Akai S900 Help Needed

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Akai S900 Help Needed

Postby matia » Mon Feb 22, 2010 10:27 am

Ok ... to make a long story short, I got totally screwed over 70 quid on an Akai S900. I've got a S950 back at home in Cali and really wanted some nice 12 bit grit for use here in the UK, found a deal on a S900, and found the issues when I got home with it. f**k wont return my cash (he stood me up saying he would refund me and then bailed before I showed up to his place) so I guess I gotto fix it.

I am really baffled by the problems. I've repaired quiet a bit of gear but the Akai has puzzled me.

When sending it MIDI notes it basically adds notes, changes decay times, and plays back notes completely differently. I know there is no problem with my MIDI set up which is why I am very baffled by this. Second, when I attempt to trim the sample (discard before start and after end point) a lot of noise gets imprinted onto the sample itself. Like the data is corrupted. It also happens when I perform a reverse of the sample. Has anyone at all had these issues on their S900? The sampler's OS is revision 1.2. I've opened it up and made sure all the ribbon cables were seated properly, made sure all the molex was nice and tight and in place and overall ... the insides look good. I don't have my volt meter to test voltages but I suspect that there are a couple of paths to follow in terms of potential sources of the issues. My gut wants to say that this is a corrupt OS. I checked that the eprom chips were seated properly and they are, but perhaps the OS has gone bust. No unfortinatly I don't have a PC with an internal floppy to put OS 4 onto it and boot it and see if these issues go away. My guess is that the eprom OS is bypassed when you boot from the floppy OS. If anyone has OS 4 and can make me a copy I will happily provide an akai formatted floppy and send it to you via post and pay for return postage.

This could also be a power issue as in something is going awry with the power supply or the regulation of the voltage. Without a voltmeter I can't test this theory.

Finally, I have heard some stories about the floppy drive causing problems with the RAM before it dies. The floppy really seems to work fine and loads, formats disks, and saves data no problem. The data gets corrupted only when you perform certain editing functions.... As for the MIDI issues I really am baffled.

In any case, any feedback at all is much appreciated. I'm seriously pissed as I am without all my tools to do a proper diagnosis and also because this is the first time I get screwed in a gear transaction ... feel like my gear karma is under attack.

Cheers,

-matia
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Re: Akai S900 Help Needed

Postby Maschinengeist » Wed Feb 24, 2010 4:49 am

Corrupted OS, maybe. MIDI circuit, maybe. I'll try to think about your problem in the next few days.
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Re: Akai S900 Help Needed

Postby peterxtian » Fri Mar 30, 2018 6:58 pm

hey matia – responding to your 8 year old problem here.

did you ever get it fixed? just got one, replaced ram and PSU capacitors, but it has the _exact_ same MIDI problem you're describing. any idea?
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Re: Akai S900 Help Needed

Postby Rasputin » Sat Mar 31, 2018 12:01 am

peterxtian wrote:replaced ram and PSU capacitors, but it has the _exact_ same MIDI problem you're describing


It passes the DRAM self-test then?
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Re: Akai S900 Help Needed

Postby peterxtian » Wed Apr 04, 2018 4:14 pm

rasputin – yes! I got a DJ03 error before and replaced IC48 – and now get "RAM OK". But it didn't solve the MIDI issue strangely. I'm waiting on an OS4 floppy to see if rebooting the OS helps.

edit:
realizing you're probably the same rasputin on the gearslutz thread re: DRAM. in any case, thanks for identifying the right location for DJ03!
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Re: Akai S900 Help Needed

Postby Rasputin » Wed Apr 04, 2018 4:26 pm

peterxtian wrote:rasputin – yes! I got a DJ03 error before and replaced IC48 – and now get "RAM OK". But it didn't solve the MIDI issue strangely. I'm waiting on an OS4 floppy to see if rebooting the OS helps.


What ROM version do you have, and do you have any way to read it out? (EPROM burner, etc.)

Since the S900 boots straight from ROM (regardless of whether it bootstraps OS4 or not) then reading out the OS EPROMs to see if they're intact might be worthwhile.
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Re: Akai S900 Help Needed

Postby peterxtian » Wed Apr 04, 2018 4:38 pm

It's 1.2 ... also not too much know-how when it comes to this stuff. Definitely no EPROM burner. But I can look into this. Thanks!
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Re: Akai S900 Help Needed

Postby Rasputin » Wed Apr 04, 2018 4:45 pm

peterxtian wrote:It's 1.2 ... also not too much know-how when it comes to this stuff. Definitely no EPROM burner. But I can look into this. Thanks!


I don't know the checksums for all the revisions, but:

IC12 - v1.2A (low, least-significant) [4B8F = checksum]
IC11 - v1.2A (high, most-significant) [EFC4 = checksum]
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Re: Akai S900 Help Needed

Postby peterxtian » Wed Apr 04, 2018 4:57 pm

Thanks – just checked and it is indeed V1.2A. Now to find someone with an EPROM programmer. I'll post results in this thread.
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Re: Akai S900 Help Needed

Postby peterxtian » Mon Apr 09, 2018 1:32 am

Hi rasputin – so I ended up using an Arduino + breadboard to get a readout of the EPROM .

