Philips India radio 15RB697 (Algemeen)
door Pandu Rajan 
, 02.12.2020, 13:05 (1235 dagen geleden)
Dear members
I am restoring the above set. I am attaching the schematic drawn for each band. I am facing problem in the SW4/SW3 band. When I am applying RF signal to the antenna socket - 17.2 mhz on the low frequency side and and 25.3mhz on the high frequency side I do not get any RF tone. I checked all the band switch contacts but they are all O.K. The coils S12/S13/S14 and S6/S7 are O.K. The capacitors C17,C20,C26,C19 are O.K. The coils are common to the SW3 band. The gang condenser is O.K. The MW band is working fine. I checked everything but no RF tone.All the connection are original Finally I shifted the connection of C17 from S12 to S13 and now the bands starts working O.K. This type of problem I have faced in some sets earlier also and shifting of the connection solved the problem. I am unable to understand as to why the RF tone does not come when the connection is to S12 and the tone comes when it is shifted to S13. What is the advantage of the double coil S12,S13 and what is the function of capacitor C20?
Best Regards
P.Rajan
![[image]](images/uploaded/202012021210115fc78423bdebf.jpg)
Philips India radio 15RB697
door Piet van der Pol , Alblasserdam, 02.12.2020, 14:03 (1235 dagen geleden) @ Pandu Rajan
Hallo Pandu,
Maybe this extra coil and capacitor is necessary for a more constant oscillator voltage over the whole band. You see also an extra resistor (22 Ohm) in the circuit, that can be necessary for the same purpose. With the changes you have made, the circuit will oscillate probably more easy, but with a more variable voltage. This can give more conversion distortion. So maybe you need a new B1 mixer tube.
Piet
Philips India radio 15RB697
door Pandu Rajan , 03.12.2020, 05:08 (1234 dagen geleden) @ Piet van der Pol
Dear Mr. Piet
I have changed the mixer valve ECH81 but the problem remains the same. I have just changed the connection of C17 to S13 instead of S12 but the capacitor C20 (395pf) is still connected.
Regards
P.Rajan
Philips India radio 15RB697
door MarcelvdG , 03.12.2020, 13:12 (1234 dagen geleden) @ Pandu Rajan
Are R3 and R4 and the supply voltage OK?
If Piet's hypothesis that the oscillator loop gain increases when you change the C17 connection is correct, then there must be something wrong that reduces loop gain to well below the design target. That could be a valve with reduced emission (but you checked that), dirty switch contacts (you also checked that), a drifted resistor or an insufficient supply voltage - and possibly other things I haven't thought about.
Philips India radio 15RB697
door Pandu Rajan , 05.12.2020, 15:07 (1232 dagen geleden) @ MarcelvdG
Dear Mr. Marcelvdg
I have checked the resistances R3 & R4 and they are O.K. The voltage at pin no 8 of ECH81 is 305V on the HF side and 102V on the LF side when the Band switch is in MW position. The set is working in MW position. When I keep the BS in SW4 then the voltage at pin no 8 of ECH81 is 105V on HF side and 65V on LF side. These voltages are when no signal applied. In the MW position though no signal is applied to the antenna terminal the ferroceptor is still connected.
Regards
P.Rajan