RETROMEDICS

RETROMEDICS

NORMAL STARTUP

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Non-faults

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CIA (U1) Common faults

The CIAs (Complex Interface Adapters) are among the most common chips to fail in a C64. They run fairly cool and don't usually fail spontaneously, but are directly connected to the I/O ports and so very vulnerable to static electricity - the joystick ports in the case of the CIA1 and the Serial port in case of the CIA2. To prevent damage, never plug anything in or out while the machine is powered on. Also make sure the connectors and cables are in good shape as they can cause problems too.

The CIA chips at U1 and U2 are identical, so if they are socketed, a useful test is to swap them with each other and see if the symptoms change. If they do, one of the chips is likely bad. U1 faults generally cause keyboard/joystick issues while a failing U2 tends to cause disk drive issues. Another test especially useful for black screen C64s is to just remove these chips altogether - the computer can produce some form of a startup screen even without the CIAs.

Another task that the CIAs handle are the TOD (Time of Day) clocks. Both U1 and U2 have their own clocks, which can be useful for diagnostic purposes. You can use software to test if one of the clocks is giving incorrect readings and possibly figure out if a CIA is bad.

One notable feature of a scrambled startup screen caused by a bad CIA chip is that it tends to have normal color and borders. It's not uncommon for a faulty CIA to produce a blue garbage screen, but if you are getting multi-color garbage when booting into BASIC, it's likely something else.
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CIA (U2) Common faults

The CIAs (Complex Interface Adapters) are among the most common chips to fail in a C64. They run fairly cool and don't usually fail spontaneously, but are directly connected to the I/O ports and so very vulnerable to static electricity - the joystick ports in the case of the CIA1 and the Serial port in case of the CIA2. To prevent damage, never plug anything in or out while the machine is powered on. Also make sure the connectors and cables are in good shape as they can cause problems too.

The CIA chips at U1 and U2 are identical, so if they are socketed, a useful test is to swap them with each other and see if the symptoms change. If they do, one of the chips is likely bad. U1 faults generally cause keyboard/joystick issues while a failing U2 tends to cause disk drive issues. Another test especially useful for black screen C64s is to just remove these chips altogether - the computer can produce some form of a startup screen even without the CIAs.

Another task that the CIAs handle are the TOD (Time of Day) clocks. Both U1 and U2 have their own clocks, which can be useful for diagnostic purposes. You can use software to test if one of the clocks is giving incorrect readings and possibly figure out if a CIA is bad.

One notable feature of a scrambled startup screen caused by a bad CIA chip is that it tends to have normal color and borders. It's not uncommon for a faulty CIA to produce a blue garbage screen, but if you are getting multi-color garbage when booting into BASIC, it's likely something else.
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U3 Basic ROM

It's not particularly common for the ROM chips on a C64 to fail, but it does happen occasionally. The kernal has the highest running temperature of the ROMs, and also tends to be the most likely to fail, followed by the Character ROM. Generally the ROM chips run hotter than the CIAs but cooler than the SID, VIC and PLA.

Since cartridges bypass the default ROMs, they are useful for diagnosing ROM faults. Ultimax type cartridges are best for this purpose, as they also bypass the kernal. Kernal ROM faults seem to predominantly cause a simple black screen, thus if you are getting a black screen but an Ultimax cartridge works, a bad kernal is one possibility. Basic and Character ROM faults are more likely to cause a blank blue screen with normal borders, a screen filled with garbled characters or just random garbage. If only the default character set graphics are incorrect but everything else works fine, suspect the character ROM.
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U4 Kernal ROM

It's not particularly common for the ROM chips on a C64 to fail, but it does happen occasionally. The kernal has the highest running temperature of the ROMs, and also tends to be the most likely to fail, followed by the Character ROM. Generally the ROM chips run hotter than the CIAs but cooler than the SID, VIC and PLA.

Since cartridges bypass the default ROMs, they are useful for diagnosing ROM faults. Ultimax type cartridges are best for this purpose, as they also bypass the kernal. Kernal ROM faults seem to predominantly cause a simple black screen, thus if you are getting a black screen but an Ultimax cartridge works, a bad kernal is one possibility. Basic and Character ROM faults are more likely to cause a blank blue screen with normal borders, a screen filled with garbled characters or just random garbage. If only the default character set graphics are incorrect but everything else works fine, suspect the character ROM.
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U4 Character ROM

It's not particularly common for the ROM chips on a C64 to fail, but it does happen occasionally. The kernal has the highest running temperature of the ROMs, and also tends to be the most likely to fail, followed by the Character ROM. Generally the ROM chips run hotter than the CIAs but cooler than the SID, VIC and PLA.

