[sdiy] Question regarding IC replacement in Akai AX80 (was: Akai AX80 weird PWM behaviour . . . )

Gordon JC Pearce gordonjcp at gjcp.net
Sun Jul 15 10:09:45 CEST 2012

On 15/07/12 07:07, james meagher wrote:
>> It's actually probably supposed to provide a squarewave to some circuit.
>> The fact that it shows as 2.1V is more of an artifact of the meter used to
>> read it.  Check with an oscilloscope.
> I was checking the Volts because that's how it was marked on the
> schematic - I didn't think to put the scope on it. When I put the
> scope on Pin 13 of the other relevant IC's I get a nice square wave on
> the scope. However, Pin 13 on IC22 shows a barely perceptible square
> wave - almost a flat line. I did notice that the square waves on the
> other IC's showed a bent, top leading edge . . . I have no idea if
> this is normal, or a sign of some larger issue with the current.

A "square wave" is never going to be perfectly square, particularly when 
you're dealing with "real world" components.

> Scope images of Pin 13 on IC's 21, 22, and 24 available here:
> http://www.flickr.com/photos/63824598@N04/

Yeah, IC22 doesn't look happy, does it?  Does it track pitch correctly? 
  It looks like the output of the chip has failed.  Just for a laugh try 
sticking a 4k7 resistor between pin 13 and 5V.  If it's the top tranny 
in the output totem pole that's died, then it will pull up the pin and 
you may see an improvement.  Is there anything else that could be 
loading the pin down?

> Admittedly I don't have much good reason Gordon. My patch memory banks
> are not holding their data and the original battery that's still in
> the board shows a solid 3.2V on my meter. I am waiting for the new
> battery with legs to arrive to replace the original one. I just

Flattish.  Remember, it's got to go through a diode first, so there's 
some voltage drop there.  What's the voltage across the power pins of 
the memory chips?

Have you tried replacing the coin cell with something like two AA 
batteries in series?  That's what I do when I'm replacing them, and they 
last foreve.

> arrive. If I install the new battery and it doesn't solve the problem
> I'll be kicking myself for getting the RAM chips at the same time. If
> I get the RAM chips but don't need them I won't kick myself at all for
> having needlessly got them.

Well, you may as well get them but I don't like to replace parts that I 
don't have to.  The more you do, the more you risk damaging the board...

Gordonjcp MM0YEQ

More information about the Synth-diy mailing list