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Old 23rd December 2012, 11:55 PM   #11
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I suspect a mis-wire - but should there be a connection to R2 and both bridges as indicated by the schematic? Seems to me that would get things warm inna hurry....
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Old 24th December 2012, 12:10 AM   #12
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I suspect a mis-wire - but should there be a connection to R2 and both bridges as indicated by the schematic? Seems to me that would get things warm inna hurry....
I think I did mine this way.

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Dan

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Old 24th December 2012, 12:12 AM   #13
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Because it was easy to reach I measured from R1 to ground and got 50VDC.

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Dan
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Old 24th December 2012, 12:17 AM   #14
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More random thoughts and measurements. I have now noticed that as my measured Ids goes up my Vds goes down. Is that correct?

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Dan
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Old 24th December 2012, 12:35 AM   #15
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Dan,

You should test your power supply section before connecting the power amp section. You can detach at the bulb, at the wiper lead out, and keep the GNDs connected together. You can then measure the 7912 Vgs biasing regulator if it does go from max value to min of -12V by varying P1.

Once you are sure the PS section is okay then you can focus troubleshooting the PA section.
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Old 24th December 2012, 12:38 AM   #16
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More random thoughts and measurements. I have now noticed that as my measured Ids goes up my Vds goes down. Is that correct?
Yes, that is correct.

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Old 24th December 2012, 01:12 AM   #17
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Dan,

You should test your power supply section before connecting the power amp section. You can detach at the bulb, at the wiper lead out, and keep the GNDs connected together. You can then measure the 7912 Vgs biasing regulator if it does go from max value to min of -12V by varying P1.

Once you are sure the PS section is okay then you can focus troubleshooting the PA section.
Will measuring from point "A" to ground be the right idea?

Click the image to open in full size.

Regards,
Dan
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Old 24th December 2012, 01:17 AM   #18
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Yes that will work...just disconnect at the bulb to PS (removing bulbs from sockets work too) or at Source/GND connection to be sure there is no influence whatsoever. And you can measure main PS unloaded too.

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Old 24th December 2012, 02:26 AM   #19
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Dan
1 Possibly have speaker connected to wrong side of c6.
2 speaker open or not grounded to the correct place.
are you measuring with a dvm? The 47,000uf 25V substitution
was too low of voltage may now be bad. With a dvm and the speaker
not loading the amp c6 would probably leak enough to show dc
voltage on both sides of capacitor.
good luck
Jim
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Old 24th December 2012, 03:09 AM   #20
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Dan,

You should test your power supply section before connecting the power amp section. You can detach at the bulb, at the wiper lead out, and keep the GNDs connected together. You can then measure the 7912 Vgs biasing regulator if it does go from max value to min of -12V by varying P1.

Once you are sure the PS section is okay then you can focus troubleshooting the PA section.
Both bias circuits range from -2.0vdc to -11.9vdc when measured from point "A" to ground. An unloaded main supply measures 57.6VDC at point "B" on both channels. I'm starting to consider a different 'style' of ground connection between my bias circuit and main circuit.

Here's a more accurate schematic.

Click the image to open in full size.


Regards,
Dan

Last edited by dantwomey; 24th December 2012 at 03:15 AM.
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