Here's another retrocomputer article: the repair of a Commodore 16. Sifting through Facebook Marketplace, I was struck by this listing for a used, non-working Commodore 16. The price was good, precisely because the computer had problems. According to the seller, the power supply was damaged: he could not repair it, so he sold the Commodore 16, with its accessories, at a low price.
I therefore decided to buy the lot consisting of: Commodore 16 with power supply to be repaired, original joystick, cassettes with games and Datassette 1531 cassette player. When I received the Commodore, I immediately went to work. The computer looked very good, even though it was definitely dirty. The power supply was disassembled: it was enough to reassemble it to make it work. But unfortunately, the Commodore 16 was not working!
Brief history of the C16
The Commodore 16 was marketed by CBM (Commodore Business Machines Inc.) in 1984. It was the cheap alternative to the Commodore 64, which I will discuss in another article and on which I have done several experiments.
You can actually see the economy of the computer from some circuit solutions adopted:
- the absence of a SID chip (audio chip);
- a smaller amount of RAM than the C64 (16kB RAM total, of which 12277 free at power on);
- a linear power supply of inadequate power, with resistor R10 to help and lack of filter capacitors.
The C16 used a CPU signed MOS 7501 which, subsequently, was updated with the most recent MOS 8501 (the one present in the computer I repaired). Unfortunately, the Commodore 16 did not meet the public's favor, so much so that production ceased the same year.
Commodore 16 Repair
Returning to the repair, I opened the Commodore's chassis and disassembled all the internal components. I realized that the fuse on the motherboard was blown. Having no fuses of that size (6.3 × 32), I used a 5 × 20 fuse, of the same amperage as the original one. I also inserted a filter capacitor, since I realized that the power supply on the motherboard did not have one.
With these simple interventions, the Commodore 16 repair was successful and the computer immediately started working. I then dedicated myself to the complete cleaning of the chassis and keyboard.
Commodore 16 recapping
I performed a full recapping of the Commodore 16. I replaced all the electrolytic capacitors: both those of the power supply and those of the motherboard. In fact, electrolytic capacitors tend to degrade with the passage of time: losses increase, capacity tends to go out of tolerance and some capacitors even go into short circuit.
In addition to replacing the capacitors already present on the motherboard, I also added a couple of capacitors which were, in my opinion, missing. The first (top right in the photo) I mounted at the input of the linear voltage regulator uA7805, in order to compensate for any voltage fluctuations. It is a 1000uF capacitor, minimum 25V, in my case 50V. I inserted the second capacitor at the output of the regulator, on the 5V line (see top left in the photo). The value of the second capacitor is 220uF, minimum 16V: I used a 25V one.
Regarding the original capacitors, I made some changes (optional):
- C2, initially from 47uF, is now from 220uF, minimum 16V;
- C3, initially from 10uF, now it is from 100uF, minimum 16V;
- C4, initially from 10uF, is now from 100uF, minimum 16V.
This change serves to increase the immunity of the computer to any sudden drops in voltage.
Warning! The capacitors are polarized, which means that the positive pole must not be inverted with the negative!
Protection of the Commodore 16 from voltage transients
Finally I protected the motherboard and the external power supply with some transil (on the 9V and 5V direct current lines) and with a varistor on the primary of the 230Vca transformer. The transil before the regulator is a 1.5KE15A. The one after the regulator, on the 5V line, is a P6KE6V8A. Again, care must be taken! Transils are polarized, so they must be welded in the correct orientation! The varistor on the primary of the 230V transformer is a SIOV S10K250.
The transils and the varistor are tasked with protecting the Commodore 16 from surges and voltage transients.
After all these repairs and renovations, the Commodore 16 is ready to run for another 40 years!
In support of this article, I also attach the Commodore 16 Service Manual. It is a technical manual useful for understanding the operation of the Commodore 16 circuits and can be useful for carrying out repairs.