[sdiy] Broken Siel Opera 6, any ideas?

[Travis Shire]

> Hi,
>
> I have a faulty Siel Opera 6 opened here, and I've removed the leaked
> battery and replaced the CD40106 (IC16) that was more or less destroyed by
it.
>
> Before this, I could see that the voice LEDs indicated that
> keypresses were assigned to voices in a correct manner, but the panel
> controls/indicators behaved strangely or not at all.
>
> Now, no voices get assigned when pressing keys. The panel
> controls/indicators are still strange.
>
> When I look on different CPU pins on the oscilloscope, many of them
> seem to "blink" in the sense that they are 0 V for a while and then
> quickly changing between 0 and 5 V for a while, and so on. This
> "blinking" occurs with approx 2 Hz. I can see similarly timed things
> going on in the multiplexing address lines, they are turned on and
> off at 2 Hz. The panel indicators can show similar blinks when
> buttons are pressed. The DAC doesn't seem to change its output value.
>
> I wouldn't really expect this to happen, unless the CPU has entered
> some quite useless state waiting for something else to happen... has
> anybody had a similar experience here?
>


The CPU is stuck in a constant reset loop....IE the "watchdog" is barking.
The CPU detects something amiss and resets to try to correct it. If corrupt
data is getting to it it continues to reset and will do so infinitely until
the situation is corrected. I'd check any data/address buffers first. Look
at what goes in and make sure what comes out matches. Look for any lines
that aren't at logic level, are stuck hi or stuck low. If the buffers are ok
check the ram/rom next and work out from there. Its possible you have an
eprom thats lost some data.