[sdiy] JX8-P Issue

[LA Synth Co]

If you have an early JX8P,  you need to implement the change shown in the
"Change Information" section of the service manual.

You can either make a the assigner ROM A a 'high speed' chip (i think its
an M5L2712K2?), or make the change to bypass an IC to make the chip enable
happen sooner.  I think it's all spelled out in the service manual.  The
CPU was faster than the original ROM ICs used. Because of this, sometimes
the CPU can skip the not fully initialized ROM and cause hangups and boot
delays.  It usually causes hangs, but it sometimes just slows the boot.
I have also seen this caused by the decoder itself (I dont have the IC
position number, im going from memory).  Its an 74LS138. That can hang or
be iffy and cause slow boot times.

As the memory cells go bad, the solder joints get corroded and age creeps
up, this problem becomes more apparent.  Decoders cause a lot of issues
often misdiagnosed as CPU problems and cause strange data line issues.  Its
not exclusive to roland.

Of course one should always check the basics (power, wiring harnesses,
clean sockets, etc) and always reflow the regs on the PSU in these old
machines when you have them open.

good luck
Boomer


On Thu, Aug 25, 2016 at 3:16 PM, Gordonjcp <gordonjcp at gjcp.net> wrote:

> On Thu, Aug 25, 2016 at 10:04:39AM -0700, Thomas White wrote:
> > Hey guys,
> >
> > My JX8-P is taking nearly 30 seconds to complete the normal 12 second
> countdown on power up and then appears to have booted but plays no sound. I
> haven't opened it to check voltages yet, but does this symptom and problem
> ring any bells for anyone? Thanks!
> >
> > Thomas
> >
>
> There's an internal thermal fuse in the mains transformer, that fails and
> lifts the centre tap for the +12/-12 rails so they end up completely wrong.
>
> It's a mindbuggeringly stupid design.
>
> Fortunately, you can usually pick the tape off the transformer enough to
> just link the bastard out.
>
> --
> Gordonjcp MM0YEQ
>
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>
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