1776 multiplex jr. - modulation doesn't work

[o_gold]

Hi again,
I made this pedal with that layout:  https://tagboardeffects.blogspot.com/2015/12/1776-multiplex-jr.html

Everything works perfect except the modulation.

The led connected with 1k from the output of the tl074 opamp to the switch is working. same with the led that is with the LDR (I made a diy led ldr inside heatshrink).

I turn the knob of the depth and change the speed rate multiple time but nothing. no effect of modulation. I change multiple led-ldr and I could see the light change inside them when turning the speed pot but no modulation was happening.

I double check all connection and everything seems to be in place. What else could it be ?


Thanks for helping!

[MikeA]

Rolling your own LED/LDR pair can be tricky, I would check the output of the LDR, since you say the LED (input) side is working....  Mike

[o_gold]

I checked the output of the ldr and the max resistance I can see at my multimeter is around 20k.

[o_gold]

So now I can here very very little change of modulation. I wondering if I will cover with black tape the leds both of the on/off and the modulation it will make a difference ?

could it be there is too much light inside the box so it affected the ldr ?   

Edit: I tried that. did not help

[ElectricDruid]

Is the Depth pot ok? A faulty depth pot would stop the mod from working.

Also the mod will only do anything at one end of the Delay range. At the other end, the Mod is connected to Gnd, at least according to the schematic (I assume the layout matches...).

[o_gold]

Is the Depth pot ok? A faulty depth pot would stop the mod from working.

Also the mod will only do anything at one end of the Delay range. At the other end, the Mod is connected to Gnd, at least according to the schematic (I assume the layout matches...).

The Depth pot is fine, I checked it with multimeter

[o_gold]

https://imgur.com/a/a3gPSdv
In the picture attached the marked resistor that are 100K I checked them and find that both have a 54k resistance instead of 100k. Could it be the reason? 

[MikeA]

Quoting the 1776 Effects build doc:

"• If you construct your own vactrol with an LDR and LED, I suggest using a LDR that is 10 to 20K On, 10 Meg Off and a diffused LED."

So 10k to 20k should be the minimum you read across the LDR, with the LED at the brightest, and 10Meg the maximum with the LED off.  If you don't see that, look at the construction of the Vactrol.  Maybe there's light intrusion so it never gets dark as you say, or a mismatch between the brightness of the LED (too bright) and the sensitivity of the LDR (too sensitive.)

Another point: You said originally that changing the Speed pot changes the cycle rate of the LED in the vactrol, true?  If so, then the problem is downstream from there, and the next component is the LDR.  I'd look there.

Also verify the Time pot wiring and position.  As ElectricDruid said, with Depth at maximum and Time at minimum, the Mod signal will have little or no effect.

Regarding the two 100k, it's hard to measure resistors accurately in-circuit unless you know what's on both ends and how that will affect the measurement.  If you're concerned about them, take them out of the circuit to measure them.   Mike

[o_gold]

Thanks MikaA

I will try to make again the vactrol. I know my ldr I've got at home can reach up to almost 10m when no light. but with bright light it went down to a few ohms. I will try to defused the led's I've got and will give it another try. I'm pretty shure now this is the issue I should solve.

[PRR]

> 100K I checked them and find that both have a 54k

If you follow the circuit, these are R15 R21, a splitter across the supply rail. The supply rail is normally a very low DC resistance, a few K ohm. So they are effectively in parallel. 100k||100k is 50k, and a few k in the supply makes 54k.

As for modulation: forget the IC2B LFO, forget the photo-resistor. Turn TIME pot to max resistance. Short across the photo-resistor, the PT2399 delay should change. (Have your cat play while you dance a screwdriver on the photo-resistor leads.) No wobble? Then short the MOD-IN/OUT point to ground. That should force a 20:1 change of PT2399 delay.

[o_gold]

> 100K I checked them and find that both have a 54k

If you follow the circuit, these are R15 R21, a splitter across the supply rail. The supply rail is normally a very low DC resistance, a few K ohm. So they are effectively in parallel. 100k||100k is 50k, and a few k in the supply makes 54k.

As for modulation: forget the IC2B LFO, forget the photo-resistor. Turn TIME pot to max resistance. Short across the photo-resistor, the PT2399 delay should change. (Have your cat play while you dance a screwdriver on the photo-resistor leads.) No wobble? Then short the MOD-IN/OUT point to ground. That should force a 20:1 change of PT2399 delay.

I short across the photo-resistor - nothing happen.  I short the Mod in/out to ground (Actually it like to short between lug 1 and 3 of the time pot?) and the delay time change from maximum (long delay) to minimum (very short)

[PRR]

> ...across the photo-resistor - nothing  .... short the Mod in/out ....delay time change ...

So probably a break between those two points, but I can't see it from here.

[o_gold]

> ...across the photo-resistor - nothing  .... short the Mod in/out ....delay time change ...

So probably a break between those two points, but I can't see it from here.

Really strange.. Could it be a cold joint somewhere ? 

[o_gold]

So I created the modulation part on a breadboard. When I connect on lug of the ldr to the gnd of the pedal and the other one to lug 3 of the depth knob -and guess what? modulation is working! I then try to connect it one step before - before the 180k and the modulation did not work. could it be the reason? the 180 k resistor just before the mod pot ?

[ElectricDruid]

could it be the reason? the 180 k resistor just before the mod pot ?

It's possible. The typical construction of a through-hole resistor is two little caps with the wires attached stuck to the ends of the resistive material. It's possible for the caps to get knocked and break away from the body. If it's only cracked, this isn't really that visible, until you tug it and the end comes away on its own. I've seen it a couple of times.

If the 180K is soldered into the board, the two joints are much more likely as a source of failure.