A while back I got my hands on my first Commodore 64 “silver label”, yet another 1980s 8-bit computer to add to my collection. The machine was in good condition and came with several accessories and its original box, however it was sold as “untested”.
The Commodore 64 “silver label” was a limited edition of the C64, which is popular with collectors due to its rarity – these machines are very early examples of the Commodore 64, featuring silver case labels as seen on the C64 box artwork (hence their nickname).
Silver case labels were typically fitted to early Commodore 64s manufactured between early 1982 and early 1983 – in the US these labels were made from metal, and in the UK these were plastic (which lifts up with age).
These early machines were typically fitted with the first C64 board revision, the rare ASSY 326298 “buggy” board – this had all kinds of manufacturing problems and was usually replaced with the later ASSY 250407 board under warranty, then scrapped. In Europe, some machines were fitted with the rare ASSY KU-14194HB transition board.
Due to their limited availability for the time, “silver label” machines can be tricky to find, but not impossible – there are usually a couple up on eBay at any one time, but they often command high sale prices even in poor condition.
This one is so early that the serial number (WGB 1465) is only four digits long, as though they expected to produce less than 9999 units per factory – the C64 is rumoured to have sold in the region of 20,000,000 units whilst it was in production between 1983 and 1994.
The C64 seemed to be all original, and featured a 1982 ASSY 326298 REV.A mainboard with 64 KB of RAM – the CPU, character ROM, PLA, CIAs, and VIC-II were socketed.
All of the ICs were dated early 1982 – the board had an early PLA and a ceramic VIC-II.
After checking that the PSU was working OK, I did a quick power-on test – the power LED would light up, but the computer wouldn’t output video. The unit required repair.
I checked the 5Vdc and 12Vdc supply rails to the video IC, the 6569 VIC-II, but neither were present – these are both “clean” rails derived from the 9Vac input from the PSU via a bridge rectifier and two linear voltage regulators, VR1 (7805) and VR2 (7812).
I traced the 9Vac input back through the board, checked the power switch (which was OK), then noticed that 9Vac wasn’t even getting to the input of the switch. I removed the power socket using my desoldering station (a Duratool D00672), cleaned all its pins thoroughly using a brush and contact cleaner, then reinstalled it.
After re-testing the system, I noticed that 9Vac was now present across the board – the computer now seemed to output video, but only displayed a blank black screen.
In the C64, a black screen fault is a common failure mode which can indicate all kinds of problems: typically, a missing or improper signal, a data or address bus conflict, an addressing problem, or a stack page fault, all of which can be caused by a power issue or a failed IC. Most of the ICs on the board are connected to the data or address bus, so there are a large number of potential problems that need to be worked through.
The first thing to check with any repair is that power is being correctly received on the board, which may indicate a problem with the power socket, power switch, or fuse – I checked for 9Vac and 5Vdc at the user port, and both were OK. I also checked the 5Vdc and 12Vdc supply to the VIC-II, and both were OK.
A constant reset can also cause a garbage screen – I checked the reset signal at the user port and it worked as expected, staying low (around 0Vdc) for about a second at power on, then going high (around 5Vdc).
I checked for signs of previous rework which could indicate a potential problem (flux residue, non-factory sockets, etc), but there was none; I also checked for physical damage on the board which could be causing connection problems, including scratches or cold solder joints, but the board was pristine.
I reseated all of the socketed ICs and cleaned all sockets, ports, and switches with contact cleaner, however there was still no change in symptoms.
I removed the SID and both CIAs, as these are not required for the computer to boot and may cause a black screen if shorted – however, there was no change in symptoms.
I tried the machine using a dead-test cartridge, which can indicate potential hardware issues (i.e. bad ROM, bad RAM) – however, there was no change in symptoms. I tried the same with a diagnostic cartridge, with the same result.
At this point I decided to aim my investigation at the ICs themselves, starting with the socketed ones as these are easy to remove and test.
One by one, I tested the original VIC-II, PLA, CPU, character ROM, and both CIAs in my C64 test board – unfortunately, the early PLA had failed.
