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

[james meagher]

Hey Adam,

The two harnesses I swapped on the Voice Board were P2 and P3. Voice
Board P1 remained plugged in. I also checked Pin 8 on the Voice 4
filter chip (marked DCO2 Mix Level on the schematic) and its level
remained consistent with those of the same pin on the other voice's
filter chips as I changed the DCO2 mix level value - so I think this
is working properly.

Thanks for pointing that out though. It's not an avenue of
investigation I would have thought of, and now I can certainly see how
it could have been the problem.

Cheers,
James

On Sat, Jul 14, 2012 at 9:36 PM, Adam Inglis <21pointy at tpg.com.au> wrote:
>
> 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
>
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