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Korg DW-8000 No Sound - Need Help

 
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prouser



Joined: 09 Nov 2019
Posts: 4

PostPosted: Sat Nov 09, 2019 7:46 pm    Post subject: Korg DW-8000 No Sound - Need Help Reply with quote

Dear fellow GearHeads,

I recently acquired a Korg DW-8000 with no sound, thinking it was just an easy fix because of the well know battery issue. Unfortunately it wasnt! Unit powers on, all leds work, you can change different patches, arp seems to work, Midi i/o works it just doesnt generate any sound.

I replaced the battery, cleared the old presents (5+8 power on) and loaded factory presets (84 MIDI channel is set to 1 and 85 Enable is set to 2 (All)... but still no sound. Having basic understanding of electronics I took out my multimeter and started testing.

First I started with testing the power supply

KLM 661

1. Gnd
2. -5.00
3. 5.00
4. -6.99
5. 4.99
6. 15.11 (Service Manual says should be 11v)

KLM 662

1. Gnd
2. -5.00
3. 5.01

KLM 663

1. Gnd
2. 5.01
3. -.5 (Service Manual says should be -.6)
4. 15.11 (Service Manual says should be 11v)

Since the 11v is unregulated i dont thing the voltage being a little over should be causing the problem I am experiencing but regardless I visually inspected the caps and they seem to be fine. My next step was to remove the caps and test them but I dont think its the problem because the next Testing step I took is very evident there is an issue else where.

Next I moved on to Test Mode 1 (5+6 Power On) - D/A Converted CV Check KLM 661 and this is where I fear the problem lies.

Testing TP- AG to TP-Ref 1 = 3.46

But when I try to adjust VR1 to get same value TP-AG to TP-CV = 0

No matter what I do its "0"

Unfortunately the service manual doesnt really say what to do if its zerp and as I mentioned before I only have the basic knowledge of electronics. I really would like to learn more on how to diagnose and fix this as I love vintage gear and buy them all the time and would like to be able to service them myself.

So if anyone can point me in the right direction and suggest what I should test next and or replace would be much appreciated.
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voip
Platinum Member


Joined: 27 Nov 2014
Posts: 3758

PostPosted: Sun Nov 10, 2019 9:43 pm    Post subject: Reply with quote

With the DW-8000 in Test Mode, look at the voltages at the various points in the chain, starting with RA1, pin 6, which presumably should be non-zero in this part of Test Mode. Then look at IC9, pin 1; does this change as VR1 is adjusted, and likewise then on to IC9 pin 7. Then pin 14 of IC12, followed by pin 4 of IC12, which should be the same as pin 2 of IC11, since these are on the same net.

.
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prouser



Joined: 09 Nov 2019
Posts: 4

PostPosted: Mon Nov 11, 2019 6:49 pm    Post subject: Reply with quote

voip wrote:
With the DW-8000 in Test Mode, look at the voltages at the various points in the chain, starting with RA1, pin 6, which presumably should be non-zero in this part of Test Mode. Then look at IC9, pin 1; does this change as VR1 is adjusted, and likewise then on to IC9 pin 7. Then pin 14 of IC12, followed by pin 4 of IC12, which should be the same as pin 2 of IC11, since these are on the same net.

.


Hi voip, thank you for your input. Putting the DW8000 in test mode 1 and with the negative lead attached to TP-AG I did the following tests.


RA1
1. 0
2. 4.96
3. 4.96
4. 4.96
5. 4.96
6. 0 (adjusting VR1 i can get this up to .018)

IC9
1. -3.46 (adjusted VR1 to get that reading)
2. 0
3. 0
4. -5
5. 4.96
6. 3.46
7. 0
8. 0

IC11
1. 0
2. 0
3. 0
4. -5
5. 4.96
6. 3.44
7. 0
8. 0

IC12
1. 0
2. 0
3. -7
4. 0
5. 0.15
6. 0.15
7. 0.15
8. 0
9. -2.85
10. 0
11. 0
12. 4.96
13. 0.15
14. 0.15
15. 0.15
16. 0.15

DAC08 (IC42)
1. 0
2. -0.48
3. -7
4. -0.48
5. 1.8
6. 1.8
7. 2.0
8. 2.0
9. -5
10. 0
11. 0
12. 4.96
13. 1.8
14. 1.8
15. 1.8
16. 1.8
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voip
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Joined: 27 Nov 2014
Posts: 3758

PostPosted: Mon Nov 11, 2019 8:52 pm    Post subject: Reply with quote

Thanks for the detailed measurements. They are helpful.

The parts are so old, getting data on some of them is not easy. A good starting point to further investigation would be to find out why RA1 pin 6 is at 0V, and go on from there. The following may seem a bit of a ramble, but it is meant to show the logical flow of reasoning behind troubleshooting circuits, and the fact that all may not be what it might appear to be at first.

