There seem to be a lot of Commodore 64s going strong today, but also a lot that have died. There are a few common causes of failure, however most of the time blindly replacing components is not a reliable way of fixing anything. I looked around and found there are “Dead Test” Cartridges which can assist with some level of troubleshooting for dead C64’s, primarily if the problem is caused by RAM.
It certainly is no magic bullet for finding problems, and may not help if the C64 any of the following issues:
– Defective power supply
– System Reset or System clock failure
– Defective MPU, PLA or VIC
However its seemed like a cool thing to have, especially when you can make one yourself.
There is plenty of great info on this available, I will bring info together that helped me.
The cartridge is a Dead Test REV 781220. This was an official Commodore diagnostic tool. A corresponding manual came with it. C64_Dead_Test_Diagnostic_Manual_(1988-Jan)
Essentially to make the cart, you need an existing cartridge (or new blank PCB), program an EPROM, make an adapter, and possibly configure some jumpers. My goal was to do this cheaply as possible using existing components. For convenience you can buy them from eBay for around $30 or so, but where is the fun in that!
I used a game cartridge with a type of ASSY # 326173-01. I had a spare Radar Rat Race that used that PCB.
The first thing to do is get the cartridge open. Firstly remove the screw in the cartridge. Now the fun part, getting the case apart. it is held together by a couple of small hinges.
Get a firm grip on it, and prise it open. It will feel like it is going to break, but it won’t. Do be careful, as when you get it open the PCB will likely fly out, and you don’t want that to break.
Lets look at the PCB. The chip needs to be removed. I used a desolder iron. There are plenty more methods to desolder if you don’t have that gear.
Pay attention to the jumpers on the PCB marked J1, J2, J3, J4 & J5. They are required to be set a particular way to put the cartridge in ultimax mode. As luck would have it these jumpers were already set as required for the Dead Test cartridge. to confirm, the required jumper settings are:
C64 Diagnostic Cart Jumpers
Closed – No connection
Open – Connected
If the jumpers need to to adjusted, simply scrape away the trace between the jumper pads, or bridge with solder.
Now unless you use a suitable drop in chip, and have the means to program it, you will need to make an adapter to use an alternative chip.
I have some spare 27128 chips, so will use one of those.
The 27128 is a 128Kbit eprom. It is twice the size of the original chip. You can actually install a switch to choose between 2 banks if you do this. I decided to do this, however at this stage I have only burned the Dead Test rom twice.
Programming the EPROM
dead test (zip file)
After unzipping the Dead Test file, it can be burned to the eprom. To burn the 64Kbit file to a 128Kbit eprom, I am simply going to put 2 copies on. To prepare the file, I used the following prompt from a Windows command window:
copy /b dead test.bin + dead test.bin 27128.bin
Basically this command appends 2 copies of the dead test file, and creates a new file named 27128.bin which contains the appended file. Note you can append more than 2 different files depending on your requirements.
My programmer is a VP-280, using this is a simple task to burn the file.
Preparing the chip adapter
The original chip removed from the PCB was 24 pin, the 27128 is 28 pin. There are some good adapters available to buy, for example http://store.go4retro.com/2364-adapter/
I decided to build my own, as I did not want to wait for postage.
There is some very helpful information at http://blog.worldofjani.com/?cat=7 about building C64 cartridges, which is where I have sourced the following diagram from:
Essentially you can see the pin comparison for the 2 chips, with traces between the connected pins. Those which match will have the trace between adjacent legs, those that don’t have traces to the corresponding legs.
Note the Resistor & Switch (J1). You probably don’t need to put these in if you don’t have different rom images on the chip.
Lets start to make the adapter. Taking a chip socket that was more than 28 pin, I cut that back to size. Then I snipped off part of the legs as indicated by the red circles, and filed them down a bit further. This is because we want enough room to solder on, but not to make a connection to the PCB.
Following the above diagram, solder on joins, resistor, switch & thin jumper wire.
When attaching to the PCB, I put some of the wires through the pcb holes where I snipped off the socket legs. this worked OK, however next time I might try something different. Make sure to install the socket the correct way!
Underside of the PCB.
So now it is time to drop the programmed EPROM in. Using the socket won’t allow the cartridge shell to fit back together, to make it fit a suitable hole must be cut in the top casing. I always like to file down any cuts to make a neat job of it.
Apologies for the poor photo, but we have success! The cartridge works fine on this working C64.
The above screenshot shows what should happen at the conclusion of a successful test – no errors.