Ampeg Scrambler problems

[John Egerton]

Hey guys...

I've built my clone of the ampeg scrambler from the tonepad site and it's acting kind of strange. I was wondering if you could help me figure out what is wrong with it...

The problem is that when the effect is engaged, i get my normal bypassed sound, however in the background, at a lower volume, i hear distortion, probably the sort of thing that the pedal is supposed to be sounding like...

Messing with the blend pot is useless, it only works in the last quarter of the turn, or else it is just totally clean...

Any help would be appreciated...

John

[Dan N]

I remember when I built one the dearligton transistors had a different pinout than the data pdf. Maybe switch the base and collector pins and see what happens?

[Elektrojänis]

Or maybe there is actually something wrong in the connection of the blend pot. The blend pot sets the mix between the clean sound and distorted sound and at least the units I have built I get plenty of the distorted signal before I even get to half of the pot's full rotation.

Hmmm... Are you sure that your blend pot is linear taper (not logarithmic or "audio" taper).

[tcobretti]

I am by no means an expert, but to me it sounds like either the blend pot is the wrong value (the schematics I've seen say 50kl) or you've used the wrong part in the immediate area of the blend pot.  My thought is that if the resisitance is too high on the pot than the useful area of the pot will shrink.

I'd start at the blend pot and work outward on the schematic.  I'd also be audioprobing like a madman.

http://www.geofex.com/FX_images/audioprb.gif

[John Egerton]

I read on the tonepad site that it is rumoured that the extra diodes on the factory schematic were left out on the actual units...

What are your views on this?

Did you include the extra diodes or not?

Thanks

John

[DDD]

I have extra diodes and it's OK.

[phillip]

The Scrambler sounds the same even if you leave those three extra diodes (highlighted in orange) off the board.  In my commercial version of the the Scrambler, I only use the two diodes that were actually installed in the real ones and it sounds terrific.

The 2N5306 darlingon transistor has a B-C-E pinout, which is different from some other common darlingtons, like the MPSA13 and MPSA14, which have the more standard C-B-E pinout.  If I remember correctly, the Tonepad layout has transistor pads that will accept either pinout very easily.  I remember that the MPSA13 would not work in mine...it sounded weak and farty, but once I dropped in the higher gain 2N5306 (after waiting a week for them to arrive) it worked perfectly.  Also try a higher gain BJT in Q2, like a 2N5089....they sound better to me than the lower gain BC169B (which also has the B-C-E pinout!).

Phillip

[sovtek50]

Here it is: If you're using the Tonepad pcb layout, you have to take a close look at  the WIRING of the pots, especially the 'blend' pot.
Make sure the wiper (hi Greg Sage) goes directly to output, and there you have it: sonic mayhem.