Roland keyboard power supply problem

Den Thu, 25 Jan 2007 09:54:35 +0000 skrev Mungo \"Two Sheds\" Toadfoot:

New capacitors. The bulge is a sign. If you mesure them with aa ESR meter you will find a high esr. It cauces all kinds of strange faults in the PSU.

On Thu, 25 Jan 2007 09:54:35 +0000, Mungo "Two Sheds" Toadfoot Has Frothed:

Start by replacing capacitors in the PSU. Having done repairs for a Roland dealer I've fixed a few SMPS that had bad caps.

Much appreciated, thank you... and to Mr Plow :)

I've put some pictures here if you wouldn't mind having a look:

The 1st, 3rd and 4th pictures show the small bulge in the top, and the 5th and 6th pictures show their position on the board, next to the transformer.

I'll get searching for new ones.

Would it be an idea just to replace all the caps on the board or is that overkill?

Si

On Thu, 25 Jan 2007 12:24:11 +0000, Mungo "Two Sheds" Toadfoot Has Frothed:

Might as well replace all 8 or 9 of them while you're at it. Don't remember how hard it is to get to the SMPS in your particular keyboard but if I recall it's not something that I would want to undertake as other caps fail :)

Apart from the... oh, 15,000 perhaps, screws holding the back on it's not too bad to get into the thing - there's remarkably little inside it either, considering what it can do!

Having said that I'll still replace all the caps I can.

Thanks again,

Si

On Thu, 25 Jan 2007 09:54:35 -0000, "Mungo \"Two Sheds\" Toadfoot" put finger to keyboard and composed:

I don't think the largest caps are your most likely culprits. Instead I think that the one near R12 is probably the root cause of your problems. As a general rule I always check the smaller electrolytics on the primary side of switchmode PSUs (eg the one near R2) as these often go high ESR. But as others have advised, change all the caps on the supply secondary for good measure.

- Franc Zabkar

Thanks Franc.

If you have a minute spare I wonder if you would look at the second picture again and tell me if you can work out what is next to the capacitor you recommend I replace; the one near R12?

I know it's difficult to make out but the power board is back in the keyboard now until I can find a source of capacitors, so I can't take more pictures yet. It looks to me like another capacitor but it's covered in moulded black plastic with a pinched top. It looks to me like some kind of sleeving to protect the component from heat, or interference maybe, as it's located next to the transformer? I will remove the component when I change the caps but at present I've no real idea what it is!

Any ideas?

On Sun, 28 Jan 2007 10:37:31 +0000, Mungo "Two Sheds" Toadfoot Has Frothed:

Maybe an MOV or thermistor? Got a schematic?

Unfortunately not. I wish I did but sites which had Roland schematics were asked to remove them.

Si

On Sun, 28 Jan 2007 10:37:31 -0000, "Mungo \"Two Sheds\" Toadfoot" put finger to keyboard and composed:

A coil? If so, it will measure close to zero ohms, and there should be an Lnn circuit reference on the PCB. I'd find out where the + terminal of the capacitor goes. It will probably go to one of the secondary outputs at connectors CN1/2/3/4. Actually, upon closer inspection the capacitor looks to be covered in fixing glue, so it may not be bulging as I first thought. BTW, when replacing these caps, make sure you use low ESR, 105 degC types.

- Franc Zabkar

Thanks. I'll have a closer look when I start replacing things.

Ok. There's nothing on the board that *looks* dead at all really; all the caps look perfect apart from just one of the big ones that has the slight bulge in the top.

Is it possible that another board is causing the power to shut down and restart? If so that opens up a whole load of new possibilities! The relay which clicks when the keyboard is switched on, or resets, is on a seperate power amplifier board which is fed from the main power board, obviously.

I wish I knew more about this stuff!! :)

Si