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"Commodore 64 powers on but..."


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Last month I picked up a C-64 which powers (red LED) yet nothing comes up on the screen. After looking into units on eBay I've noticed the C-64s seem to lack the staying power of other retro computers. Most of the C-64 systems on eBay are listed as "not tested but powers up.." (aka Broken)

 

Seriously, to me the C-64s seem to have excessive failure rates. I'm coming from TI-99/4 background where most everything ancient still seems to work.

 

Is there a quick-n-dirty thread/PDF available to help fix the Commodores?

 

Kind of an Idiot's Guide to the most common C-64 failure issues, or maybe a system troubleshooting guide?

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Yes, Sir! The canonical answer to entry level troubleshooting is Ray Carlsen's documents:

 

If your C64 is the older model, grey/brown breadbin: http://personalpages.tds.net/~rcarlsen/cbm/c64/c64-ic.txt

If your C64 is the newer model, white/grey flat C64C: http://personalpages.tds.net/~rcarlsen/cbm/c64/c64c-ic.txt

 

(if you have an early C64C, it uses the old motherboard so check the first link)

 

When it comes to C64's with the old motherboard, the single most common fault is the PLA chip, which is kind of a memory controller. There are some modern replacement solutions for that one. On C64C's with the newer motherboard, the PLA on the other hand barely never breaks.

 

YMMV but the majority of the C64's I've found, even when stored in a basement or garage for two decades, booted fine. However I have understood this may vary a lot depending on geography.

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Last month I picked up a C-64 which powers (red LED) yet nothing comes up on the screen. After looking into units on eBay I've noticed the C-64s seem to lack the staying power of other retro computers. Most of the C-64 systems on eBay are listed as "not tested but powers up.." (aka Broken)

 

Seriously, to me the C-64s seem to have excessive failure rates. I'm coming from TI-99/4 background where most everything ancient still seems to work.

You purchased a "non tested" c64 and expected it to be working, that was your first mistake. I know someone who purchased two untested a8s recently and both were DOA. Having said that and in defence of Atarians, my first two Atari purchases ( 600xl & 800xl ) were untested but both arrived working. So you can have some pretty good luck sometimes.

 

When you buy "untested" it's a lottery and that goes for any computer system especially on eBay.

 

My experience with c64s tends to differ. The early PCBs were built very well, sockets were of the high quality machined pin type, not the cheap single wipe friction design used in some of the more expensive machines like in my Apple //e.

 

I logged countless hours on my unit from 1986 with no problems. The early ones did have high failure rates, I think as high as 30%. If some users were more responsible and allowed air to circulate under the machine instead of playing on the lounge room carpet then perhaps they would have had the same experience as me.

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I appreciate your reply. It wasn't a "mistake," as you say, since there were no expectations upon purchase. I don't believe asking for a debrief on my decision? Your feedback, sir, reads slightly condescending.

 

The unit in question was found at an outdoor flea market for next to nothing. It was a roll of the dice. Only after googling and checking eBay have I noticed a large percentage of non-working "untested" C-64s. This is why I conclude they had quality issues. The three TI-99/4As and two Atari 800XLs I've purchased via untested Goodwill all work fine. Perhaps my luck finally ran out?

 

Anyway, my post is all about finding tips for C-64 repair. Though your reply was lengthy I found it less than helpful except for the exceptional tip about airflow and carpet.

 

Brilliant!

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What kind of cables are you using to connect it to the screen? When I got my first one a few months ago nothing came up on the screen when using a standard RF adapter. I had to jiggle the cable somewhat where it connects to the system until it showed up. I also found mine doesn't like to turn fully on unless the on switch is firmly pressed. What kind of power supply are you using? I've heard lots of bad things about the first party ones. Have you opened it and checked the fuse?

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Anyway, my post is all about finding tips for C-64 repair. Though your reply was lengthy I found it less than helpful except for the exceptional tip about airflow and carpet.

 

Brilliant!

The first two, most lengthy paragraphs were not about finding tips on repair, more like the typical C64 bashing rants you'd find on this forum. And there's no shortage of those. Edited by shoestring
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Good points by xenomorpher. When you say nothing comes up, are you getting a black image that is different from whichever static or placeholder image your TV/monitor normally displays? Did you connect using RF or composite video? If you've got a composite video cable that works with an Atari 800XL, the same cable should work with the C64.

