Commodore 128 Power Switch and Regulator Repair

The power input area of a 310379 board: the square power socket at the top right, the power switch SW1 below it, the square bridge rectifier, C106 (1000 µF) and the 7812 regulator U59 at the bottom

On the Commodore 128 main board, PCB assembly 310379, the +5 V and 9 V AC from the external supply enter at the 5-pin connector CN11, pass through the line filter L5 and the two-pole power switch SW1, and go two ways. The +5 V feeds the logic directly; there is no 5 V regulator on the board. The 9 V AC is rectified by the bridge CR13, and diodes CR10 and CR11 with capacitor C104 produce about +18 V for U59, a 7812 that supplies +12 V to the SID. When a supply that tests good is connected and the computer is still dead, or runs with no sound, check this path. The checks below are Commodore's, and the repairs cover cleaning or replacing the switch and replacing L5, CR13, the diodes and U59.

Symptoms

• Dead computer, power light off, with a supply that measures correctly. Ruben Schade's C128 showed no power light and no video; he measured 0.38 V at C107 and 5.23 V at C99 on the supply side of the switch, and the fault was a power switch clogged with dust.
• A machine that needs several power cycles to start, or cuts out. Schade's machine had needed a few power cycles to turn on before it failed completely. Carlsen advises checking the solder of the power connector and the power switch for intermittent faults and screen glitches.
• No sound or weak sound: the SID's +12 V supply comes from U59. Carlsen also names transistor Q2 for missing sound.
• A blown supply fuse on the 9 V AC line: Carlsen traced one to a shorted SID.

Before opening the computer, test the supply itself on a load as described in Commodore 128 Power Supply Testing and Repair. For other faults, see the Commodore 128 Troubleshooting Guide.

Tools and parts

• A multimeter. Commodore's checks are to ±10 %.
• Contact cleaner for the switch. Schade notes that GGLabs sold replacement two-pole switches for the C64 and C128.
• Desoldering tools: a solder sucker and desoldering braid, and a larger tip for the ground and supply connections, which draw heat away.
• Commodore's parts for this area, from the 310379 parts lists:

Power input parts, PCB 310379 rev. 6 and rev. 7

Position	Part	Commodore part number
SW1	Rocker switch	252182-01
L5	Line filter assembly	251878-01
CN11	5-pin square DIN socket, shielded	252167-01
CR13	Bridge rectifier, 100 V 2 A	251026-01
CR10, CR11	1N4001, 50 V 1 A
U59	7812 regulator, 12 V, TO-220
C107	100 µF 16 V electrolytic
C111	10 µF 16 V aluminium electrolytic
C104	220 µF 50 V electrolytic
C105	330 µF 50 V electrolytic
C106	1000 µF 25 V electrolytic
C101	10 µF 35 V electrolytic

Procedure

Open the computer

Carlsen's steps for the flat C128:

1. Unplug the supply and all cables, and remove the six screws, Phillips or Torx T10, from the bottom of the case.
2. Lift the top at the front edges, releasing the two plastic snaps on each side.
3. Unplug the power LED lead, then shift the top to the left and pull the keyboard plug straight up by its wires, rocking it.
4. Remove the small Phillips screw holding the ground strap on the right of the main board, and lift the top away.

Trace the +5 V

The power input section of Commodore's schematic 310378 rev. 6, sheet 2. CN11 is at the left; both the 9 V AC and the +5 V pass through L5 and SW1. CR13, CR10, CR11 and U59 are above. CN12, below, is an alternative power connector marked "high end only".

Reconnect the supply, switch on and work from the logic back towards the socket. These are Commodore's steps.

1. Measure the positive lead of C107. +5 V here means the supply path is good; if the logic still has no +5 V, look for an open track beyond C107.
2. If C107 is low, measure the +5 V side of C99, which is on the supply side of the switch. +5 V here and not at C107 means SW1 is faulty.
3. If C99 is also low, measure pin 1 (+5 V in) of CN11. +5 V at CN11 but not at C99 means L5 is faulty; nothing at CN11 means the supply or its plug.

Trace the +12 V

Commodore's steps:

1. Measure the positive lead of C111. +12 V means the regulator path is good; if the SID still has no +12 V at pin 28, look for an open track.
2. If C111 is wrong, measure the anode of CR10. About +18 V unregulated there means U59 is faulty.
3. If that is wrong, measure the 9 V AC at both AC inputs of CR13.
4. If the input fed by the line Commodore labels 9VAC1 is missing, measure the positive lead of C97. 9 V AC there means SW1 is faulty. If not, measure pin 3 of CN11: 9 V AC there means L5 is faulty, and nothing means the supply.
5. If the 9VAC2 input is missing, measure pin 5 of CN11: 9 V AC there means L5 is faulty, and nothing means the supply.
6. With 9 V AC on both inputs of CR13, measure the anode of CR11. Commodore reads the result this way:

Anode of CR11 (Commodore section 2.2.2, step 4)

Reading	Faulty part
9 V AC on a 9 V DC level	CR10
0 V DC	CR10
9 V DC only	CR11
9 V AC only	CR13

The manual's step 3.2 prints "pin 3 of Capacitor CN11" and "Defective Switch - L5"; CN11 is the connector and L5 the line filter.

Clean or replace the power switch

1. Switch off and unplug the supply. Look into SW1 with a torch while it is in the off position; Schade found a large clump of fluff inside his.
2. Blow the dust out with compressed air.
3. Spray a little contact cleaner into the mechanism and operate the switch a dozen times. Schade let it dry for an hour before powering up.
4. If the switch is broken or still fails the C107 check, replace it with a two-pole replacement.
5. Resolder the joints of the power socket and the switch.

Replace U59, CR13 or the diodes

1. Remove the board from the bottom case if the solder side is not reachable.
2. Desolder the part. Heat each joint for about two seconds, until the solder is molten on both sides of the board, and draw it out. The supply and ground connections sit on large copper areas that draw heat away; use a larger tip and braid on those, and do not hold the iron on any pad for more than a few seconds or press hard, or the pads and tracks lift.
3. Check that each lead moves freely before lifting the part out.
4. Inspect both sides of the board. A copper-coloured blob on a removed lead is torn through-hole plating; repair it with a single strand of wire soldered on both sides.
5. Fit the replacement the same way round as the original: U59 a 7812 in a TO-220 package, CR13 a bridge rectifier of at least 100 V 2 A, CR10 and CR11 1N4001 or equivalent.

Testing

Expected readings (Commodore, ±10 %)

Point	Expected
Positive lead of C107, or SID (U5) pin 25	+5 V DC
Positive lead of C111, or SID pin 28	+12 V DC
Anode of CR10	About +18 V DC, unregulated
AC inputs of CR13	9 V AC on both

1. Power up and check the readings above.
2. Check that the power light comes on and the start-up screen appears.
3. Check the sound, which depends on the +12 V at the SID.
4. Switch on and off several times; the machine should start every time.

Related pages

• Commodore 128
• Commodore 128 Power Supply Testing and Repair
• Commodore 128 Troubleshooting Guide
• Commodore 128 General Maintenance
• Commodore 128 Capacitor Replacement Guide
• Commodore 128 Diagnostic Instruction and Troubleshooting Manual (314060)