Melos Mini Fazer (1-knob) mod to change depth?

[ericohman]

I have one of these, from my dad's collection:
http://www.effectsdatabase.com/model/melos/minifazer

Where would a trimpot go if I want to change depth, it's way too much effect, I don't want to fit an additional knob, but something like a trimpot fitted inside the box would be perfect.

Schematic:
http://files.effectsdatabase.com/docs/schematics/univox_microfazer.gif

I tried running it with drained batteries to see how it would affect it, only tried a 4.5 and 6.5V but nothing happened, I didn't have anything in the 7-8.5 range to try...

(The 1 knob that's currently on the pedal changes rate)

[RickL]

The simplest thing to do would be to replace R21 with a larger value trimpot (try 50k or 100k) wired as a variable resistor. Adjusted to 22k it should sound exactly as it does now, higher values should give less effect. Once you get the sound you like you could replace the trimpot with a resistor of the same value that the pot has been set to. I'd leave the trimpot there though in case you change your mind later on.

[ericohman]

Thanks. I'll try that! I have a few trimpots lying around.

What does the R19 do?
I first thought that would be the one to replace with a trim, but I'm just a beginner...

[daverdave]

looks like R19 and C6 form a high pass filter, probably just to kill some of the lower frequencies. C6 is a DC blocking cap.

[ericohman]

Thanks everyone. It worked perfectly!





I've adjusted the trim to the lowest depth (I used a 100k trimpot) and I bent the wire so it's almost completely hidden under the board.
Going to put the lid back and continue playing

Pretty nasty solder traces on this pedal by the way!

[Renegadrian]

So it has all those 1458 inside?! Is it worth replicating?!

[ericohman]

Schematics is for a univox micro fazer. It says the Melos is the same but this is a picture of the op-amps:




All resistor leads was brown, as if they were painted. Looked really cool.
Don't know much about phasers, but after adding the trim pot to control the depth, I will definately be playing a lot more with this pedal.
The thing I didn't like was that you could hear the phase cycling "turnaround points".

After the mod it is more subtle (R21 = 110k instead of 22k) and I really like it that way.

[Renegadrian]

THX for the pics - I thought there were 6 opamps, forgetting that 1458s are dual...
It could be a nice project, I believe, And the parts count's not so high...

[ericohman]

The board it was built on is pretty transparent when held in front of a lamp.
I took one picture when holding the circuit board in front of a lamp, it shows all the traces. I'll post that picture later, in case anyone want to use it for reference when doing a layout.

[Renegadrian]

Why not, thx for your will to share! 

[ericohman]

Well, it wasn't much to see, but maybe it will be of interest/help to someone

(1.1mb)
http://djoman.dyndns.org/melos/_MG_5387.JPG