Re: Opto Coupler Help?

Harvey White

On Thu, 22 Feb 2018 06:54:46 -0800, you wrote:

To check the photocells, you just need the ohm meter.


Hi guys. Fascinating discussion regarding the curve tracer. I love how some of my questions pivot into conversations like this. It's was just a few weeks back that someone (I can't remember who) had posted about using a curve tracer for tube testing and I was so convinced, I started snooping around (and started watching this one on eBay:

But, since I don't have one and am unlikely to acquire one just yet, I'm still faced with the problem at hand and want to return to the prevailing solution prescribed earlier and some questions.

First, I do realize that you can't accurately test either the diode or the photocell in-circuit with a multimeter. But I wonder if certain I certain observations can help solve the puzzle. The guitar amp has 5 of these opto couplers (OCs). Now that I've really studied the circuit, I've figured out their precise role.

1. The amp has two channels; clean & dirty. Switching between the two is done with a push-button switch on the front panel, or with a foot switch (common setup for amps). When switched off (out), clean is selected. When switched on (in), dirty is selected. You can probably see where this is heading.
2. The OC's are essentially gates on the clean and dirty circuits. The diode/LEDs for all five OCs are on a single dedicated circuit (essentially in series). The directionality of their diodes is configured so that only half the the LED circuit is energized at any one point... either the half that cuts through the clean channel's circuit, or the half that cuts through the dirty channel's circuit.
3. When the switch is out, approximately 10 volts is cutting through the OC's LEDs on the clean channel, reducing the resistance of the photocell through which the audio signal is passing. At that same time, the LEDs on the dirty circuit are dark, therefore gating any signal cutting through the associated photocells. Switching the switch just redirects the voltage one way or the other.
4. The amp is stuck in dirty mode and the pots that are supposed to control the dirty volume have no effect. For example, I can't turn it down to "zero."

I have some more tests to do, but my gut tells me that an electrical event of some sort fried one or more of the OCs, the switch, and the pot that controls the dirty channel's level.

Just poking around with my multimeter, I wonder if any of the following observations (when the amp is off) are relevant (even though everything is being tested in circuit). I was mainly just looking to compare them to each other to see if anything was wildly different.

a. one of the OCs (OC2, positioned in the clean circuit) is showing about 35M-ohms of resistance on the photocell. Another is showing OL (shouldn't this be impossible?). The remaining three show 1.5 M-ohms.
b. OC2 is also showing continuity between the cathode of the diode and the photocell. I realize this could be associated with a shared ground, but none of the other OCs show this sort of continuity.
c. For shits and grins, I ran the multimeter's diode test on the diodes and got about 1.5 on each.

Finally, going back to the 9V battery test which seems like the easiest... I just have to rig that up (with the resistor). Seems like a reasonable test. I'd like a suggestion on how to pass some current through the in-circuit photocells to watch for the voltage drop.

Thanks everyone.

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