Digitech Jetpack modded XP400 problem

[Mick Bailey]

I posted a while ago on MEF with no luck and in retrospect may have been better posting here first;

I have a Digitech XP400 in at the moment that I need some assistance with. It has the Jetpack 'XP1000' modifications. The owner tells me it was working fine and then suddenly stopped working.

When I opened it up I could see what I thought was the problem - the stack of 4 EPROMS soldered together are a hefty lump and had pulled out of the socket so they were clunking around on the end of the attached leads. So I reinserted them, powered the unit on and did a pedal sweep calibrate in position 1 which is the XP100 setting. Worked fine. Powered off and tried position 2. No sound, but the display alternates between 0.0 and 1.1. Same with 3 and 4.

Then I tried position 1 again and it took a considerable number of attempts to initialize the pedal - maybe 20 or so. Kept getting the same 0.0 & 1.1. errors before eventually getting it to work, but it would only do so in position 1 and since then I haven't been able to get beyond the flashing error no matter how many times I've cycled it.

I've checked the rotary switch, connections, chip stack solder integrity, socket and voltages. The pull-up resistors are good. My thoughts at the moment that either the loose assembly may have caused damage when the owner was attempting to find out what was wrong, or the EPROMs have got corrupted. Because they're paralleled maybe a fault on one is pulling them all down. It could be that separating them and trying each one individually may reveal something, or it could be something else altogether.

The XP100 schematic generically covers all models - in the XP400 the ICs marked as 'omit' are populated.
Some further investigation With the selecting EPROM model for the XP100 and the single memory IC U19 inserted the pedal operates correctly in that mode only. Swapping any of the 4 memory ICs into the U19 socket works - so they're all fine. The other models select off the rotary and run through each patch, but the sound is hashy and badly distorted - presumably due to the missing memory.

Adding U18 and the unit still operates as described. When U16 and U17 are installed to fully populate the memory, I get the error and the pedal is inoperative. I notice that the flashing display coincides with the EPROM being strobed.

So, something doesn't like U16 and U17 being installed. Given that these ICs are OK, then I'm concluding that the problem is related to the data/address lines, though I can't understand why this would be a problem with the XP100 model (which only addresses U19). Maybe there's a fault with the DSP TMS57070 U15.

Any further insight would be appreciated. The problem with the DSP is it was never released as a consumer product and no data sheet is available.
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Update; Since then I separated the Eprom stack and tried each one individually. The XP100 basic configuration with default memory works, but the other models don't - with any memory configuration. I have not been able to conclude whether the programming on the models other than the XP100 is damaged, but given they are paralleled together I would expect them all to fail rather than just one. To me it looks more likely that the TMS57070 is not addressing the full memory range of U16, U17, U18.

The XP100 schematic can be found here - for the IC designations

http://experimentalistsanonymous.com/diy/Schematics/Vibrato%20and%20Pitch%20Shift/Digitech%20XP100.pdf

[digi2t]

From my experience, distorted/harsh sound is usually indicative of a memory problem. Since you've alternated all the memories around in XP-100 mode (the mode that worked), we can rule that out.

First off, it would be nice to know in what order the EPROM's are stacked, whether the switch wiring corresponds correctly to the order, and finally, whether each chips programs is corrupt or not. The 0.0, or 1.1 error generally points the blame finger at the EPROM(s).

Worst case scenario; if the EPROM's fell out under power, and the pins shorted against the board, then the problem might be more extensive.

I would start with the integrity of the switch wiring, and then go to the EPROM's. It's a PITA to have to desolder all that beehive, but what can you do? That's why the XP-ALL+ version was invented. One chip to rule them all.

[Mick Bailey]

Thanks for replying.

The switch wiring checks out fine and the pedal was previously fully operational. Interesting what you're saying about the error usually pointing to a bad EPROM. I'm thinking the next step is to try a known-good EPROM of one of the other models in the socket, along with the full memory complement. Or to attempt to re-program one or all of the separated ICs (when I can get my hands on an eraser/programer).

I've a bad feeling about the stack clunking around while the unit was powered so I need some luck with this.

[digi2t]

If you need help with the EPROM's (reading, erasing, programming), let me know. If you need .bin files/schematics, you'll find them for all models, including the XP-ALL+ .bin, in my gallery (Dino's stuff).

If memory serves me correctly, the XP-300 uses the full bank of memory.