Starting point

This Commodore 64 is in for some refurbish, and probably some repair. From the outside it is very dirty, but it appears to be most old dust and grease. The machine seems to be mechanically OK with to immediate signs of broken or not working parts. Since all the three screws at the bottom cover are missing, the whole top cover is loose.

I also notice from the serial number label that this is probably a NTSC machine: the label is not placed in the "label area" due to its size, the label refers to FCC compliance and the serial number structure itself is different from PAL machines.

Below are some pictures of the Commodore 64 before refurbish.

Refurbishment plan

The refurbishment plan for this C64 breadbin (several of them in parallell):

- Refurbish the casing (cleaning, repairing and retrobrighting)
- Refurbish the keyboard (cleaning, reviving the plungers and retrobrighting)
- Refurbish main board (cleaning, checking, repairing, replacing capacitors and voltage regulators if required, adding heat sinks etc.)
- Verify operation by testing

The plan can be updated during the refurbishment process. Sometimes I discover areas that needs special attention.

Disassembly

Disassembling the covers and PCB

The first step of disassembly is very easy. There are no screws - so it is just a matter of lifting the top cover from the bottom. Note: when lifting a top cover on the Commodore 64 it is good practice to lift it only 30 degrees first and then wiggle it loose. This will reduce the probability of braking the rear tabs. Also, notice how the serial number sticker is not placed on the "normal" center spot (for European machines) due to the US sticker size (showing FCC compliance).

Lifting the top cover the interior is revealed. There are some things to notice:

• There is another indication that this is a NTSC machine. There are ferrite rings surrounding wires (both from the keyboard and LED) which is typically to comply with FCC regulations

• None of the PCB screws are present

• The RF shield looks a bit dirty, but I think that its just a matter of some proper cleaning

The RF-shield, which also function as a heatsink, is simply lifted from the PCB since there are no screws holding it in place. The complete PCBA is now revealed, and now there is no doubt: this is a NTSC Commodore 64 equipped with the MOS 6567R8 VIC-II.

Again, no screws holding the PCBA to the bottom cover. The whole PCBA is simply lifted out of the bottom cover which looks to be in good condition.

Removing the keyboard and LED

The keyboard is mounted to the top cover with eight 3.5 x 9 mm Phillips screws which are removed.

Removing the LED is not complicated, but can be very easy if you follow these steps:

• Remove the outer ring surroundind the LED wires with a flat thin screwdriver. This will loose easily by prying it off.

• Place the top cover firmly on the table top resting only on the tip of the LED. With a firm press on the top cover (now with the LED pressing on the table top) the LED will pop out

• With a 5.5 mm hex nut driver the inner LED clip is pushed out (from the inside of the top cover towards the outside)

NOTE: The inner plastic ring is a bit damaged for unknown reasons.

Casing

As previously mentioned the casing is very dirty. There is literally dirt and grease all over it.

Removing the metal badged

Before the casing is cleaned, the "Commodore 64" and "Power" badge are removed. NOTE: these badges will likely be transferred to another C64 machine since this will be refurbished first (and I´ll get new badges for this later).

The trick to remove the badges without damaging them is to apply some hot air from a hairdryer - and at the same time carefully pry the badges off with some prying tools. Both badges are removed without any damage to either casing or the badges themselves.

Cleaning the casing

Both the top- and bottom cover are placed in mild soap water for about a week (together with another C64). This will dissolve most of the dirt and grease.

After the long bath, and with some isopropanol to remove the last stains, the covers looks way better!

Mainboard

Visual inspection

This is an Assy 250425 mainboard 251470-01 Rev A (PCB also marked with "Hi-Pric 37A"). A quite common mainboard found in mid-age Commodore 64 breadbins. Unfortunately, I am aware that these PCBs can be challenging to work with due to poor PCB quality (ripping pads and traces due to desoldering). But hopefully we will not have too much of a problem here.

Since this is a NTSC mainboard there is a large RF-shield covering the backside of the mainboard to comply with US FCC regulations. This is something you do not see on PAL machines for the European market (as far as I know).

The mainboard looks to be in overall good condition. First thing I notice is that all the thermal paste, both for several custom ICs and VR2, has completely dried out. This needs to be cleaned and replenished later. Naturally, there are some dust and grease and some oxidation/corrosion on RF-modulator, user-, datasette-, and cartridge port. But otherwise I can not see any major damage, bulging/leaking capacitors, broken traces/pads or re-work. Also, there are no screws holding the metal plate covering the control ports and power socket.

Below are some pictures of the mainboard before refurbish.

In the table below all the custom ICs are listed. And as can be seen from this table is that all original ICs were produced between week 48 of 1985 to week 05 of 1986 so I think we can assume that this Commodore 64 was produced in the autumn of 1985.

