SOLVED General Tso's comp - not 'flashy' enough?

[jamesm]

Hello everyone.

Just 'finished' a General Tso optical compressor and I'm now testing it before putting it in an enclosure. I've checked all the voltages. The audio signal flows through from input to output. When I cover the LDRs in tape, the volume increases. The audio signal is making it to the bases of Q2 and Q4. So far, so good...

The thing that's giving me pause and stopping me from shrinkwrapping the LED/LDRs is that the LEDs don't seem to be illuminating in time with the peaks in the music. If I connect the cathode of LED 1 to ground, it illuminates strongly; if I connect the anode of LED 2 to Vcc, the same. Just nothing when the guitar is playing. I was expecting flashes of light.

Is this to be expected? Is the LED illumination supposed to be so small as to be imperceptible?

These are the LEDs: https://www.mouser.co.uk/datasheet/2/427/tlhg640-1766969.pdf

Looking again at that datasheet, I think I may have picked the world's dimmest green LED...

 

[TSReppe]

Same thing with my general tso. I think its supposed to be like that because the pedal sounds great.

 

[jamesm]

The thing is that without the LEDs working, it becomes a high-quality JFET buffered op amp boost, so it probably should sound great!

I think I'll try to swap out the LEDs for something brighter and see if I can see any illumination.

 

[jamesm]

Hmm, swapped the LEDs for the Tayda ones recommended elsewhere and still no visible lightshow. Any one else built this pedal and seen the LEDs flashing?

 

[jamesm]

Or alternatively, any clever ways of troubleshooting the BC549C / BC557B part of the circuit beyond checking voltages and checking for audio at the bases of Q2 and Q4? (I don't have a scope)

 

[zgrav]

you can post pics of your board if you conclude it is not working and get some feedback on items to check. Seems like you should be able to see some changes in the LED if you are putting a loud signal through the pedal and you are in a dark room. ordinary transients like pick attacks might be too fast to see, but it should be easy to tell if the compressor is working when you are maxing out the gain and the compression.

 

[jamesm]

Grateful for any insights...

 

[jamesm]

I was hoping to be able to test it thoroughly before heat shrinking the LEDs to the LDRs and boxing it up, but if that's impossible then I'll just do that. Can't see anything obviously wrong with the build.

 

[untamedfrontier]

I think your LDRs need to be positioned differently. I believe you'll want them facing directly at the LEDs. You may want to peek at some build reports for the General Tso

 

[jamesm]

Thanks for taking a look. My plan is to bend the LEDs over and have them attached to the LDRs via pieces of drinking straw and heatshrink. I was just hoping to be able to see some signs of life from the LEDs before gluing at all together.

Maybe I should stop over thinking this and just commit...

 

[zgrav]

can you measure any voltages on the LED leads when the pedal is running? set it for a slow rate with maximum compression

 

[zgrav]

"slow rate" probably means a slow attack for this pedal.

 

[zgrav]

are your LEDs soldered in place? if not, try reversing one of them and test the circuit to see if it lights up. if so, just reverse the LEDs.
if you still do not see the LEDs light up, with a loud signal going into the pcb, ground the black lead on your meter and check the voltage on each side of the LEDs. if you are not getting any voltage readings there check for continuity with the transistors and measure voltages on each leg of them and let's see what you get.

 

[untamedfrontier]

I finished a 2nd gen tso last night where i've got the LEDs socketed, and I think this one lights up a little bit more than the 1st one, but I'm gonna try out some different colored LEDs to see if there's any major changes in behavior or sound. It definitely is more subtle compression and not your Country style ice-pick compressor.

 

[jamesm]

Thanks for the suggestion @zgrav. I don't think it's the polarity of the LEDs. If I ground/apply 4.5v to the relevant legs of the transistors then they do light up ok, so the transistors and LEDs are the right way around at least. I'll have another go at testing this evening when there's less ambient light.

 

[Dan0h]

I know on both of my Tso builds you have to really hit the strings to get those to light up, or dime the knobs on the pedal. Not sure if you already fixed it but your LDRs need to be right up against your LEDs.

 

[Dan0h]

By the way, the TS0 is one of my favorite sounding pedals I have made. I love it. Its almost become an always on while clean for me.

 

[jamesm]

Thanks. I certainly will seal the LEDs and LDRs up together. It seems to be super-sensitive to ambient light.

There's quite a high probability that it's all working fine and that I'm just overthinking this!

 

[zgrav]

seems like you should be able to see some reaction in the LEDs when you are maxing the sustain and putting in a high level signal. maybe easier to tell in a dark room.

 

[jamesm]

I feel a bit sheepish about this, but in a very dark room, with blu tack over the LDRs, the LEDs flash as expected...

Thanks to everyone for your tips.