Debugging harmonic percolator

[pinkjimiphoton]

word!!!!!!!!!!!!!!!!!!!!!!

[Bret608]

Hey again everyone,

I had only tried a couple of different ge trannies in here--one was a 2n1305 with 49 hfe and under 150uA leakage, the other was a 2n1376 with 75-ish hfe and under 100uA leakage. My 2n3565 is about 260 hfe; I tried another in there with no change as well as a 2n4441.

So this weekend, I swapped out R1-R3 for the Albini values and no change. Someone over at Madbean suggested I might have a board with a manufacturing error. I guess that would explain the non-stop theremin/motorboating thing I'm getting. My electronics instructor friend said this circuit looks like a "cascode amplifier", which is specifically designed to keep a circuit's in and out isolated. In my case, that isn't happening--in essence, the output is able to completely feed back into Q1.

I'll keep messing with it I guess--even if there is something connected that shouldn't be, I may be able to track it down. I need to check my vero stash and see if I have enough for a Juergulator while i'm at it...

[pinkjimiphoton]

bret, i've built juergulators on a 10x10 piece of vero... gotta be a little bit slick to do so, but it can be definitely done.
if you need a piece, pm me.

[Bret608]

Thanks for the offer! I am almost positive I have enough for Miro's layout though. Let me just double check my stash tonight. I love their layouts. Did their Supafuzz this summer and it rips.

[garcho]

here's an etching layout if you'd like

[Gus]

Hey again everyone,

I had only tried a couple of different ge trannies in here--one was a 2n1305 with 49 hfe and under 150uA leakage, the other was a 2n1376 with 75-ish hfe and under 100uA leakage. My 2n3565 is about 260 hfe; I tried another in there with no change as well as a 2n4441.

So this weekend, I swapped out R1-R3 for the Albini values and no change. Someone over at Madbean suggested I might have a board with a manufacturing error. I guess that would explain the non-stop theremin/motorboating thing I'm getting. My electronics instructor friend said this circuit looks like a "cascode amplifier", which is specifically designed to keep a circuit's in and out isolated. In my case, that isn't happening--in essence, the output is able to completely feed back into Q1.

I'll keep messing with it I guess--even if there is something connected that shouldn't be, I may be able to track it down. I need to check my vero stash and see if I have enough for a Juergulator while i'm at it...

It is not a cascode it is two cascaded gain stages HOWEVER they run at the same current being powered in series and divide up the 9VDC.  This have been covered in other HP threads

Both emitter voltages should be exactly the same
Check the cap at both emitters is it connected correctly?
Do you have long input and output leads?  How are the leads placed?
Do you have a power supply cap?

[pinkjimiphoton]

good call, gus...
i've found that some percs won't even run with a power supply filter 

[tombaker]

This is a very forgiving circuit.
I made the Tim Webster Vero layout not long ago and had R1 and R3 in opposite positions and had Q2 in backwards and it still made a sound. I also had different capacitor types than what are recommended on the schem.
I know this doesn't help debug but you shouldn't get no sound coming through. I had a 2n404a and the 2n3565 as listed on the layout

[pinkjimiphoton]

hey, the timelord ^^^^above makes a pretty valid statement...btw, welcome doctor! 

brett, do me a favor.

check that you are getting continuity thru your resistors. sometimes they (i call it ) "decapitate" and tho they look solid, are open on one side or the other internally and pass no signal.

it's gonna be something stupid like that, i'm positive. i've been laid low on a few projects because of that.

also try pulling q1 completely. use your meter and take some voltage readings on q2 with the pedal on and plugged into an amp.

see if the buzz you hear from the amp changes between b and c of q2. if it does, that part is probably working.
if you keep doing that back thru the circuit along whatever path the audio takes, you'll hear it get weaker as you get away from the working part. when you lose signal, however improbable, that's most likely the culprit. kinda like using an audio probe in reverse... with tube amps you start from the power tubes and work your way back.

jack darr, and uuber tech from days of yore called it a circuit disturbance test.

also, dumb mistake i STILL make. are ALL your grounds connected? is there a clear connection to the jacks between the board and power supply? sorry don't mean to sound ignorant or anything, but i've made that mistake... often.

finally for some reason, sometimes fuzzes need their pot casings grounded, or you'll get a parasitic oscillation that's so high you can't really hear it that will kill the circuit in the audio range.

just thinkin'.... there's no way this circuit should be so hard.

can you chuck a 2n3906 in q1 just to see if it'll fire?

