Yamaha CS50 Conundrum

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Yamaha CS50 Conundrum

Postby meatball » Wed Oct 03, 2018 5:39 pm

Hi new member and twenty year lurker here ... I joined hoping maybe - just maybe - someone here might have the insight to help me lick this problem. I have a Yamaha CS50 that was working FINE until it started blowing fuses - I went over the PS with a fine toothed comb - found bad diodes, replaced them and recapped ...

before even trying it out after this, violating RULE 1 of the repair code of conduct - i decided to replace all the 4000 series logic and recap on the KAS and SH boards ... so OF COURSE i'm having problems playing the thing now (just a single stuck note and no keys triggering anything but aftertouch - everything else on the front panel seems to work however) - SO - i started learning the schematics and probing around along all the 4000 series power and control lines - then through all the pins of the 26600 and 26700 chips .. though at this point i'm not honestly 100% sure of the kinds of things i should be looking for - power seems to be present everywhere I've looked (at all the supply lines for the chips etc) ... Is there anyone out there who might maybe be ablet to 'nudge me' in the right direction? I would surely appreciate it.
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Re: Yamaha CS50 Conundrum

Postby Mooger5 » Wed Oct 03, 2018 8:40 pm

Did it stop blowing fuses?
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Re: Yamaha CS50 Conundrum

Postby meatball » Thu Oct 04, 2018 5:12 am

yes thanks - i think i got that part licked!
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Re: Yamaha CS50 Conundrum

Postby Mooger5 » Thu Oct 04, 2018 6:36 pm

Did you have to desolder any wires to access the KAS and SH undersides?
The single stuck note you describe could be a sign the SH (Sample & Hold) board is working. So the problem is maybe in the KAS (Key Assigner) board.
For an old power supply of 15V the electrolytic caps seem underrated at only 16V. There should also be a tantalum somewhere but I cant find it in the KAS schematic.
There is only one logic chip on that board that if failing would affect all the control voltages and gate signals at the same time, I think. The master clock oscillator, or IC6. It provides a clock signal to the Yamaha custom digital chips.
Other logic functions seem to use two or maybe more identical chips. It could be that the unit is picky about modern versions.
Did you put chip sockets? If so, try to replace the new with old ones, one type at a time and see if that changes anything.
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Re: Yamaha CS50 Conundrum

Postby meatball » Fri Oct 05, 2018 1:09 am

Thanks for the reply!

Yes - well i pretty much rebuilt the entire power supply over two weeks of head scratching -i was almost sure i was dealing with an intermittent primary coil for a time (!) but it was really just a bad diode pair ... which i replaced with 1N4004s - seems to be very stable and holding up well (though the transformer is getting hotter than i'd like - very warm but just on the border of hot to the touch). Anyway - yeah all the caps were replaced and the 5338's were replaced in that too ...

i did not unsolder the KAS or SH boards when rebuilding them - the first thing i did was put in nice machined sockets, swap out the two e-lytics on each board etc ... honestly i THOUGHT i found the problem last night (after much chip swapping over the last week or so) when i discovered my NEW 4069 (the chip you speak of - IC6) had FOUR bad gates - a total of FOUR of my new chips tested 'bad' though the originals (surprisingly) mostly tested perfectly ... but i put another (good testing) new one in but after that - the symptoms were STILL there - exactly the same - i popped the 4069 again in IC6 and checked it again - just to see if damage is happening while in service but no - it was still good ... so ... the head scratching continues ..

the one really OMINOUS and scary thing i'm finding is that pin 4 (SC8) - the divide down clock signal from IC1 is a dead channel ... nothing ... pin3 gives me a decent 98 khz (within spec) but i guess that one's an input? i'm not getting much legible data anywhere on the KAS board ... but my scope is kind of messed up at the moment (I think it needs service as I can't seem to get the volts/div to the place i need it for some reason) and can't get it to properly make out the power supply ripple ... ANYWAY ... in going through the yamaha calibration procedure I thought i would compare my readings to their targets as a sort of shadetree diagnostic ... and i get this :

Image

hopefully that's not too much info ... i guess the crazy erroneous readings make me hope maybe the YM26600 is still okay but I'm just not sure ... just wondering if something is pulling the clock signal down somewhere outside of the chip (crossing fingers) ...
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Re: Yamaha CS50 Conundrum

