Help building the 70's Maestro MPF-1

[gigimarga]

Hi,

I found a scan of the original service manual via google: http://s77.photobucket.com/albums/j64/Spaghettaboutit/Maestro%20MPF/
It gives a wonderful explanation of the switching and the circuit in general.

Hope it helps,

Peter

Thx a lot man!!!

[jmasciswannabe]

Do you still need cleaned up trace version?

Sure! Go for it!

[SpencerPedals]

Did anyone end up pursuing this one yet?  I've been packing/moving/unpacking into a new apartment and refinishing furniture for about forever and won't have a free moment for a while.  Just curious how the project was coming along, if it still is.  I'd like to live vicariously in the meantime.

[xdissent]

I've just found the service manual and have given it a quick once over. I'm going to breadboard up a pseudo clone right now using some standard toggles as suggested previously in this thread. The benefits would be huge: not only would we be able to forgo sourcing the cd4016 and the 5 position rotary switch, but by separating the mode and filter Q switching, we add an "overdrive/broad" setting not attainable with the original unit. Of course, the 3 filter settings will require a center-off SPDT while a normal on-on SPDT should be used for the mode switch. I don't have a cd4007 hanging around either so I'll be substituting that out with a couple of discrete gain stages (probably BJT for now) so I can focus on tuning the actual filter circuit and getting the switching right. I'll report back with results shortly.

All in all this is an elegant, simple circuit when you can make sense of it. Classic Moog engineering... just with an insane switching configuration reminiscent of Craig Anderton.

[xdissent]

On second glance, there might need to be a tad more switchiness than just a center-of SPDT for the filter Q selector in order for the height control to work over the same range for different Q settings. The 330 ohm voltage divider from the height pot makes things a little hairy. Of course, it's unlikely that I'll find an S-taper 10k height pot in the first place, and if you're switching filter settings anyway it's not much trouble to adjust the height as well if it is way out of wack. I'm going to experiment to see just how dramatic the difference between settings without the divider can be, and I'll make a judgement call whether or not to deal with the extra effort based on my findings.

[xdissent]

Ok I didn't notice that the pole 3 (representing the "broad" filter setting) is always connected to the switch's throw terminal, so the entire filter Q switching could be done with a center-off DPDT with the voltage divider for height compensation switching included. Booya.

[xdissent]

Well I got the filter wired up and tested and all I can say is WOW. I didn't even prototype the buffers or clipping stage and I'm very impressed. I'm hearing a Brian May-ish, half-%^&*ed wah tone. I'm definitely going ahead with a PCB for this puppy, and I've already gotten most of the schematic redrawn in Eagle, ready for the board to be laid. Here is the initial schematic with the rotary switch and cd4016/cd4011 replaced with normal switches:


I'm going to rework the power section to include an external jack and contemplate true bypass and LED indicator options before finishing the board. This is what the filter section looks like on my breadboard. It's easy to get carried away playing with the frequency control while piping music through the filter. It's like an instant shitty house DJ in a single knob.

[fuzzo]

 

I don't even make work a simple JFET booster on my breaboard

impressive.

[SpencerPedals]

Wow.  And welcome.  I'm glad to hear someone's making great progress.  Keep us posted, please.  That's intense.

[gigimarga]

Thx a lot xdissent!!!

[xdissent]

Ok so I've come up with some clever tricks to NOT have to deal with a dual reverse log pot for the frequency control since finding a top adjust model proved impossible. My board layout has 3 different possible arrangements: top adjustable switches and pots, side adjustable switches and pots, and top adjustable but with foot wheels for frequency and height. In the foot wheel arrangement, a normal side adjust dual audio taper pot can be used for the frequency control if the it's on the LEFT side of the unit, since "up" will be "counter clockwise." I don't know HOW good old Mr. Moog let this slip past him! The height control is linear taper so it can be moved to the right without worrying about getting the correct taper since either way will work. What luck!?

Since I've found some side adjust dual reverse log pots by alpha that are side adjustable, the side adjustable configuration will behave as expected. The only compromise at this point is if you don't want the side mounted foot wheels, you're going to have to use a linear pot for the frequency. After testing, that's not as big of a deal as I expected especially considering it's not going to be something you change on the fly with your foot, but it's a little lame. Oh well, this thing is supposed to have foot wheels anyway, and it doesn't limit the sounds you can get in any way.

