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Subject: Re: [korgpolyex] Re: Dead 800

From: Michael Hawkins <korgpolyex800@...>
Date: 2009-03-04

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@yahoogroups.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
> > >
> >
>