DIY SD-1 not clipping [SOLVED]

[BramP]

Hi everyone,

Hope my questions here won't be too dumb; I'm trying out a couple of circuits on veroboard/tagboard with layouts from the tagboardeffects site, one of these is their Boss SD-1, now I have some soldering/troubleshooting experience (I've built pedals and tube gear before) but I can't seem to make the clipping section of this pedal work; it passes signal fine and the level and tone controls work as they should. But no matter where I put the drive knob it doesn't do anything, I've checked the board for any solderingbridges or dead spots but no luck as of yet. The opamp measures about 4.5 volts on pins 1,2,3,5,6,7, 0volts on pin 4 and full battery voltage on pin 8.

What am I missing? Are there any measurements around the diode section that I could check?

Hope to hear from you!

[mauman]

Welcome!

This vero layout is tagged as "verified" so it should be OK. If you'll post some photos of the front and back of your vero board, we can take a look.

Tagboardeffects Boss SD-1 vero layout

[BramP]

Ok let's see, here are a front and back pic, one thing; I haven't connected a led yet, but I can't imagine that's an issue? I've checked everything for solderbridges/continuity.

[Intripped]

Check that there are not copper residues around the holes you've drilled; those could still make contact.

[BramP]

Thanks I'll check for that, is there a prefered method of drilling these holes? I use a drillbit but it does get kind of messy...

[mauman]

Thanks I'll check for that, is there a prefered method of drilling these holes? I use a drillbit but it does get kind of messy...

I use a hand drill like this, and a sharp 1/8" bit. Tape the vero down to your bench, and it just takes a few turns of the crank for each cut. You don't have to drill all the way through, but you do have to cut completely across the copper trace. I check for no-continuity across the holes using a multimeter after I make the cuts.

Hand drill

[BramP]

Thanks for the tips! So I triple checked all the cuts for contact and any possible solderbridges; there are none, what would be the best next place to look for issues?

[mauman]

Run a knife along the insulation strip that's between each set of traces, which should clear up any invisible shorts. Focus on the first 9 rows, especially the ones carrying the signal from the op amp pins 1 & 2 to the Drive pot. For example, in the yellow circle, some of the strands of the yellow wire (to Drive pot lug 1) just might be bridging to the row above it.

If that doesn't do it, (1) make sure you've tied together lugs 2 & 3 of the Drive pot, (2) using an ohmmeter, measure the Drive pot pins 1 & 3 after disconnecting the pot wires from the vero. Should measure about 1 Meg turned fully one way, and just a few ohms the other way, (3) Reconnect the pot wires and trace the path from op amp pin 1, thru the Drive pot, back to pin 2 using a multimeter and checking resistance from point to point.

Marked vero back

SD-1 back.jpeg (96.25 KiB) Viewed 315 times

[BramP]

Ok, I followed your advice, resoldered both the drive pot connections as well as the input connection (that looked a bit shoddy too), measured the drivepot and it's ok (1M ohm one way, 1.2 ohm the other) and ran a knife through all the insulation strips to be sure.

So far no improvement so now I'm tracing, starting from pin 1 of the opamp, what kind of values are to be expected here? On the first pin (the cathode?) of the diode D3 it's about 0.7 ohms,

On pin 2 of the opamp I see the resistance varying between 32k with the drive pot all the way closed and about 900k with drive pot all the way open.

These measurements are all without a battery connected by the way.

[mauman]

On pin 2 of the opamp I see the resistance varying between 32k with the drive pot all the way closed and about 900k with drive pot all the way open.

Right, no power connected for these measurements.

Reverse the leads of the multimeter on pins 1 and 2 of the op amp, reading ohms, and you should see around 32k again with the Drive pot counterclockwise, and somewhere around 500k with it clockwise. The difference between 900k and ~500k is one diode vs. two diodes. If all that's good, your feedback loop with the Drive pot and three diodes would seem to be wired correctly.

Does the output level change at all when you turn the Drive pot? Does the distortion level change at all? Is the signal clean (like the Drive pot is set low) or distorted (like it's set high)?

[BramP]

On pin 2 of the opamp I see the resistance varying between 32k with the drive pot all the way closed and about 900k with drive pot all the way open.

Right, no power connected for these measurements.

Reverse the leads of the multimeter on pins 1 and 2 of the op amp, reading ohms, and you should see around 32k again with the Drive pot counterclockwise, and somewhere around 500k with it clockwise. The difference between 900k and ~500k is one diode vs. two diodes. If all that's good, your feedback loop with the Drive pot and three diodes would seem to be wired correctly.

Does the output level change at all when you turn the Drive pot? Does the distortion level change at all? Is the signal clean (like the Drive pot is set low) or distorted (like it's set high)?

Ok I seem to be getting the same measurements both ways, around 900k/1M ohm, so this would suggest there's something wrong in the diode arrangement?

The output level doesn't change, it passes a clean guitar signal, turning up the drive does give some additional noise but no distortion of the signal.

[mauman]

Have you tried using a higher level input signal? This pedal doesn't have a tremendous amount of gain, but you should hear a difference with the Drive pot turned up. If that's not the issue, then yeah, there's something amiss in the feedback loop of the op amp from pin 1 to pin 2, the increasing resistance of the drive pot when you turn it up is not adding gain. I can't see anything on the photos, but it has to be right there - you have clean signal in and out of the board, the tone and volume pots work correctly, it's just the drive function. I'd focus on the area outlined in the pics below, resolder those joints (the 3 diodes, 33k resistor, op amp pins 1-2-3, wiring to/from the drive pot), check again for good cuts and no shorts. Then maybe replace the diodes. (your op amp pins 1-2-3 are shown as 7-6-5 on the original schematic, whoever laid out the vero just swapped the two halves of the op amp.)

Schematic marked

Vero marked

SD-1 Front marked.jpg (83.93 KiB) Viewed 228 times

[BramP]

It's working! I resoldered and doublechecked all the parts that you marked in the drive section, turned out that one tiny strand of wire of the link next to diode D3 was connected through the not-so-tidy drilled cut above it to this diode. Resoldered and now it's working perfectly, thank you for helping me out!

So, lessons learned; make tidier cuts and cut the wires as short as possible to avoid wandering strands

This is one of a couple of 'classic' circuits I'm trying to build, including a Rat2, a Big Muff Ram's Head and a Univox Superfuzz so I might have some more questions the coming days at least I know some practical building errors to avoid now.

[mauman]

Congrats! We've all been there, it's part of pedal building.

There's lots of discussion, analysis and mods on those classic circuits on the forum, be sure to search for relevant threads here to get ideas. The search engine up in the top right corner here is quite good.