[Bret608]

Hey guys,

Yeah, I thought this circuit wasn't supposed to be this sensitive. This is really helpful...let me take these items one by one.

Gus, I am getting the same voltage at both emitters. I do have the cap that connects to both emitters oriented correctly, if you mean the 47uf tantalum. I do have nice, long in and out leads, placed just the same as on my working build. I have a 100uf power supply cap, which is part of the BOM on Madbean's board. I used the same value on my working pedal as well.

Jimi, I have not checked continuity through my resistors but will do that tonight. I didn't even know about this technique! 

I did pull q1 completely a few days ago. So, interestingly, I was not getting the oscillation anymore, but I could hear the radio. I will run through this technique tonight as well. Again, did not know about this one.

My grounds are connected. I actually don't have the circuit boxed right now--I've been hooking it up to my test box at home (which I've confirmed is working fine) and a breadboard at work. But your comment about the pot casings being grounded has me wondering. Could I somehow use an alligator clip to connect the pots to the ground clip on my test box? Since the circuit isn't boxed, I could easily see that being an issue.

I threw in a couple of different 2n3565s, and also a 2n4441 (I think that's the one--it was just a low-gain NPN silicon). Still no dice...

Cheers,

Bret

[pinkjimiphoton]

just for shits n grins, take out the 100u cap and see if suddenly it fires.

i built a perc with a nice power supply, and could not get it to work until i removed it. i don't know why, but it happened... was the juergulator in fact, when i first built it.

[Bret608]

Thanks man, I'll report back tomorrow. By the way, I have more than enough vero for a Juergulator! Will get it built within the next couple of weeks as time allows. Can't wait!

[Bret608]

Whoops, I forgot! Can someone confirm if my idea about grounding the pot casings via alligator clips to the ground on my test rig would work?

[duck_arse]

it won't hurt.

[garcho]

^ probably won't help either  

I do have nice, long in and out leads,

On the breadboard? Short leads are better, less chance for a short, no pun intended.

Have you audio probed it?

i've found that some percs won't even run with a power supply filter

as in 100R > 100u filter? Because if it's just a cap across the rails, nothing should change at all. Why do you think it's 'some' percolators but not all?

[Bret608]

Hey Gary,

I actually haven't audio probed this yet--still haven't built that feature into my test setup. If this isn't the impetus, what would be? 

Here's something I should've mentioned though. When my friend probed the whole thing on his scope last week, he noticed that nothing seemed to be happening after R3. Then he kind of backtracked and wasn't sure if he had something hooked up wrong. So this crazy waveform up to R3, then possibly nothing after. I've since swapped R1-R3 but still have the motorboating. I may scope this thing again tomorrow.

I was talking about the in/out wires from the circuit board. The leads on my test rig aren't terribly long, and when we breadboarded it at work last week, we used as short of wires as we could.

[garcho]

...audio [probe]... --still haven't built that feature into my test setup.

It's just a 1/4" jack with 2 wires soldered to it, and it's essential, don't wait! 

[Bret608]

Cool! I grabbed an old DMM lead and a .1u film cap just a moment ago--I will be adding the probe to my test rig tomorrow!

Also, I read through some of the other HP threads that Gus mentioned. Very illuminating! Dino's description of what happens when you eliminate the 47u tantalum or lower its value sounds like the main way to get oscillation/motorboating with this circuit if everything else is solid. I just swapped that cap out for a new one and will test it tonight. After I de-soldered both pads, I saw a small sliver of solder was connecting them. Since the old cap was sitting flush with the board surface, I can't tell if this was always there or happened while de-soldering. We will see.

[Bret608]

Now that I have swapped the 47uf tantalum for what looked to be a newer one, I can hear some distorted guitar signal coming through the oscillation. Will swap the other tant, maybe pop out the power supply cap, and start probing! 

[duck_arse]

damm! I wanted to be first to say "my money is on the hidden blob of solder". sounds like you need to go over all your caps with yr meter on ohms range. anything less than o/l might be another blob.

good luck