Initial testing

An initial test is performed on the mainboard. This is done with the purpose of checking the initial status of the mainboard - checking for any fault symptoms. In the table below the result are shown.

Removing the bottom RF-shield

Since this is a NTSC machine there is a RF-shield covering the backside of the PCB (to comply with US FCC regulations). To remove this shield all the soldered metal tabs needs to be desoldered and bent. This RF-shield will probably not be put back after refurbish as it has no (or marginal) function now.

Below is a picture of the backside of the PCB with the RF-shield removed. I notice that there are quite some flux residue, but this is to be expected.

Cleaning the mainboard

Before the repair, and refurbish, continues the mainboard is cleaned. This is done by first removing the socketed ICs (and the fuse), and then washing the mainboard with mild soap water and a paint brush. After cleaning the mainboard is left for drying for a couple of days. Below are some pictures of the mainboard after cleaning.

Checking the voltages

For the Commodore 64 to work flawlessly all the different voltages need to present and within acceptable tolerances. A detailed article on the subject can be found in the HOWTO - Checking the C64 voltages. In the table below all the measures voltages are listed (this list will also be updated after refurbishment). All the required voltages are present and within tolerances, so there are nothing obvious wrong in that area.

Mainboard - Repair

Faults investigation

After some checking with the oscilloscope I notice something. There are no AEC signal coming from the VIC-II. This is a very important signal as it will then when the CPU and the VIC-II are to operate on the data- and address bus. And this signal should be generated by the VIC-II I believe. Well, there is a signal, but it is incredible small. Just a flat signal about 0.2 V (measured at PIN#16 on the MOS 6567 VIC-II). See picture below.

Converting from NTSC to PAL frequencies

Before continuing the repair, the machine is converted from using NTSC frequencies to PAL frequencies. The schematics describing the clock circuit is shown in the picture below (taken from schematic #251469 in the C64 service manual).

This schematic from the service manual also shows something interesting. On the bottom right hand side of the schematic there is a table showing the NTSC frequency values and the ICs used in a NTSC machine. Notice especially U31 and U17 which refers to MOS 7701 (CLOCK) and MOS 7700 (PLA) respectively. Why? This very machine is a NTSC machine but does not use any of these - instead it already use the MOS 8701 (CLOCK) and MOS 906114-01 (PLA) found in PAL machines. I think it is a fair assumption that earlier versions of the NTSC machines did use the specialised (?) NTSC ICs, but that these were replaced with PAL ICs at some point.

As can be seen from the schematics above there are two things required to switch from NTSC to PAL frequencies:

• Replace the Y1 crystal. From a 14.31818 MHz to a 17.73447 MHz crystal.

• Close the "NTSC-INACTIVE" jumper

Before any changes are done the current NTSC frequencies are measured. This is done to confirm that they match the frequencies put forward in the table in the schematics above.

[*] Instability in frequency measurement is likely caused by the handheld oscilloscope not able to accurate trigger high frequencies.

A new Y1 crystal with oscillation frequency of 17.73447 MHz is soldered in (the crystal is sourced from Mouser).

Finally the "NTSC-INACTIVE" jumper is closed with a small wire.

After installing the new Y1 crystal the frequencies are measured again.

There is still no activity on the AEC line (pin #16 on the MOS 6567 VIC-II) so I suspect that his IC is dead unfortunately. A "new" PAL MOS 6569 VIC-II is installed on the mainboard (note that this IC has already some heat sinks on each side of the chip. If this IC fix the problem a heat sink will also be placed in the the center of the IC since this is where the heat is generated).

The old Y1 crystal is desoldered from the mainboard without any damage to tracks or pads.

[*] Instability in frequency measurement is likely caused by the handheld oscilloscope not able to accurate trigger high frequencies.

The machine is powered on and the AEC signal on pin #16 is measured on the new PAL VIC-II chip. And now there is a proper signal there! The AEC (Address Enable Control) is directly connected with AEC of the CPU. When going LOW, the CPU puts its bus lines into high impedance state and is thus totally decoupled from the rest of the system, so that the VIC-II can take control over it.

Does the machine work now? Well... not quite. The machine do start with proper startup screen (38911 BASIC bytes free) which is really nice, but there are no colour! The screen is black and white.

The black and white screen is probably caused by a slight misalignment in frequency after the new Y1 crystal was installed. A 40pF trimmer capacitor, CT1, can be used to adjust this. See schematic below.

It turns out that it is not easy to adjust the trimmer capacitor. When using a small screwdriver to adjust the capacitance, the component gives a terrible grinding sound! Some contact cleaner does help a little, but not much. So, a spare trimmer capacitor is installed. Desoldering the old trimmer capacitor - and installing the new one is done without any damage to traces or pads.

With the new trimmer capacitor (CT1) in place the color is back (after a small adjustment on CT1 with a screwdriver).

Banner picture credits: Evan-Amos