Philips radio 15RB708/00S (Algemeen)
door Pandu Rajan 
, 16.05.2015, 02:40 (3482 dagen geleden)
Dear Forum members
This stereo radio was manufactured by Philips India in 1970. The schematic of this set is on the NVHR site. Recently during restoration of this set I have faced the following problem.
With no signal at the antenna terminal, band switch in MW position and gang condenser in min cap. position. On measuring the voltage at the detector output ie. at the junction of R10 and S77 the DC voltage was coming to - 0.35 volts. At the junction of R10,R6 and C27 the voltage was - 1.5V. But on measuring the DC bias voltage at the pin no2 of ECH81 ie. g1 it was coming more than -5 volt. I replaced ECH81 and EBF89 but the effect was same.I have measured these voltages with the present day digital multimeter. During restoration of other sets using the same multimeter I have never observed the DC voltage going more than -1.5V at g1 of ECH81. I am puzzled as to why the voltage at g1 is going so high? What could be the reason?
I feel that due to this the RF output has decreased. Can somebody help me?
Regards
P.Rajan
Philips radio 15RB708/00S
door Ouwe Schipper , Sassenheim, 16.05.2015, 04:53 (3482 dagen geleden) @ Pandu Rajan
In your place
I'd remove all tubes from that radio and check this voltage again. If this -5 volt still is there check the AGC circuit as a whole. Then, to start with ECH, put in the tubes one by one, give each its warm up time and keep looking at the voltmeter
Just a tip to start with: of the ECH, short pins 9 (triode grid)&7 to pin 3 (cathode) which will stop the oscillator triode. The oscillator voltage is about -9 volt and there might be something wrong with the insulation of the tube holder
Nico
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