Digitech XP400 (Jetpack) fault

[Mick Bailey]

I have a Digitech RP400 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.

Can anyone help?

[Mick Bailey]

I've made some progress with this, but not yet resolved.

The XP100 schematic generically covers all models - in the XP400 the ICs marked as 'omit' are populated.

With the 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.

Digitech XP100.pdf

[Mick Bailey]

I still have this pedal - it's one of those jobs I keep coming back to occasionally as it's pretty much a long-term project to get an otherwise useless pedal working again.

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.

So my final call on this - does anyone have experience of working on these pedals, or can offer advice on further testing the TMS57070?

My plan if I can't rescue the Jetpack functionality is to revert the pedal back to the original XP100, but this feels more like defeat than progress.

[potatofarmer]

Have you tried posting at DIY Stompboxes? "Govmnt_Lacky" and "digi2t" seem to know more about these than anyone outside of Digitech.

Here's the main thread, they've debugged a few conversions in it, though they're referencing their "XP-ALL" and "XP-ALL+" conversions which IIRC are executed slightly differently than the Jetpack conversion: thoughts on Digitech Space Station eprom copying?

[Mick Bailey]

My base documentation for understanding the Jetpack mods came form those guys. The XP-ALL mod replaces the stacked Eproms with a single IC containing all four models - a more elegant solution. Anyhow, you've prompted me to re-post over there to see if anyone picks it up.