Commodore 128 Power Supply Testing and Repair

A Commodore 128 external power supply, part number 310416-03, made in West Germany

The Commodore 128 takes regulated +5 V DC and 9 V AC from an external power supply through a 5-pin square DIN plug. Ray Carlsen describes it as a switching supply that runs cooler and lasts longer than the Commodore 64's linear supply; its weaknesses are the factory glue, which becomes conductive and corrosive with age, and solder joints that break loose. Tynemouth Software gives its rating as 2.5 A at 5 V and 1 A at 9 V AC. The procedure below tests the supply on a load, then opens it to clean off the glue, resolder, replace fuses and the two main filter capacitors, check the over-voltage crowbar and set the 5 V output.

Several makers built the supply. Commodore's service manual prints two circuits "for reference only", a Mitsumi supply, part 252449-01, and a Dee-Van supply, part 252449-03. World of Jani drew the schematic of the PAL supplies made by Ismet, 310416-01 to -03, from six units. Labels do not always match the contents: Muldrf's Hobbytronic found a Mitsumi 252449-01 board inside a supply labelled 310416-05.

Symptoms

Carlsen lists a dead computer as the commonest power supply fault, and also blank screens with the power light on or off, freezes, garbage on the screen, hum bars moving up the picture and intermittent operation.

• No output at all: Tynemouth found a blown fuse on the 9 V AC side, and both joints of the main 5 V output inductor cracked so that it was loose, giving no 5 V.
• 5 V low: the same supply gave about 4.5 V after the inductor was resoldered, and was trimmed back to 5 V.
• 5 V far too high: World of Jani reports that on the Ismet supplies the 5 V output rises to between 12 and 15 V if the 4700 µF capacitor C9 across the 5 V rail goes open circuit; the crowbar then holds it down until the crowbar fails. On his supply the crowbar failed and he measured 12 V on the 5 V rail.
• No 5 V with nothing connected: some switching supplies need a minimum load before they start. A contributor to an EEVblog repair thread suggested testing a C128 supply with a 10-20 Ω resistor across 5 V and ground instead of the computer.

If the supply tests good on a load and the computer is still dead, the fault may be on the main board: a dirty power switch, the line filter or the +12 V regulator. See Commodore 128 Power Switch and Regulator Repair.

Safety

Warning: the supply contains mains wiring, a mains fuse and the transformer primary. Unplug it from the mains before opening it.

• The supply plug has the same shape as the Amiga 500, 600 and 1200 supply plugs, but the supplies are not compatible; using one on the other can damage the hardware. On the Amiga plug two of the pins carry ±12 V where the C128 has 9 V AC.
• Do not connect a supply of unknown history to the computer. Measure its 5 V output on a load first, because a failed output capacitor and crowbar can put 12 V or more on the 5 V line.

Tools and parts

• A multimeter, and for the load test a 10-20 Ω resistor. At 5 V a 10 Ω resistor dissipates 2.5 W, so use a resistor rated for at least that.
• A Phillips screwdriver; a dental pick or similar for scraping off old glue.
• A soldering iron and fresh solder. Carlsen applies new solder to every joint he reworks; reflowing the old solder is not enough.
• Fuses of the original ratings. The Mitsumi schematic gives F1 600 mA 250 V in the mains input and F2 1.5 A 125 V in the 9 V AC winding. Tynemouth's supply had a 1.4 A fuse on the AC side.
• Capacitors. In the Mitsumi supply the two electrolytics are C4, 2200 µF 35 V, after the bridge rectifier, and C9, 4700 µF 16 V, on the 5 V output. Hobbytronic found only these two in his Mitsumi supply and fitted 105 °C low-ESR replacements. World of Jani also replaces C4 and C9 in the Ismet supplies with low-ESR 105 °C parts rated for 10,000 hours. The Dee-Van circuit is different; its electrolytics are listed in the capacitor guide.
• For crowbar repairs on the Mitsumi circuit: V9, a 6.2 V zener, and V10, a TIC106D thyristor. World of Jani lists equivalent parts for the Ismet supply's V8 and V10.

Procedure

Test before opening

C128 power connector (CN11 on the main board)

Pin	Signal
1	+5 V
2	Shield
3	9 V AC
4	Signal ground
5	9 V AC

Individual Computers notes that the pin layout seen at the solder side of the computer's socket matches the outside of the cable's plug.

1. Measure the AC voltage between pins 3 and 5 of the plug. Expect about 9 V AC.
2. Connect the load resistor between pin 1 and pin 4 and measure the DC voltage across it. Expect 5 V; Commodore's own checks on the computer allow ±10 %.
3. If 5 V is present only with a load, the supply may be working normally; if there is no 5 V even with a load, or it is high, open the supply.

