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

[Adam Inglis]

On 15/07/2012, at 8:09 AM, james meagher wrote:

> Question about finding compatible replacements for some older  
> ICs . . .
>
> I verified last night that there's actually nothing wrong with the
> circuitry on the Voice Board causing DCO2 on Voice 4 to not make a
> sound. I verified this by flipping the two cable harnesses that run
> from the CPU Board the Voice Board (one of the cable harnesses is for
> voices 1-4 and the other 5-8). Sure enough DCO2 on Voice 4 worked just
> fine when the CPU thought it was Voice 8 . . . And DCO2 on Voice 8
> wouldn't make a sound. So I traced the problem back to Pin 13 on IC22
> on the CPU board. This pin should be providing 2.1V to the Voice 4
> DCO2, but is only mustering 0.1V. I checked all the other pins on this
> IC and noted a few other incongruities with the other four ICs that do
> the same job.
>
> I'm going to replace this IC22 which is listed in the AX80 parts list
> as UPD8253C-2. I've searched D8253C-2 and come up with some hits from
> companies I've never ordered from and eBay of course. What I don't
> know is if there is a more readily available alternative I should look
> at instead? By what method should I go about determining this for
> myself so I don't need to post here every time I'm looking to replace
> an old IC? I'm also thinking about replacing my two RAM chips,
> D446C-1.

James, did those harnesses also contain the CPU control data  
(connector "P1" on the voice block)? Because there is a control line  
from the CPU to a VCA in the Curtis filter chip that controls DCO 2  
Mix Level that probably should be checked also.

Adam