'Dead' Akai AX60 - where to start?

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'Dead' Akai AX60 - where to start?

Postby AdamAnt316 » Wed Apr 19, 2017 7:01 am

Hello everyone. I recently picked up an Akai AX60 at a swapmeet for $40. The seller told be right off the bat that it seemed to be completely dead; he'd hoped that the power supply was the issue, but its voltage outputs checked out fine when he tested them. Once I got it home, I plugged it in and have more-or-less confirmed what he said. None of the front panel lights come on, though occasionally the 'BANK' display will briefly show an '8', followed by a single segment illuminating or flashing on the 'NUMBER' display next to it. Plugging a pair of stereo headphones into the 'PHONES' jack on the back gives me a hissing sound which constantly drifts from channel to channel, but no audible response from pressing any of the keys or buttons, regardless of how any of the sliders are set.

My question is, does anyone have any suggestions as to where I should start? I will freely admit to not having had any real experience with something like this; I took some electronics courses in high school and college, can solder reasonably well, and own a fair amount of test equipment (DMMs, frequency counters, oscilloscopes, etc.). I have a printout of the service manual and owner's manual, both of which were included with the unit. I have a bad feeling that one or more of the unobtanium ICs might be to blame, but I have no real idea as of yet. Could a dead battery on the board cause symptoms like this, or would it just result in erased presets? Anyway, thanks in advance!
-Adam
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Re: 'Dead' Akai AX60 - where to start?

Postby Rasputin » Fri Apr 21, 2017 9:41 pm

Sure, send it to me and I'll likely get it up and running. Of course, you might never get it back so...

Just kidding. Kind of.

First thing I would do is verify your EPROM. Also, is the service manual a reproduction (Xerox, ya know?) or is it an actual Akai printed one?

As far as unobtainium goes, (if I recall correctly) the only part that is more-or-less irreplaceable that would prevent boot up is the CPU and I've never heard of anyone having dead CPUs on that family of Akai gear. Not saying it doesn't happen, but I wouldn't worry about it at this point.
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Re: 'Dead' Akai AX60 - where to start?

Postby AdamAnt316 » Sat Apr 22, 2017 1:24 am

Rasputin wrote:Sure, send it to me and I'll likely get it up and running. Of course, you might never get it back so...

Just kidding. Kind of.

First thing I would do is verify your EPROM. Also, is the service manual a reproduction (Xerox, ya know?) or is it an actual Akai printed one?

As far as unobtainium goes, (if I recall correctly) the only part that is more-or-less irreplaceable that would prevent boot up is the CPU and I've never heard of anyone having dead CPUs on that family of Akai gear. Not saying it doesn't happen, but I wouldn't worry about it at this point.

Thanks for the reply.

I'll keep your offer in mind. :) I have a friend who's fixed a few keyboards for me (including my ARP Solus), but he's extremely busy with his own projects at the moment, so I'm on my own for the time being. I can do basic recapping and whatnot, but I have yet to attempt troubleshooting anything nearly this complicated, so I'm a bit out of my depth, and can use all the help I can get.

The service manual I have is a printout, source unknown. Parts of it are hard to read, and the schematic is spread out over several pages, but it's there. Printouts of both the service and owner's manuals were included with this synth by the seller.

How would I verify the EPROM? It's in there; the label on top of it says "AX60 V1.2". I know you started a thread about a possible custom firmware for the AX60, and it'd be great to see it come to fruition, but I don't know if it'd fix all of this unit's possible ills. If you still need a test-bed, perhaps we could figure something out...

Good to hear that the CPU isn't a common failure point for these units. What else might cause what I'm seeing? The fact that I occasionally get some response from the lights on the front panel gives me hope, but stuff like reseating the ribbon cables on the main board and wiggling the socketed ICs haven't yet improved things. I have a feeling that it's something relatively simple, but finding it might be like trying to locate a needle in a stack of needles.......... :?
-Adam
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Re: 'Dead' Akai AX60 - where to start?

