Yamaha CS 40m Help!

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suchenderxxx
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Yamaha CS 40m Help!

Post by suchenderxxx » Sat Mar 15, 2014 2:36 pm

Hello!

i got an Yamaha CS 40m with the same problem the IC8 which is an NEC C271C voltage comparator is broken into 2 pieces. I changed it for an st microtonics LM311 which should be an replacement. I also replaced all the tantalums, some of them got shortened.

After that the Fuses are no longer blown. But the replacement for the NEC the LM311 gets very hot. The Memory Panel works but there is no output from the Synth. I checked the voltages on the lm 311 which should have +15 and -15 but all i can get is something like -0.3 and -10...

I have the service manual but i have no clue from where the pgm board is getting his power. Or from where the +15 and -15 is coming on this circuit. Could someone of you help me with that?

the schematics can be found on: http://manuals.fdiskc.com/flat/

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Re: Yamaha CS 40m Help!

Post by nathanscribe » Sat Mar 15, 2014 10:06 pm

Interesting. Because the same thing happened to mine:

Image

IC8, comparator.

This happened to me after moving the synth from floor to stand, during which I upended it, and after the smoke had cleared I found a small piece of solder that I thought had got lodged in there and shorted something out.

I haven't repaired it yet. Nor have I even started to troubleshoot it. I do have the original service manual, which is an epic tome indeed, and don't really have the time to fully investigate. Sorry.

But I do suspect something is still shorted out, or the supply rails are generally damaged, at least locally. There's a lot going on in the 40m. You need to start right at the beginning and be very, very thorough...

What I do know is that IC8 is part of the A/D convertor that digitises the control values. IC8 directly feeds IC1, which is made of unobtanium. I am hoping for both our sakes that IC1 has not died, because if it has, we can expect an extremely lucky, expensive, and lengthy search for a spare that may never be found.

As for power rails, there is a 3-pin connector labelled C1 which supplies power to this board. Pins 1 and 2 are +5V, pin 3 is -10V. The connector C3 supplies +15 on pin 4 and -15 on pin 5. If you can't see the numbering on the board, the connectors in question are the first and third along the bottom edge of that PCB, from the left. The connector directly below IC8 is C4. Pins are numbered left to right.

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Re: Yamaha CS 40m Help!

Post by suchenderxxx » Sat Mar 15, 2014 10:58 pm

thanks for your reply!

i further investigated the problem, on the pgm board there is no +15 and no-15 V at all! if i unplug c4 then the ic8 is no longer hot! i thought that C4 is the supply for the DM Board and feeds it with the +15 and -15 V but im not sure...

if you are right and C4 feeds the pgm board with +15 and -15 then the problem could come from the DM board...??? Could you check this?

i checked other ic´s on the pgm board, like ic 9, ic 10 and the ic 30 which all need +15 V on the pin 8 and on those pins i have a strange Voltage like -7...

i cant figure out from where the +15 and -15 V are coming i thought that one of the ic´s on the pgm board is feeding this voltage or one of the transistors. But this schematic is over my skills... so i hope someone with more knowledge could chime in. If you have these skills it would be great if you could help me identify the problem or tell me how to analyze it.

the question is is the +15 and -15 coming from the dm board or going from the pgm to the dm board?

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Re: Yamaha CS 40m Help!

Post by nathanscribe » Sat Mar 15, 2014 11:56 pm

It's C3 that supplies +/- 15V, not C4.

Regarding your other questions, I'll take a look tomorrow. 8-)

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Re: Yamaha CS 40m Help!

Post by fh991586 » Sun Mar 16, 2014 6:59 am

Damned IC8... I had a similar problem years ago with a CS20M (http://www.vintagesynth.com/forum/viewt ... =5&t=64297)! Sold it to a repairman without losing money instead of having it fixed. :|
Considering parting away from:
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Re: Yamaha CS 40m Help!

Post by suchenderxxx » Sun Mar 16, 2014 8:29 am

nathanscribe wrote:It's C3 that supplies +/- 15V, not C4.

Regarding your other questions, I'll take a look tomorrow. 8-)
i meant C3... If you could answer the question, if C3 is feeding +15V to or from the PGM Board this would be great! If you are right, then the Voltage is coming from the PGM Board and something there must be wrong... If im right and the +15 is going from the pgm to the dm board then there is something wrong on this board. and i still cant see a component which is suppliying the +15.

