Skrydstrup R&D - Overdrive ODR2

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Re: Skrydstrup R&D - Overdrive ODR2

Postby Intripped » 03 Dec 2012, 16:07

here is my try on reverse engineering the pedal
many hypothesis, but something to start from
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Re: Skrydstrup R&D - Overdrive ODR2

Postby Nocentelli » 03 Dec 2012, 17:36

CableBling wrote:They only ever seem to fit the case caps (10nF 1kv I think) to output sockets..... never to the Input socket which is wire grounded as normal.
Is it really just some misguided mojo?.... ie. Cornish did it (for whatever reason) and Skrydstrup followed suit?


The whole construction looks very Cornish-ish, so I'm inclined to think they just lifted it wholesale: This is from page 4 of the Cornish G2 thread -

soulsonic wrote:
culturejam wrote:
mictester wrote:(10n cap between chassis and output ground) It's a fairly common way (especially in the RF engineering world) to ground a panel. It gives DC isolation, but the screening properties of the case are preserved.

Wouldn't that mean that the cap would always been in the signal path, regardless of whether or not the G2 was bypassed?

Yes, but if you look at the schematic, you see that the sleeve connection of the output jack still connects directly to the chassis ground via the ground path of the circuit board that it's connected to. The output jack connects to the chassis via the cap, make sure the chassis screens the signal sufficiently, but the cap is used to make sure there are no DC ground loops caused by any difference in potential which may exist between the input and output jack grounds.

At least, I think that's what is going on! :lol:
brownwhopping wrote:How can I learn by reading threads an making circuits, when some day I can see a lawsuit or somebody beat me in the face for that?
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Re: Skrydstrup R&D - Overdrive ODR2

Postby Intripped » 04 Dec 2012, 13:34

CableBling - could you please make a note of the capacitors values? (or take some other detailed pics from different angles?)
also: the transistor is readable or it has been erased like the ICs?
and finally, if i'm not asking too much, the potentiometers values? ( just a measure with multimeter is ok)

what do you guys think of the two inductances at the IN and OUT of the pedal? - are they inductances or not? what is their purpose? useful\useless? -

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Re: Skrydstrup R&D - Overdrive ODR2

Postby CableBling » 04 Dec 2012, 22:28

As requested some close ups of the caps + a higher res of the whole board.
Impressive reverse engineering skills!!
I will get a meter on the pots later..
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Re: Skrydstrup R&D - Overdrive ODR2

Postby CableBling » 04 Dec 2012, 23:08

Ok, I've gone the extra mile and popped out the pots to see their values:

Gain
C50K

Tone
B25K

Level
A100K

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Re: Skrydstrup R&D - Overdrive ODR2

Postby ik6gpy » 05 Dec 2012, 09:24

Thanks Cablebling,
i've corrected the schematic as the there where some mistakes around the tone control section. Now it makes sense.
As someone else stated, this is a Cornish SS-2 clone, which is a Mxr Distortion+ derivative with a tone control and a volume recovery stage.
Enjoy!
Matteo
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Re: Skrydstrup R&D - Overdrive ODR2

Postby Intripped » 05 Dec 2012, 12:18

thanks CableBling - beautiful hi-res pics!
and thanks Matteo!
yes i was uncertain about C19 and C12 (are these the only corrections you made? - it would have been better if you had marked the corrections in red, just to point them out in a glance)

so these ground connections of C19 and C12 are in the top layer of the PCB, hidden by the capacitors themselves - in fact if you look at the bottom layer there are no ground connections. this was making me puzzled, because the 2 caps seemed connected to nothing.

the transistor has been erased... BC549 as in the G2 schematic?

one last note about the schematic: the IC type is still uncertain, 741 is only a guess.
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Re: Skrydstrup R&D - Overdrive ODR2

Postby ik6gpy » 05 Dec 2012, 12:53

Yes, C19 and C12 are the only corrections i made.
The rest of the circuit seems coherent with the Pcb.
741 is a guess but 99% i'm sure it's the right one (well, at least for the first IC). I think Cornish, and then Skrydstrup, have choosen the 741 due to its bandwidth limitation and its slew rate. But to make it simpler, i think they choose it just because the original Mxr had that opamp in it...
Not sure about the second IC, but if i had to build it, i'd try a Tl071 or any low noise single opamp.

