Faulty 128D

Mgarnett · October 06, 2010, 02:45 PM

Hi All

I've just acquired a 128D that I would like fix up. At the moment, the machine powers on fine but it just displays a black screen. I've taken the machine apart and cleaned it up (it was very dusty inside). I also took the opportunity to pull out all of the socketted chips, clean up the contacts and re-seat them all. I tried again after this but still no luck.

When the machine powers on it does not reset the internal 1571 as it should so it's not getting that far into the boot process.

Do you have any idea which chip/s could be the problem?

Cheers

Mark

BigDumbDinosaur · October 07, 2010, 02:29 AM

It could be that CIA number 1 has failed. It's responsible for a number of hardware functions that can prevent a boot, and since that CIA is connected to the joystick ports, it is vulnerable to accidental static discharge when reaching for the reset button.

When you power the unit can you hear the 1571's spindle rotating? That would happen before the attempt to boot from the floppy was made.

Mgarnett · October 07, 2010, 10:15 AM

Hi and thanks for the reply.

I don't hear any noise out of the 1571 on boot. Which numbered chip is the CIA?

Cheers

Mark

BigDumbDinosaur · October 07, 2010, 02:31 PM

Quote from: Mgarnett on October 07, 2010, 10:15 AM
Hi and thanks for the reply.

I don't hear any noise out of the 1571 on boot. Which numbered chip is the CIA?

Cheers

Mark

Its a MOS 6526. I'd have to look in my documentation to tell you the circuit ID. Perhaps someone else may already know that.

airship · October 08, 2010, 07:42 AM

U1 on both the flat C128 & C128D.

Mgarnett · October 08, 2010, 09:30 PM

Thanks guys. I'll give that a try and let you know how it goes.

Cheers

Mark

Mgarnett · October 08, 2010, 10:07 PM

Hi All,

Does anybody know where I might pick up a MOS 6526 for a 128D? I've not been able to find any for sale on the net as yet.

Cheers

Mark

BigDumbDinosaur · October 09, 2010, 10:52 AM

Quote from: Mgarnett on October 08, 2010, 10:07 PM
Hi All,

Does anybody know where I might pick up a MOS 6526 for a 128D? I've not been able to find any for sale on the net as yet.

Cheers

Mark

They're scarcer than honest politicians.

RobertB · October 09, 2010, 02:36 PM

Quote from: BigDumbDinosaur on October 09, 2010, 10:52 AMThey're scarcer than honest politicians.

Then those politicians must live in certain areas.

You can contact C= dealer, Jim Scabery, in Portland, Oregon. Comm-Jim(at)comcast.net

And our club has parts machines in storage,
Robert Bernardo
Fresno Commodore User Group
http://videocam.net.au/fcug
The Other Group of Amigoids
http://www.calweb.com/~rabel1/
Southern California Commodore & Amiga Network
http://www.sccaners.org

BigDumbDinosaur · October 10, 2010, 09:32 AM

I didn't say they didn't exist, just that they are scarce.

RobertB · October 10, 2010, 05:54 PM

I didn't say they didn't exist... just that they are at Jim Scabery's or from parts machines.

In the Los Angeles area,
Robert Bernardo
Fresno Commodore User Group
http://videocam.net.au/fcug
The Other Group of Amigoids
http://www.calweb.com/~rabel1/
Southern California Commodore & Amiga Network
http://www.sccaners.org

Mgarnett · October 22, 2010, 12:26 AM

Hi All,

I've replaced the CIA chips and the fault is still there. I've checked the fuses in the power supply and they look OK, certainly no obvious issues.

When the machines powers up, it does not reset the 1571 which I think would rule out any of the video chips or kernel ROM chips (booting with a cartridge or IDE64 makes no difference). I was thinking that it could be the Z80 or the MMU, what do you think? The Z80 in there at the moment is a Sharp branded chip, do you think there would be any problems replacing it with a Zilog? The reason I think the Z80 could be the problem is it was my understanding that it has control of the machine initially in case the user has the CP/M disk in the drive, if not, it passes control over to the 8502 but I could be mistaken on this.

I've got a friend who's teaching me how to solder, so I am going to socket any chips that I de-solder to make this an easier process in the future.

Other than the Z80 and the MMU, do you think any other chips could be causing this, such as 8502 or a memory chip? It's not the SID because I removed it and it made no difference. I've not really thought of this, but could it be a chip on the 1571 daughter board?

Cheers

Mark

Hydrophilic · October 25, 2010, 07:20 AM

I have years of soldering experience, and would NOT want to try replacing the Z80 (Zilog, Sharp, or otherwise). But that is me.

As I understand, you've tried replacing the SID and CIA(s). The next easiest thing to try would be the VIC-IIe (assuming you can get one). It is responsible for all timing and RAM refresh.

That's my 2 cents. Good luck!

Mgarnett · October 25, 2010, 04:02 PM

Thanks Hydrophyilic,

Yep, you are correct, I've replaced the CIA(s) and the SID with no change. I never realised that the VIC-IIe was responsible for timing so I will replace that and see how I go. I actually have a replacement chip on hand so I should be able to report back in the next day or so. There's also a socketed oscilator in there as well which controls timing so if this doesn't work, I will try and get hold one of one to test. I'd really like it to be a socketed chip failure otherwise there's no real option but to go down the soldering path.

I agree with you about the Z80, it looks pretty convoluted with that additional wire soldered to one of the pins (I have a 128D not a 128DCR).

Thanks

Mark

Mgarnett · October 26, 2010, 02:38 PM

Well, I replaced the VIC-IIe and still no joy. I had both a PAL and NTSC chip and tried both. The television acknowledged the different signals and knew that it was a PAL or NTSC signal but the screen was still just black.

I tried booting directly to CP/M and whilst the disk did spin, there was still no change and at no stage did the drive sound like it was reading the disk.

I'm starting to run out of ideas now so any help would be really appreciated.

Cheers

Mark