Korg SDD-1000 fault - stuck in bypass?

[Saltech]

I'm looking for schematics or any tips for repairing a faulty SDD1000.
The unit powers up but appears to be stuck in bypass mode - only the clean input signal is being output from all 3 outputs, no effects / wet signal. There is a bypass push switch and its LED lights up when it is engaged, and there is an output mixer section with "Direct" and "Effect" level pots - but the output remains the same whatever these are set to, with just the original input signal at all outputs and no hint of any delay signal.
The reason I think it might be stuck in bypass (if such a thing is possible) is the mirroring of the clean input signal to all outputs (Direct, +Mix, -Mix) at once, which is what is supposed to happen in bypass mode. If there was a signal at the Direct output and an absence of any signal at the Mix outputs, then that would suggest a fault in the fx processor, especially if it could be toggled between that state and the bypass mode (same signal at all outputs).
Inside the unit everything is clean with no sign of burns or damage. Any ideas? I'm guessing that there must be a switching circuit for the mix outputs that swaps between the clean input / through signal and the FX signals, but that's as far as I've got. Any tips on tracing the signals or finding the switching circuit would be most welcome.

 

[rock soderstrom]

Any schematics available?

 

[Saltech]

I've not found a service manual yet for the SDD1000 (I'm still looking), except a hard copy on ebay.
Here are service manuals for the SDD2000 and SDD3000, which probably (?)  use similar circuitry.

http://www.florian-anwander.de/korg_sdd3000/Korg_SDD-3000_ServiceManual.pdf

https://elsonidodeu2.files.wordpress.com/2011/02/sdd2000-service-manual.pdf

Edit, I just found the service manual for the SDD-1200, which may be a closer relative:
https://www.manualslib.com/manual/1055179/Korg-Sdd-1200.html

 

[Walrus]

Open circuit Bypass relay, or faulty switching circuitry.

 

[rock soderstrom]

Open circuit Bypass relay, or faulty switching circuitry.

Yes, or just a broken footpedal mute jack. 

I would check the PSU first and then I would start to  trace signals with the right schematic.

I have no idea how close  the different SDD models are, here are some infomation from muffwiggler forum:

"SDD-1000
no MIDI, no programmability, all knobs
single 12bit delay line 0.5 to 1024 ms 16khz bandwidth
up to 110% feedback, invertible
hi cut filter, variable
single analog LFO frequency/intensity, triangle waveshape only
sample playback with trigger

SDD-1200
no MIDI, no programmability, all knobs
dual 12bit delay lines 0.5 to 1024 ms (stereo or serial) 16khz bandwidth
up to 110% feedback, invertible
hi cut and low cut filter, variable
dual analog LFO frequency/intensity, triangle waveshape only, independent, in phase, or out of phase.
sample playback with trigger
nice rear panel patching for crazy effects like cross feedback or ping pong echoes

SDD-2000
MIDI (in and thru only), 64 programmable patches, rotary encoder with button interface
single 12bit delay line 0.1 to 1092 ms 18khz bandwidth, switchable to 4368 ms at 4.5khz bandwidth
up to 110% feedback, invertible
no filters
TAP TEMPO!
Complimentary outputs only, not true stereo
single digital LFO frequency/intensity, triangle waveshape only. May hear discrete stepping at slow frequencies.
sample playback with trigger, can be scaled from MIDI controller and tuned +/-50ct
NOTE: bypass control is a misnomer, it should be MUTE. It mutes the delay signal but the direct signal is always present in the MIX outputs.

SDD-3000 (original, not reissue)
no MIDI, 9 programmable patches, knobs
single 12bit delay line 0.1 to 1023 ms 17khz bandwidth
up to 110% feedback, invertible
hi cut and lo cut filters, switchable not variable
Complimentary outputs only, not true stereo
single digital LFO with independent frequency/intensity, waveshapes of triangle / square / random or envelope mode. May hear discrete stepping at slow frequencies.
sample playback
CV input for delay time (!)

SDD-3300
MIDI, 64 programmable patches, six sliders for editing with menu interface, MIDI remote editing
triple 12bit delay line 0.5 to 500 ms 16khz bandwidth
up to 110% feedback, invertible
hi cut and lo cut filters
each delay line has dual mixable digital LFOs with independent frequency/intensity (can be phased locked with offsets between delay lines), triangle waveshape only. May hear discrete stepping at slow frequencies.
sample playback with MIDI note trigger
digital audio routing matrix system for insane effects not possible on other multiFX. Hugely rewarding if you do your homework, don't expect instant gratification"

from  https://www.muffwiggler.com/forum/viewtopic.php?p=1613399

 

[Saltech]

There's +15V, -15V and +5V coming from the power supply (not exact, but near enough). There are no relays on any of the boards.  I've had a quick look at the schematic for the SDD1200, which I'm guessing is the closest relative - it's a dual version of the SDD1000 but on a different board, but I'm not knowledgeable enough to recognise or find the bypass switching circuit. The bypass jack works as it should - shorting tip to sleeve engages bypass in the same way as the front button bypass control i.e. it lights up the bypass LED. But it doesn't do any more than that. It's still continuously stuck in bypass mode with the same direct input signal at all 3 outputs -  the only control that does anything is the input gain which works as normal (the level changes at all 3 outputs; Direct, +Mix, -Mix and the input level LEDs indicate the different levels as expected.)
I'll try to trace the signal tomorrow - it would be great to hear a delayed signal coming off the board somewhere, at least it would show the effect is working, but just not routed to the outputs. Any tips for probing with the oscilloscope and listening at the same time - is that possible?

