Philips radio BCA628A (Algemeen)
door Pandu Rajan 
, 25.04.2022, 07:32 (943 dagen geleden)
Dear friends
I am in the process of restoring the above set. After completing the restoration and applying power following readings have been noted: The set is working properly with all the valves in position. The rectifier valve is AZ41.
1. Incoming voltage - 226V A.C.
2. Secondary transformer voltage - 275-0-275
3. filament voltage ( 4V) - 4.41V A.C.
4. Filament voltage ) 6.3V) - 4.68V
5. Voltage at C1 - 322V
6. Voltage at C2 - 299V
I am unable to understand why there is such high voltage at C1? The transformer is original and all the connections are also original. As per the schematic the voltage at C1 should be 260V and at C2 -235V for an incoming voltage of 220V. The set is similar to BCA645A. The schematic for BCA645A is in the site. In most of the set which I have restored for a 260V/255V d.c. voltage at C1 the secondary transformer voltage is 235 -0-235V or 230-0-230V. But in this set the secondary transformer voltage itself is very high. I have measured the current in the secondary winding going to AZ41 pin 2 is 22ma on load.
Kindly advise.
Regards
Rajan
Philips radio BCA628A
door Ouwe Schipper , Sassenheim, 25.04.2022, 07:53 (943 dagen geleden) @ Pandu Rajan
Well, Rajan
This 4,68 volt which should be 6,3 sounds funny to me, as all other secunary voltages
seem to be rather high. What do you measure there with all valves removed?
Anyway, measuring the current at pin 2 of the AZ 41 seems rather useless to me. The DC
current coming from the rectifier can best be measured at the center tap of the transformer
As a whole, I have my doubts about the output valve
Nico
--
een kinderhand
is ook weer gauw leeg
Philips radio BCA628A
door gertjanrhe , Gooi, 25.04.2022, 22:26 (942 dagen geleden) @ Ouwe Schipper
the heater voltage should be 6.3V but is far to low. This causes a low emission at the various tubes which makes that the high tension voltages are higher because the load current is much lower. It seems to be half of the nominal value. Maybe there is a short in a heater circuit in one of the valves? You can check it by removing them one by one.
--
De techniek was in de jaren 40 veel verder dan menigeen zich bewust is
Philips radio BCA628A
door Pandu Rajan , 27.04.2022, 09:24 (941 dagen geleden) @ Ouwe Schipper
Dear MR. Schipper
You were correct in suspecting the O/P valve. The no load filament voltage was 7.35V. With all the valves in place this was dropping to 4.68V. I removed the EL41 valve and measured the total D.C. current and it was 19ma. With the defective EL41 this was coming to 27ma. In the schematic for the BCA628A The various currents are given and the total current comes to 60ma. So the current with the old EL41 valve was very less and this was the reason for the high D.C. voltage at C1 and C2. I replaced the valve EL41 and the various voltages measured are as follows:
1. Incoming voltage -- 223V a.c.
2. Secondary voltage -- 272-0-272 V a.c.
3. filament voltage (4V)-- 4.34V a.c.
4. Filament Voltage (6.3V)-- 6.34V a.c.
5. D.C. voltage at C1 - 283V
6. D.C. voltage at C2 - 258V
7. The total d.c. current - 58.4ma.
But still the voltage at C1 and C2 is high by 23/28V
Kindly advise?
Regards
P.Rajan
Philips radio BCA628A
door Maurice 
, Dordrecht, 27.04.2022, 10:11 (941 dagen geleden) @ Pandu Rajan
Bewerkt door Maurice, 27.04.2022, 10:18
You can't lower C1 as long as Secondary voltage 272V ac is.
As found on a picture, Main transformer should be A3.141.68
EL41, I mostly check the voltage across the cathode resistor to determine the current
through output tube roughly.It also indicates a correct Bias setup for the tube.
To determine the Anode current just measure the Amp through measuring across the primary
output transformer. Indeed you shorten the output transformer and therefore silence the sound through this method, but its a correct way.
To solve the issue you could put a series resistor app 470 Ohms and couple of Watts.
EDIT: A3.141.68 also found in the BX520A, Vc1=265Vdc, Vc2=245Vdc, indicating the transformer should be 250-0-250 in my opinion.
--
"I haven't even turned this on to see if it works because as long as I don't it might"
Philips radio BCA628A
door Ouwe Schipper , Sassenheim, 27.04.2022, 12:47 (941 dagen geleden) @ Pandu Rajan
Bewerkt door Ouwe Schipper, 27.04.2022, 12:53
Allright Mr Rajan
So it was the EL41 indeed; as for these DC voltages being about 10 % too high:
There always is the solution already mentioned: a series resistor. I myself
might make a choice: put two resistors in the anode leads of the AZ41, (value
to be determined by experiment) The other choice is to leave the set as it is,
for these 10% does not seem really critical to me so I wouldn't bother
Nico
--
een kinderhand
is ook weer gauw leeg
Philips radio BCA628A
door Pandu Rajan , 27.04.2022, 14:11 (940 dagen geleden) @ Ouwe Schipper
Dear friends
Thanks all for the advise.
Regards
P.Rajan