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Wavestation distortion

 
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Emil_T



Joined: 27 Feb 2018
Posts: 6
Location: Sweden

PostPosted: Wed Feb 28, 2018 9:03 pm    Post subject: Wavestation distortion Reply with quote

I have a wavestation. Some of the sounds is have a distortion that not supposed to be there. I have searched alot in this forum and found the advice to change electrolythic capacitors in the power unit. I have done that now but the problem is still there. When I measure the power unit output I get +/-12V and the +5V sometimes dip to 4V when I start measure (with multimeter) but increases to 5V in a few seconds, somthing strange there i think... What is the next component to change? Transistors? Anyone seen this Before?
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BasedFrequency



Joined: 06 Mar 2018
Posts: 2
Location: canada

PostPosted: Tue Mar 06, 2018 4:40 pm    Post subject: noise Reply with quote

hey it may be the transformer on the board for the screen that is introducing some noise http://www.tellun.com/wavestation/wavestation.html
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voip
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Joined: 27 Nov 2014
Posts: 1643

PostPosted: Tue Mar 06, 2018 7:27 pm    Post subject: Reply with quote

The measurement point was not given, but might have been useful. The variability in the +5V supply could arise because of a number of reasons. However, because the switch mode PSU gets its feedback from the +5V supply, albeit from before inductor L5, a fall in the 5V rail ought to be accompanied by a change in the +/-12V rails. It could be due to variations in current demanded from the 5V supply, by a failing component on one of the other boards, or to dry joints in the PSU PCB or in the cabling connectors, or a malfunctioning EL backlight supply circuit, or even the backlight itself. Disconneting the backlight is an easy thing to try. An understanding of switched mode PSU operation and an oscilloscope will help to diagnose the problem. Operating the PSU with dummy load resistors might be useful in identifying if the issue rests with the PSU itself. 6.8 Ohms 5 watt across 5V, and 82 Ohms 2W for each of the 12V rails.
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Emil_T



Joined: 27 Feb 2018
Posts: 6
Location: Sweden

PostPosted: Fri Mar 09, 2018 10:45 pm    Post subject: Reply with quote

I have done some more tests now. First I disconnected the display backlight (EL) cable. Still distorsion on some of the sounds.
I built a simple test rig with 100ohm 3W resistors connected to +/-12V and a 10ohm 5W for the 5V. 0.12A and still +/-12V, 0.5A and 5V. Not max load but almost. I did the measurement in the end of the cable connected to the dummy resistors, not on the card. So I think the power supply is ok.
Now I think about find a sin-wave in the sytheziser and try to follow it from the DAC to output. Is it possible?
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T3owner
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Joined: 17 Jun 2011
Posts: 325
Location: USA

PostPosted: Sat Mar 10, 2018 4:42 pm    Post subject: Re: Wavestation distortion Reply with quote

Emil_T wrote:
I have a wavestation. Some of the sounds is have a distortion that not supposed to be there. [...] Transistors? Anyone seen this Before?

A somewhat common problem is output muting transistors becoming leaky. On the original Wavestation they are Q4 ~ Q7, 2SC2878, on the KLM-1416 DAC board.

Check for DC on the emitter leads - if measureable at all, it should be very low, certainly no more than 50 millivolts. If you're not sure, the transistors can be removed temporarily - check again for distortion. Just be aware that turn-on and turn-off noise will occur with the transistors missing, so keep amp volume level down when turning the synth on/off.

These transistors have special characteristics (see data sheet) - if replacement is needed, be careful to use the original type or exact equivalents.

https://www.electroschematics.com/wp-content/uploads/2012/11/2sc2878-datasheet.pdf
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Emil_T



Joined: 27 Feb 2018
Posts: 6
Location: Sweden

PostPosted: Sat Mar 10, 2018 10:20 pm    Post subject: Reply with quote

Ok I try to check the transistors. Thanks

What will happend if I adjust the VR1 potentiometer on the DAC-board KLM-1416? It looks like an amp after the D/A converter. Maybe the signal is to high in to the analog part?
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T3owner
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Joined: 17 Jun 2011
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Location: USA

PostPosted: Sun Mar 11, 2018 12:36 am    Post subject: Reply with quote

Emil_T wrote:
Ok I try to check the transistors. Thanks

What will happend if I adjust the VR1 potentiometer on the DAC-board KLM-1416? It looks like an amp after the D/A converter. Maybe the signal is to high in to the analog part?

VR1 is the DAC offset adjustment. I suggest that you check those transistors before attempting to "adjust" it.

Do you have the service manual?
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Emil_T



Joined: 27 Feb 2018
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Location: Sweden

PostPosted: Sat Mar 17, 2018 4:51 pm    Post subject: Reply with quote

Tried to do some measurements. Emitter to gnd on Q4 and Q5 0mV. Q6 and Q7 60mV. So there is something...
But when I connect output 3-4 to the mixer there is distorsion on some sounds, same as output 1-2 and phones.
I think I change the transistors anyway...
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T3owner
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Joined: 17 Jun 2011
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PostPosted: Sat Mar 17, 2018 6:49 pm    Post subject: Reply with quote

Emil_T wrote:
Tried to do some measurements. Emitter to gnd on Q4 and Q5 0mV. Q6 and Q7 60mV. So there is something...
But when I connect output 3-4 to the mixer there is distorsion on some sounds, same as output 1-2 and phones.
I think I change the transistors anyway...

As I mentioned previously, you can just remove those transistors temporarily. If that doesn't help, then the casue is something else, and you can put the transistors back.
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Mellontikos
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Joined: 01 Feb 2012
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PostPosted: Thu May 24, 2018 8:03 am    Post subject: Reply with quote

Those old boards are known for having Op Amp chips start to fail, and this will give you distortion across the analog signal path (after the DAC)

Might want to check those. They are a pain in the <beep> to replace.
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