The reason for 3 different arrangements on the same board is that I wanted 1 board to provide for a normal pedal, a pedal with foot wheels, and a rackmount / desktop unit. So far so good!

I've also decided to add an output volume pot, which actually has a push/pull DPDT switch attached in the rackmount configuration for an enable/bypass switch. The other versions use a normal pot since the footswitch will handle bypass. The problem is, the CD4007 inverter section used as an output buffer has relatively high output impedance (10k as shown!) so the choice is either use an audio taper pot or a linear pot with tapering resistor from wiper to ground. Unfortunately I can't find an audio taper pot with a push/pull DPDT anywhere, not to mention I loathe audio taper to begin with, so I'm going with the linear taper pot with tapering resistor. Of course a side effect is that the tapering resistor will cut the output level dramatically due to the high output impedance and low input impedance of a pot with a tapering resistor. I kind of have to wait to get my hands on my 4007 ICs to do some testing to compensate for this insertion loss since there's basically no data available for the inverter section used as a linear amp and each chip can vary pretty wildly.

Regarding true bypass - screw it, input impedance is as high as you'd have to be to claim you can tell the difference between bypass as shown and true bypass. So that's one less thing.

Ordering parts tomorrow and finishing the board to be prototyped this week as well. I'll have boards and component kits available for whatever configuration you want to build, as well as fully assembled units if you'd rather just get your hands on one without delay - and trust me - YOU DO. This thing is hands down my new favorite pedal.

[xdissent]

Oh, it should go without saying that if you're an off-board component type of dude, you can just hook whatever pot/switch configuration your heart desires. I have found that off board components are not only a major pain to wire up, they're also the number one source of failure or error in builds so I always take the time to precisely layout pads for pcb mounted stuff. It's just personal preference but either way this board will work for you.

[SpencerPedals]

This is going to be like an itch I can't scratch for a month or two, since I promised myself I would finish my half-finished pedals when I got done unpacking and organizing my new appartment, which has almost happened.  Congrats on conquering this one and thanks for sharing.

[fuzzo]

How have you changed the rotoswitch by a biploar switch ? the rotary switch has more of two selections  ?

Thanks a lot  !!

[xdissent]

How have you changed the rotoswitch by a biploar switch ? the rotary switch has more of two selections  ?

Thanks a lot  !!

The rotary switch in the original is doing double duty by selecting natural/overdrive as well as the Q (sharpness) of the filter. You can break those functions apart into 2 switches - one for Q, and one for natural/overdrive. Luckily, the "broad" setting for the filter doesn't actually require a switch at all, so that becomes the "off" position in a center-off DPDT switch. The other switch is just an SPDT to select between natural and overdrive. The bonus is you can use the broad Q setting with natural tone, which is not possible with the original switch, plus u can ditch 2 DIP14 ICs! Check the schematic above to see how it works but note that it has a couple minor errors unrelated to switching.

[fuzzo]

Ok.I get it. Thanks.

That's really nice that you share your work with us .

[xdissent]

Oh, dude, it's not MY work, it's my interpretation. MY work is just maybe trying to make sense of an idea from a genius. Not sharing would be an injustice to everyone who respects the man behind the original design. I'll take credit for modernizing *maybe* but at this point it's my duty to some how facilitate the reproduction of the legend. Maestro may not have been a succesful venture, but the dream lives on damn it! Plus I really need to own this pedal, and I don't want anyone to have the same withdrawal pains.

[xdissent]

She's done! In a stroke of luck, I remembered that I wanted to test and potentially replace a CD4007 from a Realistic Concermate MG-1 which was being flakey. I popped the chip out, ran some tests, determined it was 100% functional, and used it in the pre/overdrive/output buffer stages of the MPF-1 circuit I breadboarded up. So now I have a complete prototype sitting on my desk and it sounds great. Ordering parts and finishing up the board layout now that I'm totally confident in the accuracy of the design. My buddy is also lending me his original pedal so I can compare it to my version, which will be very helpful in comparing the old TL022 opamps which I'm replacing with TL062's, and the modern MC14007 with the stock CD4007. Progress!

The full monty:

[fuzzo]

Nice ( I don't know how you did to make a complex circuit like that on a breaboard but "bravo")

[fuzzo]

Nothing new ?