Don't jump to conclusions so quickly.

If IC48 is 74HC04, shorting pins 1 and 2 while A15 is driving pins 1 and 2 together could very well put the inverter into the linear region at 2.5V We still don't know if the A15 output is blown.

 

Don't jump to conclusions so quickly.

If IC48 is 74HC04, shorting pins 1 and 2 while A15 is driving pins 1 and 2 together could very well put the inverter into the linear region at 2.5V We still don't know if the A15 output is blown.

What about making a Reset Switch for IC1 as was mentioned previously? I could then monitor A15 at Reset. I know power reset should do the same. I say should??

Forcing a Reset after power up might see a difference?

 

Yes, I'm coming up with that next.
We will install a reset switch and will monitor both A15 and A0 simultaneously with two probes.

I will look for places for you to solder some hook up wires.

 

WITH THE POWER OFF,

wire a NO (normally open) push button across the 10μF/16V cap or any place convenient (cap, 10k or D10) to GND.

Solder solid hookup wire to A0 and A15 as shown (to allow hands free operation and no accidents).
We are looking at A0 at IC8-12 and A15 at IC45-15.

Connect A15 to Channel 1 and A0 to Channel 2. Trigger the scope on Channel 1.

Turn on power and push RESET button and release. You will have to do this with the scope at different HOR SWEEP settings.

What we are looking for:

First and foremost, that A15 is capable of going both HI and LO.
Under normal operation, both A15 and A0 should be toggling like crazy going LO and HI. Any other information (such as frequency) is not vitally important at this stage until we plan our next step.

A15 switches between accessing the program ROM and everything else (other ROM, RAM, I/O).

A0 is the lowest address bit and if this is not changing somehow IC1 is locked up.

 

If Kire is listening in, I hope you're paying attention here.
If you want to learn how a bused computer system works, this is the place to see it in action.
Re:

http://forum.allaboutcircuits.com/showthread.php?t=72694

 

Interesting to note that the said mentioned IC1 (KPU) YM3424 is still available from Yamaha.

(Nice to know)

Sorry for the delay getting back to this project, been caught up. Be back ASAP

 

Hello,

I have been unable to get to any more hands on work (its coming), however I have been able to do a little reconnaissance.

Not knowing the truthful history of this unit, I decided to contact Yamaha and was able to get some encouraging help.

Specifically, I tried to gather any known issues regarding this model. Here is what the Technician in Australia had to offer.

But before hand, please bear in mind that even though the KPU is the first component we have found issues with, I know that it may NOT be the problem. I just felt any additional information may inject some hope and optimism. Also, I am still very annoyed with myself for my Probe slip, and this is partly why my questions are based around this IC. But please note that, AFTER MY PROBE SLIP, I SAW KPU'S A15 (Address 15) BUS TOGGLING HIGH TO LOW. (If I knocked it out with my Inverter probe slip, its near impossible for that to happen directly after the slip. IMHO)

My email conversation with whom I believe was the chief Yamaha technician in Australia.


Hello,

Thank you for the information you provided regarding the availability of the KPU (YM3424)

May I please ask you a couple of questions. (Below)

I am attempting a repair on this Yamaha CVP-50 Digital Piano. (as I am assuming that the DM (Main Board) is no longer available)

Using the official Service Manual, myself and two other, qualified technicians, are working on this unit, and we highly suspect the KPU IC (YM3424). However we cannot be sure and I am hoping you might (or someone) maybe able to assist in anyway possible.


Any information you might be able to provide would be extremely helpful. (Questions as follows)






Are you in Australia?

Yes, in Melbourne, Australia.


Is there any Technicians in Australia I might be able to contact regarding this issue?

I provide the technical support to our Authorised Service Centers, but apart from answering these questions, I'm not in a position to spend further time on your problem. Sorry.


Is this KPU IC (YM3424), an upgraded/improved version from the original. (As this version has the dash 2) YM3424-2.

For your purposes, you can ignore any suffix variations of the YM3424 device number.


Can anyone tell me if there was a known issue with the original KPU in the CVP-50?

No known issues. Please remember, you're dealing with a 23-year old unit; any recent failure is unlikely to have been caused by limitations (if any) of the original design or construction.


Would anyone be familiar with this Model and know if its likely that the KPU has died?

It depends on what signals exist around the chip: clocks, data/address line activity, etc. I owned and repaired one of these about 15 years ago. It had numerous faults, but ultimately, all of these were caused by open-circuit PCB tracks damaged by surface corrosion from salt-laden environmental operating conditions.


Is the DM (Main Board) available for this model?

No


Is the Data Sheet available for the YM3424 IC (KPU) ?

No, it is a proprietary Yamaha device, intended for Yamaha's own products, and not for OEM use.

There are many parts in this unit that are now unobtainable, so the likelihood exists that even if you do diagnose and trace down to a certain IC, it may not be available anyway. In my many years of experience with such models and circuitry, and given this unit's age, I would suggest you don't immediately conclude that there has been a a chip failure. Remember, the KPU and other ICs in this system are working as somewhat inter-dependent entities, with many connections between them. As with any digital circuit, the failure of even one line could render the system unusable.

Before you consider any chip replacement, confirm the continuity of every connection, particularly those lines associated with inter-IC communication, data, address, chip-selects, etc.

Good luck.

