Radio set BCA495A (Algemeen)

door Pandu Rajan , 26.08.2023, 09:40 (454 dagen geleden)

Dear Friends

Recently I was in the process of restoring the above set. This set is identical to BX495A.
1. During taking trial suddenly Hum started coming.
2. When put on Pick-Up mode there was no Hum.
3. Replaced valve B1 and B2 one by one but no change.
4. Checked the voltages of B1 & B2. The screen grid voltages of both these valves had become
more than 200V whereas it should be around 100V as per the service manual. Earlier it was
around 95V ( before the hum problem started).
5. Also the negative voltage at the diode plate of B2 (pin 3) and pin no 6 of B1 had become
more than -10V.
5. I tried all possible checks but could not solve the problem.
6. When I disconnected C50 this problem was solved and the screen grid voltage also became
O.K.
7. On connecting C50 same problem was there.
8. Presuming C50 to be defective replaced C50 by a new one but again the same problem was
there.
9. Struggled for some days to solve the problem but no success.
10.Recheck all the wiring under the chasis but everything was o.k.
11.One day while checking the wiring on the top of the chasis found that junction of C3 and
C42 which is connected to the negative terminal of C1 & C2 had got diconnected. Connected
the same and the problem is solved.
12. The HUM , Screen grid voltages and the -ve voltage all are now OK.
13. The set is working satisfactorily.
14. Now my question is why the screen grid voltage and the -ve voltage had increased when
the capacitor C3 was disconnected? Also why the problem was getting solved by
disconnecting C50?
I am attaching the Schematic of the above set.

Regards
P.Rajan