Boss Metal Zone - Power Issue

[nickbungus]

Thanks Garcho.

And Paul, like I said before, I'm an idiot.     I assumed all op-amps have the same pinout!  Also, I bought a load of TL072s just for this project.

[anotherjim]

It's working? Like I said above, and the Boss schematic shows, 2 of the amps are mis-biased to ground instead of 1/2 supply reference.

[nickbungus]

Jim, do you think I can fix it on this board?  If so what do you suggest I do?

Its working and sounds good too but I dont have the mids control like I did when I owned on.

Heres how I wired the pots.


I dont think I'm reading the mid freq pot wired.  On the boss schematic it looks like this



I'm starting to think I need to do some voodoo with a stereo pot

[garcho]

it's a 50k dual-gang anti-log. fun

EDIT: it's concentric, not dual gang, sorry. save yourself $20, just buy two 50kC pots

[anotherjim]

Pin5 IC2 is the nearest source of 1/2vcc I can see to the affected area.
Pin3 IC3, the 100k on that and Pin5 IC4, the 1M on that. For both resistors lift the ground ends and wire to the nearest 1/2 vcc source.
If the signal is already heavily distorted before it reaches the mis-biased amps, you may well not notice this error - it would be distortion hiding inside distortion.
It isn't that obvious on the Boss drawing, but the 1/2vcc (+4.5v) is denoted by solid arrows while +9v are outline arrows.

[nickbungus]

Thanks as always Jim. I will make the changes and report back. 

Garcho, thanks too.  I will get a 50k dual gang reverse log (extra fun) pot ordered and also report back. Hopefully I'll have a bit more mids control to get some of those wackier dirty tones.

[garcho]

^ do NOT order a dual gang!!!!!


i meant to write concentric, not dual-gang, i'm very sorry. if you've already ordered that expensive dual-gang pot, i offer to buy it from you and pay for the shipping.

50k is the resistance, anti-log is the taper, concentric is referring to one independent shaft inside of another, wth a set of concentric knobs. it will be difficult to find that, and expensive. much better to find space for another drilled hole and buy two 50k anti-log pots.

[anotherjim]

Gary, as I read it the Boss is a dual concentric with one of the shafts controlling a 2x 50k wafers and the other shaft a single 100k.
This "Metano" scheme has 2 separate controls, one of which is a dual 50 or 47k. Same circuit as Boss  as far as I can see.
The Boss scheme is a better guide to how to wire that dual pot as it has the pin numbers.
As the Freq is antilog and the Mid is an "S curve" (? is that type G ?) use linear for any of those types you don't have.

[ElectricDruid]

^ do NOT order a dual gang!!!!!

+1 agree with anotherjim

The "Mid Freq" *is* a dual-gang pot. Yes, the control is concentric, and the middle part controls the "Mid" cut/boost, but that's a *third* gang on that pot. This was some special order thing that you can do if you're Roland/Boss and you're building 100,000 pedals, but which us mere mortals never see the like of.

The service notes are quite clear about it - it's a triple-gang concentric pot!

Tom

[garcho]

holy mole! i made an ass of myself 3 times over then. ah, there's me coat...

[nickbungus]

Wow, now I am really confused.

No Gary, I haven't ordered anything yet and I would have never laid any blame on you if I did.  Anyway, buying the parts is my favourite bit about building, regardless of whether I need them.

When I was planning the build I just thought I needed 2 47K pots so I do have that in stock.

Are we saying a 2 gang is the way to go?  If so I'll get it ordered. 

[ElectricDruid]

Yep, the mid freq is a dual-gang 50K pot. 47K if you can't find 50K. Same thing for pots, really, given 20% tolerance.

HTH,
Tom

[nickbungus]

Cheers guys for all the help on this one.  Dual Gang pot is on and I can really play with those mids.

Jim, I made the changes to bias the opamps and I now have a more focused sound.  You were dead right about the distortion hiding the distortion.   You can really tell with the distortion pot on the lower settings, I can now get an overdriven type sound.

[anotherjim]

That's good to hear Nick, and congratulations for another successful build.