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Old 30th June 2003, 10:29 AM   #1
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Default Problems with P3A after refitting

Hi!

I have been refitting my P3A with new resistors (RC55) and one of side with new power devices (MJ15003/15004) and new VR1, since I killed them due to slipping probes (I had connected the collector of Q7 to ground).

The side without new power devices works well, but the one with new transistors doesn't.

Tests like Rod describes with 22 Ohm resistors are OK, +- 42 V on the rails, 4 mV dc offset

But then measuring voltage across the 0.47 ohm resistors (which should be about zero), I measured 38 V across R13... as well as between the collectors of Q7 / Q8

Then I also exchanged all other transistors (the BD139/140 and BC546 types), and measured again. Voltage across R13 went down to 9.3 V, as well as between collectors Q7 / Q8, but still way too high.

Amazingly nothing gets really hot.

Has anyone an idea of what to look for? I resoldered everey solder joint, checked for shorts, did not find anything...?


Bye,

Arndt
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Old 30th June 2003, 10:38 AM   #2
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Hi,

If you have the schematic and care to share it with us, one of us would be able to help you out.

Regards,

Audiofanatic
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Old 30th June 2003, 10:42 AM   #3
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Hi!

You can find it at ESP, but I steal a picture:

Click the image to open in full size.

Just one comment: I do not use A1302 / C3281 but MJ15004 / MJ15003...

I had just an idea (but I am not home, so I can't check it), that it maybe R13 itself which could be faulty...


Bye,

Arndt
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Old 30th June 2003, 11:25 AM   #4
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Hi!

Just got back, checked R13, and it was broken (infinite resistance).

Exchanged it, and now it works again...


Thanks anyway,

Arndty
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Old 30th June 2003, 12:27 PM   #5
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Quote:
Originally posted by Cradle22
Just got back, checked R13, and it was broken (infinite resistance).

Exchanged it, and now it works again...
with the kind of voltage you got over it, it would have been generating hundreds to thousands of watts,
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Old 30th June 2003, 06:06 PM   #6
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Quote:
Originally posted by millwood


with the kind of voltage you got over it, it would have been generating hundreds to thousands of watts,

OP said infinite resistance - How does an open circuit resistor dissipate power ...
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Old 1st July 2003, 06:41 PM   #7
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It doesn't and he knows it. His suggestion was that the resistor must be open, otherwise with 38V across it, it would be dissappating over 3000 watts.
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Old 1st July 2003, 08:44 PM   #8
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Would be a nice thing, simply disconnect the resistors, and 3000 W <output> power...
But those 2 * 100 W into 8 ohms from the P3A are sufficient for me...


Bye,

Arndt
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Old 1st July 2003, 10:28 PM   #9
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NO stealing pictures!!!!!!!!!!!!hahahahaha
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