I took a look on the DM Board it is full of 4558 opamps and some YM617.

there is one Connector which is C12 which supplies the +5 and -10 for this Board from the PSU. i cant see or understand how and where this voltage is upped to +15V

Edit: I think i know where the +15 is coming from it is coming from pin1 on the DM Board which is connected to the psu. So the +15 is coming from the DM Board from C6... now i must further trace it down...


Edit: I checked the psu, if no Connector is plugged into the PSU i got the Voltages only the rail which should support +15 shows +14.30. if plug the connectors on the psu then the Voltage drops Down to +0.6

I already recapped the Psu and replaced the opamps and tantalums. Maybe is one of the transistors defective?

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Re: Yamaha CS 40m Help!

Post by nathanscribe » Sun Mar 16, 2014 10:57 am

suchenderxxx wrote:Edit: I checked the psu, if no Connector is plugged into the PSU i got the Voltages only the rail which should support +15 shows +14.30. if plug the connectors on the psu then the Voltage drops Down to +0.6

I already recapped the Psu and replaced the opamps and tantalums. Maybe is one of the transistors defective?
Yes, the power does indeed get to PGM via DM.

Try unplugging PGM and testing the power rails again - if the 15V rails work when PGM is out of the equation, then I'd guess the problem is likely to be on the PGM board.

If the power supply is regulated, 14.3V might be just what you get. But according to the service notes, it is outside tolerance. The supply should be set to 15V ±.01V. First though I'd find out if PGM is bringing it down.

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Re: Yamaha CS 40m Help!

Post by suchenderxxx » Sun Mar 16, 2014 11:15 am

i thought the same. But it makes no difference which Connector i Plug into the Powersupply as long as one is connected the Voltage drops...???

there is an VR1 on the Psu which is for adjusting the Voltage for the +15 rail but this thing does nothing... the other VR´s work, i adjusted the other voltages and they are in spec. But i cant adjust the +15 i can only get +14.30.

If i plug one of the Connectors on the Psu then the Voltages drops, maybe one of the transistors is nor working or is not fully working or got damaged...??? i suspect TR1 on the psu but im not sure if this is the transistor which makes the +15 or if there could be something else that is defective.
Last edited by suchenderxxx on Sun Mar 16, 2014 11:19 am, edited 1 time in total.

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Re: Yamaha CS 40m Help!

Post by nathanscribe » Sun Mar 16, 2014 11:17 am

OK. Maybe it's time to rebuild the power supply..?

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Re: Yamaha CS 40m Help!

Post by suchenderxxx » Mon Mar 17, 2014 5:34 pm

On my psu are some resistors which should be fuse resistors, one of them is blown. I will get a new one and test this. This Fuse Resistor was soldered on the +15 rail. So i hope this will fix my problems. I will report back for useres which have the same problem...

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Re: Yamaha CS 40m Help!

Post by SynthOSphere » Mon May 09, 2016 4:13 am

suchenderxxx wrote:On my psu are some resistors which should be fuse resistors, one of them is blown. I will get a new one and test this. This Fuse Resistor was soldered on the +15 rail. So i hope this will fix my problems. I will report back for useres which have the same problem...
So ????
Did it work ? What happened ?

I hate it how some people are so present and fast to answer when they have problems, relying on others, but once their problem is solved, they disappear. :(

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Re: Yamaha CS 40m Help!

Post by 1way2life » Sat Feb 09, 2019 7:00 pm

Same issue here, get an under-voltage on the +15 rail, with no boards connected. I get 14.58v.

Anyone ever figure this one out?

I'll have to trace through, and swap some pairs on the board to try to figure it out.

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Re: Yamaha CS 40m Help!

Post by 1way2life » Tue Feb 12, 2019 12:49 pm

Well, found the issue... and it certainly was the 0.47 fusible resistor on the +15v rail. It was noticeably dark in color, and was reading at 5k ohms.

Although these are still available, I'm going to look into an alternative solution, as fusible resistors IMO are just a bad idea, and are rarely used anymore in modern applications.

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