MXR bass octave deluxe randomness

[LukeFRC]

I picked up a secondhand broken pedal kit off someone, and he chucked in a faulty mxr bass overdrive deluxe for free.

Basically the thing doesn’t seem to be filtering the signal going into the detector- so there’s a constant noise coming out when the bass is plugged in but muted. Roll off the volume on the bass and it goes - so lack of LPF on the signal?

What would cause that?
- trace here https://www.freestompboxes.org/viewtopic.php?t=30057#top

 

[LukeFRC]

So it seems that the it's high end content coming from the bass, that's inaudible until an octaver brings it down into the audible range...

looking at the circuit ... green is a buffer of some kind? - which then goes into the two octave circuits and the envelope follower/frequency divider bit.


The odd thing is if I touch the circuit with an Allen key (so ground it via me?) at either of the yellow spots the noise goes. Caps don't show as open.

 

[LukeFRC]

sorry to ask here, it's not a pedalpPCB pcb but the combined knowledge of this forum might help!

 

[LukeFRC]

It's not the low pass on the detector circuit

 

[LukeFRC]

full schematic from freestompboxes

I even spoke to MXR who confirmed it's the high sensitivity pulling the noise floor down into the audible range....

Now this circuit is normally good, and this isn't an unusual fault on it, - so I'm guessing it's some common part being out of spec.
Grith is similar to a OC2 and Growl a EBS octave

I'll go over it with a multimeter in the next week or so...


I'm pretty sure that the bit around U2C is a High pass filter for the detector.
U2A and U5C are filtering the octave signal.

Thoughts currently are...
C33 and C35 are effecting the sensitivity of the datector - change to see what happens

 

[LukeFRC]

And also I think increasing C34 will cut treble to the detector? maybe?

But then increasing C15 should do that too

 

[LukeFRC]

decreasing R56 seems to be doing the right thing. Will test and report back.


to you all.

everyone contributing to this thread


thanks!





no seriously - it's good to write it all out on here, the pedal is all SMD, and a load of parts apparently hidden under the power jacks, I've been going through it bit by bit experimenting with ...er sticking another cap or resistor on the existing on the basis that if I could make the noise way worse or way better it would help work it out... honestly hold the component on the pads and then look up and BLOW the bass in the stand 40cm away to give enough resonance to start the noise, the octave it is that sensitive! Lots of ways to lower the noise floor by making the whole thing quieter - but it was when I randomly touched the end of R56 to a pin that I later found out was ground with a 100k resistor that it seemed to work... which would lower R56 effective value from 68k to 40k.

 

[music6000]

I only just spotted this thread today & I'm on here everyday.
This is not my area that I have a lot of knowledge in so I can't be of much help, also the fact that it is SMD.
Hopefully someone will chime in & give you some guidance.

 

[Feral Feline]

Also, with any trouble-shooting threads, posting pics helps people to help you (SMD or not).

My entry to the forum was bringing a dead Electric Mistress to bear; I posted pics and received tremendous help and now have a functioning Electric Mistress.

 

[LukeFRC]

I think I’ve solved it. Bit more testing and will report back (currently childcare responsibilities get in way)
Photos included!

If you search the web this is a common fault with these pedals. MXRs response to me and others is “it’s sensitive by design and this is normal and try a noise gate”, it’s also apparently “not a fair test” to compare with other octave pedals that are different circuits - even if one of them is the same model pedal!!

 

[LukeFRC]

Here you go!!

 

[LukeFRC]

Fiddling with C15 R24 change the HPF as expected - but degrades the octave without diving the noise issue.

R56 did solve the noise… but also stopped the octave effect!

So it’s something in the bit around U6a and U6b
Moving around the circuit sticking a 100k resistor over the resistors to see what happened - R67 increases the noise (letting more signal through so makes sense)

and R66 seemed to reduce the sound, R60 less so - so possibly something up in u6b

Next time I get 5 min I’ll play with R66

 

[LukeFRC]

Aha @music6000 - I see your reaction there to the photo!! It’s why I’m not 100% sure it helps - the power components are all on the other side too!!

 

[music6000]

Aha @music6000 - I see your reaction there to the photo!! It’s why I’m not 100% sure it helps - the power components are all on the other side too!!

I can't believe how many components are on the PCB!
Get an audio ptrobe & report back in a month or 2 or 3 or.......!

 

[LukeFRC]

So I think I have half solved it.

SOLVED Problem No. 1: Too sensitive. It seemed to be only half of the signal that was adding to the noise. Soldering a 100k reisistor over R66 decreased the gain on U6B and massively lowered the noise floor without seeming to have any adverse effect on the effect.

UNSOLVED Problem No. 2: noise on the ground or something. There's a regular (but now quieter) noise pulled into the audio which is a regular pulsing or something. Interestingly touching the ground end of R67 with my finger stops it completely. How do I recreate my finger electronically?!