 

In case of PLA fault, on rare occasions one can quickly power cycle the computer on - off - on - off - on and wait a few seconds. Sometimes it jump starts the chip and you get a display.

 

If you need tips about PLA replacement chips (if that turns out to be the problem), some of the names you can search for are SuperPLA, realPLA and PLAnkton. There may be others. Some people like to replace it with a custom programmed, fast EPROM while others strongly advice against doing so and prefer another type of chip better suited for the purpose. The original PLA is a 82LS100 but nearly nobody seems to try to install those, whether there are tools to program them properly or not. The modern replacements mentioned in the first sentence should be far longer lasting, even if they cost a little bit of money.

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I think a lot of the "failure rate" has to do with the power supplies frying chips on the computers. The heavily epoxied original "brick" is known for taking out the computer when it fails, which is why it is best to at minimum purchase a power saver from Ray or better yet a replacement power supply from Ray (which is built on good quality modern components and has a saver built in) when you find a good working C64.

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It's a combination of power supply and poor air circulation in the original model. Failures due heat stress were exacerbated by operating the machine in an environment where dust can be attracted into the machine.

 

In Australia I believe the brick shipped with the c64c. My neighbour exchanged their c64c several times over a 2 month period due bad power supply. The epoxy doesn't allow heat to escape so components inside fail prematurely and take out the computer with it, it's the 5v line which causes this.

 

Op didn't mention the model of the c64 he bought. On the early C64, if the 9v AC line is missing then the screen will be blank even though the power LED is on and 5v is present. This is because the 9VAc is used as a reference frequency for the internal clock. The c64c is a little different, it will still work without 9v AC but some things will not work.

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Yes, Sir! The canonical answer to entry level troubleshooting is Ray Carlsen's documents:

 

If your C64 is the older model, grey/brown breadbin: http://personalpages.tds.net/~rcarlsen/cbm/c64/c64-ic.txt

If your C64 is the newer model, white/grey flat C64C: http://personalpages.tds.net/~rcarlsen/cbm/c64/c64c-ic.txt

 

(if you have an early C64C, it uses the old motherboard so check the first link)

 

When it comes to C64's with the old motherboard, the single most common fault is the PLA chip, which is kind of a memory controller. There are some modern replacement solutions for that one. On C64C's with the newer motherboard, the PLA on the other hand barely never breaks.

 

YMMV but the majority of the C64's I've found, even when stored in a basement or garage for two decades, booted fine. However I have understood this may vary a lot depending on geography.

 

Very helpful links! Thank you sir! Strange thing happened today. The weather has been much cooler since I last attempted to start up the C-64. It's 60F/16C indoors now vs 80F+ (Phoenix).

 

It booted up! Blue screen with all the C-64 trimmings. So...I left it running for a few minutes and noticed a blue screen of death followed by a black screen.

 

I'm assuming something is failing and eventually quits when the unit heats up? Good news though as it's not fried.

 

I suppose my next move is to open it up and start feeling for hot ICs. Any ideas on where to begin?

 

 

Sent from my iPhone using Tapatalk

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Try re-seating the chips.

 

They've probably worked their way out of their sockets ( chip creep ).

 

Edit:

 

And while you're there, it might be a good idea to check the voltages from your PSU ( 5VDC and 9VAC ).

 

https://www.c64-wiki.com/index.php/Power_Supply_Connector.

Edited by shoestring
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The U.S. C-64s are more often than not killed by their own PSU. When they fail, they will shoot to much voltage to the computer. So, when you do fix the computer, make sure you don't use the original PSU. Get a replacement!. Ray Carlsen sells great ones, or you can change the line on a C-128 PSU.

Other than the PSU, the Commodore C-64 & C-64C are rather well made. If your CBM brick seems to work, at the very least, add one of these devices, so it will not ruin your 64. The Computer Saver, or SaV64:

 

http://www.lemon64.com/forum/viewtopic.php?t=62395

 

 

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  • 11 months later...

This topic is a year old but you never stated whether you found the problem. I'm an electronic tech (among other things) and I know the C64 schematic. Good intentioned people will give you misinformed advice. It's not their fault, they are just misinformed. This reply is a bit lengthy but it's about as definitive as you will get these days.

