Kawai KDP 80 Electronic Piano Dead

[pavb2]

Hi all

Glad to join the community. I have a Kawai KDP 80 Electronic Piano and can't get anything out of it. I've traced the 230 volt supply through the board and have 230 volts at the primary side of the transformer a Bando KT-031. The problem is I don't know what the secondary supply from the transformer should be and can't get any information I've tried Kawai and their distributors etc. The secondary is the two blue wires on the right of the picture.

I've got the service manual attached

but can't see what the secondary is any help appreciated.

 

[ChrisP58]

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

Pardon our interruption...

I would not use that particular transformer for this purpose.

Your link does go to multiple transformers, but I'm assuming that you're using the 50VA unit.

Now, I haven't looked at the full datasheet for the transformer, but the VA rating is for the entire transformer. Although it does have a 12 volt tap, it is nominally a 240 volt secondary. If the 50VA rating is for the max secondary voltage, then the VA rating of the 12 Volt tap may be as low as 2.5VA.

Now, this is a pessimistic view, and you're likely to get more than the minimum, but probably not the full 50VA. It will work for a while, but all of the I^2 R secondary heating is within 4% of the secondary winding.

Also, I'm not to keen on having a secondary referenced 240 VAC point near what should be a safe, low voltage system. (even if it is shrouded)

 

[pavb2]

Thanks I didn’t realise that the 12 volt secondary output would not be 50 va. I’ll put it on anyway to test the piano. What are the implications though is it a case that the transformer will overheat?

 

[Nigel Goodwin]

Thanks I didn’t realise that the 12 volt secondary output would not be 50 va. I’ll put it on anyway to test the piano. What are the implications though is it a case that the transformer will overheat?

Possibly, try it and see - it really depends how the transformer was made - and specifically what type of wire was used on the low voltage windings.

 

[pavb2]

Update

I’ve attached a photo of the transformers new one on right.
I took a step by step approach and found that the transformer was getting quite hot when I connected he 240 primary even before I’d connected the secondary side.

A number of times I checked the primary voltage and this was 90 volts instead of 240 the secondary voltages were half of what they should have been so I don’t really know what was happening.

I connected the secondary and all the voltages seemed to be ok and the transformer didn’t appear to be overheating. I had led lights on the piano which I didn’t have before but still no sound I also tried with headphones.

Not sure what to do now will have to have a think.

 

[Nigel Goodwin]

Perhaps you should post a picture of how's you've wired it up? - but it really seems to be an overly expensive and probably unsuitable transformer.

 

[pavb2]

Here’s a photo the wiring

 

[Nigel Goodwin]

I see on the top the transformer says "rated output current 0.2A", so it's far too small - it's nothing like a standard transformer, but a special one for a special (and obscure) purpose.

How did you come by it?, it's an unusual thing to come across.

 

[pavb2]

Seems to be a general purpose transformer 50 va

https://www.ebay.co.uk/itm/116133000144?var=416317758596&_trkparms=amclksrc=ITM&aid=777008&algo=PERSONAL.TOPIC&ao=1&asc=20250324084321&meid=25307f3011ad432e8ef3179e87b1754e&pid=101949&rk=1&rkt=1&itm=416317758596&pmt=1&noa=1&pg=4375194&algv=RecentlyViewedItemsV2WithMLRPboosterAndUpdatedFeatures_BP&_trksid=p4375194.c101949.m162918&_trkparms=parentrq:6342dcd21970a2c654ebf453ffff75f4|pageci:77605225-4767-11f0-9ad5-66b538961686|iid:1|vlpname:vlp_homepage

The fuse on the power board is rated at 630 ma What do you think the current output should be?

Unfortunately I can’t get a specification for the original transformer from Kawai or the transformer manufacturer.

