Author Topic: Pot values and possible fried pot.  (Read 416 times)

Pot values and possible fried pot.
« on: March 07, 2018, 11:01:08 PM »
Hey all. Last year sometime I really wanted a fuzz factory and couldn't afford one so I bought a DIY "fuzz machine" kit. It worked before I decided to take it apart to paint the enclosure and then I could never get it working again. I ended up trading another pedal for a vexter FF on craigslist and the DIY has been sitting ever since. I revisited it today and after some troubleshooting was able to get it working again. But at one point during the troubleshooting I must have been making contact with the DC somewhere and even though nothing caught on fire I did have plenty of heat and sparks. I think I may have fried the STAB pot because that's what was touching the DC, it acts more like a really noisy volume control now. Sorry for the long-winded set up. I just have two questions: Is that how a fried pot would sound/behave, should I replace? I looked into getting another one and I noticed the pot values on the FF schematic are not the same as the ones in this kit. Namely the "gate" and "stab" are reversed. I have a 5k for the Gate and a 10k for the Stab, and the pcb is printed with those values. Can/should I try using the values in the zvex schematic or would that have a negative effect on the rest of this particular circuit? Unfortunately this kit didn't even include a schematic so I'll do my best to provide any additional information if you need. Help me out if you can, thanks!

ElectricDruid

Re: Pot values and possible fried pot.
« Reply #1 on: March 08, 2018, 05:03:45 AM »
I wouldn't assume that the pot is fried, personally. If it's still working as a volume control, then it's still working to some degree. Scratchy noise on pots is often because they have a DC potential across them - that might suggest that something else in the circuit is fried, like a cap, or a transistor. That said, if you put too much current through a pot, it is possible to burn a hole in the track where the wiper is touching it. This makes a pot that works except for one spot where the wiper suddenly stops being connected. This may be your situation.
Debugging it without a schematic is going to be tricky. Is the Fuzz Machine kit just a version of the Zvex Fuzz Factory? Do you have a schematic for that?

If the kit recommends 5K and 10K, that's probably the best values. Swapping them over is unlikely to improve things.

Tom

antonis

Re: Pot values and possible fried pot.
« Reply #2 on: March 08, 2018, 06:41:07 AM »
As Tom said, Pots can hardly be fried into 9V circuit..
(unless they are very low power rated an of very low total resistance..)

A 10k pot set across full DC of 9V is heated due to 8mW power developed on it..

Ordinaly pots used for audio circuits are rated at 100mW and they have to exceed it for a considerable time duration to get burnt/melted/fried..
Can't follow your signature changes, Stephen..!!

Re: Pot values and possible fried pot.
« Reply #3 on: March 08, 2018, 09:32:14 AM »
I wouldn't assume that the pot is fried, personally. If it's still working as a volume control, then it's still working to some degree. Scratchy noise on pots is often because they have a DC potential across them - that might suggest that something else in the circuit is fried, like a cap, or a transistor. That said, if you put too much current through a pot, it is possible to burn a hole in the track where the wiper is touching it. This makes a pot that works except for one spot where the wiper suddenly stops being connected. This may be your situation.
Debugging it without a schematic is going to be tricky. Is the Fuzz Machine kit just a version of the Zvex Fuzz Factory? Do you have a schematic for that?

If the kit recommends 5K and 10K, that's probably the best values. Swapping them over is unlikely to improve things.

Tom

Thank you for your help Tom. Much appreciated.

This kit is basically a Fuzz Factory clone from what I can tell. The closest thing I could find online to a schematic is a photo someone labeled of their own build. (link below) when comparing to the zvex schematic (link below) I noticed two discrepancies beyond the two pot values: a 47uF and a 22uF capacitors rather than two 10uF that I see in the zvex schem. Everything else looks the same. Thinking I'll swap out those four parts when I get a chance to see if that gets me any closer to the true FF sound. It sounds close to my vexter now, but it's brighter/brittler and even more finicky. Thoughts? Again, thanks for the help and please excuse ignorance. I've built a handful of pedals without much true understanding of how the circuits work. Just starting to learn.

