Faulty Keyboard Technics KN 500 switches off all the time.

[juppie]

Hi, all!
I've got a problem with my old KN500 Keyboard from Technics. When switched on via the push button, It switches off after some time. Just goes off as if the power was killed.

In the start, this was related to loud sounds, so there might have been an overload of the power supply or just a bad connection? But now it just goes off a few seconds after turned on.

As this keyboards explicitly states in the manual that it shouldn't be switched off using switches on extension cables or plugging off the power etc, there seems to be a "fragile" part, involved with the power management. Any Ideas on how this or a typical circuitry for keyboard power works, or repair tips for this model would be highly appreciated. I'm quite ok with electronics but haven't had the time to do some measuring at the board itself...

Hope to have some of you pro's give me some hints,
cheers,
Juppie

[Jazz P Bass]

Open it up.
My guess about the circuit would be:
Switch to power transformer.
Power transformer to voltage regulation board.
Check that the switch is not at fault.
Check the soldering on the regulator board.
Check the voltages on the regulator board.

[juppie]

Thanks! I'll check that first. The fault is probably on the regulator section. I hope its just the soldering. Is there any component which is knwon to wear off on that. THe capacitors?

Can you explain what you mean with "Switch to power transformer" (probably my bad english) Do you mean focus attention to that? The switch is not mechanical but an electronic circuit with calliper one.

Thanks a lot I'll be back wit some measurements

[Enzo]

The shut down instructions are not probably about any fragile parts. it is more likely that there is some sort of internal shut-down routine, and just yanking the power plug would bypass that. Then the unit could wake up confused on its next use.

I agree with Jazz. Check the switch itself. Make sure all connections to power transformer are solid. Check solder on regulators and other power supply compopnents.

You should be able to identify the power transformer, so check to see if mains voltage is present at its primary when the keyboard shuts itself off.

When the keyboard is running, tap on the circuit boards to see if you trigger any loose connections.

With it running, poke each component in the power supply with something insulated. A wooden chopstick from a Chinese restaurant is ideal for this. See if poking anything triggers the fault.

[juppie]

Ezno, thanks for the extensive description! I'll do my best to find the fault. A big concern of mine actually was the one with the manual saying "don't switch ff by power plug", which you gave a perfect explanation for (which I wonder why this never occurred to me as a computer science student :P). Thanks!

Davailo, thanks for your recommendation. Fortunately this is not the first electrical audio device I open up, but I of course will be patient as usual. The whole device is 12V-operated, so it seems to be a matter of either replacing the external power transformer or fixing something in the internal 12Vcircuit.

One thing you actually made me curious about is how long it would take me to find a professional electrician who has experience with this kind of electronics. As all our old tube-driven TVs die out in Germany and People tend to buy new things instead of fixing old early 90s Keyboards there may be a handful of (hopefully very well paid) people who like to repair such a thing.

I take it more as a leisure challenge, as a comparable device would cost about 90euros.

[Jazz P Bass]

Technics KN6000 SM

Here is the power supply schematic for the KN6000.
It may be of some use.
(compare it to the KN500)
Here is the link to the complete SM.
Link: TECHNICS SX-KN6000-SM Service Manual free download, schematics, eeprom, repair info for electronics

[Enzo]

What you need is a competent electronics tech, not necessarily one who has worked on this thing specifically. I have never seen one, but I;d bet my lunch money I could fix most common problems on one if it came to my shop.

Yes, a computer is a perfect analogy, after all, synths ARE computers. Your computer instructions tell you never to turn it off by pulling the plug.

[juppie]

Hey, folks! I've identified a transistor in the power circuit which seems to have suffered from overload. My problem is that I cannot find an exact match in my local electronics suppliers. The transistor is more high than a standard one, and printed:
C3940AR
Japan .46
Does anyone know a part with similar or equal specs?
How would I search for a replacement?
Thanks for the specs, Jazzy!

