[sdiy] Oberheim Xpander logic chip question

el macaco elmacaco at hotmail.com
Sat Nov 17 18:28:48 CET 2018

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.


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