Radio set BCA495A (Algemeen)
door Pandu Rajan 
, 26.08.2023, 09:40 (247 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![[image]](images/uploaded/2023082607404364e9ac7b2c801.jpg)
Radio set BCA495A
door Maurice 
, Dordrecht, 26.08.2023, 10:53 (247 dagen geleden) @ Pandu Rajan
Nice digging through circuit to find the problem.
C42 is a rattling capacitor to even out spikes due to diode B5.
I think as those spikes help charging the negative more then wanted.
And due to low currents in negative circuit the negative charge is able to rise.
C50 think same reason, now disconnected there is less possibility to build up charge and so voltage.
Think all to do with heavy spikes and low current and so capacitors are able to increase voltage levels.
![[image]](images/uploaded/2023082608542764e9bdc3ed434.jpg)
--
"I haven't even turned this on to see if it works because as long as I don't it might"
Radio set BCA495A
door Pandu Rajan , 27.08.2023, 05:35 (246 dagen geleden) @ Maurice
Dear mr Maurice
Thanks your reply. What is the role of C3?
Regards
P.rajan
Radio set BCA495A
door henkk , 27.08.2023, 18:03 (246 dagen geleden) @ Pandu Rajan
Bewerkt door henkk, 27.08.2023, 18:11
C3 is used for smoothing the negative grid tension,it is always broken in these sets because
it dries out in time.
When C3 lost its capacity it gives very bad feedback problems.
Always replace it.
Henk
Radio set BCA495A
door Pandu Rajan , 29.08.2023, 09:47 (244 dagen geleden) @ henkk
Dear Mr Henkk
Thanks your reply. I also presume that the disconnection of C3 was the problem of Hum and the Increase in the screen grid voltages of B1 and B2. I thing the increase in the screen grid voltages was the result indicating low current flowing in B1 and B2 which would have happened due to the increase in the negative voltage at g1 of B1 and B2 and also the diode plate of B2. and finally this increase in negative voltage could have taken place due to C3 getting disconnected which is connected across R5 And R6. Junction of R5 and R6 is taken as the delayed AVC voltage for B1 And B2.
If somebody can give a better explanation I shall be grateful.
Regards
P.Rajan