Distorted / Noisy Output SY77 - Help Please

Yamaha SY77 is a 16 voice multitimbral music workstation first produced by Yamaha Corporation in 1989. The SY77 is a synthesizer whose architecture combines AFM (Advanced Frequency Modulation) synthesis, AWM2 (Advanced Wave Memory 2) for ROM-borne sample-based synthesis, and the combination of these two methods christened Realtime Convolution and Modulation Synthesis (RCM).

Moderators: Derek, parametric, Saul

User avatar
chazza678 Great Britain
Posts: 3
Joined: Mon May 01, 2023 11:39 am
1
Where Are You Located?: Great Britain

Distorted / Noisy Output SY77 - Help Please

Unread post by chazza678 »

Hi All,


I recently bought an SY77 which was not in the best of shape. I stripped it down and serviced it / fixed the LCD panel / battery checked over all the components - no bad caps as far as i could see. Re-seated the socketed chips.

However, although all the self- tests check out and the test tones appear fine, there is a weird kind of distortion / noise present on all voices; same on either AFM or AWM. The underlying voice is clearly there, but there is a distorted sound superimposed which follows the envelope of the voice. I checked on a 'scope and it seems to be a distorted square wave.

I thought i would post on here to try to save hours of fault finding time - here are my current thoughts - anyone got any comments / suggestions please?
- Could the AFDO D/A have a loose ground connection / is the clock breaking through to the output? (surely the chip either works or not ?!)
- Could the Mix3 chips have a floating input somehow which is getting overlaid onto the outputs
- Given the synth is all digital up to the AFDO then how can this just be some sort of noise issue??
- Could this be aliasing because there is something weird somewhere with clocks / sample rates ?!
- Presumably it's not the AWM / AFM circuits as it sounds the same on either - but then again how come the test tones work fine ?!
- Could it be the envelope chips EGM ?

Thanks in advance

- Charlie
User avatar
Rara Scotland
Global Moderator
Global Moderator
Posts: 1415
Joined: Sat Jul 07, 2012 4:34 pm
12
Where Are You Located?: australia

VIP

Re: Distorted / Noisy Output SY77 - Help Please

Unread post by Rara »

I wont be able to help much at the level your checking, however did you turn effects off?
press the effects bypass button and see if this makes a difference, if it does that might help
find where the noise is coming from.


Laura 0-)
Jokeyman123 United States of America
Member
Member
Posts: 122
Joined: Sat Nov 02, 2013 2:42 pm
10
Where Are You Located?: Milford, pa

Re: Distorted / Noisy Output SY77 - Help Please

Unread post by Jokeyman123 »

You have a scope, and can use it, good. Is this distortion present on all the outputs? Headphone jack? One place noise can appear on these-the output DACs in the final stage-the op-amps could be noisy on these and can be replaced. I don't have the schematics in front of me-but if you can back-trace the square wave to see if it is pre or post the output DACs, that might help isolate the noise. And it could be as simple as an errant connection anywhere if you've had much of this disassembled. You replaced the LCD-and backlight? did you do the LED backlight mod, or are you still using the original backlight-the inverter circuit could be introducing distortion in the signal path if there is a ground leak anywhere in that circuitry-I'm just guessing and i could be completely off-base. I would not suspect any of the IC's that are holding ROM programs but it is possible. if you know all the diagnostic testing modes, try that, and see if anything comes up. I don't remember the codes to open test mode, seems like you might be there already.
User avatar
chazza678 Great Britain
Posts: 3
Joined: Mon May 01, 2023 11:39 am
1
Where Are You Located?: Great Britain

Re: Distorted / Noisy Output SY77 - Help Please

Unread post by chazza678 »

Hi all, thanks for the comments - It's been a long time before i could get back to this, but by way of an update - and another request for help. I found the following:-

I managed to pick up a second SY77 which was in better physical shape, but had a scrambled display and the usual dead battery so no sounds on power up. Cleaned it all up, put in a new display and battery and now it's all good. So, this enabled me to try swapping boards and it turns out that the problem is with the JKAN PCB (XG530) - when i swap with the new synth, the old one works fine !

So, i resoldered the AFD0 (YM3029) and DAC (PCM56P) chip, checked the cap's around that area and still all the same. I then worked my around the circuit with the scope and it seems that the DAC is outputting a waveform back to the AFD0 which has the square wave element imposed in it and this takes a while to subside after the waveform has decayed. However, the digital data stream which the AFD0 feeds to the DAC seems to follow this same sort of pattern. So, my guess is that the AFD0 is faulty, in that it is clocking through additional data to the DAC when it should have stopped. Or it is possible that there is a fault in the DAC, where the DAC is somehow continuing to process data when it should not. [Note - i have read around the Yamaha Floating point DAC concept - ie the DAC generates a voltage (exponent) input to the AFD0 which varies it's output voltage - so this is a tricky thing to debug as it's like a feedback loop!]

On the other hand, the waveformn is distorted all the time, which implies that it is not just an "overclocking" kind of thing.

Anyway, here are my requests for help:
- Does anyone have any experience of this kind of thing and any pointers ?
- Is the AFD0 or the DAC most likely to be at fault?
- and the $6M question - does anyone have any idea where and how i might be able to get a replacement for either of these two chips ?

It is so frustrating now, because the overall circuitry is quite simple really (serial data from the DM2 board -> AFD0 + DAC -> analogue filter -> Op-amp -> jack plug) - the problem is one of these two damn chips !

Cheers

- Charlie
User avatar
chazza678 Great Britain
Posts: 3
Joined: Mon May 01, 2023 11:39 am
1
Where Are You Located?: Great Britain

Re: Distorted / Noisy Output SY77 - Help Please

Unread post by chazza678 »

PROBLEM SOLVED!!

I finally got round to replacing the DAC (IC2) - the BB PCM56P is still available for a few pounds, so i bought one and replaced the original chip. Note, the AFD0 chip (IC1) Yamaha YM3029 is not available anywhere, so i took a chance with IC2 and lo and behold my Synth is now fully functional !

So, as i surmised, the DAC chip had some noise / crossover distortion or such-like going on inside and because this provides the exponent multiplier voltage to the AFD0, the AFD0 was misbehaving.

I am posting this here, in the hope that if anyone else ever has this problem i can save you some time, effort and frustration. Please do reply to let me know if this was helpful?

Cheers

- Charlie
User avatar
SYNTHDOG Canada
Posts: 1
Joined: Tue Jun 18, 2024 8:33 pm
Where Are You Located?: Canada

Re: Distorted / Noisy Output SY77 - Help Please

Unread post by SYNTHDOG »

On my Yamaha SY-77, I am having the same type of fuzzy, noisy, static like sound coming out of all four 1/4" outputs but through the headphone jack, the sound is clear, amazing and way louder. Could that be the same DAC problem or something else? If it is an expensive repair would I instead, be able to go out and buy an all male y cable, 1/4" TRS to 1/4" TS, TR and run the SY77 headphone out to my digital hard disk recorder, left and right inputs? Thanks
Post Reply