Open the supply

Carlsen's procedure:

1. Unplug the supply from the mains and from the computer.
2. Remove the four screws from the bottom of the case. Hobbytronic found plastic plugs in the screw holes of his supply and had to drill a small hole in one to pull it out.
3. Lift off the top half. It may be stuck to the transformer by double-sided tape; pull the chassis out of the bottom half-shell to free it, without stressing the circuit board, which can crack.
4. Clean the plastic residue off the screw ends before refitting them later.

Remove the glue and resolder

Commodore's schematic of the Mitsumi supply, 252449-01. The 9 V AC output comes straight from its own winding through F2; the 5 V output is a switching regulator round IC1 with switch transistors V6 and V7, diode V8, inductor L2 and output capacitor C9. R11 sets the voltage; V9 and V10 form the crowbar.

1. Look for factory glue on both sides of the board. New glue is tan or light brown; near hot parts such as the power transistors it turns brown and then black. Black glue absorbs water from the air and causes leakage between circuits and corrosion of tracks and component leads.
2. Scrape the brown and black glue away from the wiring and components. Aged glue comes off easily; Carlsen uses a dental pick.
3. Replace any component the corrosion or the scraping has destroyed, and bridge any broken track with wire soldered across the break.
4. Resolder the terminals of the power transistors and the large parts such as the filter capacitors with fresh solder, and any other joint that looks to be breaking loose. World of Jani found cold joints in several of his supplies.
5. Check the output inductor. Tynemouth found it loose despite a cable tie, with both solder joints cracked, and resoldered it.
6. Check the circuit board for cracks from rough handling.

Fuses

1. Check F1 in the mains input and F2 in the 9 V AC winding with the meter.
2. Replace a blown fuse with one of the same rating. Tynemouth's supply had no output until a blown fuse on the AC side was replaced.
3. A fuse that blows again points to a fault in the supply or the computer. Carlsen traced a blown fuse on the 9 V AC line to a shorted SID in the computer.

Replace the filter capacitors

1. Note the polarity of C4 and C9 and remove them.
2. Fit replacements of the same capacitance and at least the same voltage. Hobbytronic's replacements were shorter and slightly smaller in diameter than the originals.
3. On later Ismet boards World of Jani found an extra capacitor, C12, and R13 fitted at 100 Ω; C12 can be retrofitted to earlier boards.

Check the crowbar

On the Ismet supplies the crowbar pulls the output down if it exceeds 6.2 V. In the Mitsumi circuit it is zener V9 (6.2 V) driving thyristor V10 (TIC106D) across the 5 V output, with R13 (1 kΩ).

1. If the supply has ever produced a high output, or blows its fuse, check V10 for a short and V9. World of Jani's failed supply had a leaking capacitor and a blown V10.
2. Replace C9 as well, since its failure is what lets the output rise.

Set the 5 V output

1. With the load connected, measure the 5 V output.
2. Adjust the trimmer to 5.0 V. On the Mitsumi supply it is R11, 200 Ω. Tynemouth set a supply that had drifted to about 4.5 V back to 5 V this way.

Reassemble

Carlsen's reassembly:

1. Set the assembly back in the bottom half. The transformer fits into a recess, and the board should sit flat on the bottom of the case. If it tilts up, a wire is pinched underneath; reroute it.
2. Press the mains and DC cords into their places.
3. Fit the top. Check that it seats fully and evenly all round before refitting the screws; if not, wiring is holding the board up.

Testing

1. Repeat the load test: 5 V between pins 1 and 4 and 9 V AC between pins 3 and 5.
2. Connect the computer and measure +5 V at the positive lead of C107 or pin 25 of the SID (U5), and +12 V at pin 28 of the SID, within ±10 %.
3. Run the computer for a while and recheck. Carlsen lists intermittent operation among the supply's symptoms.

Revisions

C128 supplies named in the sources

Label or board	Maker	Notes
252449-01	Mitsumi	Circuit in Commodore's service manual, printed p. 78
252449-03	Dee-Van	Circuit in Commodore's service manual, the page after the Mitsumi circuit
310416-01, -02, -03 (PAL)	Ismet	Schematic drawn by World of Jani from six units
310416-03		Photographed above; made in West Germany
310416-05	Mitsumi board 252449-01 inside	Found by Hobbytronic

Related pages

• Commodore 128
• Commodore 128 Power Switch and Regulator Repair
• Commodore 128 Capacitor Replacement Guide
• Commodore 128 General Maintenance
• Commodore 128 Troubleshooting Guide
• Commodore 64 Power Supply Testing and Repair
• Capacitor Failure Symptoms