Philips India radio B6CA37A/01 (Algemeen)
door Pandu Rajan 
, 26.07.2015, 13:03 (3410 dagen geleden)
Dear Members.
The schematic of this set is on the forum site. It is a 6 band set with band spread on 19m,25m,and 31m bands. The other 3 bands are MW,SW1 and SW2. The SW1 band covers 11m to 16m. I am restoring one of these sets and I am facing the following problem. I have been able to align all the bands properly except SW1. When I start the alignment I am able to get the audio tone in the speaker with pointer set at 25.5Mhz and the signal generator also set at 25.5Mhz. As the pointer is moved towards the low frequency side and the signal generator also moved towards the LF side the signal strength starts reducing and around the middle of the dial it is completely lost. So Towards the low frequency end i.e. 17.9Mhz no tone is heard. If you go back towards the HF side the tone again comes back. How I should trace this fault?
Regards
P.Rajan
Philips India radio B6CA37A/01
door ruudtx , 26.07.2015, 14:18 (3410 dagen geleden) @ Pandu Rajan
As it seems it might be the local oscillator that stops.
You can check this out by using a second receiver tuned to the frequency (lost) + 450 kHz. You must hear a carrier on the second receiver. When reception stops check if also the oscillator signal is lost.
Philips India radio B6CA37A/01
door Pieter Vos , 26.07.2015, 15:41 (3410 dagen geleden) @ ruudtx
To check wether the oscillator operates you may also measure the grid voltage with a high impedance DC voltage meter. Use a series resitor of at least 100 kilo-ohm between the oscillator tube grid and the voltage meter in order to have the capitance of the meter leads isolated from the oscillator circuit. The oscillator tube grid should have substantial negative voltage when the oscillator produces signal. When the oscillator stpos working the grid voltage returns to approximal zero.
In stead of a high impedance DC voltage meter one may also use a magic eye tube which is in fact a qualitative negative voltage indicator.
Best wishes, Pieter.
Philips India radio B6CA37A/01
door Sieme , Niekerk, 26.07.2015, 17:56 (3410 dagen geleden) @ Pandu Rajan
It looks like a mistuned antenna circuit on that frequencyband.
The antenna section of the tuning-C is not properly coöperating with the antenna-coil
and the circuit is further mistuned when increasing the required capacitance i.e. at the lower frequecies.
I would suggest checking the appropiate bandswitch contact and /or the associated fixed capacitors (trimmer?).
regards,
Sieme
--
het experiment heeft altijd gelijk
Philips India radio B6CA37A/01
door Pandu Rajan , 27.07.2015, 01:49 (3410 dagen geleden) @ Sieme
Dear friends
As per the advise given by all I will carry out further checking.
Regards
P.rajan
Philips India radio B6CA37A/01
door Pandu Rajan , 29.07.2015, 12:28 (3408 dagen geleden) @ Sieme
Dear friends
I located the fault for the SW1 band in the above set. The oscillator trimmer C42 was faulty. I replaced the 30pf trimmer capacitor.Also a shielding foil is provided under the gang condenser C17-C18 which is fixed to the chasis with a metal strip and two screws. During the restoration I had to change the copper foil and I forgot to completely tighten the metal strip and therefore it was not making proper contact with the Chasis. After tightening the two screws properly I have been able to align the SW1 band properly.
Now my question is what is the purpose of this shield and how far this copper foil shielding is important for the working of the different bands?
Regards
P.Rajan