Broken SD1

[Luke JD]

Hey all,

I picked up a Boss SD1 a while back from ebay (a bit of a gamble, I know!) and whilst it worked for a week or so, it's now bitten the dust and won't output any signal (but will output if bypassed!).

I enquired at PMT Bristol into getting it repaired, but they said the price of the repair is more than the value of the pedal (or close enough to it, to justify buying a new pedal instead).

I'm studying a HNC in electronic engineering atm, so I'm more than happy to take it apart and have a tinker. Is it as simple as powering it up, putting a voltmeter on it and seeing which component has failed, and then re-soldering it?

All the best,

Luke

[Hugh Robjohns]

Yes... It could be that simple...

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...or it might involve more investigative fault-finding skills.

[zenguitar]

Have a chat with your tutor and discuss how he/she might help you use it as a project to develop your trouble shooting skills.

Win/win.

Andy

[Wonks]

From photos on the web, it looks like it still uses discrete components, so fault finding and replacement of parts is relatively easy.

Here's just one of several sites where the circuit is shown and semi-explained.

https://www.hobby-hour.com/electronics/s/sd1-super-overdrive.php

As you can currently get one new for less than £50, I can see why it's not really viable to have it repaired by someone else.

Note that there are quite a few DIY mods published to the DS1, so yours might be one that someone has played about with. Which may be the reason that it's failed. So first check for variations against the basic circuit.

[Luke JD]

Thanks very much for your replies, particularly the circuit board schematic! I'm going to pick up a multimeter on payday and I'll see if I can isolate the problem keen to learn so I can fix my own pedals in the future if I need to

[The Bunk]

If it's any help I've got one that I'm not using and haven't done so in years...happy to offload if it comes to it. Mind you I *do* need to check it's working first of course...!

[Luke JD]

Thanks for all the replies, really awesome

I've opened this baby up (without a multimeter) just to gauge what's up and I think I've found the issue...

This is a pic of the circuit board - https://drive.google.com/open?id=1VwjKE ... l6kFeEvAnX

Compared to the schematic provided by Wonks https://drive.google.com/open?id=1lTl55 ... CTKzIM7Wb6, it looks like there's a 470 Ohm resistor and a diode that have been removed by the previous owner

If it's any help I've got one that I'm not using and haven't done so in years...happy to offload if it comes to it. Mind you I *do* need to check it's working first of course...!

That's very kind of you, if you still have it, and it still works, I'd be keen to take it off of your hands!

[Sam Spoons]

They've been bridged, they look as if they are just there to protect the pedal from incorrect DC supply. Without them it should still work but a wrongly polarised or too hot a PSU may have taken out some other components.

[Luke JD]

They've been bridged, they look as if they are just there to protect the pedal from incorrect DC supply. Without them it should still work but a wrongly polarised or too hot a PSU may have taken out some other components.

Ah okay, so some of the other components have probably blown...

[Wonks]

Q4 at the bottom of the photo looks like it may be split with an orange crack (though it may just be an orange marker). But I can't see any obvious damage to any of the other components.

[The Bunk]

That's very kind of you, if you still have it, and it still works, I'd be keen to take it off of your hands!

Will give it a test tonight and let you know...

[Hugh Robjohns]

Ah okay, so some of the other components have probably blown...

Not necessarily. Zener D1 is there to clamp the input voltage to something sensible and would offer some protection against an excessive input.

And that schematic seems a little odd to me in that the missing diode D3 appears to be shown facing the wrong direction anyway!

And those two replacement wire links look like factory fits to me, rather than a later user modification. Few user mods are that neat, and there's no sign of solder reflow/flux seepage through the holes either.

I'd get a multimeter on the circuit board and start by checking the supply rails. You should then be able to check you're getting a signal voltage through each stage of the drive circuit, and you can check the mute/bypass FET control voltages from the bi-stable.

but if the bypass is working, then you know the signal path through Q5, Q2, and Q6 is all working, and if the bypass toggles then the Q3/Q4 bistable must be working too.

So that just leaves the IC and Q1, and the level pot VR2 as the most likely points of failure.

4558s are normally pretty robust devices, and this one is well protected from the outside world, so I'd be surprised if that's the problem, unless you had a power rail spike.... but send in a steady test tone and see what rms voltage you get on pin 7 and then pin 1...

H

[The Bunk]

That's very kind of you, if you still have it, and it still works, I'd be keen to take it off of your hands!

Will give it a test tonight and let you know...

Well it works; let me know if / how you want to proceed!! (which I presume is probably best done via PM...?) No pressure...

[Luke JD]

Ah okay, so some of the other components have probably blown...

Not necessarily. Zener D1 is there to clamp the input voltage to something sensible and would offer some protection against an excessive input.

And that schematic seems a little odd to me in that the missing diode D3 appears to be shown facing the wrong direction anyway!

And those two replacement wire links look like factory fits to me, rather than a later user modification. Few user mods are that neat, and there's no sign of solder reflow/flux seepage through the holes either.

I'd get a multimeter on the circuit board and start by checking the supply rails. You should then be able to check you're getting a signal voltage through each stage of the drive circuit, and you can check the mute/bypass FET control voltages from the bi-stable.

but if the bypass is working, then you know the signal path through Q5, Q2, and Q6 is all working, and if the bypass toggles then the Q3/Q4 bistable must be working too.

So that just leaves the IC and Q1, and the level pot VR2 as the most likely points of failure.

4558s are normally pretty robust devices, and this one is well protected from the outside world, so I'd be surprised if that's the problem, unless you had a power rail spike.... but send in a steady test tone and see what rms voltage you get on pin 7 and then pin 1...

H

Damn dude, that's impressive! Thank you. I'll have a try