A dusty PET with a dark screen is not necessarily a dead PET. Equally, a machine that briefly shows a cursor is not necessarily safe to keep running. Commodore PET restoration starts with restraint: identify the exact model, inspect its condition and establish whether the fault is in the mains supply, monitor, main board, keyboard or simply a poor connection. The quickest route to a damaged board is treating a newly acquired machine as though it only needs a switch-on test.
PETs span several distinct families, from early 2001 machines with chiclet keyboards and built-in cassette decks to later 3000, 4000 and 8000-series business machines, plus the 8296. Their boards, ROMs, RAM arrangements, keyboards and displays are not interchangeable by assumption. A correct repair begins with the model number, board revision and fitted ICs, not the cabinet badge alone.
Start a Commodore PET restoration with identification
Photograph the machine before removing anything. Capture the rear label, internal layout, board markings, socketed chips, keyboard connections and any handwritten labels. This makes reassembly easier, records provenance and gives you a useful reference if a previous owner has already modified the machine.
Check whether you have an original 2001, a 2001N, a 30xx/40xx model, an 80-column machine or an 8296. Screen size is a useful clue, but not the whole story. Early PETs may use 6540 ROMs and 6550 RAM, components with very different replacement options from later 2316 or 2332 ROMs and 4116 DRAM-based boards. A repair plan that suits a 4032 may be entirely wrong for a 2001-8.
Look for corrosion around the battery area if the machine has any aftermarket additions, heat damage near regulators and transformer wiring, cracked edge connectors, missing screws and signs of rodents. Also inspect the CRT neck board and monitor wiring without disturbing them. Old plastics become brittle, particularly around case posts and cassette mechanisms.
Do not make the first power-up a gamble
A visual inspection comes before mains power. Remove loose debris with a soft brush and a low-suction vacuum used carefully around components. Do not drag a nozzle across the board or use household polish on the cabinet. Warm water with a small amount of mild detergent is usually enough for exterior plastics, provided labels, vents and openings are protected and the case is thoroughly dried.
Before applying power, check the mains lead, fuse, switch, transformer connections and power supply board. A PET is old mains equipment with a CRT attached. If you are not comfortable working around mains voltages or high-voltage monitor circuitry, keep the monitor section for a competent technician. A CRT can retain a dangerous charge after the machine has been unplugged. This is not an area for improvised tools or internet bravado.
If the mains side appears sound, bring the machine up cautiously. A lamp limiter can reveal a serious short without allowing full fault current, although it needs to be used with an understanding of the PET’s transformer supply and normal inrush behaviour. Watch for smoke, overheating, crackling, the smell of failing capacitors or an unusually bright or collapsed display. Switch off promptly if anything is wrong.
Measure the low-voltage rails at the board before chasing logic faults. Bad supplies can produce misleading symptoms and can damage replacement ICs. Check the regulated rails against the service information for your specific board revision, under load where practical. A newly fitted chip will not cure a machine whose supply is high, unstable or sagging.
Separate display faults from computer faults
A blank screen does not prove the computer board has failed. On a PET, the monitor and main board are separate fault domains that happen to meet at the video signal. First establish whether the CRT has raster: a faint glow, screen illumination or a response to brightness and contrast controls can help distinguish a monitor issue from missing video generation.
Do not turn controls aggressively. Mark their starting position and adjust only enough to test. A tired monitor may need attention to capacitors, transistors, solder joints or connectors, but that work involves high voltage. Conversely, a healthy raster with no characters may point towards reset, clock, ROM, RAM, video RAM or address-decoding faults on the logic board.
The familiar PET display of random characters often suggests that some video circuitry is alive. It is useful evidence, not a diagnosis. A stable garbage screen can indicate the CPU is not executing valid code, while changing rubbish may indicate activity without a working reset sequence or ROM. Use a logic probe, oscilloscope or known-good diagnostic setup where you can, but avoid replacing chips at random. Each lifted trace and overheated pad makes an original board harder to preserve.
Common board-level problem areas
Early 6540 ROMs and 6550 RAM devices are a particular challenge. Originals are desirable, but they can be scarce and costly. Purpose-made adapter boards can substitute modern memory devices while retaining the original chip locations and board layout. They are often a sensible reliability choice, especially when an original IC is genuinely faulty, but document the alteration and keep every removed part with the machine.
Later dynamic RAM PETs commonly bring 4116-related issues. These chips require multiple supply voltages, so test the supplies first. Replacing every RAM IC may get a machine running, yet it can obscure the real cause and needlessly discard working period parts. A targeted approach, supported by diagnostics and signal checks, is better for both the board and its value.
Socket condition matters as much as the IC itself. Tarnished single-wipe sockets can create intermittent faults that imitate failed ROMs or RAM. Reseating a chip carefully may help, but repeated insertion can weaken old sockets. If replacement is required, fit a quality turned-pin socket neatly rather than forcing a valuable IC back into a damaged original.
Treat the keyboard as a separate project
PET keyboards are often blamed before they have been properly tested. Dirt, oxidised contacts, broken solder joints, degraded foam, cracked traces and connector problems can all result in dead or intermittent keys. The correct method depends on the keyboard type. Do not use abrasive paper on plated contacts or soak assemblies that contain foam, printed traces or unsealed switches.
Photograph keycap positions before removal. Clean keycaps separately, use appropriate contact cleaning methods for the switch design and inspect the keyboard cable and header. For collectors, an original keyboard with a few honest marks is usually preferable to one that has been aggressively refinished. Replacement keycaps or complete keyboards have their place where parts are missing, but the original assembly should be retained whenever possible.
Preserve originality without preserving faults
The best restoration standard depends on the machine. A rare early PET with a complete original board deserves a conservative repair, even if a discreet adapter is needed to make it usable. A common, heavily modified 4032 intended for regular use may justify a modern ROM/RAM solution, replacement electrolytics where testing supports it, and a properly engineered video or storage addition.
Avoid blanket recapping as a ritual. Capacitors do age, and known troublesome parts should be assessed carefully, but replacing components solely because they are old can introduce polarity mistakes, poor joints and unsuitable modern parts. Test, inspect and replace with a reason. Where appearance matters, choose replacements with sensible lead spacing and fit them cleanly rather than creating a tangle of extension wires.
Keep a restoration log stating the original symptoms, voltage readings, parts replaced, modifications made and outstanding issues. Put removed components in a labelled bag. This is useful to the next owner and protects the machine’s story. It also prevents a common problem in vintage computing: buying a supposedly repaired PET only to find that nobody can say what was done to it.
Source parts by exact compatibility
When buying a PET board, keyboard, monitor assembly or IC, seek clear photographs and exact part numbers. “Commodore PET spares” is too broad to establish compatibility. Ask whether a board has corrosion, cut traces, missing chips, modifications or known faults. For CRT assemblies, collection is often preferable to post because fragile glass and heavy cabinets do not travel kindly.
SellMyRetro’s specialist audience understands the difference between a tested 4032 main board, an untested 2001 keyboard and a modern 6540 replacement adapter. That level of description benefits both sides of a sale. If you list surplus parts after a restoration, state the model provenance, testing method and cosmetic condition plainly rather than calling everything “working” after a brief power-up.
A PET does not need to leave the bench looking factory-new to be a successful restoration. It needs to be electrically safe, mechanically sound, honestly documented and as original as its condition sensibly allows. A machine with a tidy repair history and a stable BASIC prompt is far more valuable to the next enthusiast than one polished into anonymity.