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I recently acquired a Commodore 128. It worked fine for about an hour, in both C64 and C128 mode, at the exception of some coloured bars, before displaying garbage and not booting anymore into any mode, displaying very faint dark blue columns.
The failure event wass initiated when I took the fully-assembled computer off the desk and on my lap. Here is a picture of the coloured poles, please pardon the angle, I cannot really take another one now that the unit plays dead.

The once working state of the computer

The garbage was dynamic in some rows, and had the habitual borders.
The unit will not boot up in any mode. Typing SOUND commands into BASIC does not seem to have an effect, and LOAD does not seem to switch the motor supply either. The supply rails seem nominal, at 4.6 4.8 (kranked up my lab PSU a bit to compensate wire loss and enter +/- 5% tolerance) and 12.4 respectively. The 5 volts rail measures 175 ohms to ground in both directions. Volt drop mode displays 200 - 300 mV in both directions as well.

The chips' heating conforms to this troubleshooting guide page 37 (according to the internals of the book, not of the PDF) except at three oddities :

  • The character ROM is only "weakly" warm.
  • ROM4 and ROM1 are somewhat warmer than the other ROMs.
  • The MMU is not very warm either.

Note : the VIC has not been tested yet, as it is under the video shield which I have not yet taken off, but the video shield itself gets warm, which indicates that the VIC is hot.
Removing ROM4, for I believe that it is not needed for a basic boot (at least some picture) does not fix the problem.

While the unit worked, the datasette port did not, but that may be another problem.

I suspect the 8701 clock generator chip might be at fault, according to this source

U28
251527-09 (8701) Clock gen. (Master oscillator)
Dead chip: no bootup in any mode.
Partial failure: diagonal lines on monitor or TV (horizontal sync bad), no color in 40 column mode, or "barber pole" colors. Check also crystal Y2

I however do not know if those lines could be considered "barber poles". Please note that all outputs shown are from the A/V port, as I am not able to use the RF or RGBI outputs. The main board (excepted from the video and modulator boxes) can be seen here :

Picture of the motherboard Back of the motherboard

As I would like to avoid tampering with the unit any more than I need to, and as I do not have a lot of experience with it :

What could explain the given symptoms ?

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    Comments are not for extended discussion; this conversation has been moved to chat.
    – Matt Lacey ♦
    Nov 1, 2018 at 3:45
  • For future reference, you can get a black and white composite signal out of the RGBI port, useful for text apps and troubleshooting, at least.
    – nexus_2006
    Sep 30, 2019 at 12:17

1 Answer 1

Reset to default
5

Stumbled upon this question. I experience this with defective trim capacitor C20 as well and can be an easy fix :-)

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    A flaky capacitor could very well be the problem indeed. I feared ESD damage, that would be good news! For future readers, could you put a recommanded value for the capacitor ?
    – Sachiko.Shinozaki
    Jul 21, 2019 at 17:55
  • For reference until edit. According to this schematic excerpt, C20 has a range of 4 to 40 pF. Seems to explain the symptoms, given wonky clock pulses, the video chip wouldn't be able to generate proper CPU pulses, and faint colors could come from an unsychronized chroma clock causing color burst issues. Given that the colors had problems even when the unit was functionning, I guess I gave C20 the final blow.
    – Sachiko.Shinozaki
    Jul 21, 2019 at 18:09
  • C20 seems to not be shorted. I measure high impedance between it and ground, when the 7801 is removed. Maybe its capacitance is off ? But then I don't get why only 157 ohms on the rails... maybe it is nominal? I'm lost.
    – Sachiko.Shinozaki
    Jul 21, 2019 at 19:44
  • A shorted C20 would result in black screen / no image at all. You didnt mention it so I just ask: Did you trim (turn) the capacitor using a small screwdriver at all? That's why it's there - and can be totally working if you turn it a little bit.
    – jonas
    Jul 23, 2019 at 8:38
  • No I haven't tried that yet. I guess I will then. Given that it's a trim cap, moving the computer could've messed it up. Thank you for all your suggestions.
    – Sachiko.Shinozaki
    Jul 23, 2019 at 9:01

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