Ebay DIY delay pedal mod to fix volume drop problem

[reddesert]

This or related accounts sell a variety of inexpensive pedal kits. The descriptions are often a little wonky (the "fuzz face" isn't a fuzz face, but probably an opamp Big Muff - stuff like that). They seem to be all from common schematics, nothing unusual (except whether one or two violate a tonepad license). I figured out what most of them are likely to be from the parts lists. The delay is a Rebote delay.

I built one that was a Maxon OD-808 clone - a tube screamer with opamp instead of transistor buffers. It works fine, but actually fitting all the components into the 1590G box is tedious. I don't like the stomp switch mounted directly to the main PCB here, as it forces the PCB into an awkward spot at the bottom of the box.  Hopefully this won't be annoyingly obvious: you need to insulate the bottom of the enclosure because the board can short against it. Also watch your jack and pot wiring as you can't see it when the box is stuffed. One thing I would do to debug the issue is start by taking all of the circuit out of the enclosure and trying it again, then look for loose connections, shorts, parts not fully soldered, etc.

[buildAndPlay]

Hopefully this won't be annoyingly obvious: you need to insulate the bottom of the enclosure because the board can short against it.

First off - you guys are awesome!  Thanks so much for all of the information.  I have lots of things I can check now without just flailing.  As far as the board shorting out, I was concerned about that too and was going to put a hard piece of thin plastic or something  between it and the case, but it turns out the back cover has a lip all the way around the outside that extends into the case a bit.  It contacts the outside edge of the board which is all grounded and prevents the rest of the board from shorting out against the back cover.  Also, I did try the board without the cover first and had the same issue. 

I will go over everything today and report back.

Jim

[buildAndPlay]

Ok - I resoldered the suggested resistors around the op amp (510k and 24k) with no change. I didn't try pulling the 5pf cap but double checked that I had the correct on in there.  All of my solder joints look pretty good though so I didn't expect much.  I then soldered in a couple of leads to either end of the 510k that leads to the input pin (6) on the op amp so that I could easily put a 1M ohm resistor in parallel with it using alligator clips.  That way I could quickly switch between 510k and 330k.  That definitely helped.  I think I would have to maybe go to 270k or 240k to get all the way there.

So, do you think it would be OK to proceed with that plan or does it seem like something is still really wrong somewhere else?  I did check the 1M resistor between the op amp input and ground to make sure the value was correct.

Thanks again for all the help.

Jim

[GibsonGM]

My opinion:  I'd change it out, sure.  As long as you have 'played' the pedal the way you will in real life....you are increasing gain, so you COULD find some clipping somewhere due to this, maybe.   You can always reverse it if it does anything you don't like.    If it is working with no glitches with your jumpering in, then you are likely good to go.     

This is a 'typical' type of mod - you are just decreasing the value of an input resistor that works with a feedback resistor to set that op amp's gain.

[buildAndPlay]

Ok -  I didn't have any 240k or 270k resistors on hand, but I did have a 330k and a 1M.  So I replaced the input resistor with the 330k and the feedback resistor with the 1M and now the volume with the delay on is just barely less than with it off.  In fact, it's about even using the neck pickup on my Strat.  The out of phase sounds a tad quieter probably more because of the loss of highs.  I think I'll probably be OK with it the way it is.  If it still bothers me then I'll replace the 330k with something smaller.

This is my first effects pedal kit.  Now I'm afraid I may have to get more. 

Thanks again for all of your help.  Maybe I'll post a video later so you can hear what it sounds like.

Jim

[GibsonGM]

Please do, always like clips and pics!  Glad you got it working better, Build!  Enjoy your next project, this is an addiction

[Quackzed]

fwiw i'd still recommend you look for some mistake or wrong value...you're setting the gain of the opamp for 3x as loud as unity... so theres gotta be soething off somewhere, the circuit shouldn't need the input signal amplified that much (or at all really) for unity gain out... somethings gotta be off...   
    those 2 24k resistorsnear the op amp are a similar setup (input r / feedback r) might be one of them?  tho i guess if its fixed its fixed, just strange how you need so much gain just for unity...

[buildAndPlay]

It kind of bothers me too, but it's working so I'm not inclined to do anything more, but then I keep staring at the schematic that Cozybuilder posted and I'm like "well I suppose it could be..."   I'm sure by tomorrow I'll be checking some more things.  lol

[GibsonGM]

fwiw i'd still recommend you look for some mistake or wrong value...you're setting the gain of the opamp for 3x as loud as unity... so theres gotta be soething off somewhere, the circuit shouldn't need the input signal amplified that much (or at all really) for unity gain out... somethings gotta be off...   
    those 2 24k resistorsnear the op amp are a similar setup (input r / feedback r) might be one of them?  tho i guess if its fixed its fixed, just strange how you need so much gain just for unity...

