Akai S900 Help Needed

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Re: Akai S900 Help Needed

Postby peterxtian » Wed Apr 11, 2018 3:21 am

Hi again – so the MIDI thru passes fine, without a hitch.

MIDI out however, passes nothing. When I'm playing MIDI notes via keyboard, there's no activity (I'm using MIDIPipe). Also when using Page 2 of the S900'S MIDI section (MIDI Test Signal Transmit), I hit PB and also no signal in MIDIPipe.

So, I guess I'm deeper down the rabbit hole!
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Re: Akai S900 Help Needed

Postby Rasputin » Wed Apr 11, 2018 7:44 am

peterxtian wrote:Hi again – so the MIDI thru passes fine, without a hitch.

So, I guess I'm deeper down the rabbit hole!


Unfortunately, yes. But great job testing and reporting! :thumbleft:


From P105 on the mainboard, we can follow back and directly find these two chips:

IC42 - 74LS04P (hex inverter)
IC8 - Hitachi HD6850P (communication interface)

The 74LS04 is common and cheap (about $1), and it's definitely not unheard of for these to fail. The HD6850 is a bit more specialized, but they aren't proprietary and you can still get them now, so it's not game over either way (I think either a HD46850 or EF6850 should work as substitutes as well - about $10).

A logic probe or oscilloscope would probably work the best for troubleshooting here. Logic probes are cheap and easy to use, so don't let that deter you or put you off.

Received MIDI hits IC42 pin 1 and exits on pin 2 -- then to IC8 pin 2 (RXD)
Transmitted MIDI originates at IC8 pin 6 (TXD) -- then hits IC42 pin 3 and exits on pin 4


So... when transmitting MIDI notes, is anything generated at IC8 pin 6? If not, we've got a bigger problem, but if so, is anything generated on IC42 pin 4? If not, then replace 74LS04P with a socket and new chip.

When receiving notes, does anything leave IC42 pin 2? If not, replace 74LS04P with a socket and new chip. If so, we've got a bigger problem.

The thing that concerns me is that if the 74LS04 died then the received MIDI would probably not be erratic, it would likely just not work at all. C23--which is a capacitor connected to IC8--seems like the most likely suspect for erratic MIDI, but I have a feeling it's not going to be that simple.

EDIT:

IC19 - 74HC393 (dual 4-bit counter), generates MIDI Tx/Rx clock on pin 5

I'm developing a pet-theory that if something is wrong with this counter then invalid MIDI clocking could cause the communication interface to behave oddly. Any other symptoms of the S900 misbehaving would be very important. If there's a way for you to test the RS-232 serial connection, that would be advisable -- having access to an old PC would probably be necessary.
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Re: Akai S900 Help Needed

Postby peterxtian » Wed Apr 11, 2018 4:06 pm

Wow, alright time to procure a logic probe.

By the way – any precautions I should take so I don't electrocute myself on a live circuit? Apart from not touching the PSU.
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Re: Akai S900 Help Needed

Postby Rasputin » Wed Apr 11, 2018 9:08 pm

peterxtian wrote:By the way – any precautions I should take so I don't electrocute myself on a live circuit? Apart from not touching the PSU.


As far as probes go, something like this should be fine: https://www.ebay.com/itm/ELENCO-LP-560- ... SwBahVJpj9

As far as not killing or maiming yourself:

It uses a linear power supply, so the S900 is not as lethal as, say, a tube circuit or switched-mode power supply. It should be relatively safe except the section between the power cord and the PSU. Anything before the transformer is what you really have to take care to avoid.

The only "gotcha" is the circuit for the display backlight. It's a little board way down at the bottom front of the S900 which you would have to go out of your way to touch, also don't touch backlight leads themselves. Although, if you've replaced the display with a newer LED style backlight then that card has probably been removed or disconnected.

I'd also cover everything but the very tips of any probe(s) I was using with something non-conductive, such as electrical tape.
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Re: Akai S900 Help Needed

Postby peterxtian » Sat Apr 14, 2018 4:50 pm

OK, I got the logic probe and was able to test the chips you mentioned while sending MIDI signals. You're right, fairly easy.

When "transmitting" signals, IC8 pin 6 gives a "high" reading, with no pulses out, and no activity on IC42 pin 4.

When receiving MIDI, IC42 pin 2 gets pulses with the note on/offs. So seems like IC42 is fine.

I tested IC19, not quite sure what I was looking for– but i got a simultaneous high/low reading with fairly rapid beeps if that helps (around 2x / second).

As for the serial connection, I might be hard pressed to get access to an old PC. Would it be worth trying to get a serial-USB cable?
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Re: Akai S900 Help Needed

Postby Rasputin » Sat Apr 14, 2018 6:07 pm

peterxtian wrote:OK, I got the logic probe and was able to test the chips you mentioned while sending MIDI signals. You're right, fairly easy.

