Digitech Death Metal (Does not stay engaged)

Necro Mold

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First-time poster and newbie here!

So a while back I acquired a Digitech Death Metal from an old coworker to see if I would be able to fix it.
I'm getting a bypassed clean sound just fine with the guitar plugged in but the effect only wants to work while I'm pressing down on the switch.
It's basically acting like a momentary effect and won't stay engaged when I let go. At first, I thought it was just a faulty switch so I went ahead and swapped it out for a new tact switch but still had the issue. I then looked at all the components to see if anything looked off and found a 330uf electrolytic cap above the "Mid" pot that looked a little swollen so went ahead and replaced that also. I still have the same issue. I've looked around on forums to see if anyone else has had similar issues but finally decided to ask myself before sending it back to my coworker.

I'm fairly new to pedal building and have only put together a handful of clones via Pedal PCB. Replacing components shouldn't be too hard for me but I'm unsure where I should be looking or probing for.

If anyone has had similar issues or has the resources that can help point me in the right direction I would greatly appreciate it! 🤘
 

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So I am not an engineer or troubleshooting expert, but if that is a soft touch tactile switch I would suspect that the pedal has a switching relay of some sort that may have gone bad. See what you can find out about u3 and search the web for a schematic.
Edit: check this thread https://www.diystompboxes.com/smfforum/index.php?topic=120562.0
I've found that thread in the past while searching for a fix. In the thread he also has a hard time finding the actual schematic so he provided one for the DOD FX86 which I believe is the same circuit under a rebranding. I know a few of the DOD/Digitech pedals are just modified circuits of their Grunge pedal. I will attach both the schematic that he posted and one for the Grunge.

Examining u3 it reads 14007UG PAA707. Doing a quick search I was able to find this: https://www.onsemi.com/pdf/datasheet/mc14007ub-d.pdf
Being able to read and decipher a schematic is beyond my capability at the moment. Would u3 be a part of the switching relay?

Death Metal DOD FX86.gif DOD Grunge.jpg
 
OK, it looks like it is a FET switching setup, but the switching bit is mostly missing from the schematic. If you see the lower schematic of the 2, there are 2 FETs at the bottom near the output (unfortunately they are not marked with component numbers so I can't point out better than that. One of the FETs is letting through bypass signal and the other is letting through the effect signal. There is a good description here about how the system works www.geofex.com/Article_Folders/bosstech.pdf

I would guess that if you can get effected sound through then both FETS are probably working, but the mechanism that swaps from one to the other being active is probably a little wonky
 
Going on what @PJS said, U3 could very well be responsible for the FET switching since it is made up of 6 MOSFETs. Does this info help you in any way? Probably not since I wouldn’t really know how to go ab telling if the chip were bad or not and replacing it would not be an easy task. I’ll let one of the actual engineers chime in.
 
OK, it looks like it is a FET switching setup, but the switching bit is mostly missing from the schematic. If you see the lower schematic of the 2, there are 2 FETs at the bottom near the output (unfortunately they are not marked with component numbers so I can't point out better than that. One of the FETs is letting through bypass signal and the other is letting through the effect signal. There is a good description here about how the system works www.geofex.com/Article_Folders/bosstech.pdf

I would guess that if you can get effected sound through then both FETS are probably working, but the mechanism that swaps from one to the other being active is probably a little wonky
After a little more digging I was able to find a better schematic for the DOD FX69 (Grunge) with the component numbers listed. Comparing the two it looks like the two FETs you were describing could be Q10 and Q4? They're listed as a J201.
(I'm using this sheet I found as a reference: https://www.futurlec.com/Datasheet/Transistor/J201.pdf)

Probing them with my multimeter in continuity mode I was able to record these results from them:
J201Q10Q4
G to S882886
G to D873878
S to G11
S to D600+646
D to S1630+
D to G11

Should I be getting some sort of reading from Drain to Source on Q10?

dodfx69bgrunge.gif
 
Yes. Q4 is for bypass, and Q10 carries the effect signal. It is the network around U3 that is doing the switching. Does the LED turn on when the effect is working and off when the effect is bypassed?
 
Yes. Q4 is for bypass, and Q10 carries the effect signal. It is the network around U3 that is doing the switching. Does the LED turn on when the effect is working and off when the effect is bypassed?
The LED turns on with the effect while I'm holding down the tact switch and off when I let go yes.
 
It sounds as if almost everything is working as it should. Sounds like you won't need to replace U3 fingers crossed. I am a little hazey on exactly how this bypass works, but from my understanding I would be suspicious of the capacitor that is labelled C16 in the above schematic. I think it's purpose is to discharge and charge with the change of state and I think that if it is not doing that properly then the system may not stay in a steady state well. Somebody else with a bit better knowledge of this type of bypass may be able to say with more certainty
 
Sound like the cd4007 is failing. It's being used to switch but isn't "latching"
I'd check the components around it as well. C6 and the half dozen resistors for opens.
 
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