two sticks of derm + tentacle feedback issue

[hateboardlove]

hey everyone! i put together a dual 125b build the other day, feeling like it was a waste not to throw a tentacle in with the two stix of derm since they’re both such tiny circuits.. anywho the tentacle works flawlessly but the two stix has this insane feedback through most of the dial.. i thought it might be ground problem at first but after adding in the two extra grounds i dont think so.. so next i was thinking capacitor burnt? so i replaced 4 of the capacitors so far since i have experienced this insane feedback once before when i burned a cap by accident, replacing it fixed that issue.. so im thinking i have a bad cap on here somewhere as well but i only have 5 left to replace and im hoping it’s not the wimas so i dont have to waste time ripping all those out.. so i was wondering for anybody that has built the two stix of derm if you get a lot of feedback as well with this circuit or am i on the right track with a blown cap before i rip the rest out tomorrow night?

 

[DGWVI]

I've built this exact combo numerous times, and never had an issue with feedback.

Is the volume pot insulated from the board?

Beyond that, I see a ton of places for potential shorts and cold solder joints.



You don't want those spiky solder bits. You're either overheating the joints and flux, or not using a hot enough iron.


I'd also recommend redoing the wires- both shortening them and eliminating as many splices as you can (or, insulating the splices of necessary)

 

[hateboardlove]

oh yeah i wouldn’t normally splice wires but i used up the very last of my wire so was a bit desperate to keep it all the same wire haha and the spikes.. that is good to know! i had no idea those spikes were evidence of all that! i’ve heard those terms thrown around and i’ve definitely felt my iron is always a bit too hot. the volume pot is pretty well insulated so i dont think it’s there. can those solder joints be fixed by applying the right temp heat and a bit more solder or should i try and remove and replace the solder entirely in those places?

 

[Feral Feline]

I would NOT add more solder.

I'd use some copper-wick or a solder-sucker to get rid of the excess solder.

Then I'd reheat the joint making sure to heat BOTH the component-leg and the PCB-pad. Pad & Leg...
To repeat: I would not add more solder, or you'll be back to blobs/spikes — unless you leaned out the solder too much when cleaning up the excess, only then you might need to re-apply solder.

Definitely follow DGWVI's advice and put some shrink-tube or electrical tape around the wire splices to insulate them, and then test for continuity.



Lots of people use one colour of hookup wire throughout their builds, and it works for them.
I prefer to have separate colours:
black - gnd
red - power
Colour 1 - input
Colour 2 - output
Colour 3 - mods (board to switches/pots
etc

Separate colours aids me in trouble-shooting my builds, test one colour for continuity/shorts such as from footswitch to board, then move on to the next colour (input wiring), next (output), next... Helps systematically go through everything and not accidentally confuse the ground connections with a signal connection or whatever.

That's just my preference.




NEXT : audio-probe — an invaluable tool for tracking down problems

Easy to make, detailed elsewhere on the forum and Internet, an audio probe would let you trace the signal through the circuit to find the point at which the feedback issue starts.


WIRE ROUTING
On high-gain or circuits with LFOs and others, it's best not to have the wiring from the jacks running over the board to the footswitch(es). Route the input/output around the edge of the enclosure away from the PCB so the wires can't pick up any interference off the board or other wires.

 

[hateboardlove]

I would NOT add more solder.

I'd use some copper-wick or a solder-sucker to get rid of the excess solder.

Then I'd reheat the joint making sure to heat BOTH the component-leg and the PCB-pad. Pad & Leg...
To repeat: I would not add more solder, or you'll be back to blobs/spikes — unless you leaned out the solder too much when cleaning up the excess, only then you might need to re-apply solder.

Definitely follow DGWVI's advice and put some shrink-tube or electrical tape around the wire splices to insulate them, and then test for continuity.



Lots of people use one colour of hookup wire throughout their builds, and it works for them.
I prefer to have separate colours:
black - gnd
red - power
Colour 1 - input
Colour 2 - output
Colour 3 - mods (board to switches/pots
etc

Separate colours aids me in trouble-shooting my builds, test one colour for continuity/shorts such as from footswitch to board, then move on to the next colour (input wiring), next (output), next... Helps systematically go through everything and not accidentally confuse the ground connections with a signal connection or whatever.

That's just my preference.




NEXT : audio-probe — an invaluable tool for tracking down problems

Easy to make, detailed elsewhere on the forum and Internet, an audio probe would let you trace the signal through the circuit to find the point at which the feedback issue starts.


WIRE ROUTING
On high-gain or circuits with LFOs and others, it's best not to have the wiring from the jacks running over the board to the footswitch(es). Route the input/output around the edge of the enclosure away from the PCB so the wires can't pick up any interference off the board or other wires.

damn that is a lot of very useful advice! thanks so much for taking the time! this all definitely illuminates a ton.. i’ll have to make sure to re read this and go over the whole thing again for cold joints etc.. i have made an audio probe recently and havent actually used it to diagnose anything so i may try that with this. when i have nothing plugged into my pedalboard and turn on my amp, interestingly, if i turn the knob all the way up right around the peak it creates a high pitched whine. i’ll include a quick video as i think i may be evidence of a problem, which im leaning towards capacitor issue since caps filter highs and it’s failing?

