Digital piano won't power ON

[Alektron]

View attachment 38707 Hello

I've got a Technics SX-PR902C digital piano that will not power ON, only the RED LED lights up, when the ON button is pressed, but the display won't.It quit in the middle of a show.

I did not find a Service Manual online for it yet, I'm still searching. The SX-PR902C and SX-PR602M share the same User Manual. I know it's not he same piano, but I found a Service Manual for the SX-PR602M here:
https://elektrotanya.com/technics_sx-pr602_sx-pr602m.pdf/download.html

So far I checked the fuse, it was OK, then the transformer, input 110V AC OK, then I checked the 3 output AC wires the blue, black, blue (see photo). Between each black and blue, DMM measures 38V AC.

I don't see any fried components, although I noticed these 2 giant caps with the tops bulged (see photo), but I'm not sure if that is bulging or not, maybe that's how they normally look like.



What should I do next?

 

[Alektron]

Some more pics of the Capacitors and the circuit board. FYI, in the lower right corner those 6 black wires on the PCB are the speaker wires. I also checked those 8 diodes they are OK.

 

[(*steve*)]

Thanks for the photos.

Take a close look at the bottom of those large capacitors and tell me if the stuff around the base looks like glue, or something that's leaked out.

I don't like the look of those capacitors. I'd probably replace them even if the piano hadn't failed.

I would measure them with an ESR meter, but that's not something I guess you have

 

[Alektron]

My LCR-T4 ESR is on the way, I won't get it till February, but I have a DMM that does Capacitor testing. Looks like there is a glue underneath those caps for support, other components seem to have it too.

 

[Alektron]

In the meantime I took some measurements with my DMM at the 14 pin connector, that connects the two boards. (see photo)

 

[kellys_eye]

No 5V supply? Can you show the digital board in more detail - there must be a 5V regulator somewhere!

I note in an earlier pic that there is a backup battery on the digital board - does that measure ok?

 

[Alektron]

 

[Alektron]

No 5V supply? Can you show the digital board in more detail - there must be a 5V regulator somewhere!

I note in an earlier pic that there is a backup battery on the digital board - does that measure ok?

I couldn't remove that battery, I had to measure it in circuit with the power OFF. DMM showed 378 mV.

 

[kellys_eye]

Battery is flat - should read around 3.6V (or maybe 3V), certainly not 'mV'. It needs to be changed - as you;ve discovered it's soldered into place but you can get replacements from the usual suppliers (Digikey etc).

 

[Alektron]

I see. So before we proceed any further I would need to replace that battery? Is this battery used for the internal memory storage after the unit is turned off?

Thank you Kelly's eye.

 

[kellys_eye]

Yes, that is it's usual function. You can't discount any sort of 'problem' without first ensuing the battery is ok as the digital part may invoke many other states based on the backed-up parameters it's supposed to retain. Your switch-on problem may be only ONE issue controlled by it.

 

[Alektron]

I'll remove that Main board today, and see what type of battery it is, hopefully there is a number on it. I was trying to find it in the service manual that I've got for the SX-PR602m (lower model), hopefully the two digital pianos would have the battery as a common part, but I can't find it inside the document, or I'm just not looking in the right place.

 

[Alektron]

" CMOS (complementary metal-oxide-semiconductor) is the term usually used to describe the small amount of memory on a computer motherboard that stores the BIOS settings. " quote from this website.
https://www.lifewire.com/what-is-cmos-2625826

Also called Real Time Clock, (RTC battery). Wow, I'm learning something new every day.

 

[shrtrnd]

Batteries can explode when exposed to excessive heat.
Many batteries have the tabs spot-welded on, and the other end of the tab is soldered to the board.
Are you trying to desolder the battery at the wrong location?

 

[Alektron]

Battery came off leaving one leg behind. Now that leg that was left behind, gave me difficulty, the solder needed a lot of heat in order to melt, I guess this tin has a high melting point. Never encountered anything like it.

Battery is an: M GC5.5V 1.0F It measured 300 mV on the bench.
Actually it's a super capacitor.

 

[(*steve*)]

That may well be a multi layer board, and in any case one leg was connected to a fairly large ground plane.

There's probably nothing special about the solder, the board was just sucking heat from your soldering iron.

 

[Alektron]

That's something new, I thought I was doing something wrong. Thank You Steve.

 

[73's de Edd]

Sir Alektron . . . . .

GREAT SHADES of the very-very last of the late 1980's . . . . . . here we are having some state of the art digital design.

Battery came off leaving one leg behind.

DRATS ! I've even got some MORE bad news .

F.I.O. . . . . . for all you battery hounds . . . in actuality that "battery" is being a 1 farad @5.5 VDC super capacitor for short time retention of the units dynamic and static RAMs.

With the unit using a linear supply, with a power transformer whose mother was a microwave transformer and dad was an AC line transformer, I wouldn't be worrying too much about the state of those two large MAIN power filter capacitors either.

73's de Edd

 

[Alektron]

I realized that after I pulled it, that it was a super Cap, but what do you suggest De Edd?

 

[(*steve*)]

Get a new one and replace it. Be VERY sure to get the polarity correct.

Also I tend to agree with Edd. I didn't spot that it was a linear power supply. There's a far smaller chance of those caps bring bad