Since cartridges bypass the default ROMs, they are useful for diagnosing ROM faults. Ultimax type cartridges are best for this purpose, as they also bypass the kernal. Kernal ROM faults seem to predominantly cause a simple black screen, thus if you are getting a black screen but an Ultimax cartridge works, a bad kernal is one possibility. Basic and Character ROM faults are more likely to cause a blank blue screen with normal borders, a screen filled with garbled characters or just random garbage. If only the default character set graphics are incorrect but everything else works fine, suspect the character ROM.
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U6 Colour RAM

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U7 MPU

The processor in the Commodore 64 computers runs at a fairly modest temperature and is usually not considered the first suspect if problems arise. If it does fail, it can cause all sorts of symptoms, but perhaps the most common is a simple black screen. Like a semi-faulty PLA, a bad MPU can cause certain programs to work incorrectly even if the startup screen looks normal. Garbled startup screens are less common, but also possible.

The MPU is responsible for controlling the datassette, so if you are having trouble loading cassette programs but everything else seems to work fine, it could be the MPU. But before suspecting that, keep in mind the datassette has to be adjusted correctly for the programs to load properly. If you suspect the MPU and have a datassette available, you could try plugging it in and switching it to PLAY before turning on the C64. There is at least
one instance where this trick managed to help a C64 with a bad MPU to boot normally.

Since the MPU is closely connected to the datassette, make sure you don't plug/unplug the device while the C64 is powered on, as this could potentially damage the chip.
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U8 Logic

When it comes to the TTL chips (the smaller ICs excluding RAM), the model makes a big difference. Normally they run cool and are quite reliable, but the MOS versions of these chips are actually known to fail quite often. In case of a fault, check if your C64 has any smaller chips with "MOS" imprinted on them, and if so consider them possible suspects. Otherwise you may want to focus your testing on the larger chips and RAM first - unless the symptoms clearly point to one of the TTLs. If any of these chips gets particularly hot, it's a likely suspect.

A garbled or otherwise incorrect startup screen seems to be a common symptom if one of these various logic chips goes bad. The machine typically does not freeze despite the erroneous video output, though a bad TTL can also cause an unresponsive black screen. Bad TTLs tend to be difficult to diagnose since they are often hard to distinguish from other faults such as bad RAM, PLA or VIC. If the startup screen text and/or cursor is drawn twice, U14 should be a prime suspect.
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U9 - U12,U21-U24 (RAM) Common failure

It's not uncommon to see a bad RAM chip in a C64. Micron Technology RAM chips ("MT" letters printed on the chip) are known to fail more often than the other types used in C64s. There may also be some correlation between long powered-off periods and failures. The maximum voltage of the RAM chips is only 0.5 V higher than the normal 5 V, so they are among the first to go in case of overvoltage as well.

Perhaps the most common failure mode for RAM is a normal blue screen but with "Out of memory error in 0" displayed in place of the normal startup text. Often there will also be some incorrect characters on screen, and it can be used to pinpoint which RAM chip is faulty (see
here). Screen full of random multicolor characters is also a common symptom. Note that RAM faults rarely corrupt the default character set, so usually the character shapes will look normal even if they are the incorrect type and/or in wrong places. Sometimes the startup screen is otherwise normal but the amount of bytes free is incorrect. If a RAM chip is destroyed by overvoltage, it will more likely result in a black screen. In that case the faulty RAM chip(s) also tend to get hotter than normal. Power up the machine and test them with your fingertip to check this. They normally run pretty cool so any that get hot are most likely dead.

Another way to test RAM chips is by piggybacking a known working chip on top of each one and checking if the symptoms change. If they do, suspect that chip. Keep in mind that this method is not always reliable - sometimes piggybacking a working chip on top of a faulty one will not change anything, even if the faulty chip is not shorted.
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U13 Logic

When it comes to the TTL chips (the smaller ICs excluding RAM), the model makes a big difference. Normally they run cool and are quite reliable, but the MOS versions of these chips are actually known to fail quite often. In case of a fault, check if your C64 has any smaller chips with "MOS" imprinted on them, and if so consider them possible suspects. Otherwise you may want to focus your testing on the larger chips and RAM first - unless the symptoms clearly point to one of the TTLs. If any of these chips gets particularly hot, it's a likely suspect.

