Thursday, 1 March 2018

Voodoo Labs Amp Selector

 This is a 2 input, 4 output amp selector and A/B switch. One guitar can drive 4 isolated outputs, all with their own volume controls and ground lifts. There is some preset save/recall functionality, so I am expecting a microcontroller in here somewhere. This is exactly the sort of thing I would have loved to play with when I was renting a practice room and had access to lots of amps, but it will probably just be sold on nowadays.

Main PCB
No surprises on the insides - there is an P87LPC764 MCU reading switches and turning JFET switches on and off. A MAX1044 provides a -9V supply. The transformers are 6 pin devices with any labels or markings removed. There is one OPA2134 dual opamp, and a 24C01 serial EEPROM for presets.

Hacked in parts at the power input.
The back of the PCB has some factory bodges/mods at the power input - there is a series resistor and a capacitor added to the MAX1044 input.

The digital side seemed to work, everything lit up and it did switch signals, but there was lots of bleed between outputs and the volume was quite weak.

I know from experience that a MAX1044 will only tolerate ~10V input before dying, so I looked there first. The inverted voltage was measuring less than -1V. I desoldered the MAX1044 and replaced it - no change. There is a zener diode at the input pin which should limit the input voltage, I lifted this part out and now I had -8V and a working pedal.

Replaced MAX1044 (pictured here sitting crooked in a socket) and lifted diode D10
This diode was a 1n5239b (9.1V 0.5W zener) so I replaced it with a 1W part I had at hand. Now the negative voltage was pulled low again! It turned out that the cap across the zener that was added to the packside was leaking. It still measured 2.2uF but had ESR >5 ohm - this looks like a dipped tantulum part. I replaced with a similar electrolytic. It's not even technically needed, the cap is just stabilising the 9.1V zener clamp when it's conducting.


  1. This comment has been removed by a blog administrator.

  2. Hi Krivx, I live in Dublin and I’ve a faulty EHX holier grail that Id love to get working- I believe that you fixed my PDS 8000 (via moose) a while back and it’s working great so I know you’re the man to see with the digital stuff!
    Is it something you’d take a look at?

    thanks in advance!

    1. Hi. I have never seen a PDS 8000, but if it was a PDS 2000 (via moose) then it might have been the same one. There are probably pictures here somewhere.

      Unfortunately I'm not looking at any pedals at the moment, just clearing out my own stuff that needs repair.

    2. You’re right it’s a PDS2000, that was mine- thanks for saving it!

      that’s a shame about the repairs but I understand, great blog so I’ll keep reading and I’ll keep my eye out for if/when you start doing repairs again.

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