transistors.
2. The stronger signal being applied to the base of the transistors, the less resistance there
will be between the anoded of the opto-coupler LED and ground.
3. The less resistance between the anode of the opto-coupler LED and ground, the
brighter the LED will light up.
4. The brighter the opto-couler LED lights up, the lesser the resistance the LDR of the
opto-coupler will be.
5. The lesser the resistance of the opto-coupler LDR, the less the op amp will amplify
your guitar signal.
This means, the harder you strum your guitar, the more your signal gets cut. And the
softer you strum your guitar, the more your signal gets boosted. This is compression - an
effect that produces an even output signal regardless of input signal strength.
Now to use this knowledge to help troubeshoot a problem in the trigger voltage path.
Assuming that you've used a Signal Test Probe to determine that signal is reaching the
base of the transistors(BLUE 18 and BLUE 28), you need to have some sort of input
signal in order to test the Trigger Voltage Path. Set your DMM to 9VDC and touch the
red probe to any Full Positive Voltage location. Then touch the black probe to the
various GREEN test locations on the PCB. You should notice the voltage increase as
guitar signal is applied and the voltage drop as the guitar signal strength decays.
Unfortunately, we cannot see the LED because it is encapsulated in the opto-coupler. It
would be easy to tell if we had problem with the opto-coupler itself if we could see the
LED. You can install a regular LED into the LED end of the opto coupler to see it work.
*If you are getting a trigger voltage to the anode of the opto-coupler LED, and you are
getting an unaffected dry signal, first check for cold joints on Pin 6, R4, the opto-coupler,
and lug 2 of the compression knob by testing for continuity throughout all those
components on all the ends you did not check when testing for audio signal. Also test for
continuity between Lug 3 of the compression pot and R5, and continuity between the
other end of R5 and C3. If all that checks out, then you may have installed your
opto-coupler incorrectly or you may have a faulty opto-coupler.
*If you are getting input signal to the base of the transistors, but you are not getting any
trigger voltage, then you may have installed both transistors incorrectly or both transistors
may be faulty.
*If you are getting input signal to the base of the trasistors, but you are getting a very
choppy compression, you may have installed one of the transistors incorrectly or one
transistor may be faulty. Unfortunately because both transistors are in parallel, there is no
way to determine which is suspect, so take a guess. Maybe you'll get it on the first try.
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