Problem with JCM800 emulation

[tommas93]

Hi guys,
Im building jcm800 emulation (pcb designed by PCB 2005, Cabochon intermedia) now. Q1-6 are J201 (drain voltage is 4,5V). After plug the battery i hear some slight noise (which is changed if i twist with some potentiometer), but if i plug guitar, the emulation is does not work. PCB layout is probably OK, JFETs are OK too. Unfortunately I dont know, where is the problem...

[duck_arse]

welcome to the forum, tommas93.

the good thing about building on pcb's is that there is so little can go wrong, you can't put parts in the wrong holes. so, that narrows the problem down, quite often, to some off-board error. a common mistake is not showing us photos of what you have built, cause we like to look. also, you might have wired your in or out jack backwards, but we can't say until we can see. see?

[tommas93]

So okay,
I have some photos of my work. Today i did check (with multimeter) wiring (white cabels) from potentiometers board to main board, everythings are alright. :/

[temol]

Take the audio probe (link1, link2) and trace the signal from the input. You can connect mp3 player or your cellphone to the input and play any sounds.

T.

[Gainiac]

Hard to tell for sure how clean your soldering is on the trace side of things from your picture but I would double-check that there are no unintended connections between pads.  Easiest way to do this imo is to hold a very bright flashlight right up to the board on the component side and look through it from the back, x-ray style.  Anything between two pads that looks dark should be scraped clean and/or resoldered.

[tommas93]

I did fix poorly soldered places on pcb.
I made the probe (such thing is very usefull)
With support the probe I measure, where is the problem, and I find out a huge signal loss on capacitor 471 after second (from left) jfet.
I  think the capacitor is KO, so i did change them ... no sound, still huge signal loss. I did change the resistor 470K to 740K resistor.
Signal loss was not as big as before, but this signal is better, but the signal is so weak. Rebiasing all of the jfets without DMM, only with my ears support, and works great!
I've discovered several shortcomings here, The emulation does not have sound like JCM800 (unexpectedly), the master pot works like another volume pot. I will try to fix this shortcomings and build new pcb layout.

A lot of thanks for all, who did help me!

[Derringer]

It's going to be tough getting a pedal to nail that 2203/2204 tone.

That being said, if you're up for more tinkering/building, I always thought that the Marshall Guvnor pedal got reasonably close to that JCM800 sound. And, it's an easier build than the emulator you've done here.
https://www.gitaarheld.nl/guvnor.html

Lovepedal made a version with a single knob tonestack instead of the full bass/mid/treble that the original Guvnor used.
Check it out. http://revolutiondeux.blogspot.com/2012/01/lovepedal-superlead.html