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need help interpreting DC readings

T

tempus fugit

Jan 1, 1970
0
Hey all;

I'm trying to isolate sources of DC from my guitar effects pedalboard so
I've cracked open a few of the pedals themselves to check them out. on the
1st pedal, I measure 0.1 mV on the output (measuring from cap to ground). It
doesn't float around a bit 1st, it just measures 0.1 On the next pedal, the
readings are all over the place. They range from about -2mV to +3mV, and
constantly floats around, never settling. I figured the cap must be leaky,
so I replaced it with a new metal film, but I get the same readings. I
measured the next pedal, and I get the same type of reading where the DC
level is floating all over the place. I haven't replaced that cap, since
replacing the cap in the other pedal didn't seem to change things. in all 3
pedals, there is a 100K resistor at the output to ground. Given the new cap
and the resistor to ground, how can there be any DC at all there?

Also, the inputs of each of the pedals show a DC level that starts out at
around 350mV and settles down to 0.2mV over the period of about 15 seconds.
Is that normal, or should I consider these caps leaky also?

Thanks
 
T

Tom Biasi

Jan 1, 1970
0
tempus fugit said:
Hey all;

I'm trying to isolate sources of DC from my guitar effects pedalboard so
I've cracked open a few of the pedals themselves to check them out. on the
1st pedal, I measure 0.1 mV on the output (measuring from cap to ground).
It
doesn't float around a bit 1st, it just measures 0.1 On the next pedal,
the
readings are all over the place. They range from about -2mV to +3mV, and
constantly floats around, never settling. I figured the cap must be leaky,
so I replaced it with a new metal film, but I get the same readings. I
measured the next pedal, and I get the same type of reading where the DC
level is floating all over the place. I haven't replaced that cap, since
replacing the cap in the other pedal didn't seem to change things. in all
3
pedals, there is a 100K resistor at the output to ground. Given the new
cap
and the resistor to ground, how can there be any DC at all there?

Also, the inputs of each of the pedals show a DC level that starts out at
around 350mV and settles down to 0.2mV over the period of about 15
seconds.
Is that normal, or should I consider these caps leaky also?

Thanks

First you need to know what you are looking for.
Taking readings when you have no idea what to expect is useless.
Get a schematic of the unit and maybe someone can help.

Tom
 
T

tempus fugit

Jan 1, 1970
0
Tom Biasi said:
First you need to know what you are looking for.
Taking readings when you have no idea what to expect is useless.
Get a schematic of the unit and maybe someone can help.

Tom

I am looking at DC leakage on the caps. When I switch the in/outputs of
these units via a relay, there are clicks and pops, which typically is the
result of DC at the jacks, likely the result of leakage from the blocking
caps on the in/outs of the effects. What I expect is to see no DC, assuming
the caps are not leaky and the resistor from output to ground is not faulty.
 
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