Saturday 6 August 2016

Digitech Whammy 4

I have a bit of a backlog of stuff I've done but not found the time to describe here. It's probably best to do so before I forget everything.

Big red pitch shifter. Another 90s digital classic. I used to own one of these these but moved it along because I didn't really get on well with no polyphony and the artifacting didn't always sound too great. I think these older Whammy pedals are pieces that need to have music written with them in mind instead of just switching them on and playing the usual stuff.


A pedal large enough that the photos look terrible

Sold as not working and had had a true bypass mod done. I was hoping that it just needed calibration or that there was a simple issue with the true bypass work. Instead it just flashed all the LEDs once when powered up and then did nothing.

Inside we have a large PCB that fills most of the enclosure. There's another Motorola 56k DSP56362 (same as some of the Line 6 and EX stuff), an Atmel 89C55 (8051 architecture), a Cirrus CS4224 Codec, the MIDI optoisolator and some CMOS logic chips and opamps. The DSP56k/8051/CS4224 seems to be a really common choice of components for late 90s/2000s digital, it's very similar to what was in the Line 6 pedals I looked at previously.

The mod work was pretty bad, the wiring used was way too thick and was under strain, there were ferrite beads cracked off the PCB and the 3PDT footswitch fell apart when I was removing the old wiring.

Main PCB and LED/encoder PCB

Atmel 8051 MCU under the sticker

This has the annoying 80s/90s practice of including an AC power supply to create positive and negative voltage supplies instead of using switching converters and running off 9V DC. There are 3 linear supplies, +5V and -5V for the analog stuff and 3.3V for digital. The 3.3V was down near 0V and the regulator looked scorched, so the heatsink was desoldered and I replaced it. No change, so there is excessive current being pulled somewhere. 3.3v to GND measured around 40 ohms. I removed any 3.3V bypass capacitors in case they were shorted, and then noticed that the main DSP chip was running pretty hot.

At this point I gave up for a little while. The DSP is a 144 pin QFP and in it can be difficult to find reliable suppliers of these 56k series parts these days. There are also no schematics for this pedal online, so I would have to trace things out or make guesses.

I tried contacting Digitech and asking about a schematic, and they replied with a PDF within an hour! Here it is https://drive.google.com/file/d/0ByVCt2OFhXnyZS1JZFlmb0Z3VTA/view?usp=sharing&resourcekey=0-vBilBFlfU0YChMUrFsavrg

I also found that this IC is actually still stocked at a lot of distributors, just not in huge numbers. I decided to get one from Farnell for around 18 euros, Digikey have them as well.

Removed the original bad DSP was difficult, as the chip is quite big and there is a large top-side ground place surrounding it that takes a lot of energy to heat up. After not much progress with hot air I flooded all the pins with some Chip-Quik low-temperature solder that I had. This kept everything melted and worked well but made a mess, there were blobs of solder all over that region of the board that had be wicked and scraped off. I realised afterwards that I should have tried using a larger air nozzle for a chip this big.

After DSP removal, there is solder everywhere
After wicking and cleaning the board

 The new chip went on easily enough with some tacky flux. The true bypass mod removes the original momentary footswitch, but this switch is still needed to calibrate the footpedal, so it is usually replaced with a pushbutton mounted to the back of the enclosure. In this case, the switch was connected to the board using the original connector and then fixed down with a big blob of hot glue (you can see this in one of the pictures above). Unfortunately, all the rework heated the board enough that this glue melted and ran all over the board, leaving a thin layer around the MCU. Now nothing happened when powering it. I could peel off  some of the larger pieces of glue but most of it needed to be scraped. At this point I gave up temporarily again.

DSP resoldered,Glue cleaned, C52 replaced, Y1 removed, cleaned and repopulated
I came back a couple of weeks later and used some IPA and cotton swabs to get off the rest of the hot glue. C52, Y1 (crystal oscillator), the momentary switch connector and some of the legs of the MCU were the worst effected, so I removed all of those except the MCU and cleaned the board as best as I could. Y1 was pretty badly coated in glue, so I cleaned it and repopulated. I tried poking around wit an oscilloscope probe and found that every time I touched one of the pads of C52 the LEDs would flash. The schematic says that this is the reset timing cap, so presumably the MCU was never being reset. I replaced it with a 1uF film cap I had on hand and the Whammy fired up. I can only guess that swapping the DSP was successful, but that the hot glue was preventing Y1 from oscillating and the MCU from getting a clock signal, or C52 was bad, or both. After redoing the true bypass wiring everything works.

