Two dead C128

[VintageVic]

So, I acquired set of Commodores that included two C128's and one power supply
for these.

Both seem to have a black screen of death.

I checked that the motherboard gets +4,97V on the other and 5,00V on the other motherboard
on some of the big chips. Measured 9,9VAC on the psu without load. Not sure on where I could check
AC voltage on the motherboard.

I pulled SID chip away on both, but no change. Also tried without U1 and U4 VIA chips with no luck.
I let the units be powered on on the table for a couple of minutes. RAM chips get warm, but none stand
out to be more hot compared to the others.

One unit has the 8502 cpu getting quite hot already in a minute or two, almost burning hot for finger to touch. The other unit has both cpu's getting hot quickly after power on.

These are my first C128's. As I have no working unit, it is difficult to test out failed chips.
But any other test I could do here before buying a working unit ?

Tried Jupiter lander module, but black screen with it too. It has proven very useful with C64, because something often comes on the screen with it at least proving the VIC-II chip is ok. But no luck with these C128's. I still have the dead test cart that I have not yet tried out, I will try that. But in more cases, Jupiter lander has been able to produce something on the screen even when trying with the dead test cart remains a black screen.

In the end, I am probably going to need to buy a working one to make it easier to find out what chips may have failed on these units.

 

[Mr Fahrenheit]

I’ve purchased many c128 machines and I have yet to find one that doesn’t work. The fact that you have two that don’t work just astounds me. Unless the person you got them from collected dead stuff to fix?

Are you dealing with NTSC or PAL machines? What continent are you on? I have a c128 with a couple of damaged keys that otherwise works. Would you be interested in purchasing it for your quest? It’s NTSC and I’m in North America.

 

[VintageVic]

Hi Mr Fahrenheit,

and thanks for your suggestion!

Had you been in Europe and had I not already purchased third unit from another ebayer,
I would probably have made a deal with you. I'm tinkering this stuff here in Finland.

I had assumed that they are PAL units, but come to think of it now, they came with 1571 drive with
117V psu. Perhaps one or more are actually NTSC unit. But still, if they were working, the tv with scart would
probably show something instead of just black screen.

The units were sold without tv cables as untested. Well, the risk was mine to buy them anyway.

While waiting for another C128 to arrive and become the testbench, I received bunch of defective commodore floppy drives. two 1541's, one modified 1541, one 1541-II and a 1551 with heavy physical damage. Put my focus on these yesterday and got the three 1541 serviced and they now work! Something went my way there. The 1541-II has
defective mechanics. The 1551 may be too far from being rescued, but if so, it will be useful to have one for spares.

 

[VintageVic]

Mailman brought me exciting packet yesterday.
Third C128 that I could use to help debugging the two dead ones.
It has some missing key issues and based on the sales ad it may have damage in SID.

But what a surprise when I tried to boot it for the first time:



I had to check is this really with a C128 motherboard, well yes it is. But I could not
boot it to C128 mode, instead it goes directly into a C64 mode!

After opening the sealed case, there was immediately evidence of moisture history with plenty of corrosion:


And things looked even nastier under the metal shield:


So, very likely to have contact issues with that IC socket and the IC itself.
What I did was to try the IC from a dead board here. I rubbed / scraped the IC to the
socket back and forth for quite a long time in hope that it would give contact there.

and it worked! Yes,this became my first unit that I have to work in C128 mode!


Well then, why not continue to check all of the socketed chips from dead C128
in this working unit. It is usually the last chip you check... and there was this ROM
chip that did not work at U34. So; I tried a working ROM chip on dead C128 and
yessss! That was the issue there - easily found after 5 to 10 minutes of troubleshooting
and no soldering involved just yet!

Now I have two working C128's. Also checked the corroded U7 chip (8722R2),
sanded it's legs clean and tried it out on formerly dead now working C128 pcb.
It goes again to C64 mode, so that chip really seems to be defective too, not the
contact with IC socket alone.

Now, the third C128 pcb is still dead. Already tested all the socketed chips and they
were fine. When I turn on the power, CPU gets really hot in a minute or so. I'm probably
going to desolder it and to check it I have to desolder the CPU on a working C128 board also.
My only way of testing, since I have no logic probes or oscilloscope. That is to come,
when I have time to tinker again.