Akai AX-60 Keyboard repair

[frank1]

Hi,

I know very little about electronics, though I really want to learn. I have an interest in analog synthesizers.

I just bought a non-fully functional Akai AX-60 Synthesizer keyboard thinking I had an easy fix. Of course it seems there's more to it.

It powers up and produces sound, but has one problem. Not all the keys produce sound in normal mode. When in "unison" mode all keys produce sound. Specifically, this is what happens: Press 1 key -> produces sound, press next key -> produces sound, then each subsequent key does not produce sound till 5 keys down, then it repeats itself ; 2 keys right next to each other make sound etc...This pattern also changes at times, but is consistent down the keyboard. I tried changing some CEM3394 chips, but so far, that hasn't worked. Anyone have an idea of what the problem might be? Thanks!

Frank

 

[Michele]

Hi Frank

Did you ever have any luck with your keyboard? I've had mine since '87 and it just started doing the same thing. Oddly, the action that triggered it was just simply moving it. It's unlikely a component simply died. I'll investigate and let you know if you're still interested.

Michele

 

[bsco]

Not sure if this would help......but here goes...I had a Korg(I think) keyboard that had every 5th key dead all across the keybed.....There were no components bad...the problem was one of the wires on the end of a ribbon cable, (these cables have bare wires on the end), had some how became unplugged for it's connector.....how it happened I have no idea.....but I am pretty sure it was from transportation......you could check for that...check all the cables going to and from the key bed.....and see if that helps...

 

[Michele]

Hi, yes I could see that happening. In fact, some of the solder connections at the keyboard pcb were poor. But even after the repairs were made and checked for continuity, the problem remained. In my case, I discovered bit 6 on the MPU input port that reads the keyboard presses has gone bad. Unfortunately, finding a replacement IC for it is not easy. Thanks for the help!!

hugs
Michele

 

[bsco]

Do you have the IC number??

 

[kpatz]

Sounds like your synth has some dead voices. Is it 2 keys work, 4 don't, 2 do, etc? Many synths of that vintage used a round robin voice allocation scheme, so each note you strike is allocated to the next voice in sequence. Since the AX60 has 6 voices, if 4 are dead, you would experience the symptoms you're describing. In unison mode, all the voices sound together for each note, so that's why all notes appear to play in that mode, though you're likely only hearing 2 out of the 6 voices.

What happens if you strike the SAME note multiple times in a row?

This link is a PDF of the AX60 service manual: http://manuals.fdiskc.com/flat/Akai AX-60 Service Manual.pdf

It will contain a lot of info you'll need to troubleshoot and repair your synth's dead voices. You'll need a multimeter and an oscilloscope at the minimum. I'd go through the adjustment procedures in the manual to see if that brings the voices back--maybe the power supply is out of spec or a connection is loose, and that will rule out the simple causes before you get to digging into each voice's architecture (oscillator, filter, VCA, modulation inputs, mixer etc.) to figure out where the problem lies.

Are the CEM3394 chips in sockets? Try swapping them from one voice to another. Be careful not to bend any pins and observe handling precautions to avoid damaging the chips with static electricity.

Some synths have small LEDs on the voice board that light up when each voice is active. I don't know if the AX60 has them but if it does, it will help in determining which voices are working and which ones aren't.

EDIT: Just noticed the OP posted this in 2008. Holy ancient thread resurrection Batman... wonder how he did with his synth. And Michele found out what's wrong with theirs. Good luck finding that chip.

 

[Michele]

IC1 is the microprocessor: NEC UPD7811G-144, or sometimes called D7811G-144. I believe the -144 is the package type. Luckily I bought the service manual years ago. I had gone through the alignments and checked the voice outputs, they appear to be in order. I removed the pull-up resistors at the input port and took measurements. They should all be high impedance but one bit was measured very low. This might explain why it make an odd sound every time at power-up.

If you need a set of schematics I can get it scanned for you.

Michele

 

[Michele]

Hi again guys,
Yes, it seems trying to locate that chip isn't easy!
I'm trying to use one of those chip locator companies but they want me to buy several.
I'll keep you posted...

 

[Michele]

Hello guys,
I finally realized that the D7811G-144 is a custom programmed MPU for this series (denoted by the "-144"). I found a D7811G-121 overseas and installed it (the pin-out is identical), but that was confirmation that the code is embedded in the MPU. It gave some characters in the numeral display, but that was it. So,I was stuck with finding a CPU board. I got lucky and bought a non-op AX60 online. It powered up but nothing worked on the control surface. I had to fix this one before fixing mine. The problem with the non-op unit was the peripheral IC controller (IC5) had shorted out internally. I found a replacement on ebay for $6 and installed it. Now it worked again and I was able to swap the CPU board to get my unit working. A happy ending!