Electronics > Repair
Kawai CA 65 digital piano amplifier
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shockpoint:

--- Quote from: Audiorepair on August 17, 2024, 07:11:10 pm ---
--- Quote from: shockpoint on August 17, 2024, 02:16:28 pm ---
Do you think an overload on this would cause both the fuses to blow? Or just one?

--- End quote ---

Both rails and VCC+ are supplied via both fuses via the 4 diodes in the Bridge Rectifier. 
If there was a short, one fuse would blow first, the other would still be supplying both rails via the 2 diodes still in circuit, and would blow shortly after.

If you wanted to experiment, you could always deliberately short out the 2 regulator outputs, and see if it blows the fuses or not.
You shouldn't damage anything, the regulators will just do what they are designed to do and shut down.
But they may still get quite hot and draw considerable power.  Dunno, never tried this myself.

--- End quote ---

I see, OK. I'll give that a go when my dim light tester is completed, I don't want to blow anything unnecessarily!
I went and tested all the 12+ and 12- rails to all the components and again there were no obvious shorts.

I read your line about the bridge rectifier again, and I see your point. Have you ever had any experience of slow blow fuses blowing out of nowhere in isolation to another issue/short in the board?

Some updates:
- I found that the voltage regulator GND pins do not ground to the heat sink. So Pin 2 and heat sink have no continuity. However, other parts of the circuit including the ANALOG GROUND from the transformer DO in fact ground to the heat sink
- There is a capacitor there (as you can see in diagram 4 in the OP, marked C12); I highly suspect this is blown based on a quick and dirty test - it's not charging up from the multimeter leads voltage, just reads OL.
- However I don't think this would contribute to a short circuit that would blow a fuse, but I'm not that experienced. What do you think?
- Photos of the components are attached

I have gone back to test the NJM V+ and V- rails as well and in particular the opamp IC responsible for putting audio out to the speaker and the headphones read like this on a quick and dirty diode mode test in-circuit:
- red lead on V- and black lead cycles through all the pins Ainput+, Ainput-, Aoutput, Binput+,Binput-,BOutput
- the V- and Aoutput, V- and Boutput in diode mode read: 0.827V
- every other NJM4580 (identical biasing circuitry) is reading 1.087V apart from the NJM4556AD (which I have nothing to compare against)

I suspect there is an issue here; would you be able to shed light on what diode mode would be reading between V- and Aoutput given the NJM4580 has the attached circuitry?
- attached datasheet only shows the circuit for half the dual op-amp
shockpoint:
An update!

I think I may have found the problem. It was not located on the amplifier board after all!

After some systematic connection of wires, I believe when my amplifier board is connected to the mother board, the -12V and the +12 rails are connected to a 125ohm resistance (I am still trying to find out where exactly this is).
On the mainboard there are 5 op amps of the LM833 variety. I believe these are connected in parallel based on DMM readings. You can see the schematic (but there is no circuit diagram, only an assembly diagram unfortunately. Kawai is a bit stingy)
I have circled the area on the board that is powered by the -12V,+12V rails
When the entire keyboard circuitry is connected together, there is a 125ohm resistance between -12V and +12V.
It is at this point when the power is turned on the fuse blows again

So I replaced the fuse, depinned the connector to the motherboard that had the -12v and +12V rails and what do you know, the fuses stayed intact

Based on the opamps on the amplifier board also running -12v and +12v NJM4580L, but NOT having any 125ohm resistance, I can only deduce that the 125ohms possibly represents an abnormality, or a short circuit. This certainly is in line with the fact that once the -12v and +12v were depinned, the short did not occur.

Now I need to run through all the capacitors and look at the ESRs
But my suspicion is on one of the LM833s because diode mode on the pins reads differently on one out of the five, whereas the other 4 read the same.
- my dirty test was putting the positive DMM lead on -VCC rail and the negative DMM lead on the output leads - this read a different voltage in diode mode compared to the other LM833s.

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