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Commodore 128 - not working?

tingo

tingo

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Ok, I bought a Commodore 128 today.It came with three joysticks, a 1530 Datasette and about a dozen or so of tapes. With luck I will get the 1571 drive and some diskettes too (the former owner is searching for them in his storage).
I connected the C128 to the PSU, and a RF cable between RF out and my TV. When I turn it on, the power light turns on, and my TV (tuned to UHF 36) indicates that it has a signal (because the OSD briefly shows "MONO"). Nothing more happens. I also connected the 1530 (after turning off the power first), and tested with that. Nothing works (ie rewind or FF buttons do nothing). I tried searching for a C128 troubleshooting guide, but didn't find it.

What's next?
 
I would start by testing the power supply. Get a pinout (unfortunately I don't have one handy, but would suspect you could find one somewhere on zimmers.net) and use a voltmeter make sure you're getting +5 VDC where you're supposed to, 12 VAC where you're supposed to, etc. The C-128's power supply was much, much better than what they shipped with the C-64, but in my experience it's still one of the more failure-prone components, and certainly it's the easiest component to check.
 
I used the pinout here (note that the picture is of the socket in the computer).
According to that my +5 VDC (pins 1 and 4) is 5.01 VDC. And my 9 VAC is 10.78 VAC. I don't know anything about a 12 VAC. :)
 
Sorry if this is obvious, but you're not hooking the RF out to a yellow RCA video in, are you (lol :D)?

I have the same problem with my 1531 - it worked for like two seconds when I first tried it, and it never moved a muscle again. I think the motors are dead.

If you have the capacity, it's fairly simple to make a C64/128 out to S-Video adapter, since their proprietary port has separate chroma and luma - might work better.
 
Ray Carlsen has a text on the C128 as well as most of the other Commodore computers:
http://personalpages.tds.net/~rcarlsen/cbm/C128-IC.txt

You mention not even the 1530 would wind, which suggests either it is broken or it doesn't get power from the computer.

Another common mistake might be that the button labeled 40/80 is stuck, so you get 80 column output through the VDC instead of 40 columns through the VIC-II. On the other hand, I can't recall if the RF output handles both and I'm too lazy to look it up myself.
 
My bad. It's 9 VAC, not 12VAC. I'm not as well versed on my Commodore 8-bits as I used to be. The power supply sounds OK. Not perfect, but OK.

The RF output was 40-column only. Checking the 40/80 column key is a good step. Something else to try is connecting a CGA monitor from a PC to the RGB output, if you have one.
 
carlsson said:
Another common mistake might be that the button labeled 40/80 is stuck, so you get 80 column output through the VDC instead of 40 columns through the VIC-II. On the other hand, I can't recall if the RF output handles both and I'm too lazy to look it up myself.
Click to expand...

In 80-column mode you get nothing out of the C-64-style outputs (video or RF), so that's a good suggestion on the 40/80 button. And it would be normal to see an RF signal briefly on power up because the machine checks for a cartridge on startup (if it finds one it goes into C64 mode regardless of the button, I believe).

I would also measure for voltage on the pins where the Datasette connects. I'm pretty sure one of those pins should always carry +5 VDC when the computer is turned on. Lack of this voltage would be a bad sign (but this would be odd since 5 VDC is present on the power supply.
 
Raven said:
Sorry if this is obvious, but you're not hooking the RF out to a yellow RCA video in, are you (lol :D)?
Click to expand...
Fair question, but no - I am not.


If you have the capacity, it's fairly simple to make a C64/128 out to S-Video adapter, since their proprietary port has separate chroma and luma - might work better.
Click to expand...

Like this one: The Commodore S-Video converter?

Update: I actually have a Commodore video cable (story: I once had a 1084S monitor for my Amiga) with 8-pin DIN to 3 x RCA phono, and I also have a 2 x RCA phono to S-Video adapter, so now I have tested with C128 video out to S-Video on my TV. No picture there either (but the OSD indicates "SVideo" "PAL 50Hz" when the C128 is turned on).
 
