problemen Philips BX798X-02 : oplossing?

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

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Can you or anyone else ad a cirquit to this posting. That will show more info on how a problem can be solved.

--
groet Sander
[image]

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Dear Mr.Leuissen
The schematic has already been uploaded on John's radio web.

Regards
P.rajan

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Dear Pandu,

It is wisely, that you replace the original couplingcapacitors c100 (from anode b7 to grid b8) and c101 (from other anode b7 to grid b9), since they are mostly leaky and don’t hold back the higher dc voltage from the anodes from b7.
It’s also advisible to split the cathoderesistor r68 into two cathoderesistors, for each output tube one resitor, with a value twice of the value of the old resistor.
Do place an electrolytic capacitor with the value of 100 uF/25V dc across each cathoderesistor.
Then at last, measure the resistance of the windings s59/s60 from the primary windings of the output transformer. They should not differ too much.
Its is also clear, that the output tubes differ from each other.

Success.
Alco Bouwense

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It looks like a leaky valveholder B8, check the isolation between contacts 5 and 6.
rgds ,
Sieme

--
het experiment heeft altijd gelijk

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Dear Mr. Sieme
Thanks to all for the reply. I carried out further checking as follows:

1. I removed valve B8 and checked for any leakage at the terminals 6 and 7. But no leakage observed.
2. Considered the changing of the base for B8 valve as a last option.
3. The wiring to the k,g3 for both the valves B8 and B9 has been done as follows. A wire is connected to the terminal no 7 - k,g3 to the spigot of the valve holder of B8. From the spigot one wire goes to C102 and the other wire goes to pin no 7 of B9 which is again connected to the spigot of B9. R68 is connected from this spigot of B9 base to ground.
4. I suspected leakage voltage from the pin 5-g2 of the valve B8. I did not suspect leakage from pin 5 to pin 7 as it is at a good distance. I suspected a leakage from pin 5 to the spigot of B8 as both are very close.
5. I removed the wire from the spigot of B8 and directly connected pin no 7 of B8 to pin no 7 of B9 thus isolating the spigot of B8 and avoiding any leakage.
6. Now there is no Hum and all the voltages are O.K.
7.The final voltages are as follows.
a. At C1 - 301V
b. C2 - 290V
c. C99- 244V
d. C98- 211V
e.Valve B9 pin 2- 294V
pin 5- 290V
pin 6- -7.83V with respect to Cathode.
pin 7- 8.54V
f.Valve B8 pin 2- 295V
pin 5- 291V
pin 6- -7.95V with respect to Cathode
pin 7- 8.62V

Regards
P.Rajan

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