The strange case of the Commodore 64 colour bands.

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stephen_usher
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Re: The strange case of the Commodore 64 colour bands.

Post by stephen_usher »

Looking at the circuit, it's more likely to be the 74LS629 as that's the main clock generator chip.

P.S. bought one from RS
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
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Re: The strange case of the Commodore 64 colour bands.

Post by stephen_usher »

Modulator re-fitted and the SN74LS629N put into a socket. Tested and working. The display looks as it did before without the ragged vertical edges.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
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rubber_jonnie
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Re: The strange case of the Commodore 64 colour bands.

Post by rubber_jonnie »

stephen_usher wrote: Mon May 09, 2022 6:34 pm The beads are pristine. I'll have a look at the circuit to see which chips to replace.
I didn't really think the ferrite beads would be a problem, but worth a look.

Probably worth changing the 74LS193 too while you're at it.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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Re: The strange case of the Commodore 64 colour bands.

Post by stephen_usher »

The 74LS193 is part of the clock divider circuit for the dot clock (and hence the main system clock) and has no influence over the chroma output, but does take the chroma reference clock as an input. Given what I'm seeing is a variation in the chroma reference clock, tinting the colours due to the PAL colour encoding frequency shifting, it's best to start there. Once the SN74LS629N has been replaced that's all of the core clock done. Given that RS don't sell the 74LS193 I couldn't get it at the same time. Farnell do have it but the handling charge makes it rather uneconomic to get it from there.

(The other components around there are purely DC voltage components and the DC voltages are good and very clean with tiny amounts of noise, even in the MHz range, which is where I need to worry for this.)

Anyway, we'll see what happens when I replace the VCO chip.

All the ferrite cores are pristine, as can be seen here:

IMG_2659.jpg
IMG_2659.jpg (303.81 KiB) Viewed 1250 times
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
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rubber_jonnie
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Re: The strange case of the Commodore 64 colour bands.

Post by rubber_jonnie »

stephen_usher wrote: Mon May 09, 2022 10:54 pm The 74LS193 is part of the clock divider circuit for the dot clock (and hence the main system clock) and has no influence over the chroma output, but does take the chroma reference clock as an input. Given what I'm seeing is a variation in the chroma reference clock, tinting the colours due to the PAL colour encoding frequency shifting, it's best to start there. Once the SN74LS629N has been replaced that's all of the core clock done. Given that RS don't sell the 74LS193 I couldn't get it at the same time. Farnell do have it but the handling charge makes it rather uneconomic to get it from there.

(The other components around there are purely DC voltage components and the DC voltages are good and very clean with tiny amounts of noise, even in the MHz range, which is where I need to worry for this.)

Anyway, we'll see what happens when I replace the VCO chip.

All the ferrite cores are pristine, as can be seen here:


IMG_2659.jpg
Yeah, I think you're right, get the 629 done and review after that.

Everything looks pristine, be nice to get that banding fixed, it seems to be all that's left spoiling the machine.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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Re: The strange case of the Commodore 64 colour bands.

Post by stephen_usher »

Yeah, other than the banding it's working perfectly.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
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Re: The strange case of the Commodore 64 colour bands.

Post by rubber_jonnie »

I might be wrong, but C66, the left leg might have a cracked joint. Probably nothing, maybe just a trick of the light bit it seems to have a line around the bottom edge.

Not that I expect it to fix anything.

Did you say you'd swapped the xtal out?
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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Re: The strange case of the Commodore 64 colour bands.

Post by stephen_usher »

I can try reflowing C66, but I think it is a trick of the light.

Yes, I've changed the crystal and the resonant capacitor, C70.

C107 and C108 are power line smoothing caps and the power is nice, smooth and correct, so there's literally nothing else which can affect the Colour clock once the 629 is replaced.

I suppose that it could be a phase issue between the colour signal and the pixels/dot clock. If this still doesn't work I can try going through the clock divider circuit.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
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rubber_jonnie
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Re: The strange case of the Commodore 64 colour bands.

Post by rubber_jonnie »

stephen_usher wrote: Tue May 10, 2022 12:17 pm I can try reflowing C66, but I think it is a trick of the light.

Yes, I've changed the crystal and the resonant capacitor, C70.

C107 and C108 are power line smoothing caps and the power is nice, smooth and correct, so there's literally nothing else which can affect the Colour clock once the 629 is replaced.

I suppose that it could be a phase issue between the colour signal and the pixels/dot clock. If this still doesn't work I can try going through the clock divider circuit.
Bit frustrating though when nothing seems to work! It does seem like something is out of phase though.

Fingers crossed that 629 will arrive relatively quickly.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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stephen_usher
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Re: The strange case of the Commodore 64 colour bands.

Post by stephen_usher »

It should be here before 2pm. :-)
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
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