Philips radio BCA495U (Algemeen)

door Pandu Rajan , 31.01.2019, 09:11 (2124 dagen geleden)

Dear forum members
I am in the process of the restoration of the above set. The set is identical to BX495U and similar to BX505U. I am facing the following problems. To start with I have replaced all the black tar capacitors with the same values and also capacitor C3. C1 and C2 are O.K.

A. Low Anode voltage of valve B3 UAF42.
The incoming voltage is 225v ac and the voltage at C1 is 188V and at C2 it is 160V. As per the schematic the anode voltage should be 45V but I get only 26V. Since I am not having new UAF42 valve I interchanged B2,B3 But The voltage remains nearly same. I also tried to replace Valves from the set BX485U ( which is working correctly) the voltage increases to 30V. I tried all possible combinations but the voltage does not go more that 30V. Presuming leakage between pin no 2 and the central spigot of the valve base ( This I had faced in one set earlier) I remove the earth connection of the central spigot and directly connected pin 4 and 7 to ground but the result is same and there is no leakage also. The screen grid voltage is 160V. The anode current is 0.6ma and the g2 current is 1.2ma.

B. I.F. & R.F. Gain of the set low.
Since I have a analog signal generator I keep the multiplier at X1 position and the attenuation pot full for carrying out the alignment. This helps me in comparing various sets during restoration.I know that one must work with the lowest possible signal bet the comparison becomes difficult. In a good set I get a DET O/P voltage of - 13 to 14V. Sometimes it is -11V also. But in this set I get only -2.8V. and no stations received. When I interchange the valve B2, B3 the DET O/P voltage became -5.5 volt but again the gain is very less. I replaced both the IFTs but the result is same. Changing UAF42/UCH21 does not help. So the voltage does no go more than -5.5. Under same conditions this voltage in BX485U is -8.6V.
The anode voltage of B2 is 158V and screen grid voltage is 90V. The anode current is 3.8ma and the screen grid current is 1.2ma.

C. When the set is tuned to a local station on MW band at 620Khz the gain is less with the antenna connected to antenna socket.But if the antenna is connected to the antenna coil S13 (SW band 50-150M) there is a lot of increase in the gain where as this is not the antenna coil for the MW band. Also the end of coil is S13 is open as the band switch is in MW position.Similarly when the Band switch is moved to SW band (50-150m)no stations are received with the antenna connected to the antenna socket.But if the antenna is connected to junction of S14 and C10 ( the other end being grounded) all the stations start coming. The resistances of Coils S13,S14.S15 and S16 are O.K.
D. Even the gains in the other 4 SW bands is low but they are much better.
I shall be grateful if somebody can help me in solving the above problems.

Thanks and
Regards
P.Rajan