So I got my previously dead Poly back last night in fully working order! It turns out the CPU (IC24 80085)had failed somehow and it had shorted and burned out one of the D Flip Flops connected to it (IC 28). Replacing just the flip flop did not work and just immediately burned out the new chip. However, with both of those components replaced everything is back up and running.
<<aaRoN>>
--- In korgpolyex@yahoogroups.com, "selloutdesigns" <aah_public@...> wrote:
>
> I haven't given up on the Poly, just my own ability to get it working in a timely fashion. Possibly if I spent a lot more time I would eventually stumble upon the fix, but fortunately I have a local resource that does vintage synth repair who is taking a look at it now. I'm hoping to have the diagnosis soon, and hopefully a functioning unit. Then I can get back to the important business of modding!
>
> <<aaRoN>>
>
> --- In korgpolyex@yahoogroups.com, Michael Hawkins <korgpolyex800@> wrote:
> >
> > Well, you might be giving up a little too early don't you think?
> >
> > Remind me again but didn't you say it looked like it had some battery leak on it?
> >
> > If that is the case, then I would take a long, hard look at those spots that you think had those leaks on them.
> >
> > Because that acid can do it's corrosive thing without showing much damage after you clean it up.
> >
> > There is an EPROM in the EPROM socket right? ;-)
> >
> > Mike.
> >
> >
> > ________________________________
> > From: selloutdesigns <aah_public@>
> > To: korgpolyex@yahoogroups.com
> > Sent: Friday, March 6, 2009 9:53:52 AM
> > Subject: [korgpolyex] Re: Dead 800
> >
> >
> > Okay, I'm totally stumped at this point. The main issue I see now with the display is that the IO/M pin on the CPU is low 100% of the time, as confirmed with a scope reading. However, I have no idea why. SOD is also low all the time as well, which is probably related. The CPU clock is fine and I see RD,WR,S0,S1 going high and low on a regular basis so it seems memory reads and writes are occurring fine, but no I/O whatsoever. I'm wondering if there is an issue with the CPU receiving instructions from memory, which is causing the I/O cycle to never occur.
> >
> > In any case, I have just about given up in trying to figure out what's going on. I think this unit is destined to land on someone else's bench. If it does get fixed I'll be sure to find out what the cause was.
> >
> > Thanks to all for the input.
> >
> > <<aaRoN>>
> >
> > --- In korgpolyex@yahoogro ups.com, Michael Hawkins <korgpolyex800@ ...> wrote:
> > >
> > > The circuit description is correct.
> > >
> > > Interrupt RES7.5 runs at around 2400 to 3600 hz.
> > >
> > > It can't run too fast otherwise the CPU won't do anything else except service this hardware interrupt.
> > >
> > > A rate of 2.94Mhz is only half the full clock rate of the CPU. So at that speed, the CPU would not even get to fetch one instruction before being interrupted.
> > >
> > > Mike.
> > >
> > >
> > >
> > >
> > >
> > > ____________ _________ _________ __
> > > From: selloutdesigns <aah_public@ ...>
> > > To: korgpolyex@yahoogro ups.com
> > > Sent: Wednesday, March 4, 2009 4:22:11 PM
> > > Subject: [korgpolyex] Re: Dead 800
> > >
> > >
> > > Hmm, now I am starting to think I'm chasing my tail. In the schematic (pg 6 in Service Manual) for the main board, it shows CPU RES 7.5 as running at .34usec per cycle, or about 2.94 Mhz, which is what got me thinking I had a problem there. However, in the description of the Interrupt Oscillator circuit in the Service Guide (pg 18) it reads:
> > >
> > > "This oscillator cycle is used for the EG, MG, LED displays, and S/H time division processing. Oscillator frequency is 24OOHz-360OHz. Interrupt order is by priority. If this circuit fails, EG operation and LED indication may become erratic."
> > >
> > > I measured the oscillator at about .34msec per cycle, or 2941Hz, which seems normal according to that.
> > >
> > > So which is correct? The schematic or the circuit description? I'm starting to think the circuit description. I've tested all the components except the TL072, but all seem fine.
> > >
> > > <<aaRoN>>
> > >
> > > 6) -Address Decoder:
> > >
> > > --- In korgpolyex@yahoogro ups.com, "narfman96" <narfman96@ ..> wrote:
> > > >
> > > > Should work fine. Install a good socket if you want. Then you can use a TL072 when you get one.
> > > >
> > > > --- In korgpolyex@yahoogro ups.com, "selloutdesigns" <aah_public@ > wrote:
> > > > >
> > > > > Anyone know offhand if a tl082 can safely replace a tl072? They do appear compatible.
> > > > >
> > > > > Thanks,
> > > > >
> > > > > <<aaRoN>>
> > > > >
> > > > > --- In korgpolyex@yahoogro ups.com, Gordon JC Pearce MM3YEQ <gordon@> wrote:
> > > > > >
> > > > > > On Wed, 2009-03-04 at 13:34 +0000, narfman96 wrote:
> > > > > > > Definitely timing issues with IC36b the interrupt oscillator. Try
> > > > > > > disconnecting the output at R107 and see if it runs at the correct
> > > > > > > frequency. If it does the CPU is loading it down somehow. If not then
> > > > > > > suspect the timing caps C76 or C79. The resistors R102-R104 need to be
> > > > > > > verified and then the Tl072 op amp itself. This circuit needs to run
> > > > > > > in the mhz or no go for the LED display and envelope generators.
> > > > > >
> > > > > > Bah. Just change the TL072 on spec. Horrible things.
> > > > > >
> > > > > > Gordon
> > > > > >
> > > > >
> > > >
> > >
> >
>