Yamaha Keyboard P-95 no sound (2)

zerolagtime said:

Tools used: phillips screwdriver, basic soldering iron with temperature control, solder (60/40), solder suction pen, multimeter, a 6-8" piece of thin (22/24 AWG) solid wire (e.g. from a spool of Cat5 cable), brain, patience, extra patience, swear jar.
Parts: the 100uF/16v electrolytic cap of course. I had bought a variety pack of basic parts a year back, so I was lucky to have stock on hand. I'd look up ham radio clubs in your area and drop them an email asking for 2-5 of these caps. You'll not only reward their pack rat tendencies, but you'll make a new friend in the process. I'm not a ham, but I know some and they're always looking for people to get interested in their hobby.

I have the P-95B and the AM board is exactly the same as the schematic from the P-85 service manual. I had considered buying an ESR meter which is a $70 way to identify the broken cap, but I thought I might be able to reason my way through this. I had two big head starts. First, I have the schematic which Google helped me find. Some studying let's you identify likely candidate test points. Second is that this forum clearly shows that the 100uF/16V electrolytic caps fail after about 10-12 years, typically in places that get a lot of spikes (headphones, so C216/C217) or have a lot of power pass through them. My problem ended up being the latter, but I thought others here would benefit from the troubleshooting technique I used so that they can narrow the list of caps down to a few rather than replace at random, risking the pads breaking loose.

If you are rusty on removing components, I recommend getting something out of the rubbish pile and practicing your desoldering skills there before working on a board you care about. While your practicing, put the part or wires back in the hole and practice your soldering skills. It's easy to do a bad solder so do some Youtube-in.

I was able to do powered-on testing while disassembled by carefully connecting the long ribbon cables. Think VERY carefully about the directions the cables are connected and ensure that they are fully seated. Think carefully about your work setup so that nothing is going to tug on the ribbon cables because that's just game over if they break something. A soft carpet or towel will do nicely.

I had five main areas to test: power supply, audio standby, headphone connectors, audio output, audio input. The board runs off of 12v and 5v and the keyboard must be plugged in and powered on for these tests. Leave the AM board secured to the chassis. According to the LA4625 data sheet, the amplifier needs 12v on pin1 4, 5v on pin 2, and 3.2v on pin 4 (audio standby). The 5v power supply was easy to test - find IC102 or IC103 which are both NJM4580D opamps. Pin 1 of IC102 has a filled circle on the package. Numbering is counterclockwise, so ground is at the end of the row on pin 4 and 5v (Vcc) is pin 8, opposite pin 1. Set your multimeter to DC volts, around 15v but more than 12v. If your multimeter is registering 5v on those pins, then caps C408 and C404 are fine, as are the two voltage regulators. Now that you can find a reliable ground from the top side of the board, let's check 12v. Leave your ground lead to the tester on IC102 pin 4. Find C115 near the amp attached to the heat sink. The plus side of C115 is 12v (negative has a big black strip). You could get your tester on both pins of C115 and have the same effect, but it's tight quarters. If you've got 12v, then we know that caps C412 and C417 are fine - I think these are the large ones. If you've got weird behavior, like @default has, I'd start my investigation with whether or not power is stable.

Now that we know power is stable, it is on to audio standby. During power up, some self protection is used to avoid pops that damage speakers. The first protection is the standby line which is held low (to ground) until the super fancy digital board passes some initial tests and is ready to go, after which it raises to about 3.5v. Check one side of R122 for voltage. Anything over 3v is a win. If you really want, keep your probe on ground (IC102, pin 4) and R112 aand have a third hand power cycle the keyboard. You should see it stay at 0v for 1-2 seconds and then go to 3.5v. This told me that the main digital board is good.

The second audio standby test is the relay. If you've been playing this keyboard for any length of time, then you have the click memorized and would recognize it if it was different. If you hear a sharp click 1-2 seconds after power on, then the relay is probably just fine. If you never hear the relay click, then assume it's fine and check the circuit around it. Finding the exact relay is not fun and it's tough to find a substitute that has the same pin out and separation.

For these last tests, we need a steady source of sound, so start demo mode by pressing Select-Left and Demo at the same time. You can put the volume slider at mid level. If you're not sure that you're doing it right, you can just start the metronome and you get a blinking light.

Testing the headphones might be hard, but I decided to bypass the circuit to see if the dedicated headphone board had something wrong inside. As a bonus, we also work out if audio is coming out of the amp. Locate the middle connector with the speakers. Leave it plugged in. Now look under the heatsink near the amplifier IC. You should see four red capacitors and some resistors. I think R222 is closest. Use a wire to touch the right side of R222 to the white wire on the speaker connector. You probably could just set your multimeter to AC volts, say around 20V, and look for any voltage on R222 (use IC102 pin 4 for ground). If you still hear silence or have very low voltage, then the problem is at or before the amplifier. If you hear sound, then everyone's favorite caps, C216 and C217 have both probably taken a static shock that shorted them out. I mean, even if the problem is somewhere else, I'd replace those caps as part of the job if your soldering skills are good enough.
Finally, it's time to challenge the audio input to the amp. Your multimeter must be on AC volts, but this time measuring something below 5v. With one tester lead on ground (IC102, pin 4), put the other lead on resistor R112. If you get voltage, then audio is going to the amp. Double check it by stopping demo mode or the metronome and the voltage should drop to nearly zero. If there's no sound, then cap C108 is your friend to replace. If you were only getting sound from one channel, I'd check caps C108 and C208 early on in your testing as they have likely failed open, meaning a multimeter test for a short won't work.

If you got this far something is wrong at the amp. Look closely for scorch marks. If you see them, I guess you're buying a new amp IC and some thermal paste. Be sure to stick to the exact model number because a slightly different version has the same pins but more current draw which will likely exceed the design of this board and the speakers.

If there are no scorch marks, then C112 is probably keeping 5V from getting to the amp. Check the plus side of C112 for 5v (ground on IC102, pin 4), if there's wiggle room. My problem was C112 failed closed (a short) and was what finally fixed my board.

So, if you've read this whole forum and think that we're all guessing, please know that it's not true. Everyone's comments over the years in this thread gave me a strong bias towards dead caps. I looked at nothing else. I wonder how many other jobs would have worked if I'd considered that the caps might be dead. Yes, I had to spend over two hours reasoning about how the circuit worked and just as much developing tests. I've recorded some fundamental tests here that should help narrow down where your failure is at. Let's imagine that you had no voltage on resistor R112 while the metronome or music were playing. I'd go back in the circuit and see if the input to to the AM board had anything. This would be a check on R101 and R201 and no voltage on them would be a failure on the digital board. But failures around the op amps and inputs would likely only affect one channel which narrows your choice of caps too. Total failure is something that affects the whole system.

For those of you who harped on failed caps, I thank you. Someone early on mentioned an ESR tester and I'll probably get one the next time I have a repair but no schematic. I am so proud to have fixed this. I groaned with pain at the waste that would go from an instrument that's in otherwise fantastic shape. Reduce, re-use, (repair,) recycle.

Click to expand...