[sdiy] Akai AX80 weird PWM behaviour on Voice 8 when Saw wave enabled

james meagher w.james.meagher at gmail.com
Sun Jul 8 09:23:01 CEST 2012


Hello . . . I joined three weeks ago and made a post about my AX80,
but didn't realize emails had to be in plain text so it didn't get
through. I made two more posts about it today before being informed of
the HTML folly. Here's what I first wrote:



I recently purchased a decent meter and scope so I could start
performing my own calibrations and minor repairs, and in the process
learn a bit about the electronics under the hood.

After getting comfortable with my new toys by calibrating my JP6 and
SX-240 (and replacing the SX-240 battery), I bought a rough AX80 as my
first repair project. I've since got the keyboard and panel buttons
repaired, and now everything's working perfectly except for a wee
issue with the PWM on voice 8.

If I ONLY use the Pulse wave on osc 1 there is no problem with the PWM
(functions exactly as it should) - but if I activate the Saw and the
Pulse together and turn up PWM rate to anything above OFF, it suddenly
sounds as though the depth of PWM (not a parameter that's adjustable
on the AX80 - despite what the manual says) has been ramped way up to
the point that you can clearly hear the pulse collapsing completely
into silence, and then back on again. The PWM (rate parameter)
continues to successfully alter the rate of the PWM, but the depth is
off the charts compared the other 7 voices. This also happens even
with the osc 1 Pulse wave off if I combine the osc 1 Saw with the Sub
Osc on - again, if the PWM parameter is set to anything but zero, the
PWM rate modulation (which in this case should not be affecting either
the Saw or the Sub-Osc) modulates the output (not 100% sure if it's
affecting the Saw and the Sub-Osc or just the Sub-Osc here) to such a
degree that it almost sounds like a VCA being heavily modulated by a
Saw wave (in that the sound goes quickly silent, then ramps back up).
The other thing of note is that there is sometimes a crackling sound
present when this issue occurs, not unlike the sound you might get
from moving a bad patch cable around while playing. As long as I don't
activate the Saw wave on Osc 1 the Pulse wave and Sub-Osc function as
they should with or without PWM modulation. As soon as I activate the
Osc 1 Saw with PWM set above zero and either the Pulse wave, Sub-Osc
or both activated as well, things go sour. Obviously I've got an issue
with the Voice 8, Osc 1, Saw wave, but what I can't figure out is how
that's affecting the PWM of the Pulse wave . . . AND Sub-Osc!

To summarize, on voice 8 only:

OSC 1 Saw On - Pulse Off - Sub-Osc Off - PWM set above zero |
No problem with PWM modulation depth affecting things it shouldn't in ways
it shouldn't, but still some crackling noise

OSC 1 Saw On - Pulse On - Sub-Osc Off - PWM set above zero |
Overly deep PWM modulation of sound to point where Pulse collapses into
Silence then comes back again & some crackling noise

OSC 1 Saw On - Pulse Off - Sub-Osc On - PWM set above zero |
Overly deep PWM modulation of sound to point where Pulse collapses into
silence and then comes back again & some crackling noise

OSC 1 Saw Off - Pulse On - Sub-Osc Off - PWM set above zero | No problem

OSC 1 Saw Off - Pulse Off - Sub-Osc On - PWM set above zero | No problem

OSC 1 Saw Off - Pulse On - Sub-Osc On - PWM set above zero | No problem

OSC 1 Saw On/Off - Pulse On/Off - Sub-Osc On/Off - PWM set to zero |
No problem

I do have a paper copy of the AX80 service manual, and if anyone would
be kind enough to point me and my rudimentary understanding of
electronics in the right direction I'd love to attempt this further
repair myself and perhaps learn a thing or two more along the way
(especially as there is no real, dedicated synth tech here in
Winnipeg).



And here's what I added to that earlier today:

Hello again . . .

Two quick follow-up questions to my original post about the weird
behaviour of Voice 8 on my AX80.

1. As I received no suggestions from AH or sdiy on this issue I
decided forge on. After a few online tutorials on reading schematics
and basic electronic components . . . yada, yada, yada . . . I'm now
looking most suspiciously at IC704 (a 14 pin TC4011BP) and somewhat
less suspiciously at IC802 (an 8 pin NJM4558D). I'm suspicious of
these two because all of the functions mentioned (Saw, Pulse, Sub-Osc
and PWM) operate properly when isolated, but the problems start when
they are combined. If I'm reading and understanding this schematic
correctly (50/50 probably), these are the two spots where these
different elements come together. Also, if I follow the signal flow,
it seems to me that everything is alright before reaching these IC's,
but not after. So my first question is if anyone thinks I'm way off
here, or if I might actually be onto something and should replace one
or both of these?

2. If the answer to question one is replace'em I need advice on
sourcing a TC4011BP or substitute. I've never purchased anything like
this before and my initial searches on Mouser and DigiKey yielded
nothing. There are some on eBay from Taiwan, but I have no idea if
this is a good option or if there is better. I'm in Canada if it
matters. (This question has already been answered on AH.)

Again, if anyone wants a PDF copy of the AX80 Service Manual feel free
to give me a shout. It's not the best PDF of an old service manual,
but it's quite legible.

Here's a link to download or view the schematic in question:
https://www.dropbox.com/s/bhi1314hv6qxi0v/AX80%20Voice%20Schematics.jpg



Any advice will be very welcome.

Cheers, (yet again)
James



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