Fender concert 2 vactrols

[JMMP]

Posted in the wrong place, mods and tweaks. Sorry.

Dirty channel sounds like ass, as the signal fades it fizzles out. I'm looking at the opto coupler spec sheet and I dont understand how this can work with the very high VTL5C1 coupler resistance. Anyone know how this is supposed to work, and is there a unit that would work here that has less resistance? I'm looking at the NSL-32SR3 someone had talked about on another forum.
Not sure exactly what I should be reading on the LED side, 6.4 volts on one led leg, 4.8 to about 5 on the other side of the led. This does not change when I pull the vol channel switch (which is good by continuity check). I can short the sensor side of the vactrol feeding wiper on master volume and it seems to come to life. Doing this on any other one of the 3 remaining does nothing. What gives? Thanks in advance.

[g1]

Is the 6V at the vactrol a negative voltage? Should be -6V coming off the zener shown over by the AC plug.
The pull switch requires the routing through the switch of the 'pedal jack red' jack. If that section of the jack is bent, dirty, or incorrect the pull switch will not work.

[Mick Bailey]

You can check if the Vactrol is operating correctly on the LED side by tacking a red LED across the terminals to give a visual indication. There's a trick I use to test them in-circuit using two DMMs. With the amp powered off and discharged, set one to resistance check and connect it to the LDR side, then the other to diode test and connect it to the LED side. The diode test will pass enough current to just about light the LED and you should see the resistance drop to perhaps 100 Ohms. You should also see the correct forward voltage drop for an LED. If you only have one DMM, use a 9v battery and 1K series resistor to light the LED.

The failure mode of Vactrols is that either the LED shorts out, or the LDR either goes open or doesn't respond and stays at a high resistance or doesn't vary much. Many seem to fail stuck around 5K Ohm.

The light/dark resistance of a Vactrol isn't the only consideration - the slope of the resistance change is also important. I've just been doing some breadboarding using the NSL-32SR3, which is my usual preferred device. However, a cheap Chinese VTL5C1 worked much better for my VCA design due to the slope characteristics, which was somewhat unexpected.

[JMMP]

Thanks g1 and mick. I found one questionable device, replaced it, and all seems right with the amp. Gonna look at the schematic and try understanding this better. I know how the vactrols operate individually, just not quite clear in this application. Funny thing though, no two have the same "on" resistance, varied from 100 ohms to almost 500 ohms. Is there something to vactrol matching, lol! Thanks again.

[Mick Bailey]

There's been some discussion about vactrol matching on one of the synth forums, where resistance and slope are critical. If you're measuring resistance in-circuit consider parallel resistance. The characteristics can change over time, especially so if operated at increased temperatures found in a tube amp. Also if the device is operating close to the maximum ratings.