When "transmitting" signals, IC8 pin 6 gives a "high" reading, with no pulses out, and no activity on IC42 pin 4.

When receiving MIDI, IC42 pin 2 gets pulses with the note on/offs. So seems like IC42 is fine.

I tested IC19, not quite sure what I was looking for– but i got a simultaneous high/low reading with fairly rapid beeps if that helps (around 2x / second).

As for the serial connection, I might be hard pressed to get access to an old PC. Would it be worth trying to get a serial-USB cable?


IC8 pin 6 seems like it's stuck high. That's bad. So, to clarify, regardless of whether or not you're generating MIDI out from the S900, IC8 pin 6 is registering high, right? It doesn't ever drop to low at any point, no matter what else is going on?

If pin 6 is constantly high, then IC8 is quite possibly faulty.

Can you test IC8 pin 2 while receiving MIDI from MIDI In? IC42 should be passing that along, so I just want to make sure that it's actually getting to IC8 -- but when you put the probe on IC8 pin 2, can you put it up toward the top of the pin instead of down where the pin meets the PCB? Basically, the idea is to test the pin itself and not the solder joint where it meets the trace because the joint could be faulty, if that makes sense.

IC19 sounds correct. A clock will be a pulse like that, so it's probably good. You could double-check with an oscilloscope to actually look at the shape of the pulse, but I bet it's good.

My current thinking is that IC8 is bad, but I will try to devise a more definitive test because unless you know someone really good at de-soldering then you don't really want to go yanking stuff out unless it's actually known to be defective.

As far as serial-USB goes, that should work as I'm thinking something like this: http://www.yahcolorize.com/s900/index.htm
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Re: Akai S900 Help Needed

Postby peterxtian » Sat Apr 14, 2018 8:43 pm

Hi, yes, just double-checked.

IC8 pin 6 is always high, whether I'm pressing the PB button in MIDI test mode, sending MIDI signals IN, or doing nothing.

And testing IC8 pin 2, it's showing the same MIDI pulse as IC42 when I test it at the top of the pin.

So, it's looking like IC8 is the culprit?

Definitely won't attempt to desolder that chip myself. Nearly botched the board when I tried it on the RAM chip.
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Re: Akai S900 Help Needed

Postby Rasputin » Sat Apr 14, 2018 11:56 pm

peterxtian wrote:IC8 pin 6 is always high, whether I'm pressing the PB button in MIDI test mode, sending MIDI signals IN, or doing nothing.

And testing IC8 pin 2, it's showing the same MIDI pulse as IC42 when I test it at the top of the pin.

So, it's looking like IC8 is the culprit?

Definitely won't attempt to desolder that chip myself. Nearly botched the board when I tried it on the RAM chip.


Yes, at this point, I personally would be replacing it, but I wouldn't say it's a certainty that this is the one and only problem. If the 6850 chip is bad then it would certainly explain any MIDI anomalies, though -- it's the most direct and straight-forward explanation.

If you decide to remove it, micro flush cutters are highly recommended -- something like Hakko CHP-170 ($5 or so) or TR-20-50. Just snip all the pins of IC8, desolder the remaining pin stubs one at a time, then replace with a DIP-24 socket, and drop in the replacement. It's very doable that way, but of course destroys the (probably already broken) old chip, of course.

Solder sucker suggestion: https://www.ebay.com/p/Engineer-Ss-02-S ... 2020539494

That sucker works very well, especially if you put a little lithium grease or something inside the mechanism to make sure it slides buttery smooth.

Besides the Hitachi versions, these chips should work as a replacement:
https://www.ebay.com/itm/MC6850P-MC6850 ... Sw9IpXzpJX

https://www.ebay.com/itm/EF6850P/323194 ... m#viTabs_0
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Re: Akai S900 Help Needed

Postby peterxtian » Wed Apr 18, 2018 10:33 pm

Juuusst waiting for the new IC8 chip to arrive– got the MC6850P. I'm getting someone else to desolder/replace with a socket because I'm a wimp.

BTW, I tried the RS-232 port with the software you posted. Was running Virtual Machine with a USB-serial adapter my roommate happened to have, but it might've had the wrong chipset (used "prolific"). So I didn't get any communication (followed instructions closely).

We'll see what happens with the new chip. In any case, thanks a ton for all your help on this.
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Re: Akai S900 Help Needed

Postby peterxtian » Sat Apr 21, 2018 1:05 am

Hey, so I went through the motions with the S900 today. Some weird things happening.

Switched out IC8 with a new chip. I booted it up, and it had the same problems as before. Tested the same chips with logic probe, sending MIDI, same results.

