problemen Philips BX798X-02 : oplossing? (Algemeen)

door Pandu Rajan , 17.01.2016, 13:47 (3235 dagen geleden)

Dear members

Recently I have faced a peculiar problem in the above set.
1. On switching ON the set Hum was observed.Put the Radio/Gram switch to Gram position the hum was still there.
2. Removed the valves EF40 ans ECC40. Hum was still there.
3. Removed valve B9-EL41 the hum was coming.
4. When the valve B8-EL41 with B9 in Position was removed the HUM stopped.
5. Assuming that B8 had become defective put the valve B9 in the position of B8. Again the HUM started coming.
6. Fitted a new EL41 valve in B8. But Hum was coming.
7. Checked R70,R71,R69,R67,R66,R78,R68,C100 and C102 and found O.K.
8. Checked the voltages at C1,C2,C98,C99 and found O.K.
9. Checked the voltages at B8/B9 and they were as follows. Both the valves were in position.
Valve B9 -- Pin 2- 275V
Pin 3 - 7.22V
Pin 5 - 274V
Pin 6 - -5.8V with respect to cathode & with valve B8 removed
Valve B8 -- Pin 2 - 260V
Pin 3 - 16V
Pin 5 - 258V
Pin 6 - -13.25V with respect to cathode & with valve B9 removed.
The voltage at the cathode was coming very high.As per the service manual the cathode voltage should be 9.3V with both the valves B8 & B9 in position. With one valve in position it should be around 4.5V. The voltage at pin 6 in the valve B8 was also coming high because of the high cathode voltage.
10. The valve B8 was becoming red hot.
11. Disconnected the resistance R70 from B8 socket pin no 5. Now the cathode voltage became
7.2V.
12. But when the resistance R71 was disconnected from pin no 5 of the socket of valve B9 the cathode voltage was still at 16V.
13. Finally connected an ammeter between R70 and pin no 5 of B8 socket.The current was 18ma whereas as per the service manual it should be 4.8ma.
14. So the valve B8 at pin no 5 - g2 is drawing high current.

Kindly help me in solving this problem.

Regards

P.Rajan