[Jazz P Bass]

2SC3940AR transistor

I would think it is a 2SC3940AR.
Digikey has them.
Transistors (BJT) - Single | Digi-Key
MCM Electronics also.
http://www.mcmelectronics.com/product/2SC3940&t=1

[juppie]

Hey, folks!
Thank you for the great help. It was the 2SC3940AR transistor, which was quite easy to replace once we got the device in Germany.
I've got 3 of these left if anyone is interested... ;P

Many thanks and great forum!
Daniel

[Terry Douglas]

Hi Daniel

Delighted to have found this thread as I am trying to breath life into a Technic KN500 with exactly the same problem. Do you still have one of these transistors left by any chance?

Happy to cover costs of course.

Cheers, Terry

Hey, folks!
Thank you for the great help. It was the 2SC3940AR transistor, which was quite easy to replace once we got the device in Germany.
I've got 3 of these left if anyone is interested... ;P

Many thanks and great forum!
Daniel

[juppie]

Hey Terry,
cool that this comes up again.
I just had a look through my electronics boxes and can't find the transistors any more. I might have packaged them with the keyboard at some time, and won't see that as I'm far from home :/.

it seems that part is really close to extinction now. I found a similar part on ebay (SC3940A-NPN-Transistor-For-Low-Frequency-Output-Ampl-Toshiba-TO-92L) here.

Where are you based?
Note the model number is different (2SC3940A instead of ...AR), so I can't guarantee it is not going to burn away. Best to compare the datasheets (there is one for my suggestion datasheet(4).pdf
)

That disclaimer being said, you could try it. They are both 50V1A. Careful though - the pins of the 2SC3940AR could be different - if (and this is just an idea) the last R stands for reversed.

Maybe some more seasoned folks here can suggest a modern replacement part? Your local electronics dealer should know. It's just a power switching transistor and I bet you can replace it with other NPN components that can handle 50V1A...

Replacing the part was very easy, so good luck!

[g1]

MCM has a listing:
Panasonic Transistor Sub:2sc3940aq | 2SC3940AR | Panasonic

[Terry Douglas]

Hi Daniel

Thanks so much for your reply and lots of information.

Looks like the problem is going to be finding one of these transisters. I'll probably try the 2SC3940A (i.e. the same number, but without the 'R' on the end) and hope it will be the same. I note your comments about what the 'R' may mean, so I am concerned it may be different connections and I won't be able to know.

I'll continue for a while with websearches to find the right number.

I'm based in the UK. Sadly MCM (kindly suggested by G1) don't supply to the UK.

Anyway, I'm hugely appreciative for advice and suggestions.

All good wishes, Terry

Hey Terry,
cool that this comes up again.
I just had a look through my electronics boxes and can't find the transistors any more. I might have packaged them with the keyboard at some time, and won't see that as I'm far from home :/.

it seems that part is really close to extinction now. I found a similar part on ebay (SC3940A-NPN-Transistor-For-Low-Frequency-Output-Ampl-Toshiba-TO-92L) here.

Where are you based?
Note the model number is different (2SC3940A instead of ...AR), so I can't guarantee it is not going to burn away. Best to compare the datasheets (there is one for my suggestion [ATTACH]42924[/ATTACH]
)

That disclaimer being said, you could try it. They are both 50V1A. Careful though - the pins of the 2SC3940AR could be different - if (and this is just an idea) the last R stands for reversed.

Maybe some more seasoned folks here can suggest a modern replacement part? Your local electronics dealer should know. It's just a power switching transistor and I bet you can replace it with other NPN components that can handle 50V1A...

Replacing the part was very easy, so good luck!

[g1]

The R does not mean 'reversed'. Any 2SC3940A should work. Lots of other transistors would probably work too.
The R designates the HFE range as stated at the bottom of pg1 of attached datasheet. It should not matter here.
A couple UK ebay sellers, one with free shipping, one not:

2SC3940A Transistor Make Panasonic | eBay

2SC3940A TRANSISTOR TO-92 C3940A