View Full Version : I can't repair my c64 - strange failure

November 30th, 2009, 05:04 PM
Hi people !

Still trying to bring my C64 to life again.
When I bougth this one (it's the second I have) it was suppossed to be fully working, but it doesn't work at all.

The failure is very strange, since it isn't the clasic blank screen, o garbage caracters.

Instead of this, when I turn on the computer, I can only see lots of short lines. Also, the TV produces a high frec noise comming from the CRT.

Turning on and off, sometimes I get some sort of image with the commodore screen barely visible, with lots of lines. Sometimes the image contain two comodore screens fited in the CRT !!!, and of course, out of scale.

The dataset sometimes work. When I turn on, the motor runs, stops for two second and run again. Sound like the CPU is working after all.

ViC II is fine, since I tested it in another C64.

A clue for the problem ?

Lest see:

MPU frec is around 750KHZ !!!!!!!. The color clock seems to be fine (I put seems, because is very difficult to measure with my scope).
Dot clock is wrong. About 6Mhz and so is the MPU clock.
The 4044 phase detector has the 1Mhz signal coming from the color clock across the 74139 and 7474. And also the phase0 clock from the VIC II which is the MPU clock and which the dot clock divided by 8. This frec is aroun 750Khz.

So the 4044 has two signals which don't have the same frequency. The voltage value to the dot clock VCO is 2.6V.(The board is from 1982)
This control voltage is not enough to correct the VCO.

I tried to replace the 74ls629 (dual VCo) and the 4044 (pll) without any success. The same sympthom.

Just for fun, changing a bit the timing cap in the dot clock VCo, allow the C64 to display an "image", which was the area of the upper-left corner enlarged to the whole screen. The characters were enormous !!!!

Clearly, the machine works, but as long as there's no relation betwen the color and dot clock, no image is expected. Also, the MPU runs very slow.

Another question,

The C64 is PAL-N. Could somebody tell me why it has a 14.318 XTAL ?
A bit strange too.

Many thanks for reading this post and hope you can help me to fix this 64.



November 30th, 2009, 07:38 PM
Can you post pics?

The last time I saw a C64 that had short multicolored horizontal lines on it the PLA chip had failed.

December 1st, 2009, 01:36 AM

I'll try to connect the C64 to my TV this nigth and take a shot for you
But not caracter or colour are visible. The screen is dark grey with line sall arouns
The lines are horizontal ... the whole image is like those coming from a VCR which is very out os tracking or trying to read the tape recorded in different television system (VCR set in PAL, but tape recroded in NTSC).
No colors, no cracters and not border. The VIC II is working. It ouputs the croma and lum signals ... but ... I don't know. Also runs warm , but not very hot.

When I played with the VCO dot clock timing cap, some image was visible, but completely out of scale.

Forget to tell, the same sympthom in A/V, but worst, since modern TV's put a blue/balck screen if the don't found a valid signal.

What do you think about the frec on the MPU ?. It's very strange. Morover, I replaced both, the 74LS629 and the 4044. I'm lost at all with this.

Many thanks for your help


December 3rd, 2009, 07:34 AM
Lets Update ...

I measured the frec with an universal counter. The MPU was running at 730Khz, but the main osc (color clock) was fine,
I changed a resistor of the dot clock to change the range and allow the 4044 lock both, dot and color clock.
I was happy until connect it to TV ...
I turned it on and the image appeared. Very clear and nice, but IN BLACK AND WHITE !!!!!!!!, Never seen something like that.

The worst thing with this is that after 10 seconds cursor disapears, or the startup message disapears. Turning it on and off would finally result in characters all around the screen.

If I let it off for some minutes, the message appears again, but after a short time the cursor dissapears and again.

Many failures as I saw. Not as easy as locking the osc.

For me, the lack of color is the main issue now, since I'm very lost.
The rest of the story ... just pray the PLA is working. And it isn't the VIC-II because I have an spare one and do the same failures.

What do you think ?

December 3rd, 2009, 09:34 AM
Sounds like it might be at least partly heat-related; maybe try cooling the various chips with a cooling spray or a well-wrung-out cloth soaked in alcohol?

December 3rd, 2009, 11:16 AM
I like to start with the obvious things...

Are you sure the computer is connected to the television correctly?

December 4th, 2009, 04:44 AM
The TV set is quite old, very simple. It's only PAL-N and I'm connecting the computer using RF, channel 3/4.
Television works in color fine. I connetcted a PAL-N VCR and plays in cloros fine. Also, I connected the C64 to the A/V imput of the VCR and detects PAL-N signal (I put the VCR in auto and let it to guess). The VCR is connected to TV using RF and the image is still B&W.
I connected to a second TV set, that's a modern one. I conneted through A/V and outputs a B&W image too. Sadly, the image is quite perfect.
In relation to the frezzing, just noticed too much ripple in the 5V of the supply. However, the mean is 5V and the caps in the 64 filter it completely.
I fixed the supply, but the problem continue.
I would say, that the C64 lasted more time until frezzing. Let's say more than 2 minutes and before that (before fixing the supply), only 10 seconds. Just discovering if this is supply realted.
At the moment, I'm worried about the lack of colour.
This C64 is PAL-N. It was manufactured in my country (Argentina) with Commodore License by Drean.
The board was assembled outside though. It's a 1982 board ASSY 326298 with the VIC in CERDIP package (nice). The Color clock is 14,328 Mhz and MCU runs at 1,023 Mhz.

Thanks for your help


December 6th, 2009, 01:52 PM
Hi people,

I'm still working on this C64.

Update: Fized the color isue (I get working moving a pot which is connected to the VICII's color out in this board. I moved it and got color

But it still freeze.


MPU (exchange betwen e working 64)
82S100 PLA

New 74LS629 for the VCO's.
Checked power supply. I changed filtering caps. Also, the two on board regulators on the C64 are new, and the heatsink of the 7805 was improved (a large one).

But still got the same issue.


At power up, 40% of the time blank (black) screen
The other 60% get some image on power up.
If the computer was first turned on. the normal screen appears.
After about a minute, the screen clears and READY with blinking cursor is written. After 10 seconds the cursor stops blinking. Acossionally, some character appear.
If I turn off and on, the normal screen still apperas but for a shorter time until locks. Finally, If I keep insisting, it powers up with colored characters and this image comes pretty fast (less than one second). In that case, clearly, no internal test is performed, and the image is shown directly.
In normal commodore, take about 3 seconds to get the startup image. I guess that this delay is due the internal sefl test and RAM test.

Another curious point is the dataset behaviour
I pluged a C2N to the cass port with play presed. The motor remains on until the C64 locks. When cursor stops blinking, motor is TURNED OFF !!!!!!!!
A bit strange. I dcided to exchange the MPU with a working C64 because of that.

With a scope, I inpected the reset line, and it never goes down. So nobody resets the MPU. Supply power is clean.

In some cases, If I let the C64 locked on for a long time (say five minutes) the characters start to change.

Pretty strange ...

I'm lost at all. Your opinion is very welcome !!!


December 10th, 2009, 08:06 PM
Time to start swapping out the DRAM chips.