Roland Keyboard Works for 1 min... then dies

[(*steve*)]

Deigh: Voltage dropper? Lower amperage?

The original fault was almost certainly a transient short, possibly caused by heat deforming a metallic shield.

And the conversation has moved on since then...

 

[melbournesurprise]

Hello again!

Not surprisingly, the problem's reared its head again. This time it fails much quicker, within 5 seconds, and there's no real way to 'trick' it into working again.

Someone suggested I look for AC voltage on the DC outputs, and sure enough, there's about 10VAC on the 5V DC rail, and roughly 33VAC on the 15V DC rail. None on the DC -15V.

There's no dry joints on the power supply. So it seems to come down to the capacitors. To be frank, for someone (me) with only basic soldering skills, there is an intimidating amount of caps on this thing, particularly ones closely bunched together and/or glued to the PCB and surrounding components. I also have no idea which are the critical ones, so I would have to replace them all. And it's possible the power supply problem may extend beyond the caps.

I was thinking: is it possible to buy a 3rd party power supply and stick that inside instead? I've seen some units listed for around 50 bucks on ebay. However, my uneducated guess is the keyboard needs a specifically regulated and smoothed DC supply, and a run-of-the-mill 5V/+15V/-15V power supply that comes in a little box just won't cut it.

Is this the case?

Also: a while ago, just in case, I ordered a replacement Hitachi IC chip for the suspect one we identified. I plan on replacing that IC now anyway, but don't want to be replacing it in vain if the AC leakage is going to cause problems again. So something has to be done about this power supply.

In case you're all wondering, this is not my only keyboard, but fixing it has turned into a bit of a challenge/obsession! Onwards...

 

[(*steve*)]

How did you measure the AC on the DC rail?

Try measuring a 9V battery the same way and tell me what readings you get (obviously there is no AC on a 9V battery, but what your meter reads may be interesting)

For instance, a cheap meter I have here measures 1.538V on a DC range for a 1.5V battery, but 2.9 on the AC range.

 

[melbournesurprise]

Hmm, you are right. If I probe a 1.5V AA battery, I get 1.5V DC, roughly 2.5V AC.

To measure the keyboard, I was told to ground one end to the keyboard's case (which itself is connected to ground in the wall), and use the positive lead to probe the DC voltage rails on the power supply. That's where I saw DC and AC voltage out of what should've been a DC rail.

Is this an issue with cheap multimeters, or old/faulty ones? I could pick up one for 10 bucks, but if it's a cheapness problem...

At least this could be good news, in that it's one less problem to fix.

 

[melbournesurprise]

Repeated the test with a friend's industrial-quality multimeter, no AC shows up now. Going to replace that IC, if that doesn't work, it's the end of the road and I've done all I can!

Thanks again for all the help.