Postby Mooger5 » Fri Oct 05, 2018 1:22 pm

The transformer running too hot reminds of of an old scope That I bought untested. It ran for a couple a minutes before getting way too hot. I found the mains voltage selector contacts were busted, mixing up the primaries for the different country voltages. I bypassed the selector by directly soldering the mains leads to the proper primaries but it was too late. There is no doubt a short in the windings.
So I´d recommend you to inspect the mains voltage selector. In addition try desoldering the secondaries to the psu board and run the trabsformer unloaded to see if it still gets too hot.
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Re: Yamaha CS50 Conundrum

Postby Mooger5 » Fri Oct 05, 2018 2:22 pm

Speaking of scopes, have you tried switching the test leads range from 10x to 1x?

About IC1 I´m not sure about pin 3 but if there is no clock signal coming out of pin 4 it´s maybe because there is no clock signal coming out of IC6. IC6 output leads to pin 2 of IC1. That should be the clock input for IC1.
There´s also the power lines to check for IC6: 8.5V on pin 14 and -6.5V on pin 7...
Pin 6 of IC6 has to output a sqaure wave. The timing capacitor is the 100p one.
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Re: Yamaha CS50 Conundrum

Postby Mooger5 » Fri Oct 05, 2018 3:44 pm

Page 5 of the service manual tells pin 4 of IC1 is an input, for portamento and glissando. So, no signal there is to be expected since port/glis (and every keyboard related function) are not working. Not alarming. Still, pin 2 is indeed the input for the clock signal, and pin 40 of IC2 as well.

That page 5 provides clear info on what should go on during normal operation.

The custom chips run on 8.5V and -6.5V just like IC6. Can´t stress enough it could all be due to poor supply.
The transformer secondaries must provide a bit higher than 12VAC with no load, and 12VAC under load (8.5*1.4).
Another thing to consider is WHY did did those diodes fail. Seeing there is one diode in the 8.5/6.5 supply and the other in the +15/-15 we can assume there is only one thing that is common to both, and that is the primary winding.

Hope all this is of some help. Good luck!
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Re: Yamaha CS50 Conundrum

Postby meatball » Fri Oct 05, 2018 4:49 pm

Thanks for the reply Mooger - Well I am just guessing those E-frame tronsformers tend to run warm in general - as I spend a lot of time already sorting out the power supply I'm not going to worry about it for the moment - I'll assume the diodes failed by being 40 something years old (maybe?) or just defective from the start - though there could be something funny going on with the wiring loom in general since I originally purchased it as 'non working' but it arrived working 100% for whatever reason (something got jostled on the truck i imagine). But listen - maybe it's another side-story that's not going to help me sort this business out but only distract perhaps ...

hmmm weird about 'pin 4' (that's IC1 right? do you have the same manual i have? - according to mine:

"The master clock pulse ¢ is used for synchronizing the LSI operaton and counter, and SC8 with speed slowed down (period extended 10 times)- which provides the interface between the digital circuit at the timing of master clock pulse and the analog circuit. NOTE: The SC timing pulse generated in this IC is not used for Model CS-50."

top of p.13, servicing guide for the yamaha CS50
which seems more definitive than the service manual ...i'll digest p.5 further but I don't see anything that might contradict the above -

so that pin four seems MIGHTY critical for basic operation (?) - are you by chance reading your info from the CS60 service manual?? Perhaps they are different ...

also just getting constant voltages of 2.1 something volts coming out of pins 34-38 in IC1 during any kind of keypressing (or not)... and elsewhere mostly just DC. IC7 - the 4050 is showing -6.5 volts at every pin except for the unused ones and pin 8 which is 8.5 volts ... everywhere i look I just see signs of non activity and full feed current ...no responses from pressing any keys ... makes me wonder if there's a short somewhere (which might also explain the power supply fuse blowing and diodes blowing somehow ... but honestly ... there are no more fuses blowing and i have correct values installed etc .. so i'm just kind of scratching my head over this ...

would you happen to know of any sure-fire methods for verifying the health of IC1 and IC2 ...? perchance??

and ... lastly - THANK YOU - you're a mensch - i may reply further shortly as i digest this but this is my initial response
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Re: Yamaha CS50 Conundrum

Postby Mooger5 » Fri Oct 05, 2018 6:49 pm

You´re right! I was looking at IC2 pinout description! Sorry for wasting time here.

