MXR Auto Flanger and Auto Phaser schematic request

[beatnik]

Hi everybody

I have two of the old blue faced MXR modules, one Auto Flanger M-111 and one Auto Phaser  M-110

They are in need of repair and I couldn't find any schematics online

Has anyone got a copy to share ?

[Mark Hammer]

First, cool!  I'm envious.

Second, what do they not do?  In what ways are they misbehaving?

I don't doubt that they are likely a bit different than the pedal versions, since they are probably predicated on line levels.  BUt the general design aspects would be fairly similar to pedals.

(I looked in my own hard-drive holdings, but could not find schematics for them)

[beatnik]

MXR gear of that era is very cool, the 19" units like the pitch transposer and flanger doubler are awesome.

these smaller lunchbox units came to me as untested. I have quickly tested the modules with a 15v supply and it seems that there is only dry signal.

i have later realised that the modules feature control voltage input and output connection, so i though i should try linking these two, in order to close the cv path.

maybe i didn't get an effect signal because the control voltage signal is interrupted.

[Mark Hammer]

Or, it might be a trimmer setting.  Both the Auto-Phaser and Auto-Flanger need to have their bias voltages set right

[ElectricDruid]

Cripes, vactrols didn't cost then what they cost now, did they?!? Look at that board!

Don't have a schematic in my collection, sorry.

Tom

[beatnik]

I have managed to get the effects working by jumpering the CV in/out connections

When the modules will be racked up I will be making the CV connections on switching jacks to create some sort of normalisation, in order to be able to use external CV signlas from my modular synth

I also noticed these units are full of tantalum capacitors, which are known to go bad over time, my plan would be complete recap with good quality electrolytics

I am unsure about the polarity of the tantalum capacitors. They are colour coded and there is one colour band to mark the polarity, however I don't understand if that is indicating the positive or negative terminal. The pcb is not silkscreened, so that doesn't help either

[StephenGiles]

I have never seen schematics for either of these, but they must be out there somewhere. As Mark said. bias adjustment is a good starting point.

[PMowdes]

Looks prime for a trace

[duck_arse]

this might help with polarity:

[Mark Hammer]

Cripes, vactrols didn't cost then what they cost now, did they?!? Look at that board!

Don't have a schematic in my collection, sorry.

Tom

Keep in mind that MXR used a mess of them for the Phase 100.

[ElectricDruid]

Cripes, vactrols didn't cost then what they cost now, did they?!? Look at that board!

Don't have a schematic in my collection, sorry.

Tom

Keep in mind that MXR used a mess of them for the Phase 100.

Exactly! They were all over the place at one point! Have you ever looked at Buchla's synth circuits, Mark? He's famous for loving a nice vactrol for voltage-control.

T.

[Fender3D]

Keep in mind that MXR used a mess of them for the Phase 100.

I can't imagine how many were discarded when matching them...

[Mark Hammer]

You actually don't need to match vactrols for phasers.  That's one of the many reasons why they're so great for phasers.

The matching of FETs is important because it allows you to identify a range of control current that will make every FET change its drain-source resistance and not crap out or "stall".  LDRs do have a min and max resistance, but the full resistance range is never used.  So as long as each LDR is able to keep changing value as the LED gets brighter and dimmer, the phase shift keeps on-a-moving.

Keep in mind that, even when you DO match JETs for a phaser, you are matching them for Vgs, and never matching them for drain-source resistance.

I have Mike Irwin to thank for this insight.  I wish I could find out how to contact him again.

[armdnrdy]

I think I posted this a few years back. This is one page of the MXR Phase 100 factory docs.
It clearly states that the vactrols are matched. How they are matched is another question but...I would imagine resistance min/max per the LFO circuit.

https://www.dropbox.com/s/lob4ot9orky6s60/Phase%20100%20parts%20list%20pg.2%20-%20Copy.jpg?dl=0

[Mark Hammer]

That may be what they DID, Larry.  But it doesn't necessarily mean it was what was required....at least to produce a phasing effect.  That said, being able to get a pleasing musical phasing effect is different than nailing the same sound for every copy of a commercial pedal that comes off the line.  For FET-based phasers, matching of FETs is needed to get a pleasing sound in the first place, and consistency of product just happens to be a positive perk of doing that.  In the case of LDR-based phasers matching is not needed, UNLESS the goal is to maintain consistency across production.

Kinda makes one wonder what Mike Biegel did for the Mu-Tron Bi-Phase, which was also LDR-based, ar what A/DA did for the Final Phase (well, the vactrol-based issue).