Hi, just wondering if I can get some help with this diagram. The piano symptoms were as follows:
- suddenly not able to hear sound from either the headphones or the speakers (Not related to the Jacks in the following schematic, same circuit board but different jacks)
- digital screen still works
- MIDI output still works
- Fuses to amplifier board blown (which is the board shown here)
I've tested everything across the board and I've come across an oddity that I cannot get my head around. There seems to be a short across a PAIR of capacitors, and I cannot figure out whether this is abnormal or normal.
In the attached schematic, the two red circled capacitors C59 and C60 read as a dead short, 0.001ohms, and the value doesn't change over 5 seconds either, so I don't think it's charging. They both short to each other as well.
I have also taken a photo with some markup to give some physical context - as you can see there is a burn mark circled.
I have marked the pins of the Line In 1/4" jack, and the pins are:
https://www.synth-parts.com/media/image/ac/39/07/YKB21-5074-schematic.jpgAs you can see, the "burn mark" straddles the trace between pin T of JA4 and Capacitor 61.
The op-amp is a dual op amp NJM4580L, and I have marked the pins 1-8. The datasheet is here:
https://pdf1.alldatasheet.com/datasheet-pdf/view/7454/NJRC/NJM4580L.htmlIn the third image, you can see the way the 12V and -12V rails are connected on the schematic, all the way to the transformer. Fuses (slow blow) F5 and F4 were found to be blown.
Questions:
1. Can someone please have a look and explain to me how this might cause an overload on F5 and F4? (I tested the bridge rectifier and it was working fine).
I can't figure out how shorting out C59 and C60 would result in an overload; the positive terminals of each of the capacitors connect to a jack INPUT, moreover this terminal does not cause the (-) terminal of the op-amp to short to ground.
2. How is it both capacitors C59 and C60 short TOGETHER? How likely does this occur? I'm not very experienced with circuitry so I don't know how capacitors in this configuration behave but I presume that the likelihood of both failing together is low.
The only thing I can think of is that pin T of JA4 is connected to Pin B of JA3, so an overvoltage would blow both C59 and C60 together.
Incidentally if you look at the circled "?" trace of JA3, you can see a bit of a bubbliness under the PCB top layer suggestive of high temperature exposure, albeit less severe than the JA4 one.
3. What could have caused this issue? What would have failed first in the schematic, in your opinion to have led to the cascade of events and then the symptoms?
**EDIT - I was looking over the schematics again, and I guess with how the op-amp negative feedback resistor R44 is tied to the negative input terminal, there must be a parasitic drain from the +/-12V rails across R44, R41 and C61 to GND?
Noting that C61 is a polarity specific capacitor, how could this current occur?
Impression
- So my impression is that the short caused an excessive draw from the 12V or -12V rails to ground, resulting in overcurrent in fuses F4 and F5 and causing them to blow. It isn't an outright short circuit but more of an overload.
Would someone be able to provide insight, because I think I understand what's going on but I can't fully explain from cause to symptoms
Thank you!
Update 1: Turns out the jack has a clip that when connected, decouples the short, so that was a wild goose chase. I have now turned my attention to the power board - there are two voltage regulators on it - 78M12 and 79M12, which are positive and negative voltage regulators respectively.
- Is there a difference in the way the positive regulators and negative regulators are constructed? I see a very different voltage drop using diode mode on the 78M12 output->GND; This might be measuring diode D27, but my voltage drop is 0.126V, while it is 0.600ish volts for the Output->GND of the positive regulator. Alternatively maybe D27 is a mess and D26 is functioning correctly.