A garbled or otherwise incorrect startup screen seems to be a common symptom if one of these various logic chips goes bad. The machine typically does not freeze despite the erroneous video output, though a bad TTL can also cause an unresponsive black screen. Bad TTLs tend to be difficult to diagnose since they are often hard to distinguish from other faults such as bad RAM, PLA or VIC. If the startup screen text and/or cursor is drawn twice, U14 should be a prime suspect.
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U14 Logic

When it comes to the TTL chips (the smaller ICs excluding RAM), the model makes a big difference. Normally they run cool and are quite reliable, but the MOS versions of these chips are actually known to fail quite often. In case of a fault, check if your C64 has any smaller chips with "MOS" imprinted on them, and if so consider them possible suspects. Otherwise you may want to focus your testing on the larger chips and RAM first - unless the symptoms clearly point to one of the TTLs. If any of these chips gets particularly hot, it's a likely suspect.

A garbled or otherwise incorrect startup screen seems to be a common symptom if one of these various logic chips goes bad. The machine typically does not freeze despite the erroneous video output, though a bad TTL can also cause an unresponsive black screen. Bad TTLs tend to be difficult to diagnose since they are often hard to distinguish from other faults such as bad RAM, PLA or VIC. If the startup screen text and/or cursor is drawn twice, U14 should be a prime suspect.
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U17 PLA

The PLA (Programmable Logic Array) is one of the most common parts to go bad in a C64, and can cause a multitude of different failure modes. So if you have no idea where to start, the PLA is always a good first suspect. A typical scenario with a faulty PLA: a C64 has been sitting in the attic for years, and is returned back to use. Either it goes wonky right away, or it seems to work at first, but then malfunctions after some hours or days of use. Especially older PLAs suffer from inadequate passivation, making them vulnerable to moisture and oxidation. These chips also have a fairly high running temperature, and the heat is concentrated on a particularly small area of the chip, which may contribute to the high failure rate. The C64C models feature a new type of PLA chip that is known to be more reliable, so in those boards the PLA should not be your first suspect.

Some common symptoms include a simple black screen and flashing/rainbow colored startup screen characters. A failed PLA may get very hot. If the chip becomes hot quickly after switching on, it's likely dead. Generally if you get an incorrect startup with flashy/colorful/animating graphics, the PLA and VIC-II are prime suspects - especially if the borders are also affected. If you get a solid single-color screen such as violet/pink/orange on startup, the PLA should be your primary suspect as well. It can also cause more subtle symptoms, such as some specific games not working properly.

PLAs tend to fail spontaneously so it's hard to prove any of the following tips have any real effect on preventing a failure, but they may at least prolong the chip's lifespan:

  • Add heat sinks on the chip to help moderate the temperature levels
  • Store in a place that is cool and dry, a storage bag may help as well
  • Power on the computer for a couple of hours every once in a while

CAUTION: When swapping PLA chips, make sure you don't mistake a SID for a PLA. They are the same size, and the chip locations vary depending on the board revision. You will likely kill either chip if you put it in the wrong socket and switch on the C64.
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U18 SID

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U19 VIC

Although the VIC-IIs are not particularly prone to failure, they are nearly always socketed and thus easy to rule out as long as you have spares. A faulty VIC can produce a black screen, but more commonly an incorrect startup screen. In some cases the basic graphics are fine but sprites are incorrectly displayed. If everything else works but sprites in games and such are messed up, the VIC is a likely suspect. Due to its high running temperature, the symptoms of a faulty VIC often change as the chip heats up, for example garbage that gradually fills the screen from one side to another. Graphical glitches that only arise after the machine has been on for a while may be caused by an overheating VIC chip. Another factor that points to a failing VIC are graphical errors that extend to the border areas, but as these pictures prove, sometimes a failing PLA can also mess up the borders. Generally if you get an incorrect startup with flashy/colorful/animating video output, the PLA and VIC-II are prime suspects - especially if the borders are also affected.

If the power LED lights up but you get no video signal at all, check the fuse. On the older "breadbin" type C64s, a blown fuse prevents the VIC-II from working. This results in a totally dead machine with no response to blindly typed commands either. If the VIC stays cool while the other chips heat up, that's more proof that it's the fuse - the VIC is not heating up because it's not getting the voltage it needs to operate. On the newer C64C models, the VIC operates at a lower voltage and is not affected by a blown fuse. A fuseless C64C will boot up but audio will be missing and datassette won't work.

The VIC-II is usually the hottest chip inside a C64, so it may be a good precaution to add some extra cooling. They can heat up to about 70 degrees celsius, which is the specified max temperature for most chips inside a C64. Adding heat sinks and/or a fan can help if you are worried about the temperature. You can also remove the cardboard shield that covers the mainboard to help with the air circulation.

