Repairing a Yamaha CS-60 + CS-50

Hi everyone,

first time posting in years...  I've acquired a both a Yamaha CS-60 and CS-50 that need some work.  The 50 is already mostly operational save for a couple envelopes.  The 60 is in need of more attention, but I would rather get this one going as it has twice the voices and an awesome ribbon controller.  If you've got any experience with this series of synths you know they're in many ways like working on a car... tens of boards littered with proprietary parts.  I will use this thread to document my progress working on these machines and to put my questions in one place.  Please feel free to chime in if you have any ideas or questions.

Anyhow, the first issue I've run into on the 60 is actually kind of mundane.  Not getting proper voltage from the +-15 rails.  My problem is that when the PSU is attached to the circuit, I can't know if there is a short on a board somewhere down the line bringing down the supply, but if I detach it and short the +15<->+15s and -15<->-15s, (I assume to maintain a load on the supply so it can operate) I get only a couple milivolts on the pins.  Attached to the voice circuits, the PSU is somewhere around +.9v and ground-ish on the - rail.  Any ideas why I can't test it with the boards disconnected from the psu, but it generates a volt when the cards are attached?

I can easily make measurements/scope anywhere in the schematic and report back if you have an inkling what might be going on.

Here is a copy of the relevant page from the service manual:
http://therogoffs.com/cs80/manuals/CS60_Service_Manual/29%20-%20SVU%20Circuit%20Diagram%20&%20Circuit%20Board.jpg

Thanks a bunch,
Cooper

The +S and -S lines are powersupply voltage sense lines, that will ensure an exact voltage delivered to the point where they are connected.

Connect +15S to +15 and -15S to -15 to test the powersupply without the rest of the modules.

Jakob E.

Hi Jakob,

Thanks for the insight, when I gave this a try, the output went to 0v for both + and - rails.  What puzzles me is that the supply produces a small voltage when connected to the other boards, but it wont produce any on its own, even with the sense pins hooked up properly.  I guess recapping the supply should be my first effort?

Your preamps look awesome, I hope I can build one some day...
Cooper

Update 9/21

Recapped PSU with Panasonic FM's, replaced 4558s in there too.  +-15v rails are back to life! Now -6.5/+8.5 rails are erratic and have stopped working.  Weird.  Maybe a power transistor blew during service?

Replaced all of the CMOS, most of the opamps throughout, got first sounds out of the CS-60!  Distorted... sounds like crazy modulation FM on all voices, and not very predictable.  Im assuming this is from my cracked out 8.5/-6.5 rails that power voice control CMOS.

Next project... new nichicon muse caps have arrived for recapping of all the voice cards.

Still need to sort this stupid PSU problem though.   :-[

Service doc:
http://therogoffs.com/cs80/manuals/CS60_Service_Manual/29%20-%20SVU%20Circuit%20Diagram%20&%20Circuit%20Board.jpg

Should I start by making any measurements?  For a time it was changing voltages, but then I cleaned the flux off the board and now 8/-6 rails are only a +- a few mV from ground.

Hi,
I’ve been donated a Yamaha CS60 with a broken power supply. The -15V rail measures fine without load. As soon as I set both rails (+-15V) under load (two car bulbs 12V, 21W), the -15V rail goes down to almost 0V.
I’ve replaced all electrolytic caps, the opamp and TR 6,7 and 8. Still no success. Any help is appreciated!




http://therogoffs.com/cs80/manuals/CS60_Service_Manual/29%20-%20SVU%20Circuit%20Diagram%20&%20Circuit%20Board.jpg

Cheers
Bernd

19;20  minutes in is a pretty good description of the supply.....

https://www.youtube.com/watch?v=r47y4r5g66U

Hi,
Thanks for pointing to the video. I’ve already watched it and the other parts. Very informative indeed.  I’m going to swap the remaining parts. Is there another version of the schematic? My psu has got a kind of protection diode at the output of the -15V rail that isn’t drawn in the schematic.
Cheers
Bernd

Those bulbs pull a lot of current???

