Recap guide
Recap the Compaq Portable
Old power supply caps and failing tantalums are standard on these. The Portable III has the best write-ups.
Open the interactive checklist Tick off each capacitor as you replace it. It saves in your browser.
Worth a recap. Reports say it is a common and useful fix on this machine.
Which board do I have?
Boards changed during production. These notes come from the linked sources, and many are only partly documented, so check the board in front of you.
- Compaq Portable (original) (Medium confidence). How to tell: Original 1983 portable. Capacitors: Tantalum caps on motherboard and ISA cards are failure risks. Capacitor list differs from other revisions: not known. Source
- Compaq Portable II (Medium confidence). How to tell: Intel 286 luggable. Capacitors: PSU C13, a 15uF 25V tantalum, has burnt in several units. Capacitor list differs from other revisions: not known. Source
- Compaq Portable III (Medium confidence). How to tell: 286 plasma luggable sharing a PSU with the 386. Capacitors: PSU makes two 200V rails for the screen; tantalums near the PSU connector 15uF 25V and 10uF 15V fail. Capacitor list differs from other revisions: not known. Source
Where to start
Clues from write-ups and forum threads, not a diagnosis. Low confidence means one thread or one repair; high means several write-ups agree.
- Shorting tantalums (High confidence). Several write-ups report shorted or burnt power supply tantalums. A VOGONS thread names positions on a Portable III (PSU, C47, C49, C2) and suggests higher-voltage replacements. Source
- Check for a cracked trace (Low confidence). On one Portable III the power supply was mostly recapped but the real fault was a cracked trace, so check for that. Source
- Original Portable list (Medium confidence). Old Crap has a capacitor chart for the original Portable, plus teardown photos and a Meanwell MPQ-200B power supply replacement. Source
Tips, tales and maybes
Found in write-ups and forum threads. These are other people's reports, not tested by us. Check part references and values against your own board.
- Story: Portable 2 PSU with a 2Hz click (Medium confidence). A Compaq Portable 2 supply clicked at 2Hz with the PowerGood signal never asserting. The owner replaced Sprague 81D, Nippon RX and Sprague 672D/678D parts with 105C high-ripple types close to the originals in size, and replaced tantalums on the motherboard and video card with MLCCs. An ATX retrofit was also discussed. Source
- Watch out: Portable II: C13 shorts and burns (Medium confidence). An Old Crap page on the Portable II says C13, a 15uF 25V tantalum between the regulator input and common, shorted and burned on multiple units. Other symptoms were the supply refusing to restart for about seven minutes, overheating regulators and no voltages without a load. Some owners used a PicoPSU with buck converters instead. Source
- Tip: Beep and error code families (Low confidence). The original Portable page says POST error codes starting 6 indicate disk drive faults, 5 the video adapter and 1 the system board, and that the Zenith CRT flyback often fails. The Meanwell replacement needs a power-good signal buffered, with a 7409 gate suggested. Source
- Watch out: Plasma rails on the PSU connector (Medium confidence). A Portable III page says the supply has a second connector carrying two 200V rails for the gas plasma screen, and recommends replacing all caps and testing the regulators before powering on. It also notes that only drive type 04 (about 70MB) worked reliably, and that Connor drives are unreliable. Source
- Watch out: Factory reversed tantalums in the expansion unit (Medium confidence). A Portable III write-up reports shorted tantalums causing immediate shutdown and says the expansion unit had two tantalums fitted in reverse polarity at the factory, replaced with electrolytics. The Dallas chip battery fails with age and the author rewired it for an external cell. Source
- Story: Swapped tantalum, failed again (Low confidence). A Portable III supply with an exploded tantalum (about 5uF 25V, partly readable) was fixed with a 6.8uF 35V part on the reasoning that a filter cap's exact value matters little there. It passed a smoke test but later failed again with the same burning smell. Source
- Story: Humid storage, popped tantalums and NiCd crystals (Low confidence). A forum post on a stored Portable describes several 10uF 16V tantalums going pop plus exploded supply caps. A soldered 3.6V NiCd had covered the board in green crystals, cleaned with dilute vinegar then water. Source
Before you start
- Boards changed during production. Check your board number against the guides before you order parts.
- A mains power supply can hold a dangerous charge. If you are not trained to work on one, have a professional do it.
Circuits 3
Circuits we redrew in one plain style, with the parts listed. Our own drawing from the source diagram, so check it against the original before you build anything.
