Odd in n out dc voltags on mx eq

[airbug]

Good evening to all


i am relatively new to this forum but read a ton of posts.
Great stuff, great people, glad it exists.

i am building pedals since two years ( better to say copying existing schemes and tagboards, not creating by myself). But still i struggle with very basic issues. But its fun and a great hobby.
I am mainly playing guitar, but building for it is Double the fun and a deeper level of music experience from my point of view.


but enough introduction talk.

here comes my Problem

actually its not a diy pedal. so it doesnt belong here normally, aint it?
i give it a try

but i had to change some components because someone messed it up with a strange "mod".
it is a 10 band mxr eq equalizer
he modded it to have an additional 47uf elco cap on the gain pot and on the volume pot
effect was nothing but a constant error and pulsating leds. maybe he tried for a gonw-wrong highpass filter.
so i removed that. it works fine now. vers few hiss and a little random crackle. its ok.

but now the strange thing.
the output jack gives 50mv dc and the input 9 Volts. it is a 18volt pedal by the way.  i guess that is not good. and i guess it is partly a typical mxr issue. my supercomp mxr also gives 4.5v on the input jack(9v pedal, not 18). but its not showing any output jack dc.
so my maybe wrong conclusion:
i should have No or little (5mv) dc on the out jack and 9 on the input is normal but not so fine in the signal chain: i get " crackle not okay "  pots (zvex) on my guitar.

i am planning to build a cornish buffer for before the input jack and wonder if i should install a coupling cap after the eq ( for which i dont know the proper value and the proper additional series resistor ...guessing somewhat 100nf )

maybe some of you fine folks can give advice?

thank you so much
and excuse my poor english


airbug

[garcho]

But its fun and a great hobby.
I am mainly playing guitar, but building for it is Double the fun and a deeper level of music experience from my point of view.

wow, you mean you're not here to learn how to start your own boutique fuzz/BBD pedal company?     
nice to hear someone wants to have fun and learn, bravo!!

guitar pedals should never have DC voltage at the input/output jacks. Where are you tapping ground to take your measurements?

crackle

that's probably from the DC

excuse my poor english

don't worry about your English, many people here are not native English speakers, it's great to have a global forum!

[ubersam]

guitar pedals should never have DC voltage at the input/output jacks.

Agreed. Sounds like either there are no DC blocking caps at the input or the output, or they have failed.

[airbug]

thanks for the reply.
i missed to set up email notification so i found this answers quite late, sorry.
i measure tip n sleeve of a plugged in patchcable. wrong?

if there normally never is dc on in n out, how come mxr pedals like super comp have?

i also thought maybe some guy messed up that pedal but i couldnt find any other modification than the ones i named. the rest seems original/stock. but the mod he "tried" might have been for that very reason to build a coupling cap System.

do you think i can install coupling caps ...but if, which values?

[airbug]

by the way: how can i attach pics? at the attachments section there is nothing that allows attaching an Attachment

[garcho]

my supercomp mxr also gives 4.5v on the input jack

is it a stereo jack by chance?

[airbug]

no, sure it is not. normal patch. mono.

[airbug]

i found some posts where people complain about crackle on their guitarpot when using a mxr product...just saying

[garcho]

you probably know, but sometimes people use stereo jacks to switch the DC power (so you don't lose battery power while you're not using the pedal), i thought maybe there's a chance something could be wrong there, that's all

normal patch

jack, not patch

stereo, 3 terminals:


mono, 2 terminals:

[airbug]

oh sorry.
it is indeed a strange jack. not an open one like switchcraft, but closed like in some amps. it is t r s and the r has some to me nonunderstandable hook above the popping-up-when-jack-plugs-in-it contact.

how can i send a pic?!?

[ubersam]

how can i send a pic?!?

like these but with out the spaces:
[ i m g ]url of the picture you want to post[ / i m g ]

[airbug]

oh it must be online lying in www, i cannot attch it from my desktop...too sad

[airbug]

i found when i switch the pedal to bypass ( its not true bypass, but true mxr bypass only, i guess thes keep the input always connected to signalchain) them i get teh same voltage on input and output. 9 volts dc. maybe i have a short somewhere in the circuit or in the dc jack...

[airbug]

but maybe back to my starting question.
the circuit Sounds fine.
can i just add caps to stop the dc directly before the in n out and im fine? and if, what value? seems to depend on impedance of in n out, but maybe there is a good rule of thumb?

and wouldnt a buffer or actually every solid build pedal that HAS proper coupling caps stop the dc and im fine?

[garcho]

and wouldnt a buffer or actually every solid build pedal that HAS proper coupling caps stop the dc and im fine?

yes

can i just add caps to stop the dc directly before the in n out and im fine? and if, what value?

100nF are a good place to start blindly. 100nF = 0.1µF = 0.1uF

if you get 4.5V at the input jack, there's most likely some Vref (Vcc/2) that's not being AC coupled

[airbug]

thank you.
just to be shure because i think there is a typo in your farad table. you say to start with
0.1 uf polyestercap, right? input and also output.
i can listen to the tone and if i sense tonecut i raise the value stepwise?

the sentence about vref ....i dont get it.

its 9 volts.  do you mind explaining more?
so sorry that i dont understand it better. you guys are really in the topic.
and thank you very much!

[garcho]

just to be shure because i think there is a typo in your farad table.

sorry! i'll edit it

3 ways of writing the same value:

100nF = 0.1µF = 0.1uF

[airbug]

you mean half of thevoltage is used as a reference for an opamp or something similar.
but what means it is not ac coupled?
like it is not blocking dc sufficiently?

[airbug]

so i should search for that spot where the voltage divider sits and maybe there is a short or a tinbridge or something?

[garcho]

so the u and the micro symbol (u with long dash) means the same.

yes, that is correct. sorry if i confused you, not sure how

a milliFarad is 1/1000 of 1 Farad.
a microFarad is 1/1000 of a millifarad. micro is written with 'µ', often times written as a 'u' because the Greek letter isn't available or known how to be keyed in or laziness. they both mean the same thing
nano is 1/1000 of a micro
pico is 1/1000 of a nano