DSR-1000 (1987) Audio Problem / Static background noise is much too loud

[Streets]

Hello!

I´ve just bought a vintage Yamaha DSR-1000 Keyboard.
It´s overall in good shape, but has one significant problem.

There´s a static/white noise comming form all Audio-Outputs while the Keyboard is on. The Sounds/Synth/Rythm ect. works.
This static noise is much too loud. It is the same noise you can hear from any speakers with volume at max.
But here you can hear it very loud, even the Master Volume is at 50%.
It comes to all Audio Outs: AUX-Out, Headphones and even the build in speakers.
I know, a bit of noise is alwasy normal, but not on that loud.

The DSR-1000 has several Volume Controls.
One for the Master Volume and additional ones for Synth-Volume and Rythm Volume.
If you turn the Master Volume up or down, so does the volume of the noise.
But if you turn the other Volume Controls for Synth oder Rythm, the noise does not chance.

And there is one more thing. If you wait about 10-20 seconds, you can hear a short "PLOP" comming throu the audio-out.
Then, the noise is gone...until you hit a key to play a tone. The noise comes back.
Then you can wait again for 10-20 seconds....ect.

Does anyone ever heard of such a thing?

I heard about many things that might cause this problem:
- Old or bad capacitors
- Bad OP-AMP Chips
- Bad DAC Chips
- Bad Reverb Chip
and so on.

By the way. It is not the Power-Supply unit. I´ve testet 2 of them. Both are OK and working (with other Keyboards,too)

 

[happyrat1]

Most likely it is old leaky electrolytic capacitors and noisy pot controls.

The pots can be fixed by cleaning them.

The caps pretty much have to be replaced.

Gary

 

[Streets]

I´ve done a complete recap. No change
Cleaned the pots, too .

 

[happyrat1]

Only other thing I can think of is using a scope to trace the signal path to hopefully find a cold or cracked solder joint or a defective semiconductor.

Gary

 

[happyrat1]

BTW, it really doesn't take an expensive scope to do that sort of work. One of these would work.

https://www.amazon.com/JYETech-Battery-Powered-Oscilloscope-Display-DSO112A/dp/B01BMYS10S/

And you'd need one of these probes.

https://www.amazon.com/RioRand-probe-Attenuation-ratio-Oscilloscope/dp/B00JRP9DHU/

Gary

 

[Streets]

Only other thing I can think of is using a scope to trace the signal path to hopefully find a cold or cracked solder joint or a defective semiconductor.

Thanks.


I´ve resolderd all joints with no success.
Then It must be an IC. (There is nothing more left to change
Maybe the OpAmps. 4558DVs.
I´ll change them to 4558Ds. I wonder if can use 4559Ds as well.

 

[happyrat1]

Before you start randomly desoldering and replacing ICs I'd do a signal trace and see if I could locate the faulty chip.

Like you said, it could be a faulty DSP as well.

Gary

 

[Streets]

I found out somthing more!
Beside the noise, I thought the sound was overall very dull,...no trebble..flat....
So I pluged in the Yamaha PSR90, with the exact FM Chip, the OPZ.
The Instrument Sound ist the same, but the PSR-90 is much more brighter, bigger in Sound.
I think the frequence spectrum on the DSR-1000 is cutted, too.

I don´t know how to locate a bad chip with my multimeter
But I know someone who can. And then I hope I will finally learn this (for the next time)

 

[happyrat1]

Like I said earlier. Buy a $50 pocket oscilloscope and track the noise in circuit.

There's no way in hell you're going to track a noisy chip with a DVOM.

Simply not doable.

Gary

 

[happyrat1]

Gary

 

[Some-E]

It's clearly a feature. Caps don't do noise. Pots don't do noise.

And I do have one. My first search gave this thread. I want to fix the noise. I don't believe the FM chip generates noise much.

Schematics and output amplifier considerations. Need to know the schematics and then plug in your own output before the final amp. Carefully, not to break the FM or other chips.

 

[Some-E]

I opened my DSR-1000 yesterday. I did search for service manual from the net, but couldn't find, so I thought I just reverse-engineer the analog audio board.

What I found before opening was that AUX OUT has more noise than the other output (where it reads "OPTIONAL"). So I *think* that summing the AUX IN and synth sounds add another JRC 4558DV op-amp and they make (most of) the noise, as suggested before already. Not verified yet.

At first I added contact spray to the master volume slider. It didn't lose all the non-linearity, but it was not crackling. I got the feeling the master volume is done by a VCA, and yes it is (M5241L). It's a stereo VCA, control inputs 1 and 2 are tied together (no panning in this device, good for fixed lef/right balance). Another thing I found is the BBD (analog shift register) MN3206, which is used to create a tiny delay for the chorus (I believe it's for chorus, as I don't see any other reason to delay analog signal here). The 3206 is clocked by MN3102. That clock is surely modulated for the chorus purposes. I just haven't examined it, as the main goal is to find the sources of the noise.

After the VCA there is just one op-amp before OUTPUT. Most optimal test is to connect the VCA output directly somewhere for listening. If there is a lot of noise, then I have to bypass the VCA if I want to get rid of the noise.

For the op-amp noise assumption I could go easy and just replace the JRC 4558DV op-amps with NE5532 which I already have available, but I want to understand the board more first. I'll get back to this.

 

[Some-E]

I found out major cause for noisy output. The digital board shield (the shiny plate covering the whole board) was not properly connected to the ground, to the frame underneath the keyboard.

The fix: tighten the three small screws holding the shield. For a precaution, I also added some aluminium tape over the screw holes for better electrical contact. The shield is not metal, it's just covered with aluminium layer. The other side is soft and has compressed over time so the screws don't have the original pressure agains the shield and thus poor/flaky electrical contact.

The result: I got the noise down approx. 10 dB (didn't measure, just an approximate figure from experience), and now I can hear the analog chorus noise. It varies according to the clock frequency, which is oscillating to give variable delay.

The op amps don't contribute to the noise significantly enough, so I will not replace them. There's more noise coming from the digital board. I don't find trying improvements on EMI filtering, replacing capacitors, adding new shields worth the trouble. The overall grounding around the unit looks really good, there are thick wires running to the frame, so there has been some thought in the design.

For the future: having separate outputs for drums, bass and "synth"

 

[Chiemaft]

Like I said earlier. Buy a $50 pocket oscilloscope and track the noise in circuit.

There's no way in hell you're going to track a noisy chip with a DVOM.

Simply not doable.

Gary

Please kindly explain better how to trace or track a noisy chip on the circuit board.

 

[happyrat1]

Well, you don't really need a signal generator in this case. You just set the keyboard to produce a sine wave or a flute voice, latch the note with the sustain pedal and trace the audio circuit with the scope's probe until you find the stage that's outputting the noisy signal. It will appear as spikes or garbage on top of the sine wave.

Eventually it will lead to the noisy IC op amp.

It does help, however, to have a working schematic of the circuit and the test points.

Gary