Hey folks
Working on a guitar effect pedal that uses JFET switching. Was wondering if anyone can help me out. Schematic is here
http://www.freestompboxes.org/download/file.php?id=11624Issue: pedal passes AC input signal only when engaged in the effect-ON state.
So far I believe I have troubleshot the issue. This pedal uses a JFET as a series switch and the source and drain are never connecting on the unaffected signal path.
I was going to just pull and replace but I began to look at the circuit and began to wonder how it works. I am no EE and essentially have a rudimentary knowledge of circuits but mostly audio (guitar amp). I can read a schematic, solder and use an oscilloscope to trace a signal to troubleshoot no output.
That being said, I am weak on most other EE areas so no super tech answers please.
basically my audio path is good at the source. This is a 9v system run off a battery or wall adaptor. I find 4.3vDC on both the Source and Drain (which makes sense). The circuit designed to switch the gate voltage from 8v to 0.5v (service manual specs) works and I get those readings at the junction between the 1meg resistor and diode (MA150) that are in series with the gate.
So my JFET is not making the source to drain connection. Easy answer was to pull and replace but I got to wondering how the diode there is being used...can someone explain?
I assume it is there (based on its orientation) to stop DC voltage from getting on the gate. I read that the diode was there to let the gate "float" where it will but I don't get that. How does presenting either 8v or 0.5 volts on the other side of that diode cause the gate to open or close since the diode stops the gate from seeing that voltage?
JFET in question is the one in the lower right corner of the schematic D106. I get 7.6v and 0.6v at test point labeled CP9 as per service manual. I assume that is enough to open and close the gate when S and D are at 4.5vDC.
Also the service manual seems to indicate that with the controls set as directed I should be able to use the trim pot VR101 to adjust so that there are 0 volts on pints 10 and 12 of the IC. With pot fully rotated I gain only get it down to 0.8v DC. Is this issue related? Will having 0.8 volts there harm anything?
Item is an audio signal compressor