Then, on a whim I decided to load up the OS4 disk I had. At first, it seemed to be properly functioning. It was receiving MIDI notes fine. Then I noticed some things were off: the floppy disk drive wasn't able to save, and sometimes even pressing the "disk" button caused the whole thing to freeze. (The disk functions fine on 1.2). Also, probing further, the OS4 still doesn't transmit MIDI notes.

Then I found sample corruption issues in OS4. When I record a couple samples, they're fine, but after I record more, the original ones start to have corrupted endings.

So , obviously the underlying problems aren't solved by bootloading.
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Re: Akai S900 Help Needed

Postby peterxtian » Wed Apr 25, 2018 6:20 pm

So, any hope for this one?
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Re: Akai S900 Help Needed

Postby Rasputin » Wed Apr 25, 2018 7:30 pm

peterxtian wrote:So, any hope for this one?


There's always hope :)

I haven't had much time to think about it in-depth though, so I haven't come up with a good game plan yet.

Definitely an odd problem, but your extra testing might have been helpful. I think the sample corruption rules out a clocking issue, so the common thread between all the issues could be the bus data.

If the RAM tested correctly but samples are going corrupt when manipulated, MIDI doesn't work, and (possibly/probably) serial Tx/Rx doesn't work then IC18 - TC74HC245P could be messed up. Not a solid theory yet, just a straw grasp.

I'm assuming you don't have the optional ASK90 drum trigger board installed. If you did then that would help to test that theory. Have you noticed any symptoms with the encoder on the front panel though? Anything at all?

Edit: The more I think about it, the more confident I am that it's tied to this. The bus transceiver is a "bridge" between quite a few sub-systems and the CPU/RAM. If there's a problem with IC18 then it would affect MIDI, serial, floppy, envelope data, the rotary encoder, the footswitch, and more.

If your symptoms now clearly extend beyond MIDI not working then this really points to it being like the OP problem. More S900 behavior analysis is needed though -- so try out the footswitch and watch for erratic LEDs on the front panel, or anything strange at all, however trivial it may seem. If I'm right on this one (fingers crossed) then the S900 will be haywire in quite a few ways, but still semi-operational.
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Re: Akai S900 Help Needed

Postby peterxtian » Wed Apr 25, 2018 11:18 pm

Cool! I'll test the footswitch and others when I get home. No drum trigger unfortunately.

The encoder is always totally fine.

I've also forgotten to mention this: when I do the RAM test, the machine becomes totally nonoperational and I have to restart it. It says "RAM OK", but pressing buttons make weird glpyhs appear, like a string of B0 B2 C0, etc, and sometimes kanji.

I'll take this fight to the bitter end!
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Re: Akai S900 Help Needed

Postby Rasputin » Wed Apr 25, 2018 11:50 pm

peterxtian wrote:I've also forgotten to mention this: when I do the RAM test, the machine becomes totally nonoperational and I have to restart it. It says "RAM OK", but pressing buttons make weird glpyhs appear, like a string of B0 B2 C0, etc, and sometimes kanji.


That's normal. I'm not really sure why the test menu is so strange.

Edit: Thought of an actual concrete test for you to do, as well.

Put the logic probe of pin 1 of the bus transceiver I mentioned last post (IC18 - HC245) and see if it ever changes states and if there are particular instances when it does or doesn't (saving or read to disk, etc.) Likewise, pin 19.

I'm thinking pin 19 will always or generally be low. If pin 1 is stuck low then we're on to something.
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Re: Akai S900 Help Needed

Postby peterxtian » Fri Apr 27, 2018 4:39 am

Alright, got a chance to do the tests.

Footswitch: definitely not 100%. It takes a few presses for the MIDI receive light to start registering. Then it's fairly consistent, and then it will go dark for a stretch, then back again. I'd put it at 70% of presses cause it to light up.

As for IC18:

Started with pin 1. It's a consistent baseline HI/LOW, beeping at a rate of 3x / second. Slight modulation / sound change when MIDI notes are hit. Same when footswitch is pressed *and* causes MIDI receive light. (No change in beeps when the footswitch doesn't register).

All disk activity on pin 1 causes R2D2-ish sounds for a second or two– when loading, erasing, saving, writing. Also, the HI light flickers off and on for a brief moment for those.

As for pin 19, it's a consistent HIGH reading with constant beeps, at a slightly slower clip than pin 1. Similar activity across different functions on pin 1, but the MIDI beep sound modulation was much subtler.

When reading, writing, etc with the disk on pin 19, there's similar sounds to pin 1 for a moment. For this one, the LOW light flickers on and off for a split second.

I hope I did a good job of describing that!
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