Median Compressor not working
- Thread starter huntest
- Start date
falco_femoralis
Well-known member
The popping when you turn it on or off means DC is making its way into the audio path somewhere. Try touching the wires going to the switch daughterboard and see if you can get any of them to make noise. Try turning the pedal on and off while putting pressure on them in different ways to see if the popping noise goes away when you hold them at a certain angle. Any change is going to tell you where the problem is.
Vref is one of the power supplies to the op amp. It's a label that tells us it's a 4.5v power signal instead of 9v. 9v would be called Vcc. The fact that your vref is wrong is a clue.
Looking at your schematic, vref is set up differently in this pedal compared to most others. It's derived using the IC3 chip. When you look at pin 5 of IC 3, it's supposed to be receiving 4.5v (because upstream of it is a 10k resistor going to 9v as well as a 10k resistor going to ground. This is a voltage divider and bc the two resistor values are equal the voltage going to pin 5 is halfway between them. Not super important for this but that's why you're looking for about 4.5v). Checking your voltage readouts, you're getting .007v here, essentially a short to ground.
This short to ground could be why there's DC in the audio signal
I would inspect the IC3 chip and see if there's any solder blobs making connection to a nearby ground, or maybe a nick in the PCB. Look on the top and bottom of the PCB. Remove the chip and use your meter to perform a continuity test from the socket of IC3 pin 5 to R101 and R102. If you don't get continuity here, remove the socket and inspect the board further. If nothing looks amiss, then replace the socket. You'll probably destroy the socket removing it, that's ok. Better the socket than a solder pad.
Vref is one of the power supplies to the op amp. It's a label that tells us it's a 4.5v power signal instead of 9v. 9v would be called Vcc. The fact that your vref is wrong is a clue.
Looking at your schematic, vref is set up differently in this pedal compared to most others. It's derived using the IC3 chip. When you look at pin 5 of IC 3, it's supposed to be receiving 4.5v (because upstream of it is a 10k resistor going to 9v as well as a 10k resistor going to ground. This is a voltage divider and bc the two resistor values are equal the voltage going to pin 5 is halfway between them. Not super important for this but that's why you're looking for about 4.5v). Checking your voltage readouts, you're getting .007v here, essentially a short to ground.
This short to ground could be why there's DC in the audio signal
I would inspect the IC3 chip and see if there's any solder blobs making connection to a nearby ground, or maybe a nick in the PCB. Look on the top and bottom of the PCB. Remove the chip and use your meter to perform a continuity test from the socket of IC3 pin 5 to R101 and R102. If you don't get continuity here, remove the socket and inspect the board further. If nothing looks amiss, then replace the socket. You'll probably destroy the socket removing it, that's ok. Better the socket than a solder pad.
z2amiller
Active member
What do your voltages measure now? Do you get something closer to 9V on your 9V rail, or is it still ~7.7V?
Is your VREF closer to 1/2 of your positive voltage rail? When you have something seriously wrong with
the circuit that you identify and fix, you kind of have to back up and re-try your debugging again from the
start if it doesn't work right away because the thing that you fix changes the results of the tests you ran.
Do you have a way to make an audio probe? It's not too hard to do, just a couple of components,
and it gives you a good way to figure out where in the circuit the audio dies. On a circuit like this,
you can just test the outputs of the op-amps to start with. The path is IC1/1, IC1/7, IC2/7, IC2/1,
IC3/1, IC4/1, IC4/7. Once you figure out the first place where the audio is missing, check the stuff
between the last good output and the bad output. (There's some nuance here, it's not really as
simple as A->B->C but it's a good place to start)