Regards, J.



Thanks for your help
Regards
Steve

 

Steve, I believe you are making the wrong assumptions.

By investigating KPU IC1 I am not suggesting that this chip is at fault. There can be any number of reasons why the piano is not working.

In general, the usual culprits are:

Bad power supply.
Blown fuse.
Shorted decoupling capacitors.
Poor connectors, contacts, solder joints.
Shorts or open traces.

Finally, a bad IC.

The purpose for examining IC1 first is to see what is working and what isn't.
This is only an initial stage of the hunt. There's lot more to come if we find IC1 functioning.

So once again, you are off on a tangent.

At this stage I would not attempt to replace IC1 for the following reasons:

1. It is a difficult task.
2. We do not know that it is faulty.
3. Even if it were replaced we do not know that it would fix the problem.

 

Do you think it is worth removing the DM to inspect the back. Or shall I just continue on with the tasks you have assigned in the previous few posts?


I really don't want to upset it at this stage if possible.


Thanks Mr Chips.


I am trying to avoid my usual tangents, just trying to be helpful where I can in this mind boggling adventure.
Also, I thought it quite wise to ask the people in who eat, sleep and breath these products. Sure his information is nothing more than what you have been stating. On the other hand, he could have pulled out a whammy. If you don't ask questions, you will never get answers.

Will be back, hands on, over the next few days.
Thanks for your kind patiences.

 

Have you installed the RESET push button?

As I said, shorting the input and output of a CMOS inverter should make it a linear amplifier. There is still a chance that A15 from IC1 is ok.

We need to find out what is preventing KPU from accessing its ROM code.

 

Have you installed the RESET push button?

As I said, shorting the input and output of a CMOS inverter should make it a linear amplifier. There is still a chance that A15 from IC1 is ok.

We need to find out what is preventing KPU from accessing its ROM code.

RESET? Not as yet.

Can you please explain a little more about this statement "shorting the input and output of a CMOS inverter should make it a linear amplifier"

"There is still a chance that A15 from IC1 is ok." Sweet

"We need to find out what is preventing KPU from accessing its ROM code." Rodger that.

 

An inverter is an inverting amplifier with high gain. If you feed back the output to the input, you are applying negative feedback as you would in a linear op-amp circuit. With a 4000 series inverter you can in fact create a linear amplifier by feeding the output back to the input via a series resistor. The output attempts to bias the input at a voltage halfway between VDD and VSS.

With a 74HC04 inverter, feeding the output back to the input via a resistor creates an oscillator. CMOS output gates are current limited hence it is possible that the KPU A15 output could still be ok.

 

An inverter is an inverting amplifier with high gain. If you feed back the output to the input, you are applying negative feedback as you would in a linear op-amp circuit. With a 4000 series inverter you can in fact create a linear amplifier by feeding the output back to the input via a series resistor. The output attempts to bias the input at a voltage halfway between VDD and VSS.

With a 74HC04 inverter, feeding the output back to the input via a resistor creates an oscillator. CMOS output gates are current limited hence it is possible that the KPU A15 output could still be ok.

Awesome thanks. Yes I do remember Negative Feedback in Op-Amps from my former studies. I have forgotten most of this stuff now.

BTW, I also realized (well you triggered my memory) that the C in that 74HC04 stands for CMOS. I was incorrectly using my TTL probe. I do however have a CMOS version. (Its all coming back to me slowly)

 

Like I said, put the TTL and CMOS probe aside. They wouldn't be needed at this point.

 

Just thought I would let you know, that I have done absolutely nothing to the unit. Its still sitting where I placed it.

I had a Bass Player audition and s Solo Gig to take care of.

Be back soon.

Thanks
Steve

 

This piece of land fill is still sitting in my house. Today I found a mico switch.

I have lost interest and confidence, but that is normal for me.

I have had projects sitting years and had success. Please be patient. I have had other stuff to deal with.

Cheers SteveHow.

 

That's ok. I started a radio-controlled sailboat 25 years ago that I still haven't finished.
We're all in the same boat.

 

Hi guys,

Quick update. Due to major frustration and lack of room. I packed up this unit and put it in storage. I just had no room in my house to have this monster sitting around and basically it was giving the grumpys.

One day, hopefully with more experience on my behalf, I will revisit the unit. (I am waiting to get a new shed build)

Thanks
Steve.

 

Hi guys,

Quick update. Due to major frustration and lack of room. I packed up this unit and put it in storage. I just had no room in my house to have this monster sitting around and basically it was giving the grumpys.

One day, hopefully with more experience on my behalf, I will revisit the unit. (I am waiting to get a new shed build)

Thanks
Steve.

Wait 15 years, then come and revive this thread again. We will pick up right where we left off.

 

Steve, I've been working on the repair of a Yamaha CLP-550. Lots of similarities with your circuitry. I'd be interested in working it through with you again if you feel you have the patience. For example, I'd be injecting an analogue test signal at the output of the DAC to confirm that the audio amp section works OK. I know you say that the MIDI section seems dead. What did you use to in/out the MDU signals? I used an adapter I got off E-bay for just a few dollars and was able to extract keyboard signals and feed them into aa program running on my laptop. (In in fact at that point had a version of a working piano!) Anyway, if you are interested in re-engaging in the problem solving I'm out here in the ether.