Help! Restoring a preamp Yamaha CP70b

[peterkohlmetzmoller]

Hi guys.
I'm working on a beat up old CP70. It's been really fun! I'll post some pictures when I'm done with the new finish, it's something quite different!

The previous owner had done the passive mod, because the preamp/power supply was malfunctioning.
I'm trying the figure out how to get it working again. I've found a replacement 18v power adapter and soldered on the 2 pin XLR connector and it's works great. The voltages ranges from 16 - 20v DC, it's was easy enough to find one.
So far so good.

Yesterday I measured power voltages from the source voltage pre amplifier board, that is between the adapter and the actual FET preamp with the eq and tremolo fx.
The voltages should be 8v, 12v and 16v. The 12v line was 8,5v. Therefore i went to my electronics store today and bought some new electrolytic capacitors and replaced the old ones, 3x220uf 25v and one 470uf 25v. The old ones looked fine btw, but I've heard many times that that is a good thing to due to 40+ year old gear, and thought that was were the trouble was.
The voltages are unfortunately the same still.
8v line (for the tremolo circuit) =8v fine
12v line (for the preamp circuit)= 8,5v bad
16v line (for the tremolo circuit)=15,9v fine.
I'm trying still to identify the problem, and I'm having a hard time figuring out if it's the 2 semiconductors, a transistor and a low level voltage regulators that is causing the trouble. Look at the pictures and help me identifying them if you can.
Let me know your ideas.

Upon testing the unit, noise passes through the active outs, tremolo works, as I can here it changing the noise when i turn the volume up and turn the rate/depth knobs.
The 12v line for the preamp however (of course) is malfunctioning, because it only gets 8.5v from the source voltage pre amplifier board, and not 12v like it should.
Here are a couple of pictures.
By the way, the piano worked fine with the passive mod, but I'm interested in restoring it back to working condition with eq, tremolo and linelevel outs, as I dont want the play it though a guitar amp all the time. I'm not either interested in 3rd party DI's or mic preamps to buff up the signal.
the best greetings from Peter

[madtheory]

The transistor TR201 is only on the 12V line, so that could be at fault assuming you've replace C201 and R206 checks out OK. Your local piano tuner can deal with the rest of this beautiful instrument

[jxalex]

Yes... the transistor or 7812 (quite unlikely). However check the cold joints. before swapping out the details.

check the voltage levels around the transistor TR201. Does the TR201 collector gets 12.5V ? And base gets ca 12.5V too? If yes, then that transistor is faulty, unless there is some other shortcircuit at the output on emitter.
But can be cold joint due to its age.

Assuming that you replaced the capacitor on the TR201 base then that does not shortcircuit.

Is there voltage coming to TR201 collector, if not then that 7812 is faulty if before that stabilizer there is incoming voltage. Again.. resolder the joints at first.

[madtheory]

Seeing as the other V lines are working correctly, the 7812 is OK. The transistor is collecting from the 7812, emitter is connected only to 12V line, so logically tranny is at fault.

Unlikely to be cold joints, this is a Yamaha

[peterkohlmetzmoller]

Thank you so much guys! I'll keep you posted, in the middle of moving from my apartment. I'll take a look at the transistor within this week! Thank You!!

[peterkohlmetzmoller]

Waiting on a german supplier I found who could send me a new transistor for the board.
While I'm waiting I wanted to show you guys the renovation, or rather how it turned out. Unfortunately I dont have progress pictures, only the one from the seller back when I bought it, it doesn't show much though.
The piano was very beat up, "tolex" leather was missing in big spots all over, and could easily be piled of by hand. The seller had put lots of ducktape in those spots, and it was also very dirty inside out
Electronics didn't work, the piano was very out of tune and some hinges and other small part were missing. Pedals was not functioning.


So I've taken of the "tolex" leather of everything, and decided to keep it raw plywood with varnish, as i'm not going touring with this thing. Lots of hours went into cleaning all the old glue underneath the leather, very hard job actually, as you cant sand it of, it has to be done manually. I've sanded a couple of times after that, finer and finer as I went along and used some nice ship-type varnish. Pedal and legs also was renovated, and all hinges have been cleaned and are back in raw steel, as I tediously cleaned and re-finished everything.
Here's a picture, I'll post some more when I have some nice daylight in my new apartment.
I'm really happy with the result, looking forward to put some nice small full range speakers around it and play my a*s of. Cheers from Pete

[peterkohlmetzmoller]

Before varnish, after sanding and cleaning...

[peterkohlmetzmoller]

HI again!