 

[Khron]

Comparing the block diagram with the schematic of the 1200, the bypass switching seems to be performed by a pair of JFETs - F103 and E104, both 2SK381's.

https://www.datasheets360.com/pdf/-1041149361346612493
https://pdf1.alldatasheet.com/datasheet-pdf/view/109492/ETC/2SK381.html
1 = source, 2 = gate, 3 = drain

So there's a chance that either the bypass circuitry isn't able to pull their gates high (positive), or the JFETs themselves are stuck "off" (non-conducting).

C27 & C21, on page 6 of the 1200 service manual (on the bases of Q3 & Q6, 2SA733's), seem to be electrolytics - might be worth replacing those, just in case. I've seen some of those tiny ones fail in some DBX 1066 compressors wreaking a bit of havoc in the limiting circuitry.

If the bypass LEDs work, then at least the comparator stage, and the bistable after it, should still be working, but the stage after that is the one described just above, so... That's two fingers pointing at that "already".

 

[Saltech]

Many thanks for your time and advice, I will study the schematics and try to understand what you've explained and also check those parts - but it won't be for a couple of weeks as I'm on holiday now . I'll report back on any developments.

 

[Khron]

https://www.electronicshub.org/fet-as-a-switch/#FET_Used_as_a_Series_Switch

 

[Saltech]

I haven't been able to relate the SDD1200 schematic to what's inside the case of the SDD1000.
But Korg have been very kind and sent me a link to the SDD1000 service manual:

https://www.dropbox.com/s/dy2fx4kffof0dkr/sdd1000_service_manual.pdf?dl=0

But it's for a limited time only - deleted after 24 hours (approx 3pm UK time tomorrow)

 

[Khron]

Well, the switching seems to be done in a similar manner to its bigger brother. To some extent, anyway.

On page 8, top-right, are two JFETs that do the switching.

The top one (F1) is conducting, hence its gate must be pulled negative, which then routes an attenuated direct signal through IC9(1) into the two Mix+/- outputs.

At the same time, F2 is off (not conducting), so the direct signal that goes through the "Direct" level adjust doesn't reach IC9(1).

The two JFETs can't (or shouldn't be able to be) conducting at the same time, since F1 is driven by an inverted version of F2's gate signal (through Q18).

F3 (in the middle of page 8 ) is driven together with F2, which cuts off the signal going into the high-cut stage, which would then go through the "Effect" level adjust and onwards to the "Mix" outputs.


I'd work my way back from F1's gate (worth checking what DC voltage you have there, and same for F2's gate).

The "fun" part is, that signal seems to be driven from several sources:
- IC3 (middle-bottom of page 8 ), driven by the Bypass signal
- Q16/Q17 (middle of page 8 ), driven by "DOFF"
- Q19/Q20, driven by "SOFF"

These should all be DC signals, so a multimeter should suffice.

The somewhat bad news is that "SOFF" and "DOFF" both come from the main chip, so if those are "stuck", there's not much that can be done. But on the other hand, there's plenty of stuff "in the way" that may well also be at fault.

 

[Saltech]

Thanks for your guidance.
The gate voltages and signals (on my oscilloscope) are as follows:
F1 = 0 V        Input good signal  - Output is an attenuated version of the input (approx 1/8th of the input)
F2 = -2.4 V    Input good signal - Output nothing
F3 = -2.6  V  Input nothing  - Output nothing
F4 = -2.4  V  Input good signal  - Output nothing
The voltage of F3 changes to -2.37 when the bypass switch is activated (the others  are unchanged)
DOFF output on the CPU changes from 0V (Bypass switch in) to 5V (bypass switch out), and makes changes at Q16/17.
SOFF output on the CPU does not change with the bypass switch - stays on 0V, no changes at Q19/20.
So it would seem to have something to do with SOFF.
In the service manual, page 15 section 2 Muting Operation Check, it says there is 5 seconds of bypass when the unit is first switched on.  On the unit I'm working on, the bypass LED does not switch on during the first 5 seconds, and I'm not able to switch the LED on with repeated pressing of the bypass switch during the first 5 seconds, after that the LED will light up.

 

[kazper]

Anyone have the service manual handy and can post it up?

Would really appreciate it.

 

[Colman Jones]

Did this ever get fixed? I am having the exact same problem

 

[Donornit]

Did this ever get fixed? I am having the exact same problem

Check transistor f4..

 

[Donornit]

If anyone is struggling fixing this unit I can guide you through troubleshooting.

 

[jensenmann]

If the service manual is somewhere around then please upload it to our technical documents section.