[sdiy] Oberheim Xpander logic chip question

Tristan tu at alphalink.com.au
Sun Nov 18 00:43:59 CET 2018


The HC02 and LS02 perform equivalent logic functions but they are different logic families with different 
input thresholds, output drive strength and other electrical characteristics. It is not normally possible to 
interchange LS and HC logic in a circuit unless particular care is taken with the design of the surrounding 
circuitry. But if the schematic shows LS02 for all encoders and the LS02 chip works in both positions then 
it is probably safe to install LS02 in both. Note that HC series is CMOS and may be more static sensitive 
then the LS logic, which can be a source of failure due to incorrect handling.

As far as I know the Lag Processor is a software function in the Xpander so there will be no dedicated 
circuit to implement it.

/Tristan

On Sun, Nov 18th, 2018 at 4:28 AM, el macaco <elmacaco at hotmail.com> wrote:

> Hello List,
> 
> Short Version: any difference between :
> 
> HD  74LS02 quad 2 in NOR
> SP 74HC02 quad 2 in NOR
> 
> In encoder circuits?
> 
> Long version:
> 
> Yesterday I noticed that encoders 5 and 6 on my Xpander were not responding
> at all.
> 
> Swapping IC’s and testing I found that replacing the Quad 2 in NOR used by
> encoders 5 and 6 with the one used by encoders 1-4, fixed the problem with
> encoders 5 and 6, and caused problems in encoder 1, although different
> erratic behavior in the other encoders, not just 2 disabled encoders, one
> disabled and 2 others responding after several turns.
> 
> On to my question.  The 2 ICs in question are different.
> 
> The one that failed in encoders 5&6 is a SP74HC02, and the one used by
> encoders 1-4 is HD74LS02.
> 
> The schematic notes both as LS02, but the parts list lists one of each as
> found in the circuits.
> 
> Can I replace them with either type? Is there a reason they didn’t use the
> same chip besides cost for one they would only use 2 of the NORs on?
> 
> I am assuming the HD74LS02 is superior since they used it for 4 pots and
> listed LS02 for all in the schematic.
> 
> I’m not sure if I will find these locally in the Peruvian Amazon city I
> have nearby, and I would like to replace it with the best option even still
> via mail order.
> 
> Thanks for any feedback.
> 
> Also, I noticed the Lag Processor’s Legato function doesn’t work, it
> effectively bypasses the lag no matter how I play, but I can’t find the Lag
> processor circuit in the service manual.  I presume it’s on the voice board
> but the legato enable could be on the processor board and be another logic IC
> failure.
> 
> Although in this humid jungle I have cured my xpander and some other issues
> with contact cleaner on IC sockets.
> 
> Thanks again.
> 
> El
> 
> 
> 
> 






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