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Help with Commodore 128 Repair: Black Screen in Both Modes, Even with Diagnostic Cartridge

Desperado

Desperado

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Nov 25, 2017
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Hi everyone! I am desperate again because
i’m working on repairing my Commodore 128, but I’m facing an issue where the screen stays black in both the 128 mode and the C64 mode. I’ve tried inserting a diagnostic cartridge, but the result is the same: a completely black screen.

I’ve checked the voltages, and they seem to be correct, but I can’t pinpoint the cause of the issue. I’m following the diagnostic manual, but I’m struggling with it. I also don’t have much experience with this particular machine, and I could really use some help, especially since some of the ICs are rare and expensive.

Here’s what I’ve already checked:

  • Voltages are correct (12V and 5V).
  • The reset signal seems to be working fine.
  • I’ve tried both RF and composite cables, but the issue persists.
  • The diagnostic cartridge produces no result (still a black screen).
Any suggestions or guidance would be greatly appreciated! Thanks in advance.

Feel free to make further adjustments based on your needs!
 

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Clock signals?

Have you probed the internal HSYNC and VSYNC signals for any signs of life at all?

Dave
 
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These are not the only clock signals...

Have you checked the schematics for ALL of the clock sources?

I have also updated my previous post...

Dave
 
daver2 said:
These are not the only clock signals...

Have you checked the schematics for ALL of the clock sources?

I have also updated my previous post...

Dave
Click to expand...
Not… i have not checked for Hsync and Vsync signals
 
To see if you have have bad ram preventing startup you can...

Grab a destest or destest max cart, these will run with bad ram.

Or grab a copy of the c128 diag rom, burn it to a 27c256 and put in in u36 on the motherboard.

https://blog.worldofjani.com/?p=164

Or, try swapping the banks of ram.

You lift legs of the 2 resistors on the right of the ram and then run jumpers swapping the signals making bank 1 act as bank 0.
 
No video output suggests a fault in the video generation circuitry, provided that other key components and connections check out okay.

As has already been pointed out, the synchronization signal(s) (Horizontal SYNC, Vertical SYNC, and the mixture of them = Composite SYNC) is a critical element, though.

No sync means the video display (LCD, CRT) has nothing to "lock" onto and it could treat that as no input signal and not even try to interpret it.

A bad connection anywhere between the PCB and the display could also result in no signal making it to the display.

-----

IDK about the C128, but some diagnostics software for the C64 expects a "test harness" to be attached for testing of other ports/hardware.

Without a working video output, you can't really tell what it's doing unless it drives the speaker. And any user input you provide will be entered blind... making selections/menus hard to operate correctly.
 
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dabone said:
To see if you have have bad ram preventing startup you can...

Grab a destest or destest max cart, these will run with bad ram.

Or grab a copy of the c128 diag rom, burn it to a 27c256 and put in in u36 on the motherboard.

https://blog.worldofjani.com/?p=164

Or, try swapping the banks of ram.

You lift legs of the 2 resistors on the right of the ram and then run jumpers swapping the signals making bank 1 act as bank 0.
Click to expand...
I used a multi diagnostic cart ( also death test ) but i can't see any screen....
 
There are quite a lot of things on this schematic (https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-2-left-and-right.gif) to look at (assuming this schematic is what your machine is).

I would look at the following pins on U22 (8563): R, G, B, I, VSYN, HSYN, /DCLK to start with.

Is there any activity on the /RAS and /CAS pins?

U21 pins 18 and 23 should have a 1 MHz and 2 MHz clock present (respectively).

Pins 29 and 30 should also have clocks on them.

Is your machine PAL or NTSC? Pins 29 and 30 ill have a slightly different frequency depending upon which video standard you have.

Dave
 
daver2 said:
There are quite a lot of things on this schematic (https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-2-left-and-right.gif) to look at (assuming this schematic is what your machine is).

I would look at the following pins on U22 (8563): R, G, B, I, VSYN, HSYN, /DCLK to start with.

Is there any activity on the /RAS and /CAS pins?

U21 pins 18 and 23 should have a 1 MHz and 2 MHz clock present (respectively).

Pins 29 and 30 should also have clocks on them.

Is your machine PAL or NTSC? Pins 29 and 30 ill have a slightly different frequency depending upon which video standard you have.

Dave
Click to expand...
I haven't activity on pins R,G,B,I of U22
i have a waveform on pin 3 ( HSYNC) and on pin 2 ( /DCLK ) but not activity on pin 20 ( VSYNC ).
I have waveform also on /RAS and /CAS pins.
 
daver2 said:
U21 pins 18 and 23 should have a 1 MHz and 2 MHz clock present (respectively).

Pins 29 and 30 should also have clocks on them.

Is your machine PAL or NTSC? Pins 29 and 30 ill have a slightly different frequency depending upon which video standard you have.
Click to expand...
On pins 18 and 23 i have 1 MHz clock...not 2 MHz...
Pin 29= 18 MHz
Pin 30= 8 MHz

This is a PAL machine!
 
What does the HSYNC signal look like?

Is it 'sensible' for a HSYNC signal driving a monitor of the specifications for the C128?

No VSYNC signal is interesting...

According to the schematic, there is supposed to be 2 MHz on there - but I also asked exactly what machine you had to compare the schematics with...

Dave
 
This is HSync signal and the position of the Line.
 

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Do you suspect that the VIC chip is faulty and not outputting the Vsync signal?"
 
Why would I suspect the VIC chip when the VSYCH signal we are missing is not generated by the VIC chip?

Have you actually looked at the schematics for the pins we have measured?

Dave
 
daver2 said:
Why would I suspect the VIC chip when the VSYCH signal we are missing is not generated by the VIC chip?

Have you actually looked at the schematics for the pins we have measured?

Dave
Click to expand...
The VSYNC signal should come from pin 11 of U24...
 
Correct. This is why I keep on saying that you need to learn how to read schematics and device data sheets to determine what pins are inputs, outputs, tristate, bidirectional, open collector, etc. to determine the signal flow...

So, it is U22/20 that generates signal VSYN.

Before you warm up the soldering iron, I am NOT saying that U22 is faulty (yet). We are exploring basic signals to see if they are correctly present - or not - to learn things.

Can you now check U22 pin 4 to see what signal is present please.

Dave
 
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