So it looks normal; the synchro pulse at pin 3 (an output according to my version of the SM) is there because the master clock generator is working and sending its signal to pin 2. Pin 4 as is not outputting because it´s not used in the CS50. It´s probably for the extra voices of the CS60. So no worries regarding pin 4.

What about pin 5? I´m guessing Initial Clear Circuit has the same function as a Reset, right? The other half of the 4069 is probably generating a one-shot pulse to let IC1 know everything´s alright with the rest of the circuitry or something. If you look at the schematic, there should be a signal out of pin 8 of IC6. If not, there´s the tantalum capacitor next to IC21. Those from that era are notorious for being intolerant to voltage spikes, failing short. I´d replace it asap with preferably a low esr electrolytic.
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Re: Yamaha CS50 Conundrum

Postby meatball » Fri Oct 05, 2018 7:34 pm

Yes pin 5 is a reset if i understand correctly ... i can't recall if i checked it or not ...

you may have missed the 'memo' above though ...

"NOTE: The SC timing pulse generated in this IC is not used for Model CS-50."

the SC timing pulse is on pin THREE not four - but it's the SC8 (see above, the divided down signal from SC) that is used for many of the functions on the KAS board ... and that's the signal i'm missing ... presumably that's generated iNSIDE the 26600 chip ... so now i'm wondering if i could somehow reconstruct that signal from the SC pin ... or who knows ... maybe i'm wrong about that or the manual is wrong ... :(

well i'm not too worried about the tantalum as it's made it forty odd years and it seems to be doing it's job still ... but maybe i'll swap it out anyway ... but I'm still very perplexed why this entire KAS board seems to be shut down and why there isn't more going on here ...
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Re: Yamaha CS50 Conundrum

Postby Mooger5 » Fri Oct 05, 2018 7:58 pm

I forgot to mention the block diagram doesn´t show pin 4 anywhere. It shows every important pinout. Such as pin 2 and pin 3. But then the KAS schematic calls IC1 "key encoder and assigner" when it´s only encoder hence my previous confusion while reading page 5. I mean there are a few errors around. I´m going to follow the lines and see where the output of pin 4 would lead to. But that will take a while.

Please replace the tantalum even if temporarily for (my) piece of mind. If that thing failed short it´s not providing a timing constant for the reset signal. Or, well, verify the presence of a signal at pin5 of IC1 during powerup. Sorry for the stubborness. It´s my hunch something inside IC1 must be inhibiting all the encoding functions until a reset pulse shows up.

BTW I got the SM from here http://midimanuals.com/manuals/yamaha/c ... ce_manual/
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Re: Yamaha CS50 Conundrum

Postby meatball » Fri Oct 05, 2018 8:29 pm

hey thanks for the link - GUESS WHAT???

it's FIXED (well within reason) - yesterday i started tracing ALL the signals through the CMOS chips and discovered i'd mistakenly made a solder bridge on the 4069 where one shouldn't be ... i previously noticed it ... but sucked the solder out but it LOOKED as though the copper below was there bridging - when in fact it WASN'T - so wow - that was nearly a week of stress over such a dumb thing!!! ... i had a feeling it was something incredibly silly like that ...
well ... now on to the PRA and M boards!!!

if you're ever In los angeles i owe you a beer!
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Re: Yamaha CS50 Conundrum

Postby Mooger5 » Fri Oct 05, 2018 11:03 pm

Those are excellent news! Mistakes happen. It´s natural. There are two 4069. Eheh I hope it was IC6.
Thanks for the beer. I take great satisfaction when the troubleshooting goes right. It´s part of the education I never had.
Cheers :D
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Re: Yamaha CS50 Conundrum

Postby meatball » Fri Oct 05, 2018 11:12 pm

YES iC6 was the culprit! well it's educational i guess ... but a tiny bit deflating to know it was my own repair attempt that caused it - seriously - i was at my wit's end ... and about to give up on this almost ... but i guess it inspires some confidence to have solved it -and it's CERTAINLY a relief!!! thanks for the handholding at least and good suggestions - i learned some new things about the key coder etc
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