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March 25, 2023, 07:32:34 AM

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Fender Stage 112 SE

Started by Capndenny1, December 29, 2022, 02:57:30 PM

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Capndenny1

I have one of these with loud hum in the power amp.

Not filter caps, tried that.

Not preamp, with Poweramp In grounded, it's still there.

Moving green ground wire from pcb gnd to chassis had huge effects.  Both location of mounting lug and path of green wire made a huge difference.  It was best when just disconnected.

Other than this issue, amp seems to work fine.

joecool85

Before going any further, I would check for DC voltage across the speaker leads.  Disconnect the speaker and take a measurement.  It should be zero or close to it.
Life is what you make it.
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Tassieviking

There are no stupid questions.
There are only stupid mistakes.

Capndenny1


Tassieviking

Be very careful when tampering with the ground wires in this amp, they might be critical to the amp.
The amplifier is a "Flying Rails Amplifier"
Way beyond my experience, but the +45 -45V supply to the power amp has no earth on the center tap like conventional amplifiers.
The center tap is the output from the amp to the speaker, along with the chassis earth.
The +45 -45 voltage rails are moving up/down in relation to the chassis earth as the amplifier is working.
The center tap of the +45v -45v does not stay at zero volts in relation to the chassis, it moves up/down up to +45v and -45v.
This can make the +45v rail look like 0v to +90v, and the -45v rail can be at 0v and down to -90v during operation in relation to the earth on the chassis.

I'm confused just trying to explain it.
There are no stupid questions.
There are only stupid mistakes.

Capndenny1

I remember now, thanks for the explanation.

It's the same as if the CT was and, and then the output was just the common point for all the power transistors.

There is a reason this works better, cheaper, probably doesn't need the level shifting transistors or something.  It's a common design.

g1

If changing the chassis ground wire made a difference, there may be a ground loop or ground fault.
Check for any loose pot nuts, check all the pot mounting brackets (wings) solder connections as they are used to make ground connections in some places.
Check for any non-stock parts, like pots missing their 'wings' or plastic jacks that have been replaced by metal (which can cause ground loops).