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es-6 bad input (negative dc offset)
MUFF WIGGLER Forum Index -> Expert Sleepers  
Author es-6 bad input (negative dc offset)
xi-bot
hi,

chan 1 of my es-6 gets an strong negative (~ -10v) dc offset if its not (always) connected to an (buffered) source.
so if i power on my system
- with one cable plugged in to chan1 but not connected to anything else or
- if i just unplug chan1 or
- if i freshly plug in a cable to chan 1
it immediately will jump to something around -10v and ignore any signal that is send in.
but if something buffered (i guess) is connected while powered on like an dc mixer or any voltage source chan1 will behave normal as the other inputs do.
any idea what's wrong ? doesn't feel like an mechanical problem. maybe a broken opamp ?
my small system with a few modules should have enough power with my meanwell psu.


1. powered on with nothing plugged in
2. powered on with source plugged in
3. plugged in or unplugged after powered on
3. plugged in source after powered on


thanks for any help!
os
Which version of ES-6 is this?
xi-bot
os wrote:
Which version of ES-6 is this?

Version 1.

Thanks for the quick reply
os
That version relies on the normalising contact of the jack socket to ground the input when nothing's plugged in. Possibly that contact has become intermittent with time.

Two possible solutions
- try to bend the contact back
- solder a resistor across the input.

I would suggest the second option.
xi-bot
os wrote:
That version relies on the normalising contact of the jack socket to ground the input when nothing's plugged in. Possibly that contact has become intermittent with time.


i checked it with my multimeter. the normalisation is ok.

os wrote:
- solder a resistor across the input.
I would suggest the second option.


it does not help. tried it with 1k, 10k and 100k. as soon i unplug or freshly plug in a patch cable it jumps to a very negative value and stays there

but if i "buffer" the input with my doepfer cv mixer or makenoise math's attenuator it works with any input signal !
it needs to be in this configuration while powered on.

what else can i do ?
os
Sounds like you'd need to replace the input op-amp then.
xi-bot
ok. checked it with my oscilloscope. the signal (sine) gets already pulled down in the negative range at the jacks input. after the jack got plugged in and powered on ! all the others opamps work fine (measured the opamps outputs).
so its the opamp right ? any chance on faulty resistors or other parts ? there seems to be not parts between the jack socket and the opamps input. whats the opamps part number ? i'm not very familiar with smd sizes. thanks

os wrote:
Sounds like you'd need to replace the input op-amp then.
os
It's a STMICROELECTRONICS TL074CD.
xi-bot
os wrote:
It's a STMICROELECTRONICS TL074CD.


So its most likely the first opamp (inverting amplifier) right ?
Should i check the resistors first ? What are the values ?
os
It's most unlikely to be the resistors.
xi-bot
xi-bot wrote:
os wrote:
It's a STMICROELECTRONICS TL074CD.


So its most likely the first opamp (inverting amplifier) right ?
Should i check the resistors first ? What are the values ?


Sorry. Of course the values are printed on them.
Do i need to care about the capacitors ?
os
I think the op-amp is the first thing to try.
xi-bot
ok. fixed it

os wrote:
I think the op-amp is the first thing to try.
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