M800 biasing and no sound

[Grazza]

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.

 

[Grazza]

Thanks again for the info, I'll contact the guy at Taran. I think I'll just rip all the transistors out and start again. Pretty sure I used those same j201s in another build recently, most likely a cobalt drive, now I'm thinking it may not sound as good as it could.

 

[music6000]

With the Power disconnected, Check for Continuity with a DMM with matching Colours.

 

[Grazza]

Yes, I have continuity in all of these spots

 

[music6000]

Cool, Try replacing that Transistor with the spare & hopefully you can bias it down to 4.5v!???

 

[Grazza]

Ok, got the transistor out, socketed the holes and put in the only J201 I have right now, I was able to bias it down pretty close to 4.5V. The others are almost smack on 4.5V apart from bottom right tone pot. I have pass through when off, but only the faintest sounds when all pots are dimed to hear it, and almost just sounds like my clean signal. I have to turn Volume right up to hear it. Could there be another problem too?

 

[music6000]

Ok, got the transistor out, socketed the holes and put in the only J201 I have right now, I was able to bias it down pretty close to 4.5V. The others are almost smack on 4.5V apart from bottom right tone pot. I have pass through when off, but only the faintest sounds when all pots are dimed to hear it, and almost just sounds like my clean signal. I have to turn Volume right up to hear it. Could there be another problem too?

This is where you start tuning by ear! as it appears the jfets are questionable or out of spec.
Start up the top left corner & while hitting the strings, adjust the trimmer to the sound is at its Strongest.
Then move to the next one along & repeat the process for all except the Tone Trimmer which most find needs to be at 8 & 9 if 7 is fully C.C.W!
Hopfully there is some volume & gain happening!
Then repeat the process as it's changed the settings again slightly & hopefully it represents something akin to a Marshall!???

 

[Grazza]

Ok, I will give it a shot, I'ts not in the enclosure at the moment, as I've been messing with it, and I'll probably change over the transistors when the new ones arrive, that won't make a difference to me tuning it up will it?

 

[music6000]

You will get some noise not being in the enclosure but you still can tune it.
Once you can get some decent Volume from it, you may have to fine tune the Trimmers to listen for the Gain being better or worse!???

 

[Grazza]

Legend! Apreciate all the help. I will definitely put new transistors in it when the SMD ones arrive. I had just finished building a cobalt drive which has 4 J201's in it. I doesn't sound too bad, do you think it would sound better with new transistors in it too?

 

[music6000]

Does that mean you got the M800 to function somewhere reasonable?

 

[Grazza]

No not yet, away today, won't be home till very late tonight, might not get time till tomorrow, but looking forward to giving it a go. Stay posted!

 

[music6000]

If or when you start getting some Volume happening, turn down the Volume control as it will destroy your Speaker, it's got plenty of Volume on tap when working correctly!

 

[Grazza]

Ok, had a play around with it and no amount of tweaking to the trimpots gives me any more than the faintest almost in audible signal, do you think I have another problem or am i wasting my time till I replace all the transistors?

 

[music6000]

I should have asked this question earlier, do you have plastic pot covers on the 3 pots under the PCB???
Please don't tell me it's insulation tape!

 

[Grazza]

yep, plastic dust covers, learnt that lesson on the first build!!!

 

[music6000]

I would wait until you get the Genuine replacements
You can write these transistors off by cutting the black top off & leave the resistor legs in place if they are long enough so you don't have to unsolder from the PCB again so you can solder the little PCB boards on, your decision!

 

[Grazza]

That's a brilliant idea! I was dreading having to pull them out because I'd have to get rid of the pots too. So theoretically they can be soldered to the little surface mount PCB?

 

[music6000]

They are designed to the same leg spacing so you can just drop them flat down & Solder the 3 pads then you can bend them upwards if they interfere with the trimmer adjustment:

 

[Grazza]

Great! Really appreciate all the time you have put in to this and all the help you've given! Hopefully I'll have the new transistors in a week. Most likely ill be back here when they do. I've got a Stockade and a Duocast to get into in the meantime, possibly being ambitious.

 

[Grazza]

Ok, I have taken the old transisotrs out and put in the new ones. I have been able to get it working, it has quite an open amp like sound however its not particularly high gain, boosts really nicely but I would describe it as moderate gain maybe, is that true to what everyone else has got from this pedal? I did spend quite a bit of time messing with the trimpots, I think I got the best out of it, but maybe I didnt. The loudest sound was generally to be found in the rough ballpart of 4-5V is this correct? However often it didnt sound terrible at 3V.