Help fixing a Boss PH-3 Phaser

[ItsGiusto]

My friend sent me a Boss PH-3 phaser to fix, not knowing it was digital. I don't know much about digital circuits, unfortunately.


The problem seems to be with the knob all the way to the right, the "STAGE" knob. For all settings, except the "4" setting, it sounds like it's in "STEP" mode, this weird computery sound-effect. See demo of STEP mode here at time 1:30



My guess would be that this is simply a problem with the STAGE knob itself. However, I don't even know much about digital selector knobs. Where could I get a replacement? Is it easy to replace, or will it involve some crazy SMD stuff that I can't easily solder?

Strangely, here the STAGE knob looks like a normal potentiometer:
https://www.audioservicemanuals.com/b/boss/boss-ph/329737-boss-ph-3-phase-shifter-schematic
However, the knob is definitely quantized, with discrete clicks along the way. How do these sorts of digital selectors work?

[idy]

One common way to control a digital parameter is with a stepped potentiometer. A "reader" is set to read, for example, 0-10k as "1", >10k to 20k as "2", >20k to 30k as "3" etc. Cheaper to implement in the digital domain than a rotary switch... Modern humans seem to solve everything by coding, what ever that is.

So you probably can safely get the thing out of the box and look at the model of pot they used. People here love photos of things like that.

[Rob Strand]

Check the soldering on the pot hasn't got any cracks  and check the continuity from the pot wiper through to the processor pin.   Could be a soldering issue on the processor pin.

[ElectricDruid]

Check the soldering on the pot hasn't got any cracks  and check the continuity from the pot wiper through to the processor pin.   Could be a soldering issue on the processor pin.

+1 agree. Sounds like an issue with the Stage pot - and I agree with idy that it probably is a pot, not a rotary switch. A pot simpler to implement in code, since you can use one ADC input rather than multiple digital inputs.

If the pot is completely knackered, you might find that you have to replace it with a similiarly-sized equivalent. The specific value is typically not crucial but is also likely to not be hard to find (I'm guessing 10K, for the record!). What's basically impossible to find unless you order 50,000 units (I actually asked Alpha Taiwan about this once upon a time...) is getting a pot with 7 detents. That's a special order item that Boss have requested. A normal pot will work but won't have the clicks.

But before we get to that - check the pot! A cracked solder joint at one end is pretty likely and might be all it is.

Good luck!

[pruttelherrie]

Speaking of Alpha pots with detents.... I've been waiting since january for a 10k pot with 11 detents I ordered. For now there's a non-detented in the TC Flashback triple delay that I repaired for a friend, and it works, but he'd really like a detented back in there. Yay global shortages.

[cordobes]

Check or replace C47, a major part of surface mount ceramic capacitor failures is a short (low impedance path) across the part.