Bypass Digital Board in Super Champ XD (& possibly others)

[dbtech]

If you are reading this, you probably fall into one of two categories, (1) your DSP board failed (power light on, tubes light, sound comes and goes or stays gone), or (2) you want a great all-analog combo box. Well, only because the DSP board failed in our Worship Leader's XD and Fender wouldn't replace anything outside of the warranty period, did I step between the owner and the trash can.

Being an "older" type, I am against seeing a box full of wonderful working analog components thrown away - especially due to a failure in a digital component!! So, I came up with a simple and cheap (under $5 in parts) way to preserve this nice valve amp. I do find it ironic that in a day and age when solid-state gear should be far more reliable than anything with a tube in it, that it is a solid state component that brings so many of these units to their demise! How many were needlessly tossed?

Anyway, because apparently this was not an isolated case, I thought we may still be able to save some of these amps if I just write up what I did and pass the info along. There is a caveat: you end up with only Volume and Brightness controls - it is expected that you can add all the effects you want with external pedals (and if they fail, throw them away, NOT THE FENDER! Here is a link to the instructions, which includes the parts list and a frequency graph of what to expect after the mod.
http://brasherweb.com/techfiles/Fend...bypass_mod.pdf

If you have questions, you can email me (address is in the doc). Good Luck!

[Enzo]

Hi welcome.

I do hope you went back and provided some support for those large caps that are shown hanging by their wires.

[dbtech]

I think the caps came out looking a little bigger and heavier in the pic than they actually are. The axial leaded ones shown in the finished project (as opposed to the disc caps alternative) are "paper" caps and don't weigh much..in fact under 1/8-oz for the pair. They are tied to other wiring, too, giving the whole bundle additional support. Haven't had any problems.

[Enzo]

Fair enough, I just hate to see parts suspended on their own wires. The brown one scares me more than the 0.03 pair. The brown one is not stretched between two solder points, its terminals are much closer together than its ends. When I have to do that sort of thing, I usually lay it down with a dot of silicone under it - or hot melt - then run the wires up to the terminals.

[dbtech]

Enzo, thanks for the attention to detail! That 4.7 cap has leads that are actually stronger than most parts - they are nearly the thickness of the pins on the P5 connector, which I trust is fairly sturdy since it has to withstand big hands pushing and pulling the connector on and off. Just FYI, when I worked in an aerospace engineering dept. (12 years) and was dealing with 30 gauge wires connected to PCB surface-mount rework parts, we would add a speck of RTV every inch or two along a wire and glue the added parts down; this was a case where severe vibration damage (as in a helo) had to be completely prevented. In the case of these amps, I am confident that any vibe strong enough to undo this mod would undo a few other things first!

One other issue to consider... when placing caps flat down on a board, they can pick up stray (unintended) radiation from audio signal traces or power supply hum on the PCB. I like to pursue any opportunity to separate (isolate) signals to prevent hum and feedback problems.

Your issues are valid, but I think unless the player and his amp are flying on a Sikorsky, all will be fine.

[Enzo]

OK, I won't belabor it. The amp might not be a Sikorsky, but it is a combo amp, and the vibrations from the speaker can ring any chassis like a... Bell.

[dbtech]

added PDF instructions as attachment

I've added the instructions as an attachment so the file would be available on the server.

[olddawg]

Did you even try to troubleshoot the DSP board? It could be something as simple as a bad clock crystal, power supply, or a reset.

[dbtech]

Yes and no. I looked for bad solder joints on the DSP board and found none. I used heat & cold on the board, and tested the ribbon & PCB connectors; when in failed mode, none of these had an affect. After the local Fender authorized repair center had a look, they confirmed my findings, that the DSP board was defective, there was not a replacement available, and they would put it in the dumpster for us.

I checked the internet to see if this was a chronic problem and found that there were many XDs with this exact problem, and a solution (permanent fix) was not available from Fender (with the exception that if the amp was in warranty, Fender would replace it). I did not trouble shoot the DSP board further as the owner seemed to be happier with the idea of modding the amp to eliminate the board.

Also, let's say the FPGA was bad, (1) where would I get a certified new one and (2) how would I know that the replacement would be a permanent fix if it came from the same batch as the bad one? Yes, if were a crystal, we probably could have gotten a replacement, but wouldn't Fender have known that and offered to fix it for some big $$ ? Anyway, good questions, but in light of our particular situation, we just opted to bypass some bad silicon.

[Justin Thomas]

Haven't looked at a Super Champ XD, but the Vibro Champ XD has an unused triode, just SCREAMING to be wired up with a seperate input jack, bypass the digital preamp, and have a straight-up In-Preamp-Driver-Powertube setup like a real... Champ! For about 5 extra parts...

Justin

[dbtech]

No such luck in the Super. In the single 12AX7, both triodes are used to amplify (slightly) and produce the inverted and non-inverted inputs for each of the finals (6V6s). I tried an instrument-level signal into the 12AX7, but it needs at least one other prior amp stage. So, I used the the existing op-amp string, but at least they they are analog! If you haven't looked through the PDF, scroll to the schematic on page 8 to see where I re-located the pots, effectively moving them from the digital to the analog path.

[olddawg]

I would be surprised if the local repair center had anyone knowedgeable enough to to repair a DSP board to component level. It just makes me courious though. Many times I was brought units that needed a "board replacement" that had relatively simple problems. Usually a reset timing issue, an oscillator, dry SMD caps (very common) or simply fluxing and hitting some big multi pin ICs with hot air. If you have an EPROM getting corrupted or something it could be an unresolvable issue without the flash program. If it's a very common problem, someone has to have figured it out.