Keep in mind that it's
normal for these chips to run very hot. So even if you experience problems with a C64 and the VIC-II runs hot enough to burn your finger, you shouldn't automatically assume that it's the culprit. Unless it gets that hot within seconds of startup - that's abnormal for any chip.

Most C64 games have some sort of graphical glitches and quirks in them, such as flickering graphics or sprites. Make sure it's not normal behavior for the game in question before suspecting a VIC fault. If you try to play PAL games on a NTSC C64, this can also cause all sorts of graphical glitches.

CAUTION: The newer 85xx revisions of VIC chips are NOT interchangeable with the older ones. They use different voltages and you may damage a 85xx VIC if you swap it in the place of a 65xx. However, not all hope is lost even if you do make this mistake - some people have reported that the chip did in fact survive just fine.
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U25 Logic

When it comes to the TTL chips (the smaller ICs excluding RAM), the model makes a big difference. Normally they run cool and are quite reliable, but the MOS versions of these chips are actually known to fail quite often. In case of a fault, check if your C64 has any smaller chips with "MOS" imprinted on them, and if so consider them possible suspects. Otherwise you may want to focus your testing on the larger chips and RAM first - unless the symptoms clearly point to one of the TTLs. If any of these chips gets particularly hot, it's a likely suspect.

A garbled or otherwise incorrect startup screen seems to be a common symptom if one of these various logic chips goes bad. The machine typically does not freeze despite the erroneous video output, though a bad TTL can also cause an unresponsive black screen. Bad TTLs tend to be difficult to diagnose since they are often hard to distinguish from other faults such as bad RAM, PLA or VIC. If the startup screen text and/or cursor is drawn twice, U14 should be a prime suspect.
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U26 Logic

When it comes to the TTL chips (the smaller ICs excluding RAM), the model makes a big difference. Normally they run cool and are quite reliable, but the MOS versions of these chips are actually known to fail quite often. In case of a fault, check if your C64 has any smaller chips with "MOS" imprinted on them, and if so consider them possible suspects. Otherwise you may want to focus your testing on the larger chips and RAM first - unless the symptoms clearly point to one of the TTLs. If any of these chips gets particularly hot, it's a likely suspect.

A garbled or otherwise incorrect startup screen seems to be a common symptom if one of these various logic chips goes bad. The machine typically does not freeze despite the erroneous video output, though a bad TTL can also cause an unresponsive black screen. Bad TTLs tend to be difficult to diagnose since they are often hard to distinguish from other faults such as bad RAM, PLA or VIC. If the startup screen text and/or cursor is drawn twice, U14 should be a prime suspect.
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U27 Logic

When it comes to the TTL chips (the smaller ICs excluding RAM), the model makes a big difference. Normally they run cool and are quite reliable, but the MOS versions of these chips are actually known to fail quite often. In case of a fault, check if your C64 has any smaller chips with "MOS" imprinted on them, and if so consider them possible suspects. Otherwise you may want to focus your testing on the larger chips and RAM first - unless the symptoms clearly point to one of the TTLs. If any of these chips gets particularly hot, it's a likely suspect.

A garbled or otherwise incorrect startup screen seems to be a common symptom if one of these various logic chips goes bad. The machine typically does not freeze despite the erroneous video output, though a bad TTL can also cause an unresponsive black screen. Bad TTLs tend to be difficult to diagnose since they are often hard to distinguish from other faults such as bad RAM, PLA or VIC. If the startup screen text and/or cursor is drawn twice, U14 should be a prime suspect.
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U31 Oscillator

When it comes to the TTL chips (the smaller ICs excluding RAM), the model makes a big difference. Normally they run cool and are quite reliable, but the MOS versions of these chips are actually known to fail quite often. In case of a fault, check if your C64 has any smaller chips with "MOS" imprinted on them, and if so consider them possible suspects. Otherwise you may want to focus your testing on the larger chips and RAM first - unless the symptoms clearly point to one of the TTLs. If any of these chips gets particularly hot, it's a likely suspect.

A garbled or otherwise incorrect startup screen seems to be a common symptom if one of these various logic chips goes bad. The machine typically does not freeze despite the erroneous video output, though a bad TTL can also cause an unresponsive black screen. Bad TTLs tend to be difficult to diagnose since they are often hard to distinguish from other faults such as bad RAM, PLA or VIC. If the startup screen text and/or cursor is drawn twice, U14 should be a prime suspect.
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CT1 Trimmer Capacitor

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F1 Fuze

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Capacitor

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Trace

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