There is also a 1k serial EEPROM on board, but it is connected to the DSP and not the MCU, so I think it for saving calibration settings and not the program code. I would like to dump the MCU if possible, but I'm going to leave this alone. If I come across a scrap board I'll pull the chip and try to dump it. There is also a JTAG debug port wired to the DSP, I haven't looked at that at all.

Was this worth it? Replacement parts ate up a good portion of what I would have gained from selling this, and I have more than a few hours invested in it, so probably not. The newer models add polyphony and have lowered the value on these older units too. I'll add a LED (mysteriously missing? Maybe someone killed this halfway through a mod) and sell it on. I'm glad to have swapped out a QFP144 successfully and found a schematic for this, so I'm still thinking of it as a win.

20 comments:

  1. Good evening mister. I've read your post and it seems pretty interesting to me. I'd like to repair my own whammy by buying these Motorola Micros on Mouser.

    But, how about the programming? Have you asked them on Digitech about it?

    Thank you for such a helpful information.

    ivan@selis.com

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    1. No, I haven't asked. It's worth a shot as they were very easy going about sending the schematic.

      It's possible that the program code could be dumped from a working pedal but I haven't tried this.

      Delete
  2. I got a whammy 4 from ebay untested. When I received it, I popped it open to discover two of the inductors where blown. Im guessing this is from plugging a DC power supply in instead of the AC supply. Thanks to you, I have the info that I need to get replacement parts.

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    Replies
    1. Which replacment parts did you use to fix this issue. I have the same problem

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  3. Great post, really helpful, thanks!!!

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  4. Hi my whammy 4 gets no sound from the wet output. Does anyone have an idea why and how to fix it. I have no electrical experience whatsoever.

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  5. Hey! Thanks for a great post! Have you find the dump of firmware? I have the same issue!

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  6. hello,one guy gave me a whammy like this with the same problem,all leds on,3,3v regulator was toasted,output was 0,7V,removed that and there is an almost short circuit,4 ohms,removed all the caps around the dsp and its the same,i think the dsp is faulty at this point,is it just a matter of finding one and replace it or does it need any programming?
    I have a pro guy for proper soldering.
    Thanks for the answer!

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  7. update,the voltage regulator is ok,obvously the circuit pulls so much current that the string of diodes goes very hot and drops the voltage way down,at this point the only thing powered by that supply is the DSP

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    Replies
    1. You've checked the caps at the input and output of the 3.3v regulator?

      It could be the DSP, yes. You can replace them without needing any programming, the MCU programs the DSP on start-up.

      Delete
    2. Thanks,caps and regulator ok,removed the dsp,now it's not shorted anymore, ordered the chip and waiting.

      Delete
    3. That's a good sign, probably a similar DSP failure to what I saw.

      The MCU runs at 5V and appears to be driving the DSP with 5V signalling. Maybe the DSP inputs are not 5V tolerant? It would explain some of the failures.

      Delete
    4. replaced the dsp,now it works,although it is difficult to make it start at power up,tried to replace c52 but not effective.
      now the 4053 chip appears to be damaged,outputs go to ground internally,next days.

      Delete
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  9. Great blog. I accidentally also found that probing C52 brought it to life. I has been working more but after an hour of playing it shut off. I noticed that the DSP had a bunch of bridges like yours could this be causing issues? I am sorry DSP is fairly new to me. Love your blog.

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    1. If probing C52 helps then C52 is likely bad.

      I wouldn't worry about bridges on the DSP if you haven't done any soldering on it. A lot of the pins are connected together at the PCB, see the image with removed DSP for reference.

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    2. It works sporadically, it worked for about an hour and then shut off. It is weird maybe I will replace the DSP as well. Did it permanently fix the issue with yours? Also thank you for replaying

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    3. Yeah, mine was stable after a DSP swap and replacing that capacitor. I only swapped the DSP because mine was completely dead and running very hot.

      If C52 has some issue it's possible that the DSP is being held in reset after running for a while.

      Delete
  10. Have the same pedal. Different situation. Looks like Ferrite Beads L12 and L14 are fried (I'm guessing whoever owned it last plugged into DC). Any idea where I can get those? 2.5k Ohm @70mHz 0805 T&R

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