Kawai KDP 80 Electronic Piano Dead

What voltage do you have on the two blue wires from the transformer, if it's 'anything' the transformer is OK, if it's nothing the transformer is faulty.

The actual voltage is simple to guesstimate (as you've attached the service manual), it feeds through a bridge rectifier to a 25V rated reservoir capacitor - so the DC out has to be less than 25V. From there it feeds a power output IC, which wants about 18V DC, and also feeds an active smoother (Q12/Q13) which has a 16V output, so requires a similar 18V or so input.

So the AC output from the transformer needs to be about 12V RMS, or a little more.

Thanks Nigel I have 230 at the primary but nothing coming out of the two blue secondary wires a like for like transformer is no longer in stock.

In looking at the block diagram page 15 I thought the secondary voltage might be 20 volts but didn’t want to guess and potentially blow the pcb’s also what Kva rating is the transformer

pavb2 said:

Thanks Nigel I have 230 at the primary but nothing coming out of the two blue secondary wires a like for like transformer is no longer in stock.

In looking at the block diagram page 15 I thought the secondary voltage might be 20 volts but didn’t want to guess and potentially blow the pcb’s also what Kva rating is the transformer

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20 volts would be too high, it would give about 30V on the 25V reservoir capacitor - BOOM!!!.

As for the power of the transformer, you can estimate that from it's size and weight, both of which are pretty representative of the power - it's doesn't matter if it's higher power, just not lower.

Thanks again Nigel what transformer would you recommend I’m in UK so could get one from R/S Components?

pavb2 said:

Thanks again Nigel what transformer would you recommend I’m in UK so could get one from R/S Components?

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What does it weigh, and what are it's dimensions?.

This one is 20VA and weighs 550g

A chunky old-school transformer like you have is unlikely to fail unless something downstream draws excessive current. Have you ruled out all other posssible failure points? It would be a shame if a replacement transformer suffered the same fate.

The manual you link is apparently not quite correct for your keyboard - the PSU board in yours is labelled "KEP-389" and has a lot! more components on it than the KEP-348 schematic in the manual, including optoisolators and transistors etc.

I wonder if there are other differences at the secondary side of the transformer?

Where do the black & white wires, and the yellow wires, from the AC board go to?

Thanks all the transformer weight is 1.18 kg.

shroud is 55 mm (h) x 70 mm (w) x 35 mm (d).

Bobbin is 30 mm (h) x 40 mm (w) x 70 mm (w).

I’ve checked all pcb’s as far as possible.

The black and white wires go to the power on off switch which I’ve checked with a meter, the yellow go to midi input & output sockets.

On a 200 ohm setting I’m measuring 0.6 across the secondary transformer winding the primary winding is open circuit.

pavb2 said:

Thanks all the transformer weight is 1.18 kg.

shroud is 55 mm (h) x 70 mm (w) x 35 mm (d).

Bobbin is 30 mm (h) x 40 mm (w) x 70 mm (w).

I’ve checked all pcb’s as far as possible.

The black and white wires go to the power on off switch which I’ve checked with a meter, the yellow go to midi input & output sockets.

On a 200 ohm setting I’m measuring 0.6 across the secondary transformer winding the primary winding is open circuit.

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From the weight of the transformer, I'd imagine you need a 40/50VA replacement, based on the weight of the 20VA one I mentioned above.

There's almost certainly a thermal fuse in the primary, and conveniently it's a multi-tapped primary, and I'd assume that the thermal fuse is in the 0V connection?. So try checking the primary resistance from 230V to 120V, to 100V, I would expect those to read OK?.

If so, you can check the transformer, and the unit, by applying 120V between the 230V tap, and the 120V tap - if you can find a source for 120V. One source would be a builders power transformer, or if you've got a suitable sized (or larger) mains transformer with a tapped primary (120V/240V), you could use it's primary as an auto-transformer - so mains IN to 0V and 240V, and mains OUT from 0V and 120V - good enough to feed 120V to the original transformers 120V and 230V taps, just for trying it.

Failure of thermal fuses in mains transformer is often for no reason, it's a very common failure - however, it can also fail because of shorted turns in the transformer, or a short on the transformer output. Sometimes, you can see the connections to the thermal fuse, and bypass it for testing - I've often used such transformer with the thermal fuse bypassed, and instead added a suitable mains fuse feeding the transformer.

Would this toroidal transformer fit better, 80mmx33mm?

On the 200 ohm setting I’ve measured 40 between the 240 volts tap and 120 volt tap and 42 between the 240 volt and 100 volt tap.

I could strip it down and check the thermal fuse or on the assumption it is 12 volt secondary voltage simply buy the toroidal transformer suggested. I’ve got plenty of room in the piano.

