Philips main transformer for BX696A

Dear forum members
Few days back there was a thread on the above subject and I thought that this the time to put my query. Recently I have restored the above set and I have this query
1. The set is identical to the set BX695A except for some changes in the components which includes the main transformer. In BX695A the type no is A3.141.53 whereas in BX690A the type is A3.141.54.0. I do not know the difference.
2. In BX696A the main transformer has a code no is A3.141.71.0.
3. The same transformer is used in BX704A but in the schematic the voltage given are 110-245V whereas in BX696A the voltage mentioned is 90-220V but the code given is same in the service manual.
4.In the set which I have restored the Voltage adaptor plate shows 110-245V. Somebody has replaced the main transformer with a Philips transformer belonging to some other set the code no of which is not known.
5. I checked the main transformer fitted in the set and actually the primary voltages are 90-220V whereas the adaptor plate is 110V -245V and is original.
6. So I presume the set BX696A was also manufactured with a transformer having voltages 110V to 245V whereas the schematic mentions 90-110V.
7. Now my query is as follows:
In the service manual of BX696A it is mentioned that the set is identical to BX695A. The secondary voltage of 4V and 6.3V is O.K. of the transformer fitted but the secondary H.T. voltage measured is is 280-0-280V with an input voltage of 223V a.c. and the transformer is for 220V. This is very much on the higher side. As per the service manual of BX695A the voltage at C1 is 255V D.C. and at C2 it is 224V D.C. This means that the Transformer secondary voltage should be around 230-235V each half. So what could be the code no of the transformer which has the secondary voltage of 280-0-280v.
8. Does the code no give any indication the the secondary voltage?

Best Regards
P.Rajan

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Hello Pandu,

Measuring an unloaded transformer often gives a higher voltage than under load.

I doubt if you would reach 255 Vdc after the rectifier with a transformer with 230-0-230 V at the current the radio will take (about 76 mA).

These curves for the AZ1 show that the actual dc voltage after the rectifier depends on both the current that will be taken and the resistance of the transformer (see the formula).

From the curves for 300-0-300 V you can estimate what they will be for 280 V. If R of the transformer is about 300 Ohm, than 280-0-280 V will give about 255 Vdc at 76 mA.

So I think that your transformer is fine.

As far as I know the codes of Philips transformers do not indicate voltages.

Greetings,
Robert

[image]

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Dear Mr. Robert
Thanks for your reply

I have done the following measurements.
a. On Load the voltages are Pri. 225V a.c., Sec- 272-0-272 at the 220V tap
Filament voltage - 4.06V and 6.23V
D.C. voltage at C1-297V and at C2- 257V
the voltages are with the set working.

b. The no load voltages with only the transformer energised
Pri. 223V a.c., Sec.- 280-0-280
Filament Voltage - 4.22V, 6.52V

c. The measured D.C. resistance of the transformer BX696A- Pri -25.2 ohms and Sec 195 ohms whereas the service manual gives values of 15 ohm and 150 ohm respectively.

d. In the set BX695A the values are 50 ohm and 300 ohm and in the set BX704A the values are 15 ohm and 180 ohm respectively.

Lower the values of the resistance means they are designed for higher current.

So on load also the voltages are high.

Regards
P.Rajan

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Hello Pandu,

OK, that is indeed too high.

What you could do is place resistors (two in total) between the outer taps of the secondary and the anodes of the AZ1 to lower the voltage. I would start with 2 x 390 Ohm (5 Watt) and than measure again.

Greetings,
Robert

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Dear Mr. Robert

Thanks your reply. I will try with your suggestion.

Regards
P.Rajan

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[image] [image] [image] [image] [image]

Connections of transformers A3 141 53, A3 141 54 and A3 141 71. Transformers A3 141 53 and A3 141 71 have the same replacement transformer A3 141 39, transformer A3 141 54 has a different one, A3 141 37. With a replacement transformer, the secondary voltages are the same as for the original, the secondary voltages of transformer A3 141 53. and A3 141 71 must then be almost the same. Primarily voltages of a replacement transformer differs from an original one, when it was replaced it was necessary to replace the voltageselector. Two different ones were available, 90-220V and 110-245V. point 1 of fig1 and 2 is point 1 of transformer A3 141 39 etc.

Adrie

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I don't get your problem.

Is transformer orginal with this set BX696A?

Then similar to 695 Ok Both EL41's
At setting 220V and apply 223Vac you got the 4V and 6,3Vac So there is nothing
wrong with the transformer it self.
Bad transformer always end up a Lower voltage.

So at start up , Is the Anode voltage at EL41's beyond specs EL41?

As often schematics just are schematics as a handout.

It is not a rule that the schematics you found is the one exactly confirm your set.

If the set has been working for app 70yrs with this transformer,
why you think it is the wrong model and so voltage?

--
"I haven't even turned this on to see if it works because as long as I don't it might"

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Pandu measures 297 Vdc at C1 and 257 Vdc at C2, while the radio is working (so under load), while the service documentation for the BX695A (very similar to the BX696A) states that those voltages should be 255 Vdc and 224 Vdc.

Greetings,
Robert

From the sevice manual for the Philips BX695A:

[image]

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Dear Mr. Maurice
1. The main transformer of BX696A has a code A3.141.71. Somebody has replaced with one which gives higher secondary voltage for the same primary voltage. The replaced transformer is also Philips but of some other set.

2. The code no of the transformer for BX695A is A3.141.53. I think the voltages of both the transformers are same but A3.141.71 has a higher current rating as the resistance of the winding is less.

3.The service manual of BX696A brings out only the changes that have been made in the set BX695A and clearly mentions that for other works the service manual of BX695A has to be referred.

4. 4V and 6.3V windings are O.K. and only the higher voltage in the secondary. Otherwise the transformer in general is O.K.

5. The anode voltage on the anodes of EL41 valve is 280V.

6. Why I came to the conclusion that the transformer has been replaced is when I started the restoration I noticed that the transformer had been mounted 180 degree shifted i.e. the front was at the back and the back was in the front so all the connections redone. All the wires of the voltage selector tap was cut.

7. The same transformer A3.141.71) has been used in BX704A and the voltage at C1 has been given as 265V and at C2 it is 230V

8. I made a simple experiment. With a voltage stabiliser I applied such a voltage so that
at C1 I was able to get 265V. At C2 I was getting 240V and the incoming voltage for this
setting was 200V. The anode voltages at AZ1 came to 244-0-244V.

Best Regards
P.Rajan

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Ah Thanks,

Clearly indeed 280V bit over done.
MAx ratings EL41 is 300V, so it will not break down, but
to other circuity it may be a problem.

The .71 from a former repair, that is obvious.

Best way is to find an other transformer.
Resisters could be in between solution.

--
"I haven't even turned this on to see if it works because as long as I don't it might"

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Dear Members

Thanks all for your reply.

Regards
P.Rajan

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