The schematic would suggest that RA1 is probably an R2R ladder resistor array, and is being used as a 4-bit D to A converter. Pins 2,3, 4 and 5 are the inputs. Since these are all high, at 4.96V, the unloaded output on pin 6 should be 4.96V-(4.96/16), or 4.65V. However, RA1 is not in a load-free condition since its output on pin 6 passes to op-amp IC9, in inverting configuration, with pin 3 connected to ground, which means that IC9 pin 2 is acting as a virtual earth, i.e. should be at or close to 0V. The fact that IC9 op-amp "a" pin 1 output can be adjusted to -3.46V suggests this part of the circuit is working. However, IC9 op-amp "b" is also in a inverting configuration (having unity gain, since both feedback resistors are equal (10k)), so pin 6 of IC 9 should also be a virtual ground at 0V, and pin 7 at +3.46V. The voltages you have given for IC 9 pins 6 and 7 are the other way round. Could it be that these have been mixed up?

However, notice that IC9 pin 8 and IC11 pin 8 are the positive supplies for these op-amps yet both have been measured at 0V and are shown in the schematic connected to net "[F]". But net "[F]" should be at +5V. The inputs to RA1 are at 4.96V, and come from IC10, a hex buffer whose supply (pin 1) is also connected to net "[F]", suggesting IC10 is getting power, but ICs 9 and 11 are not. Or are they, since IC9 has a pin at +3.46V, which would not be possible with a negative supply of -5V and a positive supply of 0V. Op amp outputs do not normally go beyond the supply rails. It is worth double checking the +ve supply pins on both op-amps before going further.


.
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prouser



Joined: 09 Nov 2019
Posts: 4

PostPosted: Mon Nov 11, 2019 10:13 pm    Post subject: Reply with quote

voip wrote:
Thanks for the detailed measurements. They are helpful.

However, notice that IC9 pin 8 and IC11 pin 8 are the positive supplies for these op-amps yet both have been measured at 0V and are shown in the schematic connected to net "[F]". But net "[F]" should be at +5V. The inputs to RA1 are at 4.96V, and come from IC10, a hex buffer whose supply (pin 1) is also connected to net "[F]", suggesting IC10 is getting power, but ICs 9 and 11 are not. Or are they, since IC9 has a pin at +3.46V, which would not be possible with a negative supply of -5V and a positive supply of 0V. Op amp outputs do not normally go beyond the supply rails. It is worth double checking the +ve supply pins on both op-amps before going further.


.


OMG I am such a bonehead... I numbered my ICs pins wrong Brick wall

Here is the correct numbering

RA1 
1. 0
2. 4.96
3. 4.96
4. 4.96
5. 4.96
6. 0 (adjusting VR1 i can get this up to .018)

IC9
1. -3.46
2. 0
3. 0
4. -5
5. 0
6. 0
7. 3.46
8. 4.96

IC10
1. 4.96
2. 4.96
3. 4.96
4. 4.96
5. 4.96
6. 4.96
7. 4.96
8. 0
9. 4.96
10. 4.96
11. 4.96
12. 4.96
13. 0
14. 4.96
15. 4.96
16. 0

IC11
1. 0
2. 0
3. 0
4. -5
5. 0
6. 0
7. 3.44
8. 4.96

IC12
1. 0
2. 0
3. -7
4. 0
5. 0.15
6. 0.15
7. 0.15
8. 0
9. 0.15
10. 0.15
11. 0.15
12. 0.15
13. 4.96
14. 0
15. 0
16. -2.85

DAC08 (IC42)
1. 0
2. -0.48
3. -7
4. -0.48
5. 1.8
6. 1.8
7. 2.0
8. 2.0
9. 1.8
10. 1.8
11. 1.8
12. 1.8
13. 4.96
14. 0
15. 0
16. -5

CN2
1. 0
2. -5
3. 4.96
4. -7
5. 4.98
6. 11.8
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voip
Platinum Member


Joined: 27 Nov 2014
Posts: 3758

PostPosted: Thu Nov 14, 2019 10:17 pm    Post subject: Reply with quote

So, based on the measured voltage levels, it looks like the D/A converter, IC12, has all its digital inputs set low. These come from IC19, which is an octal latch, which takes its inputs from a 8-bit bus coming from the processor. The state of the databus is latched by IC19, whose outputs are permanently enabled by the grounding of IC19 pin 1. The latch is set by an appropriately-timed clock pulse coming from one of the logic gates in IC20 (pin 6) and, based on the expected value at TP-CV, I would expect IC19's outputs to all be high in this particular test mode, which they're not. To understand why this is, you will need to use a scope and look at what IC19 is doing, and whether it and IC20 are behaving themselves and, indeed, whether the databus from the processor contains any data.

The 8-bit data bus also goes to a custom gate array, which forms part of the DWGS oscillators. There is a useful article describing how this works at:

https://acreil.wordpress.com/2017/10/26/korgs-dwgs-synthesizers-1984/

.
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prouser



Joined: 09 Nov 2019
Posts: 4

PostPosted: Mon Nov 18, 2019 3:02 pm    Post subject: Reply with quote

Thank you voip, i will be ordering a new scope so I can perform the tests you recommended.
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