Thanks again for your help fantastic knowledge

Have you got a transformer you could use to feed it with 120V - or a suitable DC power supply where you could feed 18-20V DC in where the transformer connects? (like a lab power supply).

To clarify if I could put a 120 volt AC supply on the 240 & 120 volt taps this would tell me what the secondary output voltage is?

Unfortunately I don’t have a variable power supply if so would it be a case of feeding 12 volts AC into where the transformer secondary connects on the pcb? Or could I get a transformer with a number of different secondary tappings and gradually increase from 12 volts up?

pavb2 said:

No unfortunately I don’t have a variable power supply if so would it be a case of feeding 12 volts AC into where the transformer secondary connects on the pcb? Or could I get a transformer with a number of different secondary tappings.

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Yes, 12V AC in there, or about 18/20V DC (due to the voltage drop in the bridge rectifier.

I didn’t realise you could put 18/20 v dc to the AC side of the rectifier presumably the output remains the same.

Is there a relatively cheap variable 12 - 24 v ac power supply or transformer I could get to input directly on the pcb

pavb2 said:

I didn’t realise you could put 18/20 v dc to the AC side of the rectifier presumably the output remains the same.

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DC passes perfectly through a bridge rectifier no problem (the diodes steer it the correct way - and only two of the diodes pass the current), it also doesn't matter which way round it's connected. You do make a two diode voltage drop though, about 1.4V

pavb2 said:

Is there a relatively cheap variable 12 - 24 v ac power supply or transformer I could get to input directly on the pcb

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DC, not AC - there are some nice cheap switch-mode PSU's available from AliExpress etc. - you can get a 24V DC one, and modify it to the slightly lower voltage required (just change, or add, a resistor).

We did a gig a week ago, using a StudioMaster Mixer that the bass player (who runs the band - I just do the sound engineering) bought cheap, as it was faulty, and also missing the external power supply (which is no longer available). I repaired it, and got two 24V supplies from AliExpress, I then modified them to give the required 17V outputs, and wired them for +/-17V - there should have been a 48V phantom power rail as well, but as we don't use condenser mikes we didn't bother adding it. There were no issues at all using a switch-mode PSU, and it worked perfectly - and none of the band even died, youngest is 60+, oldest (the singer) is 80 in a few weeks

Biggest problem was finding the correct plug, it was five pin, and looked like a CB mike plug - we found one that looked like it on-line, so ordered it - when it came it was only about half the size So looked further at the site where we got it from, and found it was available in various sizes, so ordered the correct one.

You could easily use one of those, and cheaper than a transformer.

pavb2 said:

To clarify if I could put a 120 volt AC supply on the 240 & 120 volt taps this would tell me what the secondary output voltage is?

Unfortunately I don’t have a variable power supply if so would it be a case of feeding 12 volts AC into where the transformer secondary connects on the pcb? Or could I get a transformer with a number of different secondary tappings and gradually increase from 12 volts up?

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If you have a toroidal transformer you can adjust the voltage by adding turns of wire through the middle of the toroid. A few turns in series with the secondary winding will increase or decrease the AC voltage. You can swap round the connections to the extra windings to change between increasing and decreasing the voltage.

I made a whole additional 15 V winding on an amplifier, but it was a lot of turns. https://www.electro-tech-online.com/threads/kef-psw2500-active-base-unit-repair.159244/

On larger transformers, it takes fewer turns for each volt generated. You can put a few turns on and measure the voltage they create before connecting the original secondary to anything.

If you do use a toroidal transformer, I suggest you use the mounting plate and neoprene washers, as toroidal transformers don't have mounting lugs. However, RS only sell the plates and washers in pack of 10, but other suppliers either sell mounting kits for a single transformer, or include them with the transformers.

I was thinking of using this transformer from RS as it’s close to the original. The only problem is if the original secondary output voltage (which is still unknown )required is more than 12 volts I’m stuck.


One other thing is the on/off switch is non latching but this doesn’t switch off the 240 volt supply to the transformer so when plugged in the transformer is on all the time Is this normal?

pavb2 said:

I was thinking of using this transformer from RS as it’s close to the original. The only problem is if the original secondary output voltage (which is still unknown )required is more than 12 volts I’m stuck.

RS PRO 50VA 2 Output Chassis Mounting Transformer, 12V ac, IEC 61558-2-6 | RS

uk.rs-online.com

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In the absence of no other options, get one and try it.

pavb2 said:

One other thing is the on/off switch is non latching but this doesn’t switch off the 240 volt supply to the transformer so when plugged in the transformer is on all the time Is this normal?

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Yes, quite normal for this type of device - in fact, mains switches in general are pretty rare these days.

I've bought this transformer which I'll fit next week. I would have liked an 18 volt tapping just in case the 12 volt doesn't work, the next tapping up is 24 volts which I'd be reluctant to put through the circuit