I've replaced it and voltages are fine now.
still dont have sound though, only white noise as i turn up the volume of my speaker. Everything is connected according to the operating manual, and there are no signs of harm on the preamp board.
Tremolo seems to have an effect also, It slightly changes the hiss as I turn the depth/rate switches still.
I'm a little beaten down right now, as I thought I'd repair it simply by replacing the faulty transistor,but I was not in luck.
I just ordered 10 new FET's for the main preamp board as a guy has said on another forum I found. Do you guys have any experince with this? Could they be faulty now after the've been treated to wrong voltages over long time and god knows what. The board visually looks fine, and I've cleaned it intensively.
Any thoughts?
Here's the service manual:

[peterkohlmetzmoller]

[madtheory]

You'll need a transistor tester, a multimeter (many of them incorporate a tester) an oscilloscope and the service manual, and you'll need to go through the circuit systematically testing all elements. We tried, here, based on the available information, but it looks like not enough info was provided. Are you familiar with all of the textbook transistor amplifiers? Because the service manual probably won't tell you what the voltage and current is supposed to be, it is assumed that you're a tech who knows the basics. You'll need the data sheets for all the silicon bits too. Randomly changing transistors is a crapshoot.

[peterkohlmetzmoller]

You'll need a transistor tester, a multimeter (many of them incorporate a tester) an oscilloscope and the service manual, and you'll need to go through the circuit systematically testing all elements. We tried, here, based on the available information, but it looks like not enough info was provided. Are you familiar with all of the textbook transistor amplifiers? Because the service manual probably won't tell you what the voltage and current is supposed to be, it is assumed that you're a tech who knows the basics. You'll need the data sheets for all the silicon bits too. Randomly changing transistors is a crapshoot.

I know, It's a long shot. I'm fair at soldering, but I only own a multimeter and can only do rudimentary inspection/testing. I'm of course not satisfied with that at the moment, but I'm trying to learn as much as possible. That's why I'm gratefull you guys help, so thank you for that friends!
I guess a lot of us in the forum are amateurs trying to get to know how to repair our own stuff. In this case a 80 kilo grand piano, not movable without a big car/muscles, and no techs around in this area, you know the drill.
What do you think? At this point, could tis "fix" be anything? There was a substantial post in another forum about the preamp going dead because of dead FEt's, and heck, i ordered some for 7 bucks in germany, no big deal.
I can still go back to the bypass mod if I want to, but I'm too stubborn u know
If you look at the circuit, you seem to know your s**t, what do you think?
to recap:

No piano sound
quite a lot of white noise when hooked up
power lines from power supple and distribution board are fine after new transistor
when tremolo in engaged it effects on the noise
when shorting out the IN pin on the board a hum appears, so theres def. signal comming trough.

when i look at the schematics, here's my reasoning;
FET 1,2,4,5 and 6 should be replaced - thats where input signal form the piezo and the patchbay (not working) and eq (not working either,not affecting the noise at this moment) are.. it's the same tranny, all of them. While I'm at it I'll change 7 and 8 as well, they might be malfunctioning.
Any suggestion are SO welcome. thank you guys, I really appreciate your help. This post is awesome for the future owners of the CP pianos. Quite a lot of people seem to have the same issue with the old preamps. Wish somebody would design a new noise free board.... In the mean time let's repair some
Thanks guys, cheers, Pete

[peterkohlmetzmoller]

btw, the parts list says all the FET's are the model 2SK30AY. They where easy to find new old stock from germany.

[jxalex]

about transistors and checking if the sound goes through and there is some amplification --- make and begin checking backwards -- from the end to the beginning, by feeding the low signal in the transistor input (you need do disconnect the previous transistor output (just removing the capacitor) but still feed the transistor through 0.1uF capacitor with some signal (10mV). This way You find which stages work and which not.

Transistors may or may not be in working order.
If the voltages and currents in schematic are far below maximum ratings (check datasheet) then there is big chance that it is working.

[madtheory]

I think the best thing is to send the board (not the whole piano!!!) to a tech for diagnosis, because it looks like V and A are not specified in the circuit, it's assumed you will know? I wouldn't recommend replacing all the transistors, as I already said... that will be a crapshoot. It would take me far too long to work out what all the levels are supposed to be because I suck at numbers, and you don't want someone with dyscalculia doing the numbers on your valuable pre-amp LOL! Just pay someone who knows what to do! Doesn't need to be an MI specialist. Just give them the service manual.

I wouldn't recommend learning analogue electronics on this unit! Maybe pick up a copy of Doug Self's book "Small Signal Audio Design", and Horowitz et al "The Art of Electronics" in the meantime, and learn about transistor amp circuits. The Doug Self book is particularly accessible for someone with a small bit of background in electronics, and it's audio focussed whereas the other is general electronics. Then try building some pre-amps, or guitar pedals, or modular synth kits.

Or just bypass the pre-amp, and use a good high impedance DI box like the CountryMan, and a nice mic pre amp.

[jxalex]

Self's book "Small Signal Audio Design", and Horowitz et al "The Art of Electronics" in the meantime, and learn about transistor amp circuits.

...and when doing all the steps in practiche the result is in self-built pre-amplifier, which he knows inside out


The schematic on this amplifier is not far from some other transistor pre-amplifiers, but if being the beginner its always simpler to build from the scratch than repairing the existing.