C64 Repair Help

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fuzzybad
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C64 Repair Help

Post by fuzzybad »

Hey guys,

Looking for advice on what might be at fault on a C64 (I think it's a Rev C board).

First, a little backstory on this machine: It was mostly working, but consistently locked up when playing a specific game (Ghosts 'N Goblins). Since PLA is a common issue and I happened to have a spare, I thought I would try replacing the PLA first. However, this chip was not socketed and I removed it by using a heat gun on the motherboard (a technique I'd read about for easy chip removal but never tried before). The chip desoldered fine but after I re-installed in a socket, the computer began locking up on boot, and showing an incorrect amount of RAM - strangely 10x more RAM than is actually available (389110 bytes free). With the original PLA installed, there is a fractional amount added as well. Replacement PLA does not show the fractional amount.

My diagnostic cartridge shows screen corruption and thousands of errors in a matter of seconds. It also marks pretty much every chip as defective, which seems impossible. Game cartridge starts up with screen corruption, but locks before the game starts. Utility cartridges do not start at all - I tried Easyflash3 and Super Snapshot V5 with same result, a black screen.

My suspicion is bad RAM. I think perhaps one of the RAM chips was starting to fail, and the heat gun pushed it over the edge. I've got some replacement RAM chips on the way and will test them soon. Does anyone have another suggestion?

Bytes free - original PLA:
Image

Bytes free - replacement PLA (PLAnkton):
Image

Diagnostic cart:
Image
Image

Game cart:
Image


Thanks!
Jase


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eslapion
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Re: C64 Repair Help

Post by eslapion »

I removed it by using a heat gun on the motherboard (a technique I'd read about for easy chip removal but never tried before).
I may be wrong but I think you've doomed your C64.

Overheating any epoxy/fiberglass circuit board will cause carbonisation of the epoxy and render it semiconductive, just like a resistor. So now, all around your PLA are resistors causing small amounts of current to flow between all communication lines of the PLA socket.

If that's what happened then your board is effectively permanently damaged. This is the sort of thimng that will cause all sorts of random erratic behavior and data corruptions.

The best way to desolder through-hole circuits is to use a desoldering gun with vacuum pump which has a direct physical contact with the component to remove, not hot air. Hot air is used mostly with surface mount and the temperature has to be carefully controlled.

I use a Hakko 808 myself for desoldering on Commodore computers or any through hole PCB.

BTW, not a single one of your pictures shows your repair work.
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Re: C64 Repair Help

Post by motrucker »

I've been told by other people not to use a heat gun on older circuit boards like the C-64's. Sounds like eslapion just verified that. Sorry to hear another C-64 may have bit the dust.
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Re: C64 Repair Help

Post by fuzzybad »

Fair enough, here are some pics of the top and bottom sides of the board, showing the repair area. The area which was subjected to the heat gun is definitely darkened; I hope it didn't damage the board. I just did some testing with a ohmmeter (on the bottom side) and there were no shorts or unusual resistances as far as I could tell..

Image

Image


The heat gun seemed like such a good idea at the time, but I won't use that technique again if it damages the board. Guess I need to invest in a proper desoldering station..
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Re: C64 Repair Help

Post by Zippy Zapp »

Yikes. Yep it looks like something is cooked. What is above that dark area looks suspicious too. It is hard to tell from the photo but it looks like some of those pins are bridged with, perhaps from too much solder? I can't tell for sure and the photo doesn't enlarge when clicked.
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eslapion
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Re: C64 Repair Help

Post by eslapion »

motrucker wrote:I've been told by other people not to use a heat gun on older circuit boards like the C-64's. Sounds like eslapion just verified that. Sorry to hear another C-64 may have bit the dust.
Its not just C64 boards. Virtually all epoxy/fiberglass PCBs are vulnerable to carbonisation.
fuzzybad wrote: The area which was subjected to the heat gun is definitely darkened
That's carbonisation; your board is destroyed.

Even if the board was not rendered conductive enough to disrupt communications, the topside-bottomside connections might have gotten damaged and all surrounding components rendered unreliable (resistors, capacitors, transistors).

Repairing this board would involve replacing all components within 2 inches of the epicenter, checking all traces and all connections from topside to bottomside with no guarantee of success. Its cheaper to remove all valuable components from this board and find another one.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders
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Re: C64 Repair Help

Post by fuzzybad »

Zippy Zapp wrote:Yikes. Yep it looks like something is cooked. What is above that dark area looks suspicious too. It is hard to tell from the photo but it looks like some of those pins are bridged with, perhaps from too much solder? I can't tell for sure and the photo doesn't enlarge when clicked.
The sloppy soldering above the scorched area was a pre-existing condition. Looks like someone replaced the SID at some point and installed a socket. I checked for shorts and it was ok, just a lot of resin left behind.
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Re: C64 Repair Help

Post by fuzzybad »

eslapion wrote:That's carbonisation; your board is destroyed.

Even if the board was not rendered conductive enough to disrupt communications, the topside-bottomside connections might have gotten damaged and all surrounding components rendered unreliable (resistors, capacitors, transistors).

Repairing this board would involve replacing all components within 2 inches of the epicenter, checking all traces and all connections from topside to bottomside with no guarantee of success. Its cheaper to remove all valuable components from this board and find another one.
That's not exactly what I wanted to hear, but thanks for the advice. I sure won't use my heatgun on Commodore boards again. :(

I'll update this thread if I ever do get it working again..
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Re: C64 Repair Help

Post by motrucker »

fuzzybad wrote:
eslapion wrote:That's carbonisation; your board is destroyed.
Even if the board was not rendered conductive enough to disrupt communications, the topside-bottomside connections might have gotten damaged and all surrounding components rendered unreliable (resistors, capacitors, transistors).
Repairing this board would involve replacing all components within 2 inches of the epicenter, checking all traces and all connections from topside to bottomside with no guarantee of success. Its cheaper to remove all valuable components from this board and find another one.
That's not exactly what I wanted to hear, but thanks for the advice. I sure won't use my heatgun on Commodore boards again. :(
I'll update this thread if I ever do get it working again..
You don't need an expensive set up to de-solder chips or sockets. You can get away with a $10.00 solder sucker and a copper braid. I did it that way for years. I only recently bought a "good", variable heat station.
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eslapion
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Re: C64 Repair Help

Post by eslapion »

@motrucker
I have a more expensive solution because I do a lot of desoldering and there are many good yet inexpensive solutions out there; A hot air gun is NOT among them unless it is equipped with an accurate temperature control system which ensures you reach an adequate temperature for soldering/desoldering without causing damage to the circuit board and components.

Using hot air is fantastic with surface mounted components, not so with through-hole components which may have mechanical hold on the PCB which encourages the technician to overheat the board until the component drops-off even when held in place by something other than solder.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders
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