To me the buffer transistor type isn't much important to know. You can put in there any low noise Npn available (BC550C, BC549C, Mpsa18...).
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Re: Skrydstrup R&D - Overdrive ODR2

Postby Intripped » 05 Dec 2012, 15:04

ok thanks

i've double-checked the new caps' values in the corrected schematic and they are ok (you missed C2 220uF)
*C11 and C19 are still uncertain * can't read the values from the pictures

what do you think of L1 and L2?

PS: the clipping diodes should be AA117 germanium
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Re: Skrydstrup R&D - Overdrive ODR2

Postby Dirk_Hendrik » 05 Dec 2012, 16:00

Could you recheck the output stage? I don't get how C3 is involved in there and how it connects to R14 and 15. I find R15 hard to understand in that position anyway. I'd expect it to go to ground instad of R14.
Sorry. Plain out of planes.

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Re: Skrydstrup R&D - Overdrive ODR2

Postby Intripped » 05 Dec 2012, 16:15

this is the picture of my tracing
BLUE lines are bottom traces
PINK are top traces (with some guesses)
GREEN are caps
BLACK dots are ground connections
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Re: Skrydstrup R&D - Overdrive ODR2

Postby ik6gpy » 05 Dec 2012, 17:02

C11 and C19 are Wima MKS4 just like C12. In my experience Wima caps with the same dimensions also share the same values. We can see that C12 is 10nF from the pictures, so C11 and C19 must be 10nF too.

To Dirk: the output stage is fine. It's just a bootstrap circuit. C3 is there just to raise up the impedence loss with the bias path (R14 and R15).
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Re: Skrydstrup R&D - Overdrive ODR2

Postby ik6gpy » 05 Dec 2012, 17:18

Here's the output stage AC Analysis...
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Re: Skrydstrup R&D - Overdrive ODR2

Postby Dirk_Hendrik » 05 Dec 2012, 17:30

ik6gpy wrote:To Dirk: the output stage is fine. It's just a bootstrap circuit. C3 is there just to raise up the impedence loss with the bias path (R14 and R15).

Then why do I see C3 running to R4 and R5 on the PCB pic?
Personally I would consider bootstrapping an opamp rather bullshitty.
Sorry. Plain out of planes.

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Re: Skrydstrup R&D - Overdrive ODR2

Postby ik6gpy » 05 Dec 2012, 17:36

http://www.hqew.net/circuit-diagram/Op$ ... _4769.html This is the first example i've found on google...

The Toptone Light Drive (a Cornish SS2 clone too) has got the same output stage. It can't be a coincidence.

If you look carefully, you'll see that C3 do not run to R4 and R5... The traces are very thin on the Pcb, you must enlarge the pictures
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Re: Skrydstrup R&D - Overdrive ODR2

Postby triplesevener » 05 Dec 2012, 18:24

Could L1 and L2 be ferrite beads? I'd assume they're in place to reduce high frequency noise, as this sucker has an inordinate number of RF noise reducers.
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Re: Skrydstrup R&D - Overdrive ODR2

Postby Dirk_Hendrik » 05 Dec 2012, 18:57

ik6gpy wrote:If you look carefully, you'll see that C3 do not run to R4 and R5... The traces are very thin on the Pcb, you must enlarge the pictures


Ah! Got it!... odd.
Sorry. Plain out of planes.

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Re: Skrydstrup R&D - Overdrive ODR2

Postby silverface » 05 Dec 2012, 19:17

Intripped wrote:PS: the clipping diodes should be AA117 germanium


Could also be AA112, not sure.
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Re: Skrydstrup R&D - Overdrive ODR2

Postby Intripped » 05 Dec 2012, 19:54

triplesevener wrote:Could L1 and L2 be ferrite beads? I'd assume they're in place to reduce high frequency noise, as this sucker has an inordinate number of RF noise reducers.


i think you're right
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Re: Skrydstrup R&D - Overdrive ODR2

Postby CableBling » 05 Dec 2012, 19:57

silverface wrote:
Intripped wrote:PS: the clipping diodes should be AA117 germanium


Could also be AA112, not sure.


AA117's :D
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