SOLVED I was on a roll... :( ... General Tso

[SteveF]

I was doing so well. Guardian, Paragon and Paragon Mini, Pyrocumulus, all working perfectly, barring a minor shorting issue when boxing my Paragon Mini, but General Tso is misbehaving.

Sound bypassing no problem. No effect when engaged. LED comes on, Dry signal still comes through. When in blend mode, the dry signal is off with the balance pot at full CCW and 100% at CW as it should be, but no wet signal. When in “Juicy” mode, the signal is just dry, same as bypass. Treble control has no effect as far as I can tell and nor does the sustain.

Unfortunately, these things always happen to me when I have tested my wife’s patience to the point where I am summoned away from the bench, so I will update with pics tomorrow. Very frustrating. I was looking forward to this one. Any ideas off the bat based on the symptoms?

 

[SteveF]

Measuring an LDR from the same bag. In front of a white LED lamp it is about 150 ohms, cover with a cushion and it climbs to about 6.8M (although it seems to take a while to get there - is that normal?)

EDIT - I put a thick walled box over it and it got up to about 9M

 

[SteveF]

I have a bag from a different batch and they only seem to range from 100-ish ohms to 4.5M

 

[Chuck D. Bones]

Yup, LDRs respond to light very quickly and to dark slowly. The more light that hit them, the longer it takes for the resistance to go up when they're in darkness. Try partial light, then dark and you'll see what I mean.

 

[Chuck D. Bones]

I have a bag from a different batch and they only seem to range from 100-ish ohms to 4.5M

That is more than enough range. As long as they get below 5K and above 400K they will work fine in this board.

One last check, put you board in total darkness and test it one more time with SUSTAIN and BALANCE maxed.

 

[SteveF]

That is more than enough range. As long as they get below 5K and above 400K they will work fine in this board.

One last check, put you board in total darkness and test it one more time with SUSTAIN and BALANCE maxed.

As in play test, powered, IC back in etc?

 

[Chuck D. Bones]

Yep. If that's a bust, then do another visual inspection and if you don't find anything, then I think we're down to removing one of both of the LDRs.

 

[SteveF]

Yeah, nothing doing. Exactly as before. With the switch in the up position, the balance knob goes from silence to normal volume. In the down position none of the knobs do anything at all. The bypassed sound is exactly the same as the sound with the pedal engaged.

 

[Chuck D. Bones]

Sure seems like it's one or both LDRs. I think you need to take out one or both and check their resistance. Sometimes there's a bad one in the bunch. I check mine before installing and so far I've found one bad one; its resistance never got higher than 20K.

 

[SteveF]

So, Before I started desoldering, I cut away the heat shrink to check the LEDs were working - I assume they should be lighting up when I play? They're not. Once I have connected the pedal up and engage the foot switch, they don't light at all when playing. If I take a lead from the battery -ve to the cathode and another via a resistor to the anode, they both light up, so the LEDs work.

Should I still desolder the LDRs?

 

[SteveF]

OK, so I desoldered one and it's not getting above 0.6k ! Off to do the other now

 

[SteveF]

The other measured fine.

 

[SteveF]

Have removed both LEDs and LDRs and will remake the optos. Still confused as to why the LEDs weren't lighting though.

 

[Chuck D. Bones]

I was going to ask you to test with only the good one hooked up, but you got ahead of me. Test the LDRs before and after applying the heat-stink tubing and before you put them on the board. You know the heat-stink is not necessary, right? Also, the LDRs are very sensitive, so if your LEDs are diffuse, you can point the LDRs at the side of the LEDs. I used to think more light was better. I found out that too much light on the LDR will make the release way too slow. You saw that when testing the LDRs and the transition from low to high resistance was slow.

The LEDs weren't lighting because with one of the LDRs essentially shorted, there was not enough signal on IC1 pin 1 to turn on the LED drivers.

FYI, this circuit will run pretty well with only one LED/LDR hooked up, in case that facilitates testing in any way.

 

[SteveF]

Thanks Chuck, I'll report back tomorrow when I have redone them. So you think it will be dark enough without the shrink and crossover won't be an issue? I have the external LED in a metal bezel with a rubber stopper on the bottom, so I shouldn't get bleed from that. I'll have a go with just the led's mounted normally and the LDRs next to them bent to 90 degrees so they're facing the side of the LED then. At least I'll be able to confirm that the LEDs are working etc.

 

[Chuck D. Bones]

Both LEDs are doing the same thing, so cross-talk, if there is any, won't be a problem. If you're going to watch the LEDs, then the room has to be very dark. You may be surprised how little light is needed to get the LDRs to lower the gain.

 

[SteveF]

Happy days! It now seems to be working as it should. It seems to be a bit more subtle than my other compressor but that's not necessarily a bad thing. It seems quieter if that makes sense? I haven't tried it on a heavily distorted guitar but it doesn't seem to amplify the noise/hiss as much.

@Chuck D. Bones Thanks so much for your help.

 

[SteveF]

Turns out a few in the bag of LDRs I got from Amazon were dodgy. I ended up discarding 4 today, plus the faulty one originally fitted. 5 from 10 isn't great!

 

[GenoBluzGtr]

Chuck is a damned wizard, I tell you. Kinda scary how tenacious he can be. Glad to see this one sorted out!

 

[Chuck D. Bones]

I like a challenge! As long as you guys do your part, we succeed.

Happy days! It now seems to be working as it should. It seems to be a bit more subtle than my other compressor but that's not necessarily a bad thing. It seems quieter if that makes sense? I haven't tried it on a heavily distorted guitar but it doesn't seem to amplify the noise/hiss as much.

@Chuck D. Bones Thanks so much for your help.

You are welcome.
The Tso does not have a lot of gain, so yes, it is pretty subtle. Less gain = less noise, all other things being equal. The Delegate has a lot of sustain available, the Tso a moderate amount and the Closed Circuit is very mild.

 

[Chuck D. Bones]

Turns out a few in the bag of LDRs I got from Amazon were dodgy. I ended up discarding 4 today, plus the faulty one originally fitted. 5 from 10 isn't great!

I'd send the bad ones back for refund.
Electronics Goldmine sells all manner of LDRs. They are a surplus dealer; the parts are pretty good prices and usually good quality. I've found only one stinker in a batch of 20 LDRs from them so far. I received some leaky tantalum caps from them once and they issued a full refund, no argument.