Maxon PT-909 True Bypass

[DJPsychic]

Is the Maxon PT-909 True Bypass?

I did the battery/power test. Unpowered it passes a bypass signal. I also took an ohm reading from input jack to PCB input, but got 0's. Not sure if I was doing it correctly.

Here I think I've accurately laid out the switch configuration. I also include the schematic for the Ibanez PT-909 which I believe is the same.

Any help as to how to properly TB (if needed) would much appreciated as always.

[PRR]

If you can't tell, why would it matter?

Does it tone-suck?

[DJPsychic]

Good question 

I like running all TB on my rig. And I enjoy soldering things

Also the switch is a little wonky I was going to replace anyway. I didn't know if it's TB the way it is or not. Still fairly new to the game my apologies

[Rob Strand]

Is the Maxon PT-909 True Bypass?

With a DPDT switch *and* an LED it cannot be true bypass.

It will be like the older MXR pedals where the switch center contact (the pole) connects to the output socket.   The switch wires the output socket either to the input socket (the zero ohms you measured) or to the output of the phaser.

The thing that makes it not true bypass is the input socket is always connected to the input of the effects pcb.  That will add a permanent 510k load or so via an input cap.  The input cap prevents multimeter measurement seeing the 510k to ground/Vref.

Some options:
- ditch the LED and use the other contact for true-bypass
- get a three pole foot switch and wire for true-bypass with LED. 
  The switch may or may not fit in the space.  It will fill the battery space for sure.
- use a Millenium bypass type circuit and corresponding switch wiring.
  With the low value of R138 on the Ibanez PT-909 schematic the switching click/pop will be very low with
  the Millenium bypass.

FWIW, the Ibanez PT-909 circuit uses JFET switching and pushbutton switch.   That era of Ibanez pedals is a little messy and I wouldn't be surprised if there's a slightly earlier Ibanez PT-909 with the exact same wiring as the Maxon.   You might find an older schematic on the FSB forum.  It won't help your problem though.  You are pretty much stuck with the options.

[DJPsychic]

Yep I was hoping to get away with a DPDT, but I'll just go with a 3PDT...I usually power Phasers with a jack anyway.

Thank you so much for answering my question!

[DJPsychic]

FWIW, the Ibanez PT-909 circuit uses JFET switching and pushbutton switch.   That era of Ibanez pedals is a little messy and I wouldn't be surprised if there's a slightly earlier Ibanez PT-909 with the exact same wiring as the Maxon.   You might find an older schematic on the FSB forum.

Interesting. Yeah there's an Ibanez PT-909 with a switch and looks to be the exact same circuit board. I have a request for analysis on FSB. I would love to make a clone of this thing. It's a sleeper (don't tell anyone  )

Ibanez PT-909





Maxon Pt-909

[Rob Strand]

Interesting. Yeah there's an Ibanez PT-909 with a switch and looks to be the exact same circuit board. I have a request for analysis on FSB. I would love to make a clone of this thing. It's a sleeper (don't tell anyone  )

Well done finding that!

Yes, most of the early Ibanez pedals where exact replicas of the Maxon.  IIRC some of the PCB's were even marked Maxon.   The PCB's usually had numbers which were codes for the circuits and the Maxon and Ibanez boards had the same markings.

Your pics sparked my memory a bit.   The units with the square buttons were all JFET switching.   The early units like yours had the round buttons and had the old school bypass circuit.   However, there were some transitional Ibanez pedals which had round button but JFET circuits.     There's a whole lot of details like wide boxes, flying fingers logos, presence of  LEDs, presence of DC jacks, types of knobs.   I spent ages working it all out at one point but it's so confusing the memory has frayed away.

[Rob Strand]

There's something puzzling about those pics.

The Ibanez (1979) has red dot JFETs whereas the Maxon (1978) has yellow dot JFETs.

I'm inclined to think the dot color means 2SK30AR and 2SK30AY which have quite different specs.  If that's the case the Ibanez unit will sound different to the Maxon, well at least the pedals in the pics will sound different.

IIRC, the PT999 Phase mostly used yellow dots but I would have to check my notes.



[EDIT:  The painted dots do not seem to match the JFET suffix.   Ibanez have red dots on 2SK30AY parts.  See reply #10 below.]

[DJPsychic]

Here is my crude analysis of the Maxon. I still haven't documented the resistors. Hope this helps! In all honesty I know nothing about JFETS

[DJPsychic]

Btw I'm not sure my layout of the stock switch is accurate. I attempted to TB with a 3PDT but not getting effect signal.

Which technique for wiring  a 3PDT using the stock LED do you recommend? I know  there's more than a few ways to configure.

I used this method

[Rob Strand]

Here is my crude analysis of the Maxon. I still haven't documented the resistors. Hope this helps! In all honesty I know nothing about JFETS

So your unit has the 2SK30AY (and yellow dots to match the Y in the part number ... or maybe not ... ).

I had a look at my old 1975 Ibanez PT900 (PCB marked 999) and it also uses 2SK30AY.   The interesting thing is the JFETs have *red* paint dots!  I can only assume the paint dots are some additional sorting marks done by Ibanez/Maxon.

Btw I'm not sure my layout of the stock switch is accurate. I attempted to TB with a 3PDT but not getting effect signal.

Hmm, the way you have drawn it looks OK to me.

Which technique for wiring  a 3PDT using the stock LED do you recommend? I know  there's more than a few ways to configure.

I'm in favour of shorting the input like the "better" version on this link,
http://stinkfoot.se/archives/2233

For one it prevents the need to add a de-pop resistor but I also like to short the input as a floating input can sometimes cause problems.

[DJPsychic]

BTW I was referring to the photo of the stock Maxon switch I drew arrows on. Might not be labeled correctly.