 

[Feral Feline]

We've all been there.

(Some of us are still there, but ... Hey! I try! Some say I'm very trying...)

You seem stuck on the high-pitched whine being a cap issue — I'm not saying it isn't, but ... keep your mind open to other possiblities.

You don't want to replace all your caps only to find that it was NOT a cap issue. That's a lot of wasted time and effort (well, apart from learning the hard way=time-well-spent).


Some circuits are just designed with a given amount of noise or whine or whatever (high-gain stuff, I'm looking at you); or how some modulation circuits can't be totally free from LFO ticking — the ticking can be minimised with careful wiring — and the original layout, but a poorly laid out vero / perf board with no thought about separating the LFO from the signal path... well that'll just tick you off!

There's lots of diagrams, illustrations, videos, clips, visual-aids for proper solder joints, soldering-technique, tip-choices etc online... Have a gander at them yourself, but here are a few:

I can't imagine cleaning each lead, as recommended by this guy , on a big build — it would add hours to a build. However, I do clean older components and especially old PCBs/vero/perf — I use a bicycle-rim eraser, it cleans off the oxide and leaves the pads/traces shiny clean with just a few swipes.

The guy in the vid above starts on the lead and works down to the pad. I don't. Too easy to damage the component heating the lead first, in my much less experienced opinion. I heat the pad and the component lead at the same time, wedging the tip between the two for a moment then add solder to the pad and draw the iron up the lead away from the pad and the component that's on the other side of the board, which seams to help get that volcano-shape.
IDK, maybe I'll try the lead-down-to-pad idea.

Lots of other good advice in the video. Watch some other vids, check out NASA's best soldering practices and try things and compare them ... you'll figure out what works best for you.


Yeah, the info, like the truth, is out there.

Whine might not be cap's fault.

 

[hateboardlove]

We've all been there.

(Some of us are still there, but ... Hey! I try! Some say I'm very trying...)

You seem stuck on the high-pitched whine being a cap issue — I'm not saying it isn't, but ... keep your mind open to other possiblities.

You don't want to replace all your caps only to find that it was NOT a cap issue. That's a lot of wasted time and effort (well, apart from learning the hard way=time-well-spent).


Some circuits are just designed with a given amount of noise or whine or whatever (high-gain stuff, I'm looking at you); or how some modulation circuits can't be totally free from LFO ticking — the ticking can be minimised with careful wiring — and the original layout, but a poorly laid out vero / perf board with no thought about separating the LFO from the signal path... well that'll just tick you off!

There's lots of diagrams, illustrations, videos, clips, visual-aids for proper solder joints, soldering-technique, tip-choices etc online... Have a gander at them yourself, but here are a few:

I can't imagine cleaning each lead, as recommended by this guy , on a big build — it would add hours to a build. However, I do clean older components and especially old PCBs/vero/perf — I use a bicycle-rim eraser, it cleans off the oxide and leaves the pads/traces shiny clean with just a few swipes.

The guy in the vid above starts on the lead and works down to the pad. I don't. Too easy to damage the component heating the lead first, in my much less experienced opinion. I heat the pad and the component lead at the same time, wedging the tip between the two for a moment then add solder to the pad and draw the iron up the lead away from the pad and the component that's on the other side of the board, which seams to help get that volcano-shape.
IDK, maybe I'll try the lead-down-to-pad idea.

Lots of other good advice in the video. Watch some other vids, check out NASA's best soldering practices and try things and compare them ... you'll figure out what works best for you.


Yeah, the info, like the truth, is out there.

Whine might not be cap's fault.

you’re def right! cause i heard it once before my brain is totally stuck on the caps thing. that is a lot of very good points, i’ve def learnt over time my brain is a bit hardwired to doing things the hard way and wasting time proving itself wrong is time well spent to it. i’ll just go through the whole thing again tomorrow night since there’s so many questionable spots. thanks for the super informative response! i have a ton to work on now haha

 

[szukalski]

I'm in the too heavy camp.. but it's a conscious decision! Great advice from Feral.

Remember, you want to join the pad and the component, so you need to heat both (the pad may need a little pressure to help transfer heat) and then apply the solder to the heated components, not the iron.

 

[yazooligan]

Based on a thorough search of the forum, a lot of people have run into this issue and it’s most likely the Q2 transistor having too much gain. I’m currently dealing with the same thing on a breadboard because I only had two 2N3904’s on hand, so I’m following the forum advice and trying the P2N2222A’s and BC109’s I do have. Haven’t found one that works yet, but I’ll keep trying until I do or get another pack of 3904’s. Hope this helps.

EDIT: I invested in a transistor tester from Amazon recently and used it for the first time to find the hfe on my stock. The two 2N3904’s were 296hfe and I used a 2N2222A at 176hfe for Q2 and now the volume pot doesn’t whine or oscillate.

 

[Feral Feline]

Right on! Hard numbers for hFE. Thank you Yazooligan.