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Paul, the reverse breakdown voltage on a diode is simply the voltage at which it reverse conducts. It's not a "fail point" in the sense that you'll break something. Zener diodes, for instance, are designed have specific reverse conductances like 3.3V, 4.7V, 9.1v, etc.

Since antiparallel diodes in a clipping stage are already conducting at a Fv drop in both directions (almost all the time), you will never, ever see a 10V swing to result in reverse conductance in a distortion pedal ... never mind that you'd need a 24v supply to produce a signal that large without clipping the amplifying device even if you only had a single diode.

Cortexturizer: 9V > 100R > Anode > Cathode. Measure the voltage at the anode. Subtract from the supply voltage. It's approximate. Does your multimeter not have a continuity setting? That's usually also the diode tester. I'm not sure I've ever seen a multimeter for sale that didn't do that, not even the super cheap ones. Even my analog meter has it.

Tagboard Effects made a vero layout, for those of you who don't etch and hate teh perf!

http://tagboardeffects.blogspot.com/2013/12/sam-ash-fuzztainer.html

I suggest the EchoCzar as a simple, beginner-friendly single-sided etching project. 

Might as well make it a 1590A since its a simple circuit. lol

Challenge accepted!*




not really, dur.

You'd get better results in this case by measuring the "good" range of the bias pot and then altering R5 to get you close to the lower end, then using a smaller value pot (say, 10K) for the bias. This will give you some interesting sounds but no dead spots. Bumping a bias knob without noticing and wondering why your signal is dead isn't fun.

Ahh, screw it, the board's on sale with free shipping ....

Sure thing on getting the measurements.

As far as you seeing more moving voltages on the OTA's, I see that in my measurements that pin 5 isn't moving. That doesn't really make any sense. It should be the same as pin 7. My guess is that I just didn't leave the probe on long enough to see the movement on the first IC I measured and when the others hit the same value, my brain was set to too quickly accept it. (Confirmation bias of a sort). I could also see that there might be some small movement on pin 4 and 13 and maybe even 3 and 14 from the feedback paths.  They seemed pretty steady to me when I measured them, but it could be that my meter just didn't pick it up well or you would see it more with a different setting of the trimmer and/or speed. If the movements you are seeing on those pins are small and around the values I saw, they may be fine.

Ah. Most of the movement was small. But e.g. pin 7 on IC3 for me is super low (like 1V), and several other voltages were off.

I can't really explain the 7v on my TL074's pin 8, either. I can't think of what could pull it up that high; even if there was a solder joint under the chip that I can't see, nothing near that pin has voltage higher than Vb.

Rob, thanks so much.

Yeah, several of my LM13700 pins are very wrong, and more of them are moving than on yours. Also found that I have 7V on pin 8 of the 074.

I might just rebuild this. It's possible that I did damage the board in some way, and tracking down a couple wrong voltages isn't too bad, but tracking down a dozen is just a pain.

Off to work now, so I guess I'll fiddle more when I get home if I haven't taken advantage of the sale.

double sided foam 3M tape (or heavy duty variants).

Also keeps the PCB from moving around.

Not having a good run here ...

My multimeter battery died so I don't have voltages on the LM13700s.

IC1 voltages look "normal" but there's no signal on pins 6-8.

LFO is working -- the LED flashes and I can get woosh woosh noises. But I had to sub a BJT chip to get it to modulate at all. Neither TL072 or 062 would work.

My only subs are 27K and 33K for the 30K resistors. I'm not sure it can make much of a difference since I think to the electrons it's same thing, right? They're two resistors in series.

Okay, audio probe. Dry signal passes at about unity. But this is super weird: I can get weak signal at pin 7 of IC3 (before the buffer) but nothing anywhere else on any of the 13700s or anything attached to them. It is weakly modulated. However, if I pull IC3 and jumper pins 4 and 8, I can get stronger signal at IC5, with some very chewy phase ... but no signal on ANY other pin of the 13700s, not even after the next inversion! I tried jumpering the other half of IC3 on the assumption that there was something wrong in that side of the board, and then I can't get anything but LFO noise on any pins on the 13700s. I tried bypassing all the stages at one point or another (though I did not pull any resistors, just jumpered 13700 pins).

A few other things: I got better dry signal with a 274 in place of the 074, but nothing else changed. I've tried about 10 different 13700s from three different suppliers, and a couple 13600s from Smallbear. All tested as working in an engineer's thumb.

One last thing: this is the board that I desoldered all the chips from. There was one lifted pad in the LFO chip. I double checked everything with my multimeter for continuity and the only bad spot I found was that somehow the base of Q1 was not connected to the R49/R50 junction for some reason (no damage to the board there, though). I'm unable to locate any other PCB damage and as noted above I've tried bypassing various stages.

Pretty mystified right now. 

I am getting varying voltage that changes with the speed control on LFO IC pin 7.

Have not considered the 4.7v zener? I assume if it was faulty I would not get any signal at all.

Now check resistance and voltages on the FET. In other words, make sure that the control voltage is getting to the FET. Work backwards if it isn't. If it is working, then something's wrong in the audio path.

The trimmer has a really narrow range, by the way.

Probe the LFO IC and make sure the voltage is varying.

Looking at the schematic, I suspect transistor distortion is involved -- there are two, one of which serves as (I guess) a sort of phase inverter. Otherwise it's just cascaded transistors. I'd look into just copying the schematic up to Q3 (before the speaker transformer) with one of the small common transformers, even the cheap ones from Radio Shack.

I do recommend looking into a desoldering gun for the amount of builds you do.

I've thought about it a lot*, but in this situation, I feel like it's a bandaid.

Rolly, you're right; the workshop does not have three-prong outlets. I guess I've got multiple reasons to upgrade them now.

*I would be mortified to tell you guys what I use for really drastic desoldering.

Jon, what temp do you have it set at.

It may not sound intuitive, but you may have better luck setting the temp higher.  It can often reduce the dwell time (although it doesn't sound like you dwell that long as it is).

I've tried a lot of temps between ~400F and 750F. I don't go above 750 on anything because the solder does not react well (it dries out and gets cruddy). Last night was something like 600F, which was the temp I could get a sort of stringy pull of solder and get it to adhere to the pad almost immediately upon contact.

I don't want to make it sound like I fry EVERY chip, but I don't seem to be able to bat 1000 on it. I managed 5 out of 6 on a Lowrider over the weekend for instance ... but fried the 074. Last night I fried at least the TL074 in the Stage Fright (I'm not sure what else I fried because I didn't think properly and clipped everything, instead of just replacing the TL074 first ... I didn't realize that one of the op amps was the Vb for everything, so I thought they were ALL fried when I measured voltages. The 13700s might have been fine in other words).

I know that you are way too experienced for this comment

Not at all. I didn't even consider it, and I fried a couple CMOS chips (including a 3101) not too long ago because I wasn't careful.

My soldering is actually not very good, except when I'm building for others or working on something that will be on my board ... I tend to be a lot more meticulous when I know it's leaving my house.

Are you melting the solder to the pad then, re-heating it while slipping the IC leg into the pad hole?

no, no, no.

... Would anyone actually do that? Seems like a nightmare ...