a value and local oscillator won't oscillate.

Your set may work if C413 is replaced with a new 110pF component.

Paula
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Old 15th Aug 2022, 5:37 pm   #3
unitelex
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Default Re: PHILIPS B5X44A VHF Tuner head wrong LO frequency

Hi Paula.
Thanks for your information, that makes sense and is consistent with my investigations.
Usually these tubular ceramic capacitors are very stable and reliable however I will try your suggestion

Regards
Chris
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Old 15th Aug 2022, 9:18 pm   #4
unitelex
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Default Re: PHILIPS B5X44A VHF Tuner head wrong LO frequency

Unfortunately replacing C413 did not improve matters...
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Old 15th Aug 2022, 10:26 pm   #5
unitelex
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Default Re: PHILIPS B5X44A VHF Tuner head wrong LO frequency

13MHZ oscillation has been stopped by adding a ceramic plate decoupler from the ECC85 heater to ground.

But still no 80MHz local Oscillator signal...
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Old 15th Aug 2022, 10:41 pm   #6
Techman
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Default Re: PHILIPS B5X44A VHF Tuner head wrong LO frequency

Quote:
Originally Posted by unitelex View Post
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.
I think that's a wise decision for the time being.

The problem is with a lot of these sets is that people will twiddle them to try to get them to go beyond the 100MHz top frequency, so you could well be right in that those trimmers have been fiddled with. You may have to throw caution to the wind and twiddle them yourself once all other possibilities have been exhausted.
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Old 17th Aug 2022, 11:37 am   #7
orbanp1
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Default Re: PHILIPS B5X44A VHF Tuner head wrong LO frequency

Hi Chris,

On the radiomuseum web-site there is a good analysis of tube type FM-tuners:
https://www.radiomuseum.org/forum/gr..._analysis.html
It has a good explanation how those typical circuits work.

Regards, Peter
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Old 21st Aug 2022, 11:22 pm   #8
unitelex
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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; 21st Aug 2022 at 11:24 pm. Reason: correct typo
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Old 21st Aug 2022, 11:32 pm   #9
unitelex
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Default Re: PHILIPS B5X44A VHF Tuner head wrong LO frequency

So as not to confuse, The set I am repairing is B5X43A however I am using a schematic from the B5X44A which is practically identical but the reference designators are different.

If you are looking at the 43A schematic the cap in question is C87, in the 44A it is C407

Schematic below is the 44A (with removed capacitor also in the photo)

Regards
Chris
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