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carlsson said:
Another common mistake might be that the button labeled 40/80 is stuck, so you get 80 column output through the VDC instead of 40 columns through the VIC-II. On the other hand, I can't recall if the RF output handles both and I'm too lazy to look it up myself.
Click to expand...
Well, the key operates normally (mechanical). I haven't checked it electrically yet.
 
mark66j said:
I would also measure for voltage on the pins where the Datasette connects. I'm pretty sure one of those pins should always carry +5 VDC when the computer is turned on. Lack of this voltage would be a bad sign (but this would be odd since 5 VDC is present on the power supply.
Click to expand...
+ 5 VDC is present on the cassette port (A-1 is GND, B-1 is +5V). + 5V is also present on the user port (pin 1 is GND, pin 2 is +5V).

Pins 10 and 11 is supposed to carry 9 VAC, but I couldn't measure that voltage there. Interesting. I wonder if there are any fuses inside the machine?
 
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Update: I found SAMS Computerfacts CC18, and using this excellent resource, I have figured out that 5V is OK at CR15, but that U59, the LM340T12 voltage regulator, doesn't have voltage in or out. Now I just need to figure out what's broken. Ok, there is approx. 10 VAC over C96, which is good. However, there is no voltage over C106, so perhaps the rectifier CR13 is broken. Must check.
 
OK, when measured with my trusty ohm-meter, rectifier CR13 appears to be ok. However, there isn't 9.6 VAC present on the rectifier input? pins 3 and 5 of CN11 carries 10.6 VAC (ok, so it is one volt higher that 9.6 VAC), and the power switch (SW1) is ok. The filter choke (L5) is also ok. C96 and C97 also seems to be ok. What could be wrong? Suggestions? I might trying to change CR13, just in case.
 
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I got a replacement for CR13, but then summer, vacation and life in general kicked in, so I haven't done much with it since. Now that autumn is here, there will be more time to work with indoor stuff again.
 
Wow, the years fly by!
Back to this old thing. The PCB says Rev. 7. Replacing CR13 didn't help.
As part of the repair, I bought a capacitor replacement kit, so the following capacitors have been replaced: C61, C62, C63, C80, C85, C88, C91, C92, C101, C104, C105, C106, C107, C111.

Now:
General checking of voltages: with my C128 connected to the power supply (and power switch on, the power LED lights up red) I measure on the components in the C128:
over capacitor C107 - 4.91 V DC, over capacitor C98 - 5.007 V DC. This should be 5.2 V, according to schematics in Sams Computerfacts CC18. Is my voltage here too low, or is it ok?

for the 11.5V, I measure as follows:
over C95 - 10.53 V AC
over C96 - 10.53 V AC
over C106 - 184 mV DC

So obviously, the 11.5V isn't ok. Will the machine mostly work without it (example: it is only needed for the serial port) since the 5V is (maybe) ok?
 
Look at this..
Also, 11.5 should be fine for the 12v. and as long at you have close to +5 you should be fine.

Common cause of black screen on the 128 is the z80 not getting a clock or power, since the 9vac supplies its power.

http://personalpages.tds.net/~rcarls...28/c128-ic.txt

Also, check the z80 input voltages and make sure the reset line is toggling on boot, that's actually the main boot processor, and after its done, it passes control to the main 8502.
 
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ok, so you answered my question; since the 11.5 V line isn't working (it is only 184 mV over C106) the machine isn't going to be working at all, since the z80 gets no power.
Unsoldered one leg and tested CR11 with my cheap chinese component tester, it checks out ok.
 
Update: verified that CR10 and C102 are ok. Measured the pins of U59 (LM430T12) with a ohm-meter (both ways) pin 1 - 2 and pin 1 - 3 in the Mohm range (five or more), pin 2 - 3 about 200 ohm. Does this sound about right?
 
Another (small) update: there was a broken trace between C96 / L5 and AC input of CR13. Fixed by soldering in a patch wire on the solder side of the PCB. The trace between + out of CR13 and C102 / C106 seems to be broken also (no continuity when measured with an ohm-meter). I'll need to double-check and fix that.
 
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