Electronics > Beginners
Question on replacing diodes
Mp3:
Hi,
I've got an old (25 years) music keyboard which has a series of diodes between the keyboard and the IC which interprets the note data from when the keyboard is played.
Suddenly three of the keys are not working, so i looked in the service manual, found the relevant diodes and tested them to see the had failed.
If i jump the diodes, those three keys work again.
Is it safe to replace the diodes with replacements or should i be worried about another issue that may have caused them to fail? or could it just be age that they went bad?
I will check the power supply voltage to make sure it is good but i have not replaced a diode before.
Many thanks in advance !!
http://www.synfo.nl/servicemanuals/Roland/W-30_SERVICE_NOTES.pdf
exe:
On which page of the manual are those diodes?
Anyway, I don't know how three diodes suddenly failed. As I understood, they failed open, which even more suspicious. Most of the time overloaded diodes fail short (unless bound wired melted https://electronics.stackexchange.com/questions/77712/can-a-diode-fail-to-open-position). But if it's a diode in keyboard matrix, it's not supposed to carry any significant current. What's even more suspicious is that it seems only diodes failed. I'd say diodes more robust than a typical IC.
I'd replace diodes and checked the power rails (and may be installed a current limiting resistor, but that's a hack). May be there is a problem with one of the rails so somehow voltage on one of the rails got lower/higher than needed and there was significant current through these diodes. But if it's a keyboard diode then diodes can only conduct when a key is present...
May be there was an ESD zap, and diodes are/were too sensitive, but so far I like power rails theory more as power rails have more energy to destroy diodes open.
Mp3:
--- Quote from: exe on October 07, 2018, 08:10:50 am ---On which page of the manual are those diodes?
Anyway, I don't know how three diodes suddenly failed. As I understood, they failed open, which even more suspicious. Most of the time overloaded diodes fail short (unless bound wired melted https://electronics.stackexchange.com/questions/77712/can-a-diode-fail-to-open-position). But if it's a diode in keyboard matrix, it's not supposed to carry any significant current. What's even more suspicious is that it seems only diodes failed. I'd say diodes more robust than a typical IC.
I'd replace diodes and checked the power rails (and may be installed a current limiting resistor, but that's a hack). May be there is a problem with one of the rails so somehow voltage on one of the rails got lower/higher than needed and there was significant current through these diodes. But if it's a keyboard diode then diodes can only conduct when a key is present...
May be there was an ESD zap, and diodes are/were too sensitive, but so far I like power rails theory more as power rails have more energy to destroy diodes open.
--- End quote ---
Thanks for your insight, as you can tell i'm still pretty inexperienced with this stuff, by "fail open" you mean doesn't allow current in either direction?
The keyboard is known to have a problem where an electrical spike (not sure what kind) towards the floppy drive at power-on has the potential to damage a disk in the drive. I don't know if it's a problem with the power supply i could come up with a hack to prevent it from happening. I was wondering if there may have been some power spike with it, or possible there was ESD damage....
Tomorrow i will measure the voltage on the power rails and order some replacement diodes.
exe:
--- Quote from: Mp3 on October 07, 2018, 08:32:33 am ---by "fail open" you mean doesn't allow current in either direction?
--- End quote ---
Yes.
--- Quote from: Mp3 on October 07, 2018, 08:32:33 am ---Tomorrow i will measure the voltage on the power rails and order some replacement diodes.
--- End quote ---
I'd say you need an oscilloscope to see for any voltage spike during turn on/off. I'd also check if capacitors are still fine, esp. electrolytics. But this may require desoldering as measuring in-circuit is tricky and never 100% reliable. Sometimes caps can absorb voltage spikes/etc (think of rc-snubbers), so I can imagine a broken cap can cause such issues.
PS oscilloscope channels have common ground, be sure not to short rails together and cause more damage. (Also, I hope your music keyboard has an isolated power-supply)
David Hess:
I could not find the diodes you describe in the service manual you linked but it was probably an age related failure. I would just replace the diodes and perhaps replace all of the similar diodes as well.
Navigation
[0] Message Index
[#] Next page
Go to full version