I wound up with 16,384 byte hex files for LSB and MSB, but I'm really not sure how to go about finding the checksum. There seem to be multiple algorithms with different results.

Here they are if that's any help:
https://pastebin.com/nmKytKJF
https://pastebin.com/ughs1jA6

Let me know if those look right at all, if you can!
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Re: Akai S900 Help Needed

Postby Rasputin » Mon Apr 09, 2018 3:18 am

peterxtian wrote:Hi rasputin – so I ended up using an Arduino + breadboard to get a readout of the EPROM .


Wow, good job!

And yes, your ROMs have the correct checksums, so the problem lies elsewhere. I have to think about this more, but the MIDI board is a daughtercard, so there may be an issue with the connectors from the mainboard, or some type of grounding problem with it.

Just to be clear: The symptoms you have are only flaky MIDI notes and not the issue with samples sounding weird after they're manipulated, correct? I believe the OP was experiencing multiple problems with separate root causes and that they aren't all caused by the same thing, but I want to make sure that theory is right by comparing your exact symptoms.
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Re: Akai S900 Help Needed

Postby peterxtian » Mon Apr 09, 2018 4:35 pm

OK, looking at OP's symptoms again, the MIDI symptoms match, but I don't have much in the way of sample corruption like he describes.

But: before I fixed the RAM, I noticed some samples loaded from floppy disks had corrupted endings. I wasn't sure if this was due to the fact that the sample disks were ~30 years old that the guy on Craigslist threw in, or a fault with the system. I didn't test this as much after fixing RAM, PSU capacitors.

Samples recorded directly are unaffected, and trim, reverse and play fine (when the right MIDI note strikes).

I'll also note that every channel out 1-8 plays fine when I hit the P.B. button.

I was hoping the EPROMs were messed up, and that would have fixed it! Thanks for doing the checksum though.
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Re: Akai S900 Help Needed

Postby Rasputin » Mon Apr 09, 2018 7:07 pm

Okay, that's pretty much what I figured.

Since you have good RAM and good ROMs (and the OS loads from the ROMs) then you should not be seeing the sample corruption issues that the OP was -- at least when sampling directly; a corrupt disk may very well give messed up samples even on a properly working machine, but that's to be expected. Your description does seem to match that idea and appears to be a MIDI problem exclusively, so...

There's a 4-pin connector on the mainboard named "P105" which connects to the MIDI daughterboard. It's right by IC6 on the mainboard, if that will help you find it.

On the MIDI daughterboard, there's a connector named P504, and also a chassis ground (a wire leading from the MIDI board to a sanded/unpainted spot on the S900 chassis).

What I would do is make sure that P105/P504 and the chassis ground spot are all nice and shiny -- no corrosion.

The 4-pins of P105/P504 provide four things:

VCC (+5VDC, pin 1)
RXD (received MIDI data, pin 2)
TXD (transmitted MIDI data, pin 3)
GND (digital ground, pin 4)

It's easily seen that those two connectors or the wiring between the two connectors are high suspect points. Let me know what you discover.
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Re: Akai S900 Help Needed

Postby peterxtian » Tue Apr 10, 2018 3:06 am

Well, I opened it up and the midi board was pretty clean.

I tested continuity between chassis & midi board solder points, and that checked out.

From what I could tell, the 4 pin connector seemed fine too. I tested solder points beneath the midi board to the CPU board's socket connector , the exposed strips of metal - and got continuity. (I didn't take CPU board out or test below on solder points - not sure if that's a big deal)

I guess the next order of business is to get a readout of the midi board from arduino or something?
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Re: Akai S900 Help Needed

Postby Rasputin » Tue Apr 10, 2018 7:12 am

peterxtian wrote:I guess the next order of business is to get a readout of the midi board from arduino or something?


There's not much on the MIDI board, just a basic opto-coupler (PH1) and hex inverter (IC1), both of which we can pretty well rule out if MIDI Thru works properly.

In other words, connect a synth to the S900's MIDI Thru and if the synth does not act erratically when receiving MIDI data passed into (and then out of) the S900 then we can be reasonably sure that the MIDI daughterboard itself is good. If MIDI Thru is sending data properly (no corruption between the S900's MIDI In and the synth's MIDI In) then both PH1 and IC1 should be fine because MIDI data has to pass through them to get to the MIDI Thru.

Also, try monitoring the S900 MIDI Out by using MIDI OX or Bome's MIDI on a computer to capture MIDI traffic sent from the S900. You'll have to do something on the S900 which generates a known MIDI event and compare that to what you see coming in on MIDI OX.

IC8 (HD6850P) on the S900 mainboard is a significant point of interest. It's basically the communication brain which translates between the MIDI board and the CPU. If MIDI Thru works fine, but MIDI Out is corrupt or flaky just like MIDI In then we might suspect something is wonky with IC8.

To break it down:

If MIDI Thru is also erratic; MIDI daughterboard is suspect.
If MIDI Out is also erratic; IC8 or supporting circuitry is suspect.
Last edited by Rasputin on Wed Apr 11, 2018 12:46 pm, edited 1 time in total.
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