Dropping from 15V AC to 9V AC?

[Ian.M]

The only issue with that is you then end up using the signal wiring as the ground return for the VU meter board power, as there is one diode drop between the AC in you connect to the PSU 0V and the VU board 0V.   This risks coupling hum into the audio.     A DC blocking capacitor between the VU board input ground, (+ towards te VU board) and wherever you are feeding it from would prevent any issues.

[madires]

Yes, I'd simply jumper straight across the bridge on the track side.  You can leave it in situ as the diodes in it that you haven't shorted out are reverse biassed. No track mods required and easy to reverse near-invisibly if you need to make a warranty claim.

May I suggest some improvements? Just a single jumper wire between the pin of CN6 used for gound and the negative pin of the big filter cap.  And keeping the bridge rectifier for passing the +20V (rectified 15V AC) to decouple the filter cap from the main circuit.

[Ian.M]

Yes. that could work nicely.  One jumper across the bridge, pins 2-3  ([-]-AC), or even, without *ANY* board mods, a jumper at the AC in connector to a nearby ground.  It looks like the - side of CNs 2-5 go top the ground plane as there's no signs of any other tracks to them, but check that with a continuity tester.

[LektroiD]

I guess that *would* work nicely, but I already removed the rectifier and did the mods I posted this morning, and that didn't go as sweetly as imagined... It was welded in with that awful lead-free solder, and thus was quite a messy removal, only one leg survived.

The PCB location of the minus pin (-) on the rectifier is already tied to ground, so the 1000µF cap is still working as a filter.

I guess I could buy another rectifier if it doesn't work as expected, I may even have one kicking around in the drawer...

[Ian.M]

No worries - what you've done will work OK.

[Zero999]

Yes. we need to see the VU board schematic or at least its PSU section to detemine whether it can run from a higher voltage or if it really needs its own isolated 9VAC supply.

If it does need its own isolated AC supply, assuming the toroidial transformer has an open center (i.e not potted). try the overwind experiment Brumby suggested, only to make the maths easy, wind TEN turns of hookup wire through the center of the toroid (not six as suggested).  Measure the voltage from that  and you can calculate how many turns are actually required for 9V AC. 

Anything under about 100 turns is no particular problem to wind. If its going to need more turns (unlikely) I'd reconsider.

I apreciate this has already been solved but for future reference.

As there are two 15V windings and only one is needed, how about converting one of them into 9V? To cut down on the number of turns you need to make, an additional 6V winding could be made and connected in anti-phase with one of the 15V windings to give 9V.

[Ian.M]

No.  both windings were used to feed bridge rectifiers feeding LM723 based positive regulators and the resulting 12V DC supplies were stacked to get +/-12V so neither could be freed up for modification and as the bridge rectifiers hold them at a mean DC voltage of half the unreg +ve rail and that voltage -12V respectively, they couldn't be tapped for aother bridge with a common 0V rail.    A capacitively coupled bridge could have been used off the  15V AC feeding the +12V rail and the caps could have been under-sized enough to drop the VU meter board input voltage to 9V AC when it was drawing its maximum current.

[Zero999]

No.  both windings were used to feed bridge rectifiers feeding LM723 based positive regulators and the resulting 12V DC supplies were stacked to get +/-12V so neither could be freed up for modification and as the bridge rectifiers hold them at a mean DC voltage of half the unreg +ve rail and that voltage -12V respectively, they couldn't be tapped for aother bridge with a common 0V rail.    A capacitively coupled bridge could have been used off the  15V AC feeding the +12V rail and the caps could have been under-sized enough to drop the VU meter board input voltage to 9V AC when it was drawing its maximum current.

Oh I see, I missed the links to the schematics.