Yes, 12V, 21W car bulbs are perfect to test the psu with a dummy load. The +15V rail works and measures fine, the bulb lights up, the -15V goes down to almost 0V.

Are you testing / loading both +15 and -15 at the same time? You'll notice at 23:30 in that video, it is explained that the negative sense is not connected to ground like the positive side. The negative sense is connected to the positive sense so that if either rail goes lower the other will too. So if you only load the negative rail, it might not work as expected.

Yes, I’m testing both rails at the same time.
Here are some voltages.


Cheers
Bernd

I think your pass transistors (Tr7 Tr8) have to be toasted. You say there's -28.3V on their bases and 0 on their emitters? They've given up the ghost.

Note that think people recommend that you replace the same transistors on the other side (positive side in your case) as well. Not sure why, this is just what I've read in forums here. I'm not a repair tech.

Personally I would replace the whole thing with a Mean-Well ACDC SMPS. You would need at least two regulators for making the +8.5 and -6.5 though. But I have no experience with this sort of thing. I know people care very much about things being original but it will absolute have zero (0) effect on the sound. And the reduction in heat will increase the life and reliability bigly.

That’s what I thought as well and replaced TR7&8. Still the same.  Without load I get -15V there.  I’m going to check if the collector is properly isolated from heatsink/ground.
Cheers
Bernd

Maybe it's those emitter resistors are failing open only under load (is that a thing?). Try measuring the emitter voltage under load. Your drawing only shows the outside of the resistors.

Otherwise, according to your drawing, you have 0V -> resistor -> emitter -> base -> -28.3V. So someone correct me if I'm wrong but if something is not failing open, it would have to start smoking.

Try testing the transistors you removed.

;D



1,5 Ohm resistors burned!!
I’m going to do further tests board by board with a bench power supply.
Thanks!
Bernd

bernbrue said:

Yes, I’m testing both rails at the same time.
Here are some voltages.


Cheers
Bernd

Click to expand...

As often happens hand drawn schematics are less accurate. As drawn that - rail would never work, because there is no base drive for the pass transistors.  The + rail has a 330k positive base drive. I suspect a similar resistor exists for the - rail if it ever worked.

The 1.5 ohm emitter degeneration resistors force sharing between the two pass devices. If the base TR7 is hard low (-28.3) and components are still intact there would be some 18A in each 1.5ohm resistors (extremely unlikely).

I see you have already identified 1.5 ohm Rs as open, check TR7 and TR8 also.  If the resistors were sacrificial (flameproof) designed to act as fuses, the transistors may be OK.

Note: something may have shorted that rail causing the failure.

JR

Thanks for pointing that out, John. I’m going to disconnect -+15V from each board and test for shorts and then reconnect one by one using a bench power supply. In fact there is no corresponding 330K resistor on the pcb for the negative rail.
What a beautiful instrument!


Cheers
Bernd

bernbrue said:

I’m going to do further tests board by board with a bench power supply.

Click to expand...

Be super careful here. Maybe use a resistor in series if your bench supply doesn't have current limiting. You probably don't need much current to find the short. And remember that the guy in that video torched all of the CMOS chips poking around. If you have one of those grounding straps, now is the time to use it.

Some progress:
After changing the 1,5 Ohm resistors the psu works fine now.  I’ve tested it with my getto style car bulb dummy load. Then I checked each board individually first testing for shorts and then with my bench power supply. I used +-15V with about 150mA . No signs of shorts or excessive current draw. The SUB board had two loose wires, otherwise no signs of broken or burnt components. I’m tempted to test the whole synth with the original power supply.
Any further advice is appreciated.
Cheers
Bernd

Progress: psu works fine! After replacing a few 4000er CMOS ICs the Synthesizer came back to life!! It’s completely out of tune and needs calibration.  Two voice cards are not working correctly. So still some work to do but getting closer.
Just a note to people who think that their Yamaha custom chips (YM26600 and 26700) are broken: these extremely rare and expensive IC are clocked by a 4069 IC which is prone to fail.
Cheers
Bernd