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Old 22nd Jan 2019, 11:33 pm   #1
Cathode Ray
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Default Audio trafo open cct? any fix?

Folks,
This is from my Philips BX548A which is almost there.
But I suspect the AF trafo is a gonner.!
Can anyon suggest some options?

In the diagram below winding S39 is open.
The other windings are good.
Is there any clever way to bypass or diy a fix?

I'm not sure what purpose S39 secondary serves, any ideas?
Going by the diagram, it dosent look to be a large number of turns.
Could I just bridge it, whats the worst can happen?

Thanks
Ray
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Old 23rd Jan 2019, 12:02 am   #2
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Default Re: Audio trafo open cct? any fix?

It's a negative feedback tap (typical Philips) and appears to be used as part of the tone control design. I would expect the radio to work after a fashion with the winding open circuit. You would need to modify the circuit to use a more conventional NFB arrangement if you wanted to use the radio permanently like that, it shouldn't be too difficult.
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Old 23rd Jan 2019, 12:08 am   #3
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Default Re: Audio trafo open cct? any fix?

It's a Philips and in this case even the circuit diagram is impossible to follow. It looks like feedback to me. Have you checked for dry joints to the windings?
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Old 23rd Jan 2019, 12:17 am   #4
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Default Re: Audio trafo open cct? any fix?

Thanks fellas,
Yes I checked the solder tag joints and they are fine.
Negative feedback seems to be it alright.

When I power the radio up there isn't anything from the speaker, and the audio drive valve glows blue at the base!
There may be another fault somewhere perhaps.
Ray
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Old 23rd Jan 2019, 12:21 am   #5
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Default Re: Audio trafo open cct? any fix?

Yes, you almost certainly have another fault in addition to the o/c winding.
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Old 23rd Jan 2019, 1:09 pm   #6
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Default Re: Audio trafo open cct? any fix?

There is a DC connection to the output valve via R35 and R40. Without the transformer connection, does this shut off the output valve?
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Old 23rd Jan 2019, 1:29 pm   #7
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Default Re: Audio trafo open cct? any fix?

Good question, I may just have to measure the grid bias to see if the valve is shut off.

What might cause the blue glow ?
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Old 23rd Jan 2019, 3:04 pm   #8
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Default Re: Audio trafo open cct? any fix?

Hi Ray, you seem to be tackling quite a few different sets lately; nice work. You might wish to contact Ed Dinning on this Forum for a transformer rewind quote. He rewound an output Tx for me from a Mullard MAS281/15 and did an excellent job. I have had more o/c output transformers on Philips/Mullard sets than any other make. Alternatively you should be able restore some negative feedback using a simple resistor and capacitor between the "live" speaker terminal and the cathode of the EABC80. It might need a bit of thought because the EABC80 is a triple diode triode so the cathode biasing might be an issue for its other functions. The following link might help: http://valveradio.net/audio/smart-ne...-feedback.html . Blue glow inside a valve usually indicates it is "gassy" i.e. loss of vacuum. If by the audio drive valve you mean the EABC80 it isn't a particularly reliable valve in my experience. Keeping one hand in pocket if you touch the blade of a screwdriver and touch the tip on the junction of R34/R35 you should get a 50Hz buzz from the speaker. If not either there is no HT or something is wrong in the EL84 circuit. As said above an o/c S39 alone would not prevent the EL84 stage from working. Next step is measure the voltages compared to those shown in the diagram. Cheers, Jerry
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Old 23rd Jan 2019, 3:24 pm   #9
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Default Re: Audio trafo open cct? any fix?

Hi Jerry,
That's a massive help, thanks.
I'm at work now, but can do some testing tonight.
It's the EL84 which is glowing blue, and oddly it seemed to stop when I switched band from FM to MW, I need to recheck the switching.

Just to be extra sure, when you say touch the screwdriver blade, do you mean for my body to make direct contact with R34,35 junction?
I'm using an isolation trafo so I expect this will be ok.

And as it happens, I purchased the isolation trafo from Ed Dinning!
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Old 23rd Jan 2019, 7:01 pm   #10
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Default Re: Audio trafo open cct? any fix?