 

Background info:

The red LED runs straight from the "brick" power supply 5 volt line. The rest of the C64 could be in flames and as long as the supply was putting out 2.1 to 5.7 volts, that LED will happily give off it's glow. Even then it's only indicating that the supply is putting out ?something? and that it's less than 10 to 12 volts. It could even be low voltage ac and the LED will still burn in a 50% duty cycle (60 flashes per second).

 

There are two voltages coming from the power supply; 9Vac and 5Vdc(+Vc)

 

The 5 volt line that runs the LED is considered (+Vc). It is not (+Vcc). The (+Vc) line is power for all the analog chips like the VIC, SID, and various clocks/oscillators around the board. It also goes out on the cartridge connector to power the ICs on any cartridge. This 5 volts from the "brick" never, never, never touches the digital circuits on the mother board, i.e. CPU, memory, CIA, PIA, or various logic gates. Even the ground returns for each are separated

 

The 9 volts alternating current or 9vac. does several things. It sets the vertical sync frequency on the VIC chip, is the input frequency for the TOD clock on both CIAs. and finally goes out on the RS-232 connector as power for external devices. Next it goes through a diode/capacitor "ladder" that doubles the voltage (ac) and gets both rectified and regulated to 12 vdc (VDD) with a 7812 regulator. This is power for both the SID and VIC internal, output amplifiers.

 

Finally the 9Vac goes through a bridge rectifier, and is regulated by a 5 volt 7805 regulator. This is (+Vcc) and is the power for all digital ICs on the board. The early versions of the C64 mother board used an inadequate heat sink for this chip and was prone to heat failure over time. Later versions of the board used a large aluminum plate to dissipate the heat but could cause what they call a "cold solder joint" on the ground pin of the regulator when assembled at the factory. A cold solder joint behaves exactly as you describe; it works properly until it heats up.

 

Misinformed info: The power supply can not send high voltage through the digital circuitry. Aside from the fact that the 9Vac is fuse protected before it goes anywhere, it would take out the bridge rectifier and 7805 first. It might (and that's a slim-to-none "might") get past the 12 volt regulator because of the way the voltage ladder works, and that would damage both the SID and VIC causing a silent computer and black screen. The 6502 may be happily giving you the READY prompt, just no way to show it on the monitor. In any case, if the LED is glowing you know that the power supply did not short to line voltage because this (+Vc) 5 volts comes from the same transformer as the 9Vac inside the "brick" and the LED would have popped like a firecracker. 99 times out of 100, a transformer burns out it's secondary winding (low voltage side) and power simply goes away.

 

Trouble shooting your specific problem: Check the two regulators 7812 and 7805 with a volt meter. The input voltage should be ~18 to 24 Vdc for the 7812 and ~9 to 10 Vdc for the 7805. the output voltage should be +/- .5 volt for each. 4.7V is actually ideal for (+Vcc). While the unit is cold, physically wiggle these two chips and see if it causes the computer to hang up or the video to go crazy. Be especially careful not to touch the leads with your fingers. It won't hurt you but it may cause the voltage to rise past tolerance for the TTL chips. If the wiggle test hangs the computer, you can get away with re-soldering it, but because of the age of the C64 I would replace these two chips just for good measure - about $1.50 for each at any electronic supply, even Radio Shack if there is still one in your area.

 

 

The other, more obscure trouble that can cause a black screen (among other stranger video effects) are two chips, labeled on the board and schematic as U13 and U25 located left of the SID and right of the memory chips.These are 74LS257 tri-state multiplexers. They are used to allow the CPU to access or address memory on the high side of the system clock, and the VIC chip to access memory on the low side of the system clock. If one goes bad, or both go bad , or even one goes partially bad, (or one leg has a cold solder joint on either chip) the CPU may still appear to work but the monitor will display random garbage; the CPU will lock up on boot up while the VIC may or may not try to operate but will give a blue or usually a black screen. If the CPU cannot address memory location $00 and $01, the entire system locks up. You can find these common TTL ICs at most electronic supply like Jameco, AVEC, American Electronics. etc on line - also for about $1.50 each

Edited by CaptPKelly
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Hay motrucker. Understand there are differences in the power supplies made for the European market over the ones made for the U.S. The European power supplies that operate on 240 vac and a split primary transformer, if about 5 different conditions are met within the brick supply and on the mother board, could put >12 volts out on (VDD) line or the 12 volt regulated line that feeds the VIC and SID chips. This most likely won't cause them to cease internal function but it would burn out their output amps (where this 12V goes) so it effectively makes the whole chip useless. It is possible for the 9Vac on the European PSU to rise to between 18 to 20 volts if the 5 volt brick regulator fails, but unless the fuse on the mother board has been tampered with this will instantly blow the fuse protecting the 5 and 12 volt mother board regulators. Any momentary spike of high voltage has to get through about 5 safety and filter components before it would ever reach the chips on the board.

