View Full Version : Commodore 128

August 20th, 2014, 02:06 PM
So i received a brand new old&used commodore 128. Arrived in the mail last week. Unfortunately the sends only included the power supply, so I ordered a S-video and composite cable off ebay. I received the cable today and thought i'd give it a shot. Connected the cable to my 128 and powered it on and set my TV input to s-video. There's only a black screen with vertical stripes (the vertical lines is due to the video signal i believe?). Alright, so i changed the input channel om my TV to check if the composite signal looks any better. But this is black as well. I notice that there is a signal change when powering the c128 off / on, and so the channels are correct.

So I figure perhaps the cable is damaged. I also have a c64 and I connect the same cable to this computer which boots up just fine on both TV inputs.

I have done the following so far:
1. Video cable is OK (works on another system)
2. Power adapter is OK (tested using a voltmeter - sends +5 DC and +9 AC) (I did open the adapter anyway just to clean out the dust from it).
3. Opened up the C128 to clean out the dust from the inside. Also checked that the chips are properly inserted into the sockets.

From what I could read out of this document : http://personalpages.tds.net/~rcarlsen/cbm/c128/c128-ic.txt
The problem could be with the video chip. I have inspected the chip but there are no visible damage to the chip as far as i can see.

So now i'm just a bit sad ... and stumped ... was so looking forward tinkering with this great system from my childhood.
Any ideas or suggestions are appreciated (or if anyone got a new chipset to fix the problem, that would save the day aswell!)

August 22nd, 2014, 08:07 AM
IIRC, you can hook the 9-pin RGBI port up to a CGA or Multisync monitor and it will work fine. Might be a good way to check if you have one on hand.

August 22nd, 2014, 11:00 AM
Don't forget to check the 40/80 column switch.

August 22nd, 2014, 11:06 AM
First off, make sure the 40/80 button on the top row is not pushed, if it is, then the 128 defaults to RGB mode. (But you should still have a border.)

Also try powering on while holding the commodore key to boot straight into 64 mode.

If neither of these work, then your machine has a hardware fault, and my next suggestion is to check the fuse in the power supply that provides the 9v supply.
This is used to power the z80 processor, which is the bootup processor for the 128, so no 9v, no post.


August 22nd, 2014, 02:00 PM
Does the on-screen display change at all (i.e. a different pattern of lines) when you type on the keyboard? If it does, then that's a sign that the CPU is probably working, and the problem lies somewhere in the video circuitry. But if the display does not respond at all to keyboard input, then the fault might be more serious, because the CPU isn't getting far enough to accept keyboard input.

You can try typing this in "blind":

POKE 54296,15:POKE 54278,240:POKE 54273,100:POKE 54276,33

If you have the audio output connected, if the computer is working, when you hit RETURN you'll hear a continuous tone from the speaker. (Hit RUN/STOP + RESTORE to stop it.)

August 22nd, 2014, 02:57 PM
It shouldn't be the 40/80-column switch, as the VIC-II will display a grey background with a green border, not a black screen, when the 128 is in 80-column mode. Nevertheless, checking it with an RGBI monitor would be a good way to make sure that the rest of the computer is in working order.

August 22nd, 2014, 08:19 PM
Where are you located? Could be someone nearby who could help.

August 23rd, 2014, 08:25 AM
Thanx for all the great suggestions. The 40/80 switch is not pressed. Unfortunately i do not have a commodore monitor to test with. I am located in Rogaland, Norway. It would be great If someone in here who lives nearby have some spare parts, or could hjelp me troubleshboot the problem

September 19th, 2014, 10:31 AM
I've troubleshooted the circuit some more and it seems that the problem is with the VIC chip on U21. I'm having a hard time finding a donor c128 or new replacement ICs, so if anyone have pulled ones laying around, a donor board or can point me in the right direction to where I can get my hands on new ones, that would be awesome!.