M800 biasing and no sound

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Grazza

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Just finished an M800 build, photos attached. I have sound pass through when the effect is not on, however when I turn the effect on I have no sound. Maybe a switch problem, I thought I had continuity in the right places?
Also I can bias transistors 1, 2, 3 and 4 to within of 4.5V, however, not matter what I do with the trimpots for 5 and 6, they never change from 8.5ish Volts. 5 and 6 circled in red so you know which ones I mean. Please help, I was really looking forward to this one.
 

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Very possibly eBay. I'm in Australia, a lot of parts can be hard to source here, especially the j201s I've noticed. Not really sure where to look that doesn't cost 10 times the price of transistors in shipping. Do you think the j201s are no good? Or at least the one in position 5?
 
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Yes, though they generally still let sound through albeit not the sound you expected. If you're getting nothing out the other side, then it's time to break out the audio probe.

1712059351958.png
 
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Grazza said:
Just finished an M800 build, photos attached. I have sound pass through when the effect is not on, however when I turn the effect on I have no sound. Maybe a switch problem, I thought I had continuity in the right places?
Also I can bias transistors 1, 2, 3 and 4 to within of 4.5V, however, not matter what I do with the trimpots for 5 and 6, they never change from 8.5ish Volts. 5 and 6 circled in red so you know which ones I mean. Please help, I was really looking forward to this one.
Click to expand...
One of the transistors doesn’t have a bias control
That leaves the other trimpot to be another tone control
 
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I noticed that in other posts, some people just said to leave trimpot 6 dimed. I will see if I can replace transistor 5, not sure if I have any left but I might, It'l be from the same place through. Is there a way of testing the transistor in situ to see if its the culprit and will this transistor be the reason that there is no noise at all? I'm pretty new to this, I don't actually have an audio probe, is that something I buy or make?
Thanks guys.
 
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I have been lucky enough to find one j201 here at home, I put this one in the tester, probably should have done that with all of them. But I don't know what the results mean anyway. Does the picture of the test below show a working jfet?
 

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Grazza said:
Just finished an M800 build, photos attached. I have sound pass through when the effect is not on, however when I turn the effect on I have no sound. Maybe a switch problem, I thought I had continuity in the right places?
Also I can bias transistors 1, 2, 3 and 4 to within of 4.5V, however, not matter what I do with the trimpots for 5 and 6, they never change from 8.5ish Volts. 5 and 6 circled in red so you know which ones I mean. Please help, I was really looking forward to this one.
Click to expand...
This circuit is fussy about the J201's being in spec, My first lot worked but it didn't sound right, They were out of spec when I removed & tested them.
The next lot were all in spec & it was how the pedal was suppose to work.
Robert sells an Adapter board with SMD J201 attached, that maybe you other option!, Out of Stock coz their so popular!
www.pedalpcb.com

MMBFJ201 JFET (Pre-Soldered) - PedalPCB.com

JFET
www.pedalpcb.com www.pedalpcb.com
 
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Here is my build from 5 years ago:

M800 Overdrive

Awesome JCM800 Emulation !!! All the Trimmers were adjusted to 4.5v & it was meh, 4.75, 5v, 5.25v & 5.5v, Louder with no Gain! Old School, Strummed a Chord & tuned each Trimmer by ear. Critical Trimmer is the Tone trimmer, This effects overall EQ & gives it the Power of The JCM800 sizzle! The...
forum.pedalpcb.com forum.pedalpcb.com
 
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music6000 said:
This circuit is fussy about the J201's being in spec, My first lot worked but it didn't sound right, They were out of spec when I removed & tested them.
The next lot were all in spec & it was how the pedal was suppose to work.
Robert sells an Adapter board with SMD J201 attached, that maybe you other option!, Out of Stock coz their so popular!
www.pedalpcb.com

MMBFJ201 JFET (Pre-Soldered) - PedalPCB.com

JFET
www.pedalpcb.com www.pedalpcb.com
Click to expand...
Just found this!!!
Here is the local Australian Company contact info:
Here is the Part:
 
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Wow, I didn't even consider that they could be out of spec. I should have socketed them now that I'm thinking about it. I'm fairly new to this, gaining a little eperience each pedal I build. I will contact that australian company for sure. Is there a way to test the J201's I have in the circutboard already or do they need to be removed? I think 4.75-4.8V was the furthest away any of the other j201s were from 4.5V. Does that mean it's a good transister? So the surface mounts are easier to get these days? If an advertised J201 says its original fairchild, is it to be believed?

Thats a lot of questions, sorry.
 
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