And that's interesting your Ibanez uses the same JFETS. I'd be curious what the color paint coding means!

[Rob Strand]

BTW I was referring to the photo of the stock Maxon switch I drew arrows on. Might not be labeled correctly.

No problem.   That pic isn't for you unit is it?  If so I'm very confused.   I can see 4xJFETs for the phaser but there's three other transistors or two transistors and one JFET.    If you look at the schematic for the PT909 there should be 4xJFETs for the phaser, three transistors and one JFET for the switch.   For your unit with no JFET switching I wouldn't expect those parts at all.

That's what I was getting at early on in the thread,  Ibanez/Maxon made changes through that period.

And that's interesting your Ibanez uses the same JFETS. I'd be curious what the color paint coding means!

Back in the early days manufacturers sorted some parts in the subcategories and marked them with a dot.  For example transistors (with the same part number) were sorted by gain into ranges.   Instead of using colored dots the Japanese put the sorting category in the part number using a letter and didn't use dots.   The letter is the name of a color.  So you might see 2SC1815Y (yellow), 2SC1815GR (green),  2SC1815O (orange).  You get the same with JFETs,  see the bottom of the datasheet,

https://imgv2-1-f.scribdassets.com/img/document/380787885/original/5eb14b72db/1607017630?v=1

Anyway that's what I thought dots were.  However, now I don't think that's the case.   Phasers need JFETs to be sorted into tighter categories than the manufacurer's category.    That means some more testing.   The dots might simply mean they have been sorted (and were placed in the appropriate sorted bins).   Alternatively the color of the dot might be significant in that it represents some sort of sub-category within Maxon.

It's hard to work out this still without having a lot of samples to see the pattern.

[DJPsychic]

No problem.   That pic isn't for you unit is it?  If so I'm very confused.   I can see 4xJFETs for the phaser but there's three other transistors or two transistors and one JFET.    If you look at the schematic for the PT909 there should be 4xJFETs for the phaser, three transistors and one JFET for the switch.   For your unit with no JFET switching I wouldn't expect those parts at all.

Yep the pictures of the labelled components and the labelled switch wiring are my Maxon.

I assumed the schematic for the Ibanez was similar because of the speed/depth/feedback pots. I'm not very familiar with phase circuits yet 

Hoping someone from FSB traces it

[Rob Strand]

Yep the pictures of the labelled components and the labelled switch wiring are my Maxon.

Well that puts a different light on things!   It might even explain why your true bypass isn't working.

Maybe the circuit has JFET switching but the LED switching is done with the switch.  That might explain
why there's one C1815 missing.

If they have done something funky with the switching it might explain why you perfectly OK true bypass wiring doesn't work in effects mode.  Maybe the funky switching is disable and the effect stays off.

However, I'm very much struggling to come-up with an arrangement which matches your wiring and also somehow switches a switching circuit.   The main problem being the center contact of the switch connecting to both the input and the switching circuit.  Also there can't be a flip-flip circuit because the footswitch is on/off not a push button.

There's a missing piece of the puzzle somewhere!

I assumed the schematic for the Ibanez was similar because of the speed/depth/feedback pots. I'm not very familiar with phase circuits yet 

It's not a bad assumption.  I'm sure the bulk of the circuit *is* like the schematic you posted.  It's the small differences that are stuffing things up.

Hoping someone from FSB traces it

I'll trace it but I'll need a bit more info.   Some more photos will help:
- photo of the bottom of the board
- photo of the wires to the pots, and the input/output sockets
- I can see a few part values on the Ibanez board pic you posted earlier but if you can take a few side shots
  it might let me see a few more values.

[DJPsychic]

I'll take some more detailed pics today...

I attempted to replace the jacks and rewired the switch so it's kind of a mess right now 

Here are the pics I took before

[Rob Strand]

I'll take some more detailed pics today...

I attempted to replace the jacks and rewired the switch so it's kind of a mess right now 

Here are the pics I took before

Cool, thanks.   I actually found the pic of the bottom of the board you posted on another forum.

Those will do for now.   Some pics of the part values might be helpful soon.

Already I can see the extra JFET is an input buffer, as opposed to the opamp buffer on the other schematic.   I'm not seeing any weird JET switching.   It's more or less looking like what we thought it was in the first place.  So now I'm puzzled about what's gone wrong with your true bypass wiring.   Perhaps you could hard wire the input socket to the pcb input (the old yellow wires) and wire the pcb output to the output socket.   That will hard wire the effect to the sockets. It should work.  The only thing you can't do is switch back to bypass.   At least you know the effects part is still working.   If it is then maybe there's something wrong with the switch.

[DJPsychic]

Effect worked great before, although I was having problem with input jack, would have to wiggle to get sound before. So I replaced both, had to sand the holes slightly to fit newer jacks. Jacks barely fit, so might not have been the greatest idea 

I'm getting bypass, LED turns on, but no effect.

Output jack is now unstable. I think there is loose connection somewhere. Or maybe jack is shorting on enclosure (although not sure what material is) Might put old jacks back if they still fit. Please excuse the mess 

[Rob Strand]

You might want to check the wiring of the yellow and white wires on the top row of the switch.  I think they are reversed.  That would stop the effects mode but not bypass.



Oh forgot to mention if the old sockets have star washers you should use them on the new jack, on the inside surface.  It gives a better ground connection.

[DJPsychic]

You might want to check the wiring of the yellow and white wires on the top row of the switch.  I think they are reversed.  That would stop the effects mode but not bypass.

oops will fix that asap thank you sir

Oh forgot to mention if the old sockets have star washers you should use them on the new jack, on the inside surface.  It gives a better ground connection.

Don't think I saw any star washers but may have some laying around. thanks Rob