M800 - Solved - PBCASI

Willybomb

Active member
So, here I am with two M800 pcbs populated but not working. I can't see any soldering issues, and on one of the boards only 2 of the trimmers are working. So, I figure that I must have a bunch of dead trimmers or this particular style is grounding or something somewhere. I replace one with a different trimmer and find I can adjust the voltage. Great!

Following on from that logic, I replace the other trimmers and I can finally bias the j201s - but there's still no sound from the board. Oddly, one of the trimmers will bias to 3.5v or so, but then the most minute of adjustments will see it jump to 9v. Weird. Still no joy.

Anyway, I start looking at the board again and notice that one of the jfets doesn't have the same style of markings as the others. I look closer and find it's a 2n3609 or something (I didn't take any real notice) so I replaced that, adjusted the bias and we now have a working pcb. Now to replace the trimmers on the second board.
 
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It was completely populated with the style of trimmers (100k) in the yellow circle. Those two are the only ones that worked so I left them in and replaced the rest with the 50k you see there. I'm certain I've used them in other builds, so.... anyway... 289018675_588169252730376_3320370064152061917_n.jpg
 
You may or may not have the range to bias the jfets. I haven't actually measured the trimpot resistance, but I doubt you'd need the upper range (50k+) to bias to 4.5v (I didn't in this case anyway).
 
I will say that the last trimmer (the treble tone trimmer at the bottom right, looking at the components) absolutely needs to be 100k, otherwise it just sounds muffled. I had a 50k in one (sounded terrible), and a 100k in the other (sounded great) and I couldn't work out why until I checked those trimmer values. FWIW, I have that trimmer all the way to the left on both pcbs and couldn't get a decent sound until I replaced the 50k.
 
That' odd..
TRIM4 (100k) and C14 (10n) - it's a low pass filter. Trimmer acts here as a variable resistor. When you increase resistance of the trimmer you attenuate the treble more and more. So with 100k trimmer there's more attenuation than with 50k - at max setting. On min setting both trimmers should "sound" the same.

1656141802415.png
 
Well.... it didn't make sense to me that a two trimmers turned all the way left would sound any different, but... this was the case. I absolutely couldn't get the same amount of treble out of them until I replaced it. It was like taking a blanket off the amp.
 
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