Vintage Synth Repair

Darn, all that work desoldering the CPU...

Do you have the 500khz coming from the CPU? Could the sub-CPU be interrupting the main-CPU? Also have you checked the eprom for the proper checksum? Disconnect all the peripheral circuits like the keyboard, switch boards, etc. You may need the display connected because in some synths like the Alesis QS a bad or missing display can actually keep the synth from booting. The other frail chips on the board are the SRAM memory chips. You did say you have good power supply voltages with very little AC ripple.  

That's the problem with trying to troubleshoot the digital circuitry. You have no idea what kind of surge or what conditions actually lead to the failure. It may actually be that every processor and voice chip is fried. I have a SY85 that had a bad solder joint on the ground leg of the 7805 regulator. The entire digital section was exposed to 8 volts or more. This SY85 has all sorts of flaky digital issues including rebooting every 10 minutes or so.
 
Hope this helps,  Fran

--- In vintagesynthrepair@yahoogroups.com, "hnrgaardmadsen" <helge1961@...> wrote:
>
> And it still refuses working. I've changed the MPU now, and at least that helped a little - I don't get traces of 8 MHz anywhere it is not supposed to be. But the rest is the same: Everything that can light up at all does light up, and nothing more happens.
> Any ideas anyone?
> 
> Sincerely,
> Helge
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, "hnrgaardmadsen" <helge1961@> wrote:
> >
> > 
> > Hello All,
> > I measured my DX7 with a scope, that's how I found traces of the 8 MHz at places where I didn't expect it at all. And yes, I did check the PS, it gives out +/- 12V and 5V and all. Even the battery is ok ;)
> > Thank you to all of you for the effort so far; it shure would be great to get the old bugger playing again. By the way, getting a new MPU shouldn't be a problem - if only I order at least ten!
> > 
> > Sincerely
> > Helge
> > 
> > 
> > --- In vintagesynthrepair@yahoogroups.com, "Doug" <dougslocum@> wrote:
> > >
> > > Hi,
> > > You don't say if you are measuring with a meter or scope.  That would help knowing before going further.
> > > Have you checked all power supplies for correct voltage?
> > > Doug
> > > 
> > > --- In vintagesynthrepair@yahoogroups.com, "hnrgaardmadsen" <helge1961@> wrote:
> > > >
> > > > Hello,
> > > > I'm currently trying to make a DX7-IID work. At power-up, all LEDs and the display lights up, and nothing more happens.
> > > > Measuring at the MPU, it does get the 8 MHz clock signal, but it does not output the 500 KHz clock signal at pin 15. Besides, most of the adress pins (34-50) have a weird output of constantly 1.5V. - and at a few of the pins above 50, there is a slight trace of the 8 MHz clock signal. I have measured the TC40Hxxx circuits directly connected to the resonator and the crystal; they seem to work ok. And none of them run hot. As a matter of fact, nothing in the machine runs hot.
> > > > 
> > > > All in all, I'm suspicious that it is the MPU that has given in. Does anybody have any experience with this problem? Frankly, I hope it is something else than the MPU, it seems to be quite a job to desolder it and replace it with a new one. Any ideas will be appreciated!
> > > > 
> > > > Best regards,
> > > > Helge
> > > >
> > >
> >
>