PLEASE HELP! My preassambled XoX is broken!

[mario1089]

So now, more accurate measurements.

Voltages at D34 and D35 are all fine. 0V for no note; 12V while note held.

D37 rises to maybe 12 V (cant say exactly) on note-trigger than falls down to zero during seconds while note held. At least it looks like this.

D36 looks weird. When triggering, it does not go up to 12V. If triggering fastly, it may go up to 0.5v or something, but not higher. Not sure if thats the meter averaging pulses or what it means. It`s looks much more like zero volts than like 12V, thats sure.

Then I checked for audio signals again. Osc Pin works fine, and also I found the same signal audibly at Q12, but that`s it. No signal at all at VR4 (the pin you told me). really not.

I`ll now setup my oscilloscope, hope it still works.

[mario1089]

so, oscilloscope runs good. I can see on D36 there is a dirac pulse for any note.

Also I can see the audio signal at Q22, but not on VR4.

[MMM]

so, oscilloscope runs good. I can see on D36 there is a dirac pulse for any note.

That means that things are probably ok. What's the situation like on the cathode of D27? (This is where the VCA envelope is mixed with the accent envelope; but it is also a good test point for the VCA envelope in general.)

Also I can see the audio signal at Q22, but not on VR4.

What does the signal look like at Q22? Very small amplitude and very "fuzzy? Does it react to the cutoff knob? Does it react to the envelope?

[mario1089]

Very small amplitude and very "fuzzy? Does it react to the cutoff knob? Does it react to the envelope?

small aplitude yes, because of r62 I assume. Not fuzzy, not affected by anything - it`s the same signal as on the osc pin (only with R62 and c17in between). pure clean sawtooth. It`s the input into the filter section I think.

Ich checked every other pin of Q22 and all pins of Q12 and Q21 and Q13. Oscilloscope at 10 mV - there is no audio signal nowhere in the filter section. just nothing.

What's the situation like on the cathode of D27?

NoteOn: Goes to 12V then back to zero in maybe 50ms
NoteOff: goes to -12V then back to zero in maybe 50ms.

[mario1089]

Very small amplitude and very "fuzzy? Does it react to the cutoff knob? Does it react to the envelope?

small aplitude yes, because of r62 I assume. Not fuzzy, not affected by anything - it`s the same signal as on the osc pin (only with R62 and c17in between). pure clean sawtooth. It`s the input into the filter section I think. I mean the pin closest to the nearby trimmer TM3.

Ich checked every other pin of Q22 and all pins of Q12 and Q21 and Q13. Oscilloscope at 10 mV - there is no audio signal nowhere in the filter section. just nothing.

What's the situation like on the cathode of D27?

NoteOn: Goes to 12V then back to zero in maybe 50ms
NoteOff: goes to -12V then back to zero in maybe 50ms.

[MMM]

It's a bit hard to see where exactly you're really measuring. This sounds as if your are actually measuring at Q12, not Q22. Q22 is at the end of the ladder, Q12 at the start. You could also check at C18, C19, C23 and C26 (which side doesn't matter).

As far as the envelope goes: This sounds as if you did measure at D37, not D27. Or is Accent active?

[mario1089]

Sorry sorry sorry, yes, Q12, I`m at Q12, all the time, at the start of the ladder.

D27 is right, the C close to r120. I measured that one and it showed the behaviour described. If accent on or off doesnt seem to matter.

Checked C18, C19, C23 and C26, no signals there, nothing.

[MMM]

D27 is right, the C close to r120. I measured that one and it showed the behaviour described. If accent on or off doesnt seem to matter.

Quick question: Did you measure this with the oscilloscope still AC coupled? If so: Please do it again DC coupled.

What's the DC offset through the ladder (from C18 through C19, 23 and 26)?

[mario1089]

damn, you are right again. Osci was still in AC mode, this is why I got the curves. Now in DC mode I see the D27 just go up and down. square-like. I think that is just what it should do, right?

[mario1089]

DC in ladder:

c18: 9.6
c19: 10.13
c23: 5.3
c26: 11.1

[MMM]

damn, you are right again. Osci was still in AC mode, this is why I got the curves. Now in DC mode I see the D27 just go up and down. square-like. I think that is just what it should do, right?

Yes, I think this means that the various envelope sections basically work.

DC in ladder:

c18: 9.6
c19: 10.13
c23: 5.3
c26: 11.1

My mistake: I meant C24, not C23. But I think this will have a value halfway between C19 and C26 anyway (those values look fine). This probably means that Q22 is working. Which leads to the question what else could go wrong - which of course points in my opinion to Q12 and/or its current control. What's the voltage at the middle pin of Q12?

[mario1089]

Q12, middle pin: DC 5.9V
Q12, 2nd and 4th pin: 9.6V DC
Q12, right pin (at TM3 side): audio signal
Q12, left pin: 5.3 V DC

[MMM]

Please measure the voltage at the bottom side of R73 while turning the Cutoff knob (it should change significantly).

[mario1089]

R73 bottom (towards CPU) side:

Cutoff 100% : 10.1V
Cutoff 50%: 7.0V
Cutoff 0%: 5.8V

[MMM]

That also looks ok.
I'll work on this problem a bit more tomorrow; but the remaining options are not really easy to debug. I'd probably check very closely Q9 to Q11 for any soldering problems first.
But, as I said, it could also be that Q12 is the problem. Or it could be something completely different.