YAMAHA CS70M PSU repair

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Hydroblaster
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YAMAHA CS70M PSU repair

Post by Hydroblaster » Thu Jun 03, 2021 9:40 am

Hello everyone - it is my first post here!

I need your help repairing my beloved CS70M.

I was playing it the other day and suddenly it started to behave strangely - like both LFO diodes stuck, weird program sounds, weird square wave sound etc.

After checking the PSU terminals (with all the boards connected) it appears that -15V reads -20,1V. All the others are normal.
Following the troubleshooting instructions I changed the IC3 and Tr23 - but the problem is still there.

Do you have any ideas?

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Yekuku
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Re: YAMAHA CS70M PSU repair

Post by Yekuku » Thu Jun 03, 2021 2:31 pm

Hi and welcome.
I think that ZD6 is fried, it is a 15V zener diode , it should regulate voltage to 15V but as it looks like it doesnt.
I have a few other ideas but you should rule out the zener first.
Good luck !
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Re: YAMAHA CS70M PSU repair

Post by madtheory » Thu Jun 03, 2021 2:41 pm

I think a failing capacitor is also likely? But with the Zener there it is not the cause of the voltage increase... although it might have pushed the Zener out of range (damaging it) because one failure mode of electrolytic is to offset the DC voltage.

You can check the caps visually, or remove them and measure them. If they're the original ones they might be operating within spec, but visually leaking so should be replaced.

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Re: YAMAHA CS70M PSU repair

Post by Hydroblaster » Thu Jun 03, 2021 3:01 pm

Thanks a lot for your responses.
The caps all look okay and fresh. There is one film capacitor that looks older but looks okay (the 0,047uF).
I do not know if it helps but the voltages going out of IC3 (4558) are +14 and -15v.
Can I test the diode in the circuit?

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Re: YAMAHA CS70M PSU repair

Post by Yekuku » Thu Jun 03, 2021 9:52 pm

Hmm, I had a better look on the schematics and the zener is probably for overvoltage protection.
You can measure voltage at its cathode it should be aprox -15V.
On IC3 pin 1 you get -15V ? If yes , can you test TR21 and TR20 ?
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Re: YAMAHA CS70M PSU repair

Post by Hydroblaster » Sun Jun 06, 2021 6:38 pm

Well all seem fine…
Zd6 emits -15
Ic3 -12 on leg1
Tr 20 and 21 also seem fine…
Weird! I can’t find where the problem might be.

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Re: YAMAHA CS70M PSU repair

Post by Yekuku » Mon Jun 07, 2021 7:38 am

TR25 and ZD6 act as voltage regulator, to provide -15V to IC3.
You should rule out TR25, what is the voltage on the emitter ? It should be -15V, if not replace TR25.
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Re: YAMAHA CS70M PSU repair

Post by Hydroblaster » Mon Jun 07, 2021 5:41 pm

There is -14v on tr25 emitter.
-15v on tr21
And ….-1,0 on tr23 emitter. Maybe that is a problem? But i have checked the tr23 and it’s fine.

Edit:

Tr20 readings are:
BE -20,5v
RE -22v
WH -15v

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Re: YAMAHA CS70M PSU repair

Post by Yekuku » Mon Jun 07, 2021 9:56 pm

-14V is not in spec, the culprit could be TR23/ZD6 but there are chances that another component is faulty and it is causing this voltage drop.
What puzzles me is how this is related to the overvoltage that you are getting on the -15V line.
Anyways , since you checked all mentioned transistors and you confirmed proper operation, there are only a few components left that could be causing this , TR22 and the bridge rectifier D3, everything else are passive components.

Edit: It would be interesting to compare transistor readings with those in the +15V power rail. (TR-20 VS TR-15)
Can you post the voltage readings of TR15?
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Re: YAMAHA CS70M PSU repair

Post by Hydroblaster » Tue Jun 08, 2021 7:51 am

TR15 is:

BE 15v
RE22,8v
WH 16v

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Re: YAMAHA CS70M PSU repair

Post by Yekuku » Tue Jun 08, 2021 9:01 am

well by comparing TR20 vs TR15, it looks like both power transistors are fed with aprox 22-23V , which rules out the bridge rectifier. Aprox 15V are applied to both transistor bases but TR20 does not output the expected voltage.
I think you have a problem with TR20 but also check TR22 as advised previously.
let us know how it goes !
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Re: YAMAHA CS70M PSU repair

Post by Hydroblaster » Tue Jun 08, 2021 10:55 am

I think that I will buy some transistors and simply switch them out. Problem is that a1015 and tr20 are ancient and hard to get so I'll have to search for replacements.

Yekuku - I really appreciate your help.

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Re: YAMAHA CS70M PSU repair

Post by Yekuku » Tue Jun 08, 2021 2:33 pm

You are welcome Hydroblaster.
Please have in mind that I am no pro, all the assumptions are based in my personal limited experience , so I might be wrong in some cases .
I think that i have a few 2sa1015 in my drawer, if you are having trouble finding some , let me know.
Last edited by Yekuku on Tue Jun 08, 2021 9:46 pm, edited 2 times in total.
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Re: YAMAHA CS70M PSU repair

Post by Hydroblaster » Tue Jun 08, 2021 9:10 pm

That’s very kind. I’ll let you know.

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