Build - it wasn't you that left the comment on Ebay, was it?  Other people (person) are complaining of the same issue.  Some circuits DO have a bit of loss, mainly because of poor design done hastily.   It IS a good idea to look, look again, look 20 times, but in the end if you have found a solution, I'd go with it, personally.   
It is a $20 pedal... Sure it could be other Rf's, another section...but at least this 'works' without having the new guy dig too deep, LOL...if you break it, then you don't have it anymore.   Only caveat, as I mentioned, would be if it clips with the increased gain.

The only way to really know what's up would be to measure your "stock" input with an O scope, and compare it to the output.  Do some math, figure out the loss of each network the signal encounters...is it accounted for?   Or go thru each section that way, see where the signal is 'eaten'.

I'm inclined to think it's just the way it was designed (hastily...), as you are not the only one noticing it.  I could be wrong, of course.  I'm glad it works better now, tho. 

[duck_arse]

is it too late for a photo of the component side of the board, with the parts fitted? even if I say please? and if you have socketed the PT, (powe off, of course) pull it out, and see if the volume level jumps to normal/expected. if there is a fault with the pcb layout, you may as well find where/what, maybe save the next guy the hair loss.

[GibsonGM]

is it too late for a photo of the component side of the board, with the parts fitted? even if I say please? and if you have socketed the PT, (powe off, of course) pull it out, and see if the volume level jumps to normal/expected. if there is a fault with the pcb layout, you may as well find where/what, maybe save the next guy the hair loss.

What are you thinking, Duck - some kind of uber-sonic oscillation in the PS, maybe?

[PRR]

> Build - it wasn't you that left the comment on Ebay, was it?

The review on ebay is mine.

[buildAndPlay]

Earlier today I took it back apart because it's still bothering me.  I double checked all of the resistors by measuring them in circuit, looked for bridged solder points, etc.  I still can't find anything wrong.  The ICs are socketed so I pulled the PT chip per Duck's suggestion and the volume still drops a little when the delay is turned "on" as opposed to off.  I assume that means I should be able to rule some things out.   I don't have a scope or any other equipment besides a VOM.  I really wasn't expecting this to be any more than a simple build.  Electronics aren't exactly my first hobby.  I have an Associates degree in electronics technology from about 30 years ago so am not afraid to dabble, but I've certainly forgotten most of what I learned back then. 

I can try to upload a pic of the component side of the board tomorrow (I'm headed to bed right now).  Thanks again for all of the help.  At least I'm learning some things.

Jim

[Armo]

I built one of these recently with no issues so I would try reflowing your solder joints.

[duck_arse]

I was thinking that pulling the PT would rule in/out all the various filters that attach around the pt's opamps, if a rong value was pulling signal to ground or something. but I guess we've now kinda ruled the PT out as the prob, now.

perhaps injecting signal into the first opamp stage, and monitoring it's output, and then injecting sig into the input of the second oppy stage, and monitoring it's output may prove illuminating.

here's hoping we spot something in the component pic, and don't have to go to a board trace-to-circuit comparison.

[buildAndPlay]

Here's a pic of the component side of the board.  It's not quite in focus.  The two large resistors are the ones I substituted because it's all I had. I'll try to take a better pic tonight when I get home.  I'll probably re-flow all of the solder joints related to the op amp stages when I get a chance.  The other problem I've got is I started looking online for a cheap scope.  I already have too many hobbies as it is.    So much for an inexpensive effects pedal.

[Quackzed]

solder flux. thats my guess. if its slightly conductive it could easilly be sucking volume to ground here and there... you could try gently scrubbing it w/ an old toothbrush / paintbrush and a little dish soap... but you gotta really make sure its dry dry dry afterwords... put it in /under a bowl of rice to absorb all moisture is an often recommended method for drying electronics... thats just a quick glance opinion didn't check all values or anything...

[buildAndPlay]

Reflowed some of the solder  joints tonight and tried to clean up some of the flux.  No change.  Will need to sit down with it when I have more time.  May have to see if I can borrow a scope from someone.  Will keep posting updates until I fix it or I give up.

Jim

[Rob Strand]

How sure are you that the 47pF is correct?
You could try pulling that out like you did with the 5pF.
The reason I point to these is if the values are larger than expected you will get the dark sound you described.

There's plenty of other areas where issues could be but to get a volume drop I'd expect to see lower frequency cut rather than the high frequency cut you are seeing.

You could also try moving the connection of EC3 (1uF) from U1 pin 1 to U1 pin 7; for example by lifting one leg an wiring it through.   This way you are only listening to the first stage. If you still hear the volume drop then the problem might be around U1b.   If not then the problem is likely to be around U1a.

If you have a multimeter measure the DC voltages on all pins of U1.

[duck_arse]

just for fun, check again the value of the cap at pin 13, the board suggests 1nF, your fitted looks larger than the lowest 82nF.