Postby Rasputin » Sat Apr 22, 2017 5:54 am

There are two routes to take with the EPROM verification. The ideal way is to pull yours and use an EPROM programmer to read it out to compare its checksum to another copy of the binary. The second way is to buy a preburned EPROM or have someone make you an EPROM from another copy of the binary.

The v1.2 EPROM binary has been posted various places, I'm quite sure a search here on VSE will turn it up, but I'll repost a link if you can't easily find it.

As far as troubleshooting goes, think about it this way:

You've got the power supply, you've got the digital logic chips, and then you've got the voice chips. All the voice stuff is inconsequential at this point because the synth clearly isn't thinking properly. Assuming the person that sold it to you was actually correct about the PSU being good (a rather dubious assumption, truthfully) then all that's left is the digital logic itself. The most important part of that is the CPU (obviously) and also the program ROM. Since the CPU is fairly robust and EPROMs are known to decay after time (UV light leaks and so forth) then I'd suspect the EPROM first. If the CPU is working but has a bad program ROM then there's no telling what weirdness it could be telling the CPU and you'd be getting the nonsensical behavior that you're seeing. Other possibilities might be multiplexer chips dealing with peripheral addressing and the like.

But I'd recheck the voltage and ripple on the PSU first. Then EPROM checksum second. Even if the EPROM checks out, it could be something as simple as being in a bad socket -- could have some cockroach legs in there, or god-knows-what.

Then take a logic probe to the CPU to see what state its pins are in and that it's getting good clock. Spend some extra time looking at the reset line as that will give you a pretty good indication if the CPU is stuck in a reboot loop, not trying to boot at all, or booting and staying up.
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Re: 'Dead' Akai AX60 - where to start?

Postby AdamAnt316 » Tue Apr 25, 2017 1:19 am

Again, thanks for the reply! I have an old EPROM programmer (Heathkit ID-4801), but don't know if it works, or how to use it. It has a socket for the EPROM, plus a socket above it for a "Personality Module", which I don't have. I'm not sure if said module would be necessary for just reading the contents of the EPROM (according to this forum post, said module routes the 25V programming voltage to the right pin(s), so perhaps it isn't needed for just reading them).

Thanks for the troubleshooting advice! I will keep it in mind. I have yet to get around to the other tests you mentioned, but I did check the power supply voltages. Using my HP E2378A digital multimeter, I got the following readings on the pins of P4 from left to right:

Pin 1: 14.96V
Pin 2: -15.10V
Pin 3: 5.75V
Pin 4: -5.66V
Pin 5: 5.21V
Pin 6: -5.16V
Pin 7: 0V (ground?)

I also checked each pin for AC, and got little to no voltage at all seven pins (is there a better way to check for ripple?). All of these readings were taken with the P4 connector pulled, but the other connectors still in place. The heatsinks were slightly warm to the touch. Other than the +/-6V pins, everything seems fine to me, though I'm not yet sure how the PSU acts under load. The service manual mentions a procedure (item 6-1) for balancing the 6V line, but given that they both seem pretty close in how low they are, this doesn't seem to be the cause. Any thoughts?
-Adam
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Re: 'Dead' Akai AX60 - where to start?

Postby Rasputin » Tue Apr 25, 2017 6:46 pm

To verify the EPROM you'd need to use the "Verify" option in the Heath programmer which requires an appropriate personality module. If you can't get the module then I'm pretty sure you could manually jump the right configurations to make your own. The manual tells what is required for that EPROM type (2764, if I remember correctly):

http://www.vintage-radio.info/download.php?id=766

Look on page 28 of the "illustrated manual" for how to wire up the personality module.

Grab the Akai AX60 v1.2 ROM from: http://dbwbp.com/index.php/9-misc/37-synth-eprom-dumps

Use a hex editor to generate the 16-bit checksum in hexadecimal. Compare that to the checksum generated by the Heath 4801 in "Verify" mode using the original AX60 EPROM. If those values match then you should have a good EPROM.

Hint: The checksum should work out to be "0920", I believe, so wiring up your own personality module is really all you need to do.
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