 

[Nigel Goodwin]

Seems to be a general purpose transformer 50 va

Nothing of the kind, it's a very specialised item, for a very specialised purpose (what ever it might be?), hence it's high price tag, and complete unsuitability.

https://www.ebay.co.uk/itm/116133000144?var=416317758596&_trkparms=amclksrc=ITM&aid=777008&algo=PERSONAL.TOPIC&ao=1&asc=20250324084321&meid=25307f3011ad432e8ef3179e87b1754e&pid=101949&rk=1&rkt=1&itm=416317758596&pmt=1&noa=1&pg=4375194&algv=RecentlyViewedItemsV2WithMLRPboosterAndUpdatedFeatures_BP&_trksid=p4375194.c101949.m162918&_trkparms=parentrq:6342dcd21970a2c654ebf453ffff75f4|pageci:77605225-4767-11f0-9ad5-66b538961686|iid:1|vlpname:vlp_homepage

The fuse on the power board is rated at 630 ma What do you think the current output should be?

There's no 'think' about it, it's printed on the top - a mere 0.2A

Unfortunately I can’t get a specification for the original transformer from Kawai or the transformer manufacturer.

You don't need it, it needs to be about 12V AC out, or a LITTLE higher (but 12V is the nearest standard value), and the size of the old transformer is about 50W, so about 4A (not 0.2A)

 

[pavb2]

I've looked online and can't seem to find a suitable transformer if anyone can help

You don't need it, it needs to be about 12V AC out, or a LITTLE higher (but 12V is the nearest standard value), and the size of the old transformer is about 50W, so about 4A (not 0.2)

 

[Nigel Goodwin]

What's wrong with the suggestions in post #10 and post #18?.

 

[pavb2]

Update

I connected the two 12 volt secondaries of the shrouded RS 50 va transformer in parallel, connected it and the piano worked. The only thing is the piano is permanently on as the on/off switch (Manual page 25) doesn't turn the piano off. I think this is an unlatched button I tested it no continuity, then continuity when you hold it down and then no continuity when you release. I don't think this is the reason for the original transformer failing but I'll leave the piano on and see if the transformer overheats and report back.

I've attached the manual.

 

[Nigel Goodwin]

The manual shows a proper mains switch, plugged in to CN15 - if you unplug CN15 does that turn it OFF?.

 

[pavb2]

No unplugging the mains switch CN15 from the pcb has no effect

 

[Nigel Goodwin]

No unplugging the mains switch CN15 from the pcb has no effect

Well according to the circuit, there's a capacitor across the socket - C54 4700pF - if that was S/C it would bypass the switch. Check C54.

 

[pavb2]

At the moment the piano is permanently on and after a couple of hours is warm to the touch. I’ll remove the pcb to check across the capacitor.

Previously the piano switched itself off after a period of inactivity. If the auto power off has failed would it explain why the original transformer failed?

Edit: There is a power auto power off programmable function via the keys & buttons for 30 minutes I tried to program it but had no luck.

I’m convinced the transformer shouldn’t be permanently on, if all else fails could I break the primary transformer feed & put a timer through the original on off switch ? Or put a timer on the mains plug whatever I do needs to be permanent as I don’t want the risk of the piano being inadvertently left on.

 

[Nigel Goodwin]

Assuming the schematic is correct, then it already has a mains switch, and it seems likely that the capacitor I mentioned is S/C. When you take the board out, have a good look - make sure that the capacitor hasn't been replaced by a piece of wire, and the mains switch discontinued.

 

[pavb2]

I’ve had the piano from new so there shouldn’t have been any modifications to the circuitry. On other models you can program the auto power timer switch off so I’ll have a look at that. I’ll check the pcb but maybe it’s a case of replacing that as well.

 

[Nigel Goodwin]

I’ve had the piano from new so there shouldn’t have been any modifications to the circuitry. On other models you can program the auto power timer switch off so I’ll have a look at that. I’ll check the pcb but maybe it’s a case of replacing that as well.

I was referring to modifications when it was made, it's common for production changes to take place.

An auto power timer isn't going to help, as it's not switching the mains, just preventing the unit working. Looking at the circuit the only connection to the power stages is a MUTE line, so it's not even switching the DC power output, just shutting the output chips etc. down.