Boss DD-20 troubleshooting

[mckenzie]

Hi, I have a DD-20 with only a dry signal, it looks like the wet/dry switching signals from the CPU are messed up?

All V rails are good. Pedal powers up ok & functions appear to work.

Problems appear at base of Q10, Q11 & Q14 which are always stuck at a few mV no matter if pedal is on or bypassed. Q10 & Q11 for instance should be toggling on/off right ?
Q12 & Q13 however do appear to work ok as they toggle on/off when input B jack is connected (activates stereo in/out)

Diode junctions base>emitter & base>collector of Q10 - Q14 all test good at around 0.6v

It looks like the CPU isn't firing out voltage to the bases of Q12,13 &14 when it should. Is my thinking correct ?

Schematic is on page 27 attached
Thanks for any help

[g1]

There are lines coming off the various jacks that seem connected to the 'JK IN/No INPUT' line at IC8 CPU (according to block diagram).
I don't see a line called that on the actual schematic, but it would be a good place to look. If nothing else, verify that jack switches are closing when nothing plugged in to them.
(edit: looks like they do not use switching jacks, ring gets grounded to sleeve when mono plug inserted)

[mckenzie]

Thanks, yes those 3.3v lines are working. For example Q12 & 13 do toggle ok when input b is plugged in

[g1]

Do you have effected signal at IC18 pin 11 ? Pin 10 ?

[mckenzie]

Do you have effected signal at IC18 pin 11 ? Pin 10 ?

Thanks for looking at this. No, no signal at IC18 pin 11/10. the signal actually stops at pin 6 of IC3 - no signal coming out at pin 7. It looks like all the 4066 switches are turned on with about 1v which is coming from Q7 area - Q7 is always off no matter if pedal is engaged or not.
When those 4066 switches are bypassing those four 39k resistors I'm not quite sure what the purpose of this is ?

So looks like here as well the control signal from the CPU doesn't appear to be working as there is no voltage change on Q7 base when pedal is turned on/off.


cheers

[g1]

1v which is coming from Q7 area - Q7 is always off no matter if pedal is engaged or not

If Q7 is off, collector voltage should be about 8.6V. What is Q7 collector voltage when pedal off, and when on?

[mckenzie]

If Q7 is off, collector voltage should be about 8.6V. What is Q7 collector voltage when pedal off, and when on?

Oh yeah of course it should! Collector voltage stays at 1v with the pedal on or off. I took Q7 out of circuit & the 1v still remains after R11.

So possibly a faulty 4066 with the 8.6 voltage being dragged right down ?

Incidentally Q7 seems faulty also as it's diode drop across base > collector doesn't read anything. Base > emitter is good.

thanks

[g1]

So possibly a faulty 4066 with the 8.6 voltage being dragged right down ?

Incidentally Q7 seems faulty also as it's diode drop across base > collector doesn't read anything.

Yes, sounds like both IC18 and Q7 are bad, replace both,.

[mckenzie]

Yes, sounds like both IC18 and Q7 are bad, replace both,.

IC18 & Q7 replaced & all is working again Thanks a lot for the help