• Please review our updated Terms and Rules here
  • Exhibitor application for VCF West 2022 is now open! If you are interested in exhibiting, please fill out the form here.
  • Here are the results of the VCF East 2022 Post Event Survey: Survey Results

C128 Display problem at switch on

StowComputers

Member
Joined
Aug 11, 2011
Messages
35
Location
Stowmarket, UK
IMG_0318.jpg
I got this really nice condition C128 about 3 weeks ago. I didn't pay too much for it (for 128's at least), but as you can see it doesn't work. The display you see here is what remains when you power it up, on both rf and composite. Strangely if I switch it off & on again I just get a black display, but if I leave it off for a while and try it again I get the display on the picture again. I thought the PSU may be on the blink, as I didn't get a 9v reading. I later had a homer doh moment when I realised unusually the PSU is putting out 5v DC and 9V AC. When the 9V AC was checked it seems fine.

I'm suspecting this could be a bad rom to do with the display, which there are 2 in an enclosure on the motherboard. You guys may know better though? I'd really like this one working, I think of all the vintage computers I own, it could be the one I use the most.

Thanks as always, in advance.
 

commodorejohn

Veteran Member
Joined
Jul 6, 2010
Messages
3,135
Location
California, USA
Check the 40/80 key on the keyboard, IIRC when the 128 boots up in 80-column mode it doesn't bother clearing the video RAM of gibberish values.
 

StowComputers

Member
Joined
Aug 11, 2011
Messages
35
Location
Stowmarket, UK
Check the 40/80 key on the keyboard, IIRC when the 128 boots up in 80-column mode it doesn't bother clearing the video RAM of gibberish values.

Well I have tried switching on with the 40/80 switch on or off, but it doesn't seem to make any difference; unless I'm missing something. It doesn't respond when typing blind a sound test program, and nothing changes or flickers when I go64 either.
 

commodorejohn

Veteran Member
Joined
Jul 6, 2010
Messages
3,135
Location
California, USA
If even a sound test doesn't respond I'd guess it's not a display issue. Does it attempt to read the disk on boot? I'd try gently but firmly re-seating the socketed chips and see if that changes anything.
 

StowComputers

Member
Joined
Aug 11, 2011
Messages
35
Location
Stowmarket, UK
Ah, well spotted sir and it looks right on the money. Good old RTFM, wins again. Where on earth am I gonna get one of those 8721's? Guess I will have to hunt down some 128's for spares.
 

StowComputers

Member
Joined
Aug 11, 2011
Messages
35
Location
Stowmarket, UK
I just thought I'd update you on where I am with this.

I received the 8721 for U11 today, one week from elettrosurplus.com Italy which is not too bad. I had ordered it along with an 6532 I needed to fix a 8250 LP. Both chips are original MOS and look unused with no signs of ship pulling or soldering.

I'd already removed the old 8721 and socketed U11 in preparation. The result with regret is exactly the same. It's unlikely to be U11 so I will focus my efforts on U18, the character ROM next.

On the positive side the other 6532 worked perfect and repaired another 8250 LP! So I can recommend this guy.
 
Top