MXR M-126 flanger/doubler noise troubleshooting

[beatnik]

i have one of these on my bench because has a noise problem

the flanging effect and all the controls work but there is a loud motorboating noise too

no niose in bypass or when the mix knob is in dry position

i tried to probe the circuit and i found the noise is on the +12V rail and almost everywhere in the circuit

on U10 pin 1 there is noise when the regen pot up is turned up. with regen at minimum there is no noise in that point of the circuit

i am trying to narrow the area where the noise builds up but this is a complex circuit for my skills and at the moment i can't figure out a solution

i have recapped the unit but didn't solve the issue, now I am suspecting a bad IC.

the BBDs are the only socketed and i have tried pulling them out with no results. at least these seem ok, it would be impossible to find replacements

i would really appreciate if anyone can help me understanding the possible areas where the noise starts and which components trying to replace

schematics are in the attachments

thanks !

 

[JohnRoberts]

i have one of these on my bench because has a noise problem

the flanging effect and all the controls work but there is a loud motorboating noise too

no niose in bypass or when the mix knob is in dry position

OK, that suggests noise is in wet delayed path.

i tried to probe the circuit and i found the noise is on the +12V rail and almost everywhere in the circuit

on U10 pin 1 there is noise when the regen pot up is turned up. with regen at minimum there is no noise in that point of the circuit

Does noise sound different in Flange or Double.  While BBD or CCD parts are not known to last forever. Both should not fail at same time.

Is input to delay devices clean audio?

Does output of delay device clip symmetrically demonstrating that DC input bias is well centered?

i am trying to narrow the area where the noise builds up but this is a complex circuit for my skills and at the moment i can't figure out a solution

I would keep regeneration down while troubleshooting. If you regenerate poop, you get more poop.

i have recapped the unit but didn't solve the issue, now I am suspecting a bad IC.

the BBDs are the only socketed and i have tried pulling them out with no results. at least these seem ok, it would be impossible to find replacements

Does the output waveform change as you vary the input DC bias? 

i would really appreciate if anyone can help me understanding the possible areas where the noise starts and which components trying to replace

schematics are in the attachments

thanks !

Good luck, not rocket science... analog shift registers are clocked and shift an input sample voltage through the delay line one clock period at a time. If input bias is adjusted wrong you could get nothing out or a signal pegged to a rail.  A bad  delay chip is always possible.

JR

 

[beatnik]

thanks for your reply

the input to the sad1024 bbd (pin 2 and 15) is clean only with regen pot at minimum.

same thing on pin 4 of the r5101 ccd which should be the input - i couldn't find a datasheet for this one so i'm just guessing

noise is the same on both flanger and doubler modes

DC input bias you mean the one one the bbd or the ccd ? but i don't have a scope so i can't make these measurements

i tried pulling these two chips out and the noise was still present on the +12V rail so i thought these two are not the problem. do you think they could be ?

 

[JohnRoberts]

thanks for your reply

the input to the sad1024 bbd (pin 2 and 15) is clean only with regen pot at minimum.

same thing on pin 4 of the r5101 ccd which should be the input - i couldn't find a datasheet for this one so i'm just guessing

The input will be the one with the DC trimpot.

noise is the same on both flanger and doubler modes

That is actually optimistic because it suggests the esoteric parts may be good.

DC input bias you mean the one one the bbd or the ccd ? but i don't have a scope so i can't make these measurements

Generally you can set the bias by ear... with a pure tone adjust for symmetrical clipping or centered between top and bottom waveform clipping.

i tried pulling these two chips out and the noise was still present on the +12V rail so i thought these two are not the problem. do you think they could be ?

There appears to be two different +12V rails, one just for the flanger chip.

With no chips the chorus output will be pegged to +12V

If the +12V is noisy look at the PS for excessive ripple and/or sagging unregulated V.

For the noise to be the same on both delay paths, and unaffected by removing the chips (carefully) suggests that the problem is more systemic and common to both, so look at the PS the +12V is regulated so should be reasonably clean.

JR

 

[beatnik]

the power supply was my first suspect, but i've changed the regulators and the capacitors with no improvement at all.

but actually i haven't tried probing before the voltage regulator

can the transformer itself or even the bridge rectifier be noisy ?

i try that

 

[beatnik]

the noise is on the regulator's input as well !

maybe the diode bridge is the problem ?

 

[JohnRoberts]

the noise is on the regulator's input as well !

maybe the diode bridge is the problem ?

There will always be noise in the input of the regulator, that is the regulator's job.

The bottom of the ripple needs to be higher than the minimum input voltage for the regulator output to be clean.

JR

 

[beatnik]

the -12V regulator hasn't got the same motorboating noise on output neither input

 

[beatnik]

the power transformer has output secondary at 9V. after diode bridge is about 13V.

it's too low for the voltage regulator to operate properly ? is that what you mean ?

thanks a lot for your help

 

[Iliketurtles]

Well???