Quote:
Originally Posted by Raymo99 View Post

Just to be extra sure, when you say touch the screwdriver blade, do you mean for my body to make direct contact with R34,35 junction?
I'm using an isolation trafo so I expect this work...
Hello, I’m concerned , why do you think it will work with an isolation transformer?

It doesn’t so much mean your ‘body’ as using just one finger on the blade, well clear of any other components ... or use the distal end of a little screwdriver with an integral neon bulb. The other hand should be behind your back and not touching anything. If you are touching something with your other hand you are at risk of getting a shock across your body , which is potentially deadly .

The isolation transformer doesn’t keep you safe. It avoids one particular kind of hazard only , a live chassis. You can still easily get a belter if you don’t know what you are doing .

Without even doing this screwdriver test there should be some noise and some hum from the speaker. Are you getting this, at least at full volume?

For the screwdriver test, safer is to do it at the centre connection of the volume pot with C62. Only still beware , there are often switches and exposed mains voltages or higher around where you are testing ...
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Old 23rd Jan 2019, 7:47 pm   #11
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Default Re: Audio trafo open cct? any fix?

Hi Astral,
I guess should have said "I expect this will be safe..." indeed it will inject mains hum with or without an isolation trafo.
And yes to everything you said about Isolation transformers, well understood here.

Back to the radio, there was no sound at all, not even a crackle at full volume, which suggests something is open.
The speaker reads 2 Ohms.
Ive measures all passives in the area and all seems correct.
I will measure voltages as suggested above and see what's what.

Thanks
Ray
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Old 23rd Jan 2019, 8:34 pm   #12
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Default Re: Audio trafo open cct? any fix?

So tests complete.
EL84:
+310v DC on pin 7, this is a fair bit higher than the stated 230v in the diagram.
+1.4v DC on pin 2 (grid)
and zero volts on pin 3, diagram states 6.3v
Heater on pin 4 is correct at about 6.2v AC
R37 reads correct at 150 Ohm.
And the blue glow is gone.

EACB80:
93v DC on pin 9, diagram states 75v

So what gives here, is the EL84 faulty?
I expect this may explain the high anode voltage, due to no current flowing.


Ray

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Old 23rd Jan 2019, 8:45 pm   #13
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Default Re: Audio trafo open cct? any fix?

Did you measure the voltage on pin 9 of the EL84 (screen grid)

Cheers

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Old 23rd Jan 2019, 9:14 pm   #14
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Default Re: Audio trafo open cct? any fix?

yes sorry, forgot to add it.
240V DC.
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Old 23rd Jan 2019, 9:33 pm   #15
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Default Re: Audio trafo open cct? any fix?

It seems your right there no or very little emission on the EL84 just a thought has the valve gone soft ?

The getter hasn't started going white has it.

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Old 23rd Jan 2019, 9:51 pm   #16
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Default Re: Audio trafo open cct? any fix?

If by getter you mean the normally dull tarnished look to the top of the valve.. the answer is yes, it has a cloudy white appearance.

New valve needed I'm thinking.

Ray
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Old 23rd Jan 2019, 10:01 pm   #17
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Default Re: Audio trafo open cct? any fix?

Yes that ones a goner

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Old 23rd Jan 2019, 10:06 pm   #18
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Default Re: Audio trafo open cct? any fix?

Thanks Mike and all.
Ordered one.

Cheers
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Old 1st Feb 2019, 2:46 pm   #19
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Default Re: Audio trafo open cct? any fix?

Progress update folks...
The new EL84 works fine and I now have reasonable audio output

But the radio howls with feedback when both the volume plus tone are turned up, low tone with high volume seems ok though!

I expect this must be due to the lack of negative feedback from the open turns in the trafo.

I see two options:
a. Leave the radio as it is, and perhaps come add some capacitor / resistor feedback circuit. It only seems to be unstable under certain conditions, so it might not need much negative feedback.

b . Have the trafo re-wound, but I'm not sure the radio is worth spending much more money on to he honest, it's not especially rare or sentimental.

Anyone got suggestions for option a?

Thanks
Ray
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Old 1st Feb 2019, 3:03 pm   #20
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Default Re: Audio trafo open cct? any fix?

The simplest option is remove the cathode bypass capacitor, this will introduce some negative feedback, perhaps increase the cathode bias resistor to 220 ohm.

Both will reduce the output but may be enough.
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