MXR M-126 Flanger/Doubler Power Supply - Stumped

[captntasty]

Hi guys,
I could use some help... working on an mxr flanger doubler that isn't working - no flanging or doubling or anything.  In troubleshooting I haven't gotten past the power supply.  It is a 12v bipolar PS.  When the 18v PT is unconnected from the board I see 18v-0v-18v AC.  Once it is connected to the board things go bad.  By the time it gets through the BR I'm only seeing ~ +2.5VDC /-2.5VDC where it should be a +/- 12V supply.  Checked the BR and it seemed OK, and the VR's seemed OK but subbed in new ones just to eliminate them as the culprits.  Subbed in new filter caps to be thorough - no improvement.  The transformer gets pretty hot as well as the VR's even running it on a current limiter.

I'm wondering if I need to be checking components further down the line?  I'm hoping someone can shed some light to point me in a new direction as far as troubleshooting goes because I'm stuck.  One thing I did notice in the schematic is that the pinouts of the VR's don't match what's happening in the real world on my board.  I've seen an application note on 78xx/79xx / LM340Txx VR's that suggest that in certain situations, pinouts can be switched around a bit - does that make any sense?  Probably not the best wording...

Schematic

http://i27.photobucket.com/albums/c163/gill23/MXR_m126_flanger-doubler.jpg

[theehman]

First off, you should pop the VRs in a breadboard and verify the pinout. 
Assuming they check out OK and are installed correctly in the board, the problem is a component on down the line.  I'd check any tantalum capacitors for shorts.  You could also cut the power supply lines at several places on the board and see if the voltage goes back up, starting with the components farthest from the power supply.

[Fender3D]

Without VRs, what voltage have you, across the filter caps?
Maybe those 2 caps (C1, C2) around the bridge...
(cap on bridge is falling down, falling down, falling down....) 

[rustypinto]

Just a clue, but i can't tell you how many units from the 70's/80's i've repaired that have bad electrolytic caps as the culprit(s). Its always a good to replace all of them anyway.

[captntasty]

Thanks for the replies guys...
EH - the pinouts of the VR's (either the new ones or old) are as they should be in relation to what they're connected to on the PCB.  However, the pinout numbers on the schematic do not relate correctly to what's going on in the circuit board.  It just seemed a bit odd... or I'm not seeing something properly?
Good call on the tantalums - C5 and C6 are tantalums and there are a dozen or so throught the circuit - I forgot tantalums tend to fail...  I'll check that.

Fender - C1 and C2 seem to have no bearing - in or out voltage remains the same.  To be thorough I have subbed in known good caps and no change.  I think they are there as a safety measure or as snubbers?  BUT! with the VR's removed I'm seeing +24.0VDC / -23.3VDC across filter caps C3 & C4.  That's a bit much for the VR's to regulate right?  What does this indicate - improper pinout or is my placement wrong (i put the new ones in the same as the originals)? or are they the wrong VR's?  The originals were LM7912 for -VR and LM340T12 as +VR(same as a 7812, right?)  Is the transformer a problem?
A short after the VR's?

thanks for getting me a bit further along.

[Fender3D]

VRs should work up to 35V...
You have a short down the line... darn tantalums or an IC maybe...

[amptramp]

I suspect the tantalums will be the problem.  When a tantalum cap fails, usually the tantalum pentoxide dielectric layer turns into conductive tantalum dioxide.  If current is unchecked, this can result in overheating causing a firestorm within the capacitor as the heat causes the pentoxide to turn into dioxide.  Current in this application is limited by the short-circuit current limiter built into the regulators, so it may have prevented a fire.  However, the short would still be there.

Tantalum pentoxide is a wonderful material - it can be made virtually pinhole-free, meaning there will be very little leakage current.  And its dielectric constant is 27, much larger than aluminum oxide, so a tantalum capacitor is much more compact.  But it does have the disadvantage that whereas an aluminum oxide capacitor will fail open, a tantalum capacitor will fail short.

[captntasty]

  Success!  Thanks for the help guys!  It was a tantalum - I couldn't say which one but it was off one of the op amps and as best I could tell straddled the +/- rails.
That is why I love this forum - straight-up sharing of knowledge and collaboration.  Sometimes I get stumped and need another pair of eyes.
Again, many thanks!
Patrick

p.s. and both the 5101 and 1024 are in working order!

[CV In]

Does anyone have the M-126 Schematic? The image from the first post have been deleted and I can't find it anywhere else...

[ORK]

Does anyone have the M-126 Schematic? The image from the first post have been deleted and I can't find it anywhere else...

Maybe your browser fools you?

[captntasty]

It's still there.... on photobucket.

[CV In]

Does anyone have the M-126 Schematic? The image from the first post have been deleted and I can't find it anywhere else...

Maybe your browser fools you?

Yes, it's exactly what's happened. It works great with safari. Sorry about this. But anyway, I'm a little bit disapointed about quality of the image. Does anyone have the schematic in better quality. Thanks.

[captntasty]

If you can find one of better quality out there, let us know - as I remember, this schematic was a rare bird...

[CV In]

I just found this: http://....org/viewtopic.php?f=19&t=2025&start=0

on "free stomp boxes.org" (The forum setting don't want to display the link...)

It contains the same image than the first post but in better quality, plus a PDF than contains another version of the schematic and a section with components position on the PCB. Very interesting. Now, I can build one by myself...