Question on replacing diodes - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: October 07, 2018, 07:36:47 am
content: 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

author: exe
author_rank: Unknown
timestamp: October 07, 2018, 08:10:50 am
content: 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 meltedhttps://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.

author: Mp3
author_rank: Frequent Contributor
timestamp: October 07, 2018, 08:32:33 am
content: Quote from: exe on October 07, 2018, 08:10:50 amThanks 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.

author: exe
author_rank: Unknown
timestamp: October 07, 2018, 10:31:55 am
content: Quote from: Mp3 on October 07, 2018, 08:32:33 amYes.Quote from: Mp3 on October 07, 2018, 08:32:33 amI'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)

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2018, 01:47:27 pm
content: 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.

author: flynwill
author_rank: Regular Contributor
timestamp: October 07, 2018, 03:27:04 pm
content: I would expect that they were small signal diodes, 1N914, 1N4148 or similar.Given their location in a keyboard (and likely subjected to considerable vibration) I would bet the failure was mechanical.I would not worry about it unless additional parts start failing.

author: ArthurDent
author_rank: Super Contributor
timestamp: October 07, 2018, 04:50:58 pm
content: flynwill - "I would expect that they were small signal diodes, 1N914, 1N4148 or similar."I agree with flynwill except I'd lean toward ESD causing the diode failure so don't play the keyboard while standing on nylon carpet with rubber soled shoes. I'd also only replace the dead diodes but if more of the old ones continue to fail, I'd then replace them all.

author: Mp3
author_rank: Frequent Contributor
timestamp: October 07, 2018, 07:11:14 pm
content: Thanks for the help everybody!The diodes are listed on page 5, 1SS-133. i found some of the same kind of mouser.If i am not mistaked the diodes are between the keyboard and IC6.

author: _Wim_
author_rank: Super Contributor
timestamp: October 08, 2018, 07:25:12 pm
content: Quote from: Mp3 on October 07, 2018, 07:11:14 pmYes, the diode are used to make a switching matrix. This way more buttons can be read in with a fixed number of inputs (and a few outputs). A good description on how this works can be found here:http://blog.komar.be/how-to-make-a-keyboard-the-matrix/