[sdiy] Oberheim Xpander logic chip question

Michael E Caloroso mec.forumreader at gmail.com
Sun Nov 18 05:20:25 CET 2018

Portamento (aka lag propcessor) went software beginning with the later
version of OBXa.  You won't find the circuit in the service manual.


On 11/17/18, el macaco <elmacaco at hotmail.com> wrote:
> I found the HD version in town today!
> Since they are handling the same thing ( encoders) I would suppose it
> shouldn’t matter unless the other half of the encoder 5-6 is used in some
> other part of the circuit. I’m thinking it may be cost cutting for the one
> they’re only using half of HC02, assuming it is cheaper as it is more static
> sensative CMOS?
> Thanks!!
> There is a hardware lag processor in the xpander that lets you patch some
> modulation direct to the osc and filter, well I assume it’s hardware... I
> saw it written in an AH post once. The other lags are cap and resistor and
> software controlled.
> Will try the logic I got and see if t solves the problem.
>> On Nov 17, 2018, at 6:43 PM, Tristan <tu at alphalink.com.au> wrote:
>> 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 ICâ?Ts 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 didnâ?Tt 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.
>>> Iâ?Tm 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 Processorâ?Ts Legato function doesnâ?Tt work, it
>>> effectively bypasses the lag no matter how I play, but I canâ?Tt find the
>>> Lag
>>> processor circuit in the service manual.  I presume itâ?Ts 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
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy

More information about the Synth-diy mailing list