 

The only possibility of this happening on a U.S. intended power supply (and if you are in the US and your C64 works you have a U.S. power supply - the European version in the US would put out 4.5 vac and insufficient amperage and unregulated 5Vdc) is if the secondary winding over heats, burns it's insulation coating and melts through and contacts the primary winding. Most of the transformers used in these PSUs have primary and secondary winding isolation which means the secondary would have to melt through about a quarter inch of nylon and/or phenolic to get to the primary. This is next to impossible.

 

What I'm saying is that these devices in your link are like putting training wheels on a tricycle. It doesn't hurt anything, but it's unnecessary here in the U.S.. They may help the rare European PSU over voltage spike, but in the U.S. it's just an added LED, more complexity to go wrong, and a psychological comfort for non-electronic tech C64 users that believe their power supply is waiting for the right opportunity to fry their mother board. If it gives you peace of mind and you want to buy one, go ahead, it won't hurt the C64 at all.

 

None of this is refuting that the original "brick" power supplies were/are totally inadequate. If you have one it will self distrust one day. I recommend instead of buying a "Saver" cable, built primarily on a misunderstanding (to put it diplomatically), investing in a new supply with at least a 2 amp, fused 9 volt output. The 5V dc@1.5 amps is more than sufficient unless you plug in a really power hungry cartridge. There were a few out there that didn't pay attention to the power limits published by Commodore / CBM.

 

Everything I've stated in this and my previous post come straight out of the C64 schematic. It's all there for anyone to see if you know how to read it. It also comes from first hand experience and knowledgewith the C64 that began when I first designed a new, beefed up PSU back in 1986 to replace the original, burnt out unit. Nothing inside the computer was damaged or fried, or blown to bits, it just had no power.

Edited by CaptPKelly
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I read about Ray having 3 c64s on his bench destroyed because of a single black power brick before he took a sledge hammer to it :) It's been known for quite some time that when the voltage regulator shorts out in these, the 5v line can reach as high as 11v ( unloaded ). DRAMs are particularly vulnerable.

 

The black 240v brick that I've across have heat dissipation issues exacerbated by our hot Aussie summers. They work fine in winter conditions but over voltage when they heat up.

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bradhig1; Be careful of the eBay units. Most of these (in your link) are simply recovered original power supplies. 99% chance they will eventually fail. You will be better off in the long run to buy a new supply made with more modern voltage regulating circuits. No one out there (in their right minds) are entombing their heat producing electronics in hard plastic. Allowing them to air cool, some even use "pancake" fans to help with that, makes them far less likely to cause troubles down the road. The bare minimum output to look for are 1 amp on the 9V line and 1.5 amps on the 5V line. Some list their output in wattage. The minimum wattage is 10 watts on the 9V line and 7.5 watts on the 5V line (voltage x amps = watts). Most have designed their PSUs to meet the output of the original PSU but a higher amperage on either or both outputs means the PSU runs cooler.

 

The "saver" devices are an extra precaution - with circuitry similar to the GFI wall plug in your bathroom. Like I said, it isn't impossible for the 9Vac to rise to double the voltage and cook the VIC and SID output amplifiers. The effect is to make the C64 blind, deaf, and dumb. In the last week I have been hunting for a replacement VIC chip and although they exist, they are becoming more and more rare. You don't want to have to replace it if you can prevent it. The saver may never come into play, but it's there for an added level of protection if you want.

Edited by CaptPKelly
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Carlsson:

I agree. My main "soap box" is based on a technician's view point. The notion that "the power supply will cook everything on the board" is one of those sensational statements that get's substantiated and passed on by people that experienced some problem when their PSU failed, automatically assumed they experienced the exact same thing, and pass the story on to others as fact - when in fact, if after replacing the PSU, they had themselves or someone who knew how to do it, had opened the C64 enclosure and replaced the buss fuse protecting the 9Vac line would have had a working computer once again. There's no telling how many perfectly good C64s lie buried in the city land-fill because of a blown fuse that not many even knew was there.

Edited by CaptPKelly
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