9 posts / 0 new
Last post
commodore 128dcr
Hey everyone I have a problem with a commodore 128DCR. I can't get it to boot in 64 or 128 mode or in monitor mode. I've checked Ray Carlsen's site about this problem other than checking U11 (8721) , U10 (z80), and U6 (8502). I cant switch these because their not socketed and I tried to desolder U11 off of a different board but not having any luck. I also found that the left most ram chip U38 was getting hot and was able to get that one off and U11,U10 were warm but U6 was extremely hot burn you hot. Is there any thing else to check. I Have a good U7,U21,U22 wich I know are good in it right now. I checked psu voltages 5vdc,12vdc,9ac read at 5vdc,11.75vdc and 9vac. could other ram chips gone bad? and It gets a black screen in 40/80 mode drive does not go thru the normal boot where drive flashes looking for an auto load disk. when I turn it on. If I turn it on the red light on power comes on like normal and drive light comes on for a second and no video and drive does not stop spinning (the internal 1571 drive). I tested an external 1571 and 1581 separately on this computer and they all reset and settle down when I turn power on. I don't have any game cartridges other than the 1764 reu to test that with. I do have clock signals and 5v votage on the chips. running out of ideas. I looked at Ray Carlsens list of things to check but i'm stuck because I cant get the 8502, z80 and pla of the boards that I have to switch them. Thanks for any help in advance. Eric commadore129@hotmail.com
Well, no chip should get

Well, no chip should get "burn you" hot, so there's a big chance of the 8502 being shot.
What kind of equipment do you have access to? Do you have an oscilloscope?  If so, you can probe the address-lines of the 8502 and see if there's any activity. If there isn't, either the 8502 is shot, or there's something holding them high or low..
When you say "I do have clock signals and 5v votage on the chips.", which chips are you talking about?
As for getting the chips out, you can either get a solder-sucker like this: https://amzn.com/B00BG62F6E
If you see yourself doing this more than once, and have an extra six bucks to spend, I'd go for something like a solder-sucker with a heater: https://amzn.com/B00B88FRME

I have checked for clock
I have checked for clock signal at both 8502 and z80 for clock signal and 5v on those chips. I forgot to mention when I say it was getting that hot was after about 10 mins or so. Imeant the 8502 was hot. Also I have a solder sucker and I added a little new solder to help extract the solder but it looks like it almost has been soldered on both side and I cant get the solder completely off cant figure out why and I added flux to it as well. Is there a way to check the 8502 and z80 out of circuit? I have another z80 and 8502 and pla out of another dead 128dcr that the motherboard is not to great of shape but both chips are socketed on that board. and having same problem getting ram chips of of that board as well. also the pins are bent over on back of the board how to I go about bending them strait to get them out of the board? thanks in advance.
The solder sucker I have has
The solder sucker I have has a bulb on the back and a new tip on the front. I was hoping the two cpus being replaced would fix the problem I hope I can't afford to do much right now. I was trying to get this one running because it's excellent condition. I also want to move the control board from the Lt. Kernal onto the board I don't have it on there yet but it goes on the mmu socket then the mmu gets piggy backed to it. Then I can hook up the LT. Kernal to the C128DCR as well basicly trying to switch from a regular 128 to the 128dcr to save space lol.
Yea, it will look like they

Yea, it will look like they're soldered on both sides. It's from the wave-soldering process, and the solder gets "sucked" up through the holes because of the capillary forces. It's definitely possible to desolder them from the back of the board only. It's the way I do it. Thats how I know.
You seem to have a good routine, with adding new solder and fluxing it as well, so you're on the right track. Just keep trying. A common mistake is to not heat the pin long enough. My weak-ass desolderer (actually, the exact same one I linked to before) needs me to hold it on the pin for a good three seconds before I can suck out all of the solder.
As for the chips where the pins are bent, the technique I usually use is to take my regular soldering-iron, melt the solder, and bend the pin with the tip of the soldering-iron whilst the solder is wet. This isn't super-nice to the tip, so don't use your best one. :)
I don't know of any hacky ways to check the 8502 or Z80 out-of-socket, but that doesn't say much. I'm sure some people on here knows some neat ways of diagnosing them by measuring their resistance or somethings. If you have a clock generator and some EE know-how, you could build the support circuitry needed to make it free-run on a breadboard in a few minutes. A bit overkill though..
You never said how you checked the clock-signals. If you have an oscilloscope, check the reset-line and address-lines on the 8502 and Z80 to see if there's life in them.

yes I forgot I have a scope
yes I forgot I have a oscilloscope and I was getting clock signals. The reset line on both were hi I forgot what voltage and what clock freq was but I was getting clock but I have not checked the address lines yet clould the pla also have some thing to do with it? i'll try to desolder a chip on the bad board for practice and see what happens later tonight thanks for the help.
The PLA could definitely have

The PLA could definitely have something to do with the whole "computer not working" part.
The PLA is responsible for mapping in the different ROMs, so that the 8502 have something sensible to execute. However, as the state of the machine is, many things are possible suspects.
The more methodical approach to this is to verify the (at least basic) functionality of each subsystem. Well, at least that's my approach..
If you want to look at the PLA, you could look at the ROM1-ROM4 signals out from the PLA. I'm a bit out of my water here, but I belive only one of these should be low at any time.
HOWEVER.. If the outputs from the PLA doesn't "make sense", it doesn't mean that the PLA is broken. The PLA acts on what it sees on the address-bus (and a few other pins), so if the 8502 is dead, the PLA won't act normally.

what i did sofar
okay I need to get another desoldering iron and some more solder wick. My desoldering iron with the bulb on it broke the tip on it is corroded solid but it's pretty old minor setbacks lol. yeah I can see a black screen on the coax out put where you hook it up to the cable jack on the T.V. But composite output I get nothing. so I think at least the vic works. voltages are good crystals are right freq. that's printed on them. I've already changed mmu. I took out a bad ram chip U38 it was too hot. external drives reset like their supposed too. Internal drive keeps spinning. I took the sid out. Tried directory trick typing blind. So I guess it boils down to Z80 and 8502 or supporting chips that's all I know lol. I'm typing this to keep track of what I did lol, So I can keep all of this strait in my mind. yes will also test pla and see what happens. If I get it working i'll be sure to post it onhere what I did to fix it. Who knows maybe some one else might make good use of this because it's not a very common problem from what I researched. that's for the help.
resoldered different 8502 and z80
I just got done resoldering a different 8502 and a z-80 chip in today still no composite out put I do get a black sreen in rf output with some random white dots and if I hook up an external 1571 to it the external drive green access light flashes at random for 1-5 seconds constantly I swaped the mmu took out the sid switched vic and other chip inside the can swap both rom chips and only thing different is external drive is blinking randomly reset button works. drive reset works. tried both 40/80 col. mode on all video out puts and just get black screen on rf output. also tried c64 mode monitor mode and 128 mode both 40/80 screens I tried directory f3 key on 128 mode and nothing. I'm out of ideas. Maybe bil might know.
Log in or register to post comments