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Author Topic: yamaha CP30 repair  (Read 1649 times)

ilyaa

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yamaha CP30 repair
« on: February 08, 2017, 04:14:35 AM »
hey!

fixing up this old analog keyboard -

here's a link to the schematic: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwicodKMlIDSAhUH_4MKHfZIAzAQFgglMAE&url=http%3A%2F%2Fwww.bleeps-and-peeps.com%2Fimages%2Fstories%2Fhardware%2Fyamaha%2Fcp30%2Fservice_manual%2Fyamaha%2520cp30%2520servicemanual.pdf&usg=AFQjCNE9RQwQYVl8lBlOrCX86TcliSv4Zg&sig2=KwFP1a67oFJjWmA7ISqsdw&cad=rja

im not getting the -30V from the DC supply (see page 12). other voltages look fine - the AC coming into that section (the one that feeds TR7 and TR8) looks okay - about 30VAC - but im only getting 0.8V coming out of the bridge. thats my first suspect.

just a quick question: is there anything wrong with replacing this: http://www.sansui-vintage.com/product_info.php?products_id=98 with two 'discrete' diodes? say, 1N4007s?

Enzo

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Re: yamaha CP30 repair
« Reply #1 on: February 08, 2017, 05:38:08 AM »
No reason at all for not using two diodes.

You have 30vAC from the transformer?  neither end is ggrounded, so is ther 30vAC measured right at D5 and D8 on their leads?   In other words is that good AC reaching the bridge?   I would find it unusual to see both sides of a dual diode like that to be open.  Is eather cap shorted?  If you suspect the bridge, did you pul it and check it with diode test?  For that matter have you done that where they sit?

Power off, what resistance measured across the 1000uf cap?  And what measured across the ground and -30 output terminals.

Disconnect the wire from the -30 to the rest of the system.  Does the voltage come back or still missing?

In other words FIND the bad point or part instead of jumping right to hopeful parts replacement.   This is a very simple circuit.

ilyaa

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Re: yamaha CP30 repair
« Reply #2 on: February 10, 2017, 09:25:35 PM »
yeah i had traced it to the bridge -

replaced it with some 1n4007s and we were ALMOST okay. still a lowish voltage (like -3V). disconnected one card at a time - traced it to one of the lower octaves - there was some bad connection there - found a weird errant solder hair - put it back together and we are back in business keyboard works.

now im trying to decide if the noisiness of this thing is normal or not - its not a hummy noise more staticy kind of 'ssssssssss', quieter than the signal but the signal to noise ratio seems a bit low for my liking. thinking to just try recapping the power supply and take it from there - not too many components in there so wont be crazy.

joecool85

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Re: yamaha CP30 repair
« Reply #3 on: February 11, 2017, 07:02:07 PM »
yeah i had traced it to the bridge -

replaced it with some 1n4007s and we were ALMOST okay. still a lowish voltage (like -3V). disconnected one card at a time - traced it to one of the lower octaves - there was some bad connection there - found a weird errant solder hair - put it back together and we are back in business keyboard works.

now im trying to decide if the noisiness of this thing is normal or not - its not a hummy noise more staticy kind of 'ssssssssss', quieter than the signal but the signal to noise ratio seems a bit low for my liking. thinking to just try recapping the power supply and take it from there - not too many components in there so wont be crazy.

Typically power supply noise is more of a hum and grounding issues in the signal chain are what cause the ssssss noise.  At least that has been my experience.
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ilyaa

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Re: yamaha CP30 repair
« Reply #4 on: February 12, 2017, 02:42:11 PM »
any tips on tracking something like that down in a long, complicated circuit like this one?

Enzo

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Re: yamaha CP30 repair
« Reply #5 on: February 12, 2017, 06:55:02 PM »
Same as any other circuit - isolate the problem.