Start with the power supply. It is the cheapest thing to test and the likeliest thing to be wrong. Then work through reset, clocks and video before you touch a chip. Commodore’s own diagnostic sequence runs that way, and swapping chips at random is how a one-fault machine becomes a three-fault machine.
The symptoms, and where they point:
- A black screen with no border: a CPU, KERNAL ROM or VIC-II problem.
- A blank screen but a visible border: the BASIC ROM is one candidate.
- A coloured screen or flashing garbage: the PLA can cause that.
What does ‘no picture’ actually mean?
Be specific, because the answer narrows the hunt. RetroTechCollection’s guide, built on the Commodore service manual, pairs each failed chip on the long boards with a symptom:
- U7, the 6510 CPU: blank screen, no border.
- U17, the PLA: blank screen, and sometimes a coloured screen or flashing garbage.
- U3, the BASIC ROM: blank screen with a border.
- U4, the KERNAL ROM: blank screen, no border.
- U19, the VIC-II: blank white screen, no border.
Those numbers are for the long boards, 326298, 250407 and 250425, which have socketed chips. Short boards use other numbers, so check yours before you reach for a screwdriver.
What do you need?
- A multimeter that reads both DC and AC volts.
- An oscilloscope or logic probe for the clocks.
- Another TV or monitor, and the right cable for each.
- Known-good replacements for the socketed chips, if you have them.
- A chip puller and an anti-static mat.
How do you find the fault, step by step?
- Check the picture path. Try another TV, and try the monitor socket as well as the RF lead, to rule out the display path.
- Measure the supply with the computer connected. On the supply’s connector, RetroTechCollection puts +5 V DC on pin 5, 9 V AC on pins 6 and 7, and ground on pins 1 to 3. Use the DC range for the 5 V and the AC range for the 9 V.
- Stop if the 5 V is high. The guide says readings above 6 V correlate with RAM failure. If your brick reads that high, don’t connect it again, and look at the supply first.
- Check the 9 V AC. The board makes its +12 V from that AC, through diodes CR6 and CR5. No AC means no +12 V.
- Measure the rails on the board. On the long boards you want +5 V (plus or minus 10 per cent) at U5 pin 24 and +12 V (same tolerance) at U19 pin 13. The VIC-II’s own pin 13 and pin 40 want +12 V and +5 V respectively.
- Check the clocks. You are after a 14.3 MHz colour clock at U19 pin 21, an 8.18 MHz dot clock at U19 pin 22 and a 1.0 MHz phase 0 clock at U7 pin 1. This is scope work.
- Move on to the chips. Commodore’s sequence puts the RAM next, then the PLA, ROMs and CPU. With no picture you can’t test the RAM from the keyboard, so the practical route is to swap in known-good socketed chips one at a time, starting with the one the symptom table points to.
What if you have a 250469 board?
RetroTechCollection says that variant connects the VIC-II’s VDD pin to +5 V, not +12 V. The +12 V check at U19 pin 13 would then give you the wrong answer. Know your board revision before you start.
What if the supply is the problem?
Replace it. A supply that has drifted high may already have killed RAM, which the guide names as the main casualty, so test the rest of the computer again once the new one is fitted. Our guide to the Commodore brick power supply covers the supply itself.
What should you check afterwards?
- The rails again, under load, with the new supply connected.
- That the screen reaches the BASIC prompt and holds it.
- That the colour and sound work.
- That the machine still runs after a longer session.
