Akai S900 / 36yo power supply - Page 1
Forum section: electronics / repair

author: t0mt0m
author_rank: Contributor
timestamp: December 24, 2022, 01:32:17 pm
content: Hi,I've bought a non working Akai S900 and the first thing I did was to check the power supply voltages.The unit generates +5V and +12V referenced to DGND and +-15V and +-6V referenced to AGND.All voltages measure correct except the -15V rail is putting out around -20V. Not good.I tried only running the CPU board with 5V and after some fiddling I got it to boot up and show correct stuff on the lcd. So it might be fixable.But before moving on I probably should address the power supply to make sure I have correct and reliable voltages.On other units I have successfully swapped out the power supplies for switched mode ones (e.g. meanwell).But I'm lacking knowledge on how to deal with the DGND/AGND scheme.I could also order parts for the existing power supply and try to fix it.And of course I am going to recap the whole unit if I get it working.Attached is an image of the PSU schematics.Would be very happy to get some advice.cheers

author: TheMG
author_rank: Frequent Contributor
timestamp: December 24, 2022, 05:52:04 pm
content: Check TR902, it is the pass transistor responsible for regulating the -15V. Based on the transformer secondary voltage, -20V sounds like the full unregulated voltage, meaning the transistor may be shorted.As for the AGND, DGND thing, basically the two halves of the power supply are galvanically isolated (note the use of separate windings on the transformer secondary). The idea is to help keep noise from the digital circuitry out of the analog audio circuits.Pretty simple linear power supply really, no reason it couldn't be fixed. Given the age, do check all the electrolytic capacitors.

author: t0mt0m
author_rank: Contributor
timestamp: December 24, 2022, 07:22:17 pm
content: Thanks for the reply2SD1189 is available for reasonable money on ebay, so I might order one.And you are right, the S900 has a 220/240V voltage selector at the back and when i change the setting it does change the voltage, so it must be full unregulated then.The only reason why I would prefer swapping the whole PSU out for something modern is the fear that something else will fail sooner or later possibly destroying some unobtainium parts. The PSU does not seem to have any kind of overvoltage protection, crowbar whatever ...Could I just get a +5V smps and a +-15V smps and -+6V linear regulators and simply hook the 5V one to DGND and the -+15V one to AGND?(I don't need the +12V because I will be using a floppy emulator)

author: TheMG
author_rank: Frequent Contributor
timestamp: December 24, 2022, 09:07:41 pm
content: Quote from: t0mt0m on December 24, 2022, 07:22:17 pmKeep in mind there's nothing particularly special about that transistor, there's a plethora of transistors still in production that could be an adequate substitute and even exceed the specs and robustness of the original.Quote from: t0mt0m on December 24, 2022, 07:22:17 pmThe vast majority of SMPS do not have such protection either. With a linear power supply like the original one, the worst that can happen is that it outputs full unregulated voltage. With an SMPS if the feedback loop fails, it can easily output 2x its rated voltage or more! Yes, I've seen this happen. If this is something you're particularly worried about, I would recommend adding an external crowbar circuit regardless of what type of power supply you go with (except in the rare case that you do find one with a built in crowbar).Quote from: t0mt0m on December 24, 2022, 07:22:17 pmYes, you could.

author: t0mt0m
author_rank: Contributor
timestamp: December 24, 2022, 09:32:46 pm
content: I really appreciate your reply. This has made me rethink my strategy. I'm going to do some research on what modern parts I could use to rebuild the power supply. Thanks a lot!

author: t0mt0m
author_rank: Contributor
timestamp: December 29, 2022, 01:57:38 pm
content: Just wanted to report back that I repaired the original power supply. Works fine now.I've replaced all electrolytic caps and the M5230L with its two transistors.For transistor replacements i used a BD437 for the 2SD1189 and a SA1357 for the 2SB891.I'm sure these are not a perfect fit, esp. the BD437 has way less fT, but I wanted to order all the parts from the one shop that had a M5230L so this is what I went with. Also I found a forum post from another guy using a BD437 successfully.As expected the sampler has more problems. It wants to boot and is showing correct messages on the lcd, but then halts at a certain point. Sometimes earlier, sometimes later. Will have to investigate further. Sadly the service manual I have is lacking the CPU board part.Since the RAM ICs are known to fail very often, I'll check them next.thanks for the helpcheers

author: t0mt0m
author_rank: Contributor
timestamp: December 29, 2022, 11:12:35 pm
content: In case anyone stumbles across this trying to fix an S900.Apparently it's normal for an S900 to hang if the digital part of the voice board is not connected.With the 3 ribbon cables and the 5V plug attached, it will boot.Found the info here:https://gearspace.com/board/geekzone/519042-fixing-dead-akai-s900.htmlSo, yay, mine is successfully booting now. but it will freeze/display garbage/display "divide error" on some menus.So I'm suspecting a RAM issue. Parts ordered.Edit: RAM issue it was. Works again.