X68000 (first one) repair

[aje_fr]

Hello,
Well, I own two of this beauty and both have nearly the same defect.
Obviously, I wanna repair them !
I isolated the defective board, and problem comes from the motherboard.
The motherboard did not start at all, only leds comes on.
There is even no video sync on monitor.
I tried to watch cpu signals, looks like roms is read for few seconds and cpu comes in idle.
Is there common issue with this first generation motherboard ?
I tried to swap cpu with a working one, same effect.
Battery was dead but no traces were broken.
Thanks for any help
Bye

[neko68k]

If it hasn't been recapped, do that first then get back to us. If it has been, then I don't know

[caius]

Which model are?Did you already try to replace PSU?

[aje_fr]

Which model are?Did you already try to replace PSU?

The very first one, CZ-600C.
Obviously, I made my test with a known working psu.

But after few measurement, I may have found my problems.
On both motherboard, the 69Mhz oscillator is not running.

The main challenge now will be to find two new one... 

[caius]

You could generate the right clock frequency using a DDS :

https://en.wikipedia.org/wiki/Direct_digital_synthesizer

[JulBS0]

Hi everyone,

I had a similar case (original X68000 from 1987, with rust everywhere and a leaking battery), with (among other problems) a nonfunctional 69MHz oscillator.

I ordered a Cardinal Components' CPP series programmable oscillator, clocked at 69.552MHz, and after butchering my motherboard, my X68000 now starts and displays the "Please insert system disk" message 

The oscillator replacement process wasn't simple, because on these programmable oscillators, the pin 1 is not the ground, but a positive Output Enable signal. So you'll have to find a clean way to let the pin 1 float (an internal pull-up will enable that signal). If someone knows how to properly do it, please share your knowledge.

I ended up using a cutter, a soldering iron at 350°C and several hex keys to file the hole on the motherboard and prevent the pin 1 from touching it, this is a complete mess but it works.

There MUST BE a cleaner way lol.

EDIT: here is a link to the oscillator data sheet: http://www.cardinalxtal.com/uploads/files/cpp.pdf.
The "field oscillator programming instrument" is expensive, but Digikey (and probably others) can program them for you when you order the oscillator, without charge.

[kamiboy]

Does anyone have a cap chart for the PSU of the OG X68000? The closes one I can find is one for the ACE, but I imagine they made changes to the PSU in that revision.

[JulBS0]

Here are the caps found in the original X68000 PSU (model number UADP-0047CEZZ):

• C714 - 4700µF 10V x1
• C721 - 2700µF 35V x1 (no more available? replaced by 3300µF 35V, seems to be working)
• C715, C716 - 3300µF 10V x2
• C718 - 2200µF 6.3V x1
• C719 - 680µF 16V x1
• C706 - 470µF 200V x1
• C725 - 470µF 35V x1
• C724 - 470µF 16V x1
• C720 - 47µF 6.3V x1
• C709, C726 - 22µF 25V x2
• C710 - 4.7µF 50V x1


• C717 - 2.2µF 50V Non-polarized/Bi-polarized x1
• C728 - 47µF 10V Non-polarized/Bi-polarized x1

[kamiboy]

Thanks a buncha muncha.