Bringing a Yamaha CS70M back from the dead (hopefully)

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andy_uk
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Bringing a Yamaha CS70M back from the dead (hopefully)

Post by andy_uk » Tue Oct 06, 2015 6:11 pm

Hi and thanks for reading.
As the title hints at, I have been tasked with resurrecting a CS70M.
From what I can gather (by using the troubleshooting chart here : http://manuals.fdiskc.com/tree/Yamaha/ ), it appears that part of the PSU has died.
When checking the voltage taps on the PSU I got the following readings:
Pin label Measured voltage
+5v +5.07v
+10v +0.16v
+15v +0.57v
-5v -0.76v
-15v +0.06v

After checking the fuses, I replaced the bridge rectifier D3...which made no difference at all!
I then read the "small print" regarding the voltages should be read under a no load condition! (well...I'd had "one of those" days!)
After unplugging all the multi pin connectors I re-measured the voltages and got the following :
Pin label Measured voltage
+5v +5.01v
+10v +8.20v
+15v +16.87v
-5v -4.10v
-15v +0.75v

Does this look like there's more to it than just a dodgy PSU or would connecting the boards cause those sort of voltage differences if some of the power lines have failed?

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Yekuku
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Re: Bringing a Yamaha CS70M back from the dead (hopefully)

Post by Yekuku » Tue Oct 06, 2015 7:52 pm

Hi Andy and welcome to the board.
I am no expert but here are my thoughts :
Obviously there sth wrong on the negative side of your power supply. Firstly, you have to fix the PSU and then check if other boards had been affected by the failure.
Is zener ZD6 ok ?
Are you getting -15V from the TR25 emitter ? ( maybe you will need to pull the emitter's leg off circuit)
Is diode D7 ok ?
A usual suspect could be TR20 but you should check also TR21 & TR22

Caution while handling the psu , it looks that there is sth shorted, so even with the unit turned off, the caps can kill you.
It is all about sharing...

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andy_uk
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Re: Bringing a Yamaha CS70M back from the dead (hopefully)

Post by andy_uk » Tue Oct 06, 2015 8:03 pm

Hi Yekuku,
Thank you for the reply (and the welcome).
I already have TR20 & TR21 on my 'watch list' but will also check/replace the others you've mentioned.
I'll test it without plugging it back in to the other boards and once it's giving out satisfactory voltages I'll plug the other boards back in one-by-one - assuming it's safe to do this? (obviously powering the whole thing down each time)

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Re: Bringing a Yamaha CS70M back from the dead (hopefully)

Post by Yekuku » Tue Oct 06, 2015 8:21 pm

Hi again Andy,
if you get the psu running up to specs then connect all cables and check the voltages again but this time on load.
Hmm... no need to go one by one, I remember reading a yamaha cs service manual noting that that the psu should not be tested when partially connected. It might not be the case with your synth but I would not risk it.
Let us know how it goes, good luck!
It is all about sharing...

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Re: Bringing a Yamaha CS70M back from the dead (hopefully)

Post by sneakthief » Wed Oct 07, 2015 3:42 pm

That PSU is convoluted junk. I had to buy a second meanwell switching PSU (+/-15V stepped down to +10/-5V) to power the +10V and -5V lines which are mainly used for the digital section. Those are originally tapped and stepped down from the +/-15V section. 8.2V instead of 10V is going to cause nothing but trouble as a whole bunch of digital circuits will start to behave weirdly and bleed into the audio path. This is what I was fighting with.

The whole board seems to suck more current as it ages, likely due to hundreds of decoupling caps getting old and putting more resistance across the power lines.

The main +5V for the digital section should be fine, but restoring the +/-15V section (at roughly 1.5A) could be a challenge. Recapping the whole PSU is a good start, but at some point you may have to replace a bunch of those transistors and ic's on the PSU board.

A big fat +/-15V 2A switching PSU might be in order here. Luckily I didn't have to go that route and disabling the +10/-5V taps was enough to stabilize everything.
sneak-thief: raw electrofunk!
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andy_uk
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Re: Bringing a Yamaha CS70M back from the dead (hopefully)

Post by andy_uk » Wed Oct 07, 2015 5:55 pm

Thanks sneakthief, I was actually thinking about a full rebuild...otherwise I can see the synth reappearing on my doorstep after a few months ;)
I'll make a list of caps and semiconductors, price it up and compare it to the cost of a whole new PSU.

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