With all the ICs reinstalled including a working PLA, the system still exhibited the same symptoms as before – a stable video output but with a blank black screen. This meant that there was still a problem with a soldered IC (or ICs) or the board itself.
Again, I tried the machine using a dead-test cartridge, which can indicate potential hardware issues (i.e. bad ROM, bad RAM) – however, there was no change in symptoms. I tried the same with a diagnostic cartridge, with the same result.
I pulled out my IR thermometer to measure the temperature of all of the ICs, to check for any outliers. IC failures often lead to gates shorting or pulling on inputs or outputs, which draws more current, which increases die and package temperature – this can range from a subtle difference to something that could quickly burn your finger (i.e. RAM ICs damaged by overvoltage).
In this case, I found that four 4164 RAM chips (U10, U11, U12, and U24) became burning hot very shortly after power-on, same with the Kernal ROM – RAM ICs should run cool, and the Kernal ROM may run warm, but certainly not as hot as it was.
Interestingly, three of the four failed RAM ICs had been marked with a permanent marker, so perhaps someone had been inside this machine at some point in the past.
I removed all of the suspect ICs using my desoldering station (a Duratool D00672).
I tested the Kernal ROM in my C64 test board, and it had indeed failed; I tested the four 4164 RAM ICs in my RAM tester, and all had failed also.
I installed a set of sockets on all of the desoldered ICs – these allow the quick removal of ICs (i.e. for testing) without the need for rework.
I always use high-quality double-sided sockets, which are more reliable than cheap single-sided sockets as they contact the IC legs on both sides – a lot of people push the use of turned-pin sockets, but I don’t like using them as they make swapping ICs difficult, they are difficult to desolder, and they are visibly obviously non-standard.
I then installed a spare known-good Kernal ROM and four new-old-stock 4164 DRAM ICs, all available to buy from Retroleum. After a power-on test, the computer now seemed to boot correctly (with flashing cursor and correct amount of RAM showing), and passed all tests with a diagnostic cartridge and loopback harness installed – except for the SID analogue input test, but with this being such an early unit, I didn’t want to replace it.
At this point I would normally perform some preventative maintenance, starting with replacing the electrolytic capacitors – these are commonly used for filtering, smoothing, and decoupling in both high- and low-voltage electronics, and suffer from ageing.
However, the board was so old, rare, and original that I didn’t want to mess with it.
At this point, the computer could be reassembled.
After all this work was performed, I did some finishing up: I thoroughly cleaned the mainboard with compressed air and an ESD-safe brush; I thoroughly cleaned the case inside and out using Cillit Bang and a microfibre cloth (or a toothbrush for all the nooks and crannies); I cleaned all IC sockets, ports, and switches with contact cleaner; I also dismantled and thoroughly cleaned the keyboard.
The computer still seemed to boot OK following all of this work. However – and I know this well – just because a computer boots, doesn’t mean it’s working properly. Thorough testing is necessary to verify operation, so I did as much testing as I could.
- All keys registered correctly; shift-lock mechanism worked OK.
- Power LED worked OK.
- Luma/chroma and composite video outputs worked OK.
- All diagnostic tests passed correctly with diagnostic harness and loopback harness.
Functionally, the original PSU was working fine – I usually install a modern mains plug (3A fused) on any PSUs that I use (not possible in this case), and check the output voltage(s). Aside from that, the PSU casing and cabling just needed a good clean.
I’d generally recommend using a modern PSU with most vintage computers, as some of the originals can be prone to failure. The C64 PSU is a power brick potted with epoxy, making it difficult to service and meaning that the internals run hot – it has a 7805 linear regulator inside, which can fail in such a way that its input voltage starts to leak onto its output, causing all kinds of damage to the computer due to overvoltage.
As such, I would recommend regularly checking the output voltages on these kind of PSUs with a multimeter before using them, both when cold and hot – the 9Vac output is unregulated so can range from 9Vac to 12Vac when not under load, and the 5Vdc output should really range between 5.1Vdc and 5.3Vdc when not under load.
Personally, I use a modern-made Commodore 64 PSU which is reliable and safe to use – I keep the original PSUs as a part of my collection, but generally don’t use them.
Adam's Vintage Computer Restorations 