Links:

https://imgur.com/a/DbyzS

http://basicaudio.net/Fuzz%20factory%20schem.PNG

vigilante397

Re: Pot values and possible fried pot.
« Reply #4 on: March 08, 2018, 10:20:06 AM »
Welcome to the forum :)

I've built more fuzz factories than I care to mention, and I don't think I ever had two that sounded exactly the same, largely due to the differences in NOS Germanium transistors. The reason "matched" transistors is sometimes a big deal is that two of the same type transistor even from the same manufacturing lot can have vastly different specs, and therefore sound completely different.

What transistors were in the kit you got? Have you opened up the Vexter to see what's inside? I've used AC128's in all my builds, but I've heard ZVex sometimes uses something else.
"I'm not sure what "serious design flaws" you see. Does it explode or poison your dog?" - PRR

"Tayda's Whipping Boy" "Most Detestable Forum Member 2018"

http://www.sushiboxfx.com

Re: Pot values and possible fried pot.
« Reply #5 on: March 08, 2018, 10:41:10 AM »
Welcome to the forum :)

I've built more fuzz factories than I care to mention, and I don't think I ever had two that sounded exactly the same, largely due to the differences in NOS Germanium transistors. The reason "matched" transistors is sometimes a big deal is that two of the same type transistor even from the same manufacturing lot can have vastly different specs, and therefore sound completely different.

What transistors were in the kit you got? Have you opened up the Vexter to see what's inside? I've used AC128's in all my builds, but I've heard ZVex sometimes uses something else.

Thanks for the welcome! I've been lurking for long enough anyway. I think what I have in there now are 3ax7? I'm at work right now so I can't check but that sounds right if memory serves. Thinking about trying something else, but honestly I'm less concerned about the brittleness of the tone than I am about the functionality (or lack thereof) of the stab pot. Is it possible that changing the trannies or the aforementioned pot/cap values could fix that? I'll probably try it regardless. Since I already have the vexter I don't really have much to lose.

vigilante397

Re: Pot values and possible fried pot.
« Reply #6 on: March 08, 2018, 12:33:48 PM »
Changing the transistors will definitely affect the tone but shouldn't affect the function of the Stab pot. Can you post pictures of your build? That would make it much easier to help figure things out. I will agree with those that have said it's unlikely (though not impossible) the pot is fried, but obviously something is wrong, so let's find it 8)

Question though: do you own a multimeter? Even the $5 ones you get from places like Harbor Freight can be invaluable in debugging misbehaving circuits.
"I'm not sure what "serious design flaws" you see. Does it explode or poison your dog?" - PRR

"Tayda's Whipping Boy" "Most Detestable Forum Member 2018"

http://www.sushiboxfx.com

Re: Pot values and possible fried pot.
« Reply #7 on: March 08, 2018, 12:55:14 PM »
Changing the transistors will definitely affect the tone but shouldn't affect the function of the Stab pot. Can you post pictures of your build? That would make it much easier to help figure things out. I will agree with those that have said it's unlikely (though not impossible) the pot is fried, but obviously something is wrong, so let's find it 8)

Question though: do you own a multimeter? Even the $5 ones you get from places like Harbor Freight can be invaluable in debugging misbehaving circuits.

Yeah, I'll have to see if I can snap a couple quick photos tonight. And yes I have a DMM. Thanks for the help! I'll be back with the photos asap.

thermionix

Re: Pot values and possible fried pot.
« Reply #8 on: March 08, 2018, 06:37:26 PM »
FWIW...10k Gate and 5k Stab, according to every FF schematic I've seen.
FDH

Re: Pot values and possible fried pot.
« Reply #9 on: March 09, 2018, 11:08:30 AM »
Changing the transistors will definitely affect the tone but shouldn't affect the function of the Stab pot. Can you post pictures of your build? That would make it much easier to help figure things out. I will agree with those that have said it's unlikely (though not impossible) the pot is fried, but obviously something is wrong, so let's find it 8)

Question though: do you own a multimeter? Even the $5 ones you get from places like Harbor Freight can be invaluable in debugging misbehaving circuits.

Here are a few photos of my build. I did end up swapping the gate and stab pots. Still not much difference. But the 10k pot I just moved to the gate position is still behaving the same way it did as the stab pot. Sounds like a wave crashing in when I turn it and seems to only work in certain positions. Anyway, have a look. Hope I didnít burn up the pcb in those two spots. Getting multi-pin components out of a board is downright frustrating.