[sdiy] Oberheim Xpander logic chip question

el macaco elmacaco at hotmail.com
Tue Nov 27 19:57:59 CET 2018


Thanks to all for the knowledge!

The hd74LS02 worked and encoders 5 and 6 work well now.

> On Nov 19, 2018, at 4:40 AM, Roman Sowa <modular at go2.pl> wrote:
> 
> Looking at the schematics, NORs are used to latch the direction bit of the encoder in LS367 (U5). So whenever encoder rotates, the step event propagates thru the NOR, D-register and then goes to latch input of LS367. If you put HC in there, which is faster, it will make latching edge to appear sooner than data bit to be latched in the other D-register. That's very close timing.
> You may either put new LS02 in there or load the HC02 output with a capacitor to GND to slow it down by a few ns.
> Or replace all those LS74, LS02 and possibly LS367 too with HC.
> 
> Roman
> 
> W dniu 2018-11-17 o 18:28, el macaco pisze:
>> Hello List,
>> Short Version: any difference between :
>> HD 74LS02quad 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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