From the diagram: R42 feeds the +5VR rail from a point marked 17.8 V, and Q7 holds the rail at about 5.1 V. R41 and R40 divide the rail down to Q7's 2.5 V reference, and C48 smooths it. PC is the power-good line. R56 pulls it up to the rail, and Q10 can pull it down. The diagram marks Q10's gate at -16.7 V and PC at 5.0 V.
| Qty | Part |
|---|---|
| 1 | 2N5639 n-channel JFET Q10, marked DRIVER on the diagram. Its gate is marked -16.7 V and its drain 5.0 V. |
| 1 | TL431C adjustable shunt regulator Q7, marked REG and 5.1 V. On the diagram, pin 3 is on the +5VR rail, pin 2 is at ground and pin 1 is the reference, marked 2.5 V. |
| 1 | 2.2 kΩ resistor R56 (2200 on the diagram). Pulls PC up to the rail. |
| 1 | 1 kΩ resistor R42 (1000). Feeds the rail from the 17.8 V point. |
| 1 | 2.32 kΩ 1% resistor R41 (2320, 1%). Upper half of Q7's reference divider. |
| 1 | 2.32 kΩ 1% resistor R40 (2320, 1%). Lower half of the divider. |
| 1 | 15 µF capacitor C48. We drew the + side toward the rail. The polarity marks on the diagram are hard to read, so check the original part before you replace it. |
The scan has small circled marks beside R41, R40 and C48 that we could not read, so we left them out. Values are as printed on the diagram, and the resistor and capacitor values are for the original supply.
Drawn from the Compaq Portable power supply diagram that oldcrap.org shows on its Compaq Portable page. Redrawn by us.
From the diagram: R33 holds Q2's base up toward +5VR, and CR2 and CR1 run from that base to the RESET line. R32 joins Q2's emitter to the PC line. Q2's collector feeds Q1's base through R24, with R31 to ground, and Q1 can pull the RESETDRV line down while R25 pulls it up to +5VR. U17, one gate of a 74LS04, turns RESET into /RESET. Where RESET crosses the PC line, the diagram hops one over the other without joining them. The diagram has red arrows showing PC and +5VR as inputs and RESET, /RESET and RESETDRV as outputs.
| Qty | Part |
|---|---|
| 1 | 2N2907 PNP transistor Q2, marked SWITCH. The diagram marks 1.3 V at its base. |
| 1 | 2N2222 NPN transistor Q1, marked DRIVER. The diagram marks 0.7 V at its base. |
| 1 | 74LS04 inverter, one gate U17, marked SN74LS04N, part of U17. Pin 1 is the input, pin 2 the output, pin 14 the supply and pin 7 ground. |
| 1 | 1N914-type diode CR2. The scan prints IN9148, so check the marking on yours. Drawn with the banded end toward CR1, as read. |
| 1 | 1N914 diode CR1. Drawn with the banded end toward the RESET node, as read. |
| 1 | 51 Ω resistor R24, from Q2's collector node to Q1's base. |
| 1 | 10 kΩ resistor R31, from the same node to ground. |
| 1 | 10 kΩ resistor R32, from Q2's emitter to the PC line. |
| 1 | 10 kΩ resistor R33, from Q2's base to +5VR. |
| 1 | 2.2 kΩ resistor R25 (2200 on the diagram), from RESETDRV to +5VR. |
The diagram marks 1.3 V at Q2's base, 0.7 V at Q1's base and 5.0 V (A) at U17's supply pin. The scan is rough, so check R24's value (we read 51) against your own board.
Drawn from the Compaq Portable reset circuit diagram that oldcrap.org shows on its Compaq Portable page. Redrawn by us.
From the adapter drawing: the power supply's power-good signal goes to both inputs of one 7409 gate. The gate's output is pulled up to +5 V by a 2.2 kΩ resistor and goes to the motherboard as PC, on pin 2. The supply's +5 V also goes to the motherboard as +5VR, on pin 1, and powers the chip.
| Qty | Part |
|---|---|
| 1 | 7409 quad 2-input AND gate, open-collector outputs IC1. Pins 12 and 13 are the inputs, pin 11 the output, pin 14 +5 V and pin 7 ground. |
| 1 | 2.2 kΩ resistor Pull-up from +5 V to the gate output. |
The 7409 has open-collector outputs, so the pull-up resistor is what makes the high level. The source drawing leaves the other three gates in the package unconnected.
Drawn from oldcrap.org's adapter drawing for replacing the original Compaq Portable supply with a Meanwell MPQ-200B. Redrawn by us.
Guides we used
This page and the interactive bench are built from these. They have the photos and the full detail, so please read them and support their authors.
- Compaq Portable III (Remi's restoration) (Write-up) by rclassiccomputers.com. Shorted tantalums replaced; no values
- Compaq Portable III PSU trouble (Forum) by TinkerDifferent forum. Recapped most PSU caps but the fault was a cracked trace; a good reminder to check for it
- Compaq Portable - Old Crap Vintage Computing (List) by oldcrap.org. Has a capacitor list. Has a capacitor list chart for the original Portable, teardown photos, jumpers and PSU replacement with a Meanwell MPQ-200B
- Compaq Portable II - Old Crap Vintage Computing (Write-up) by oldcrap.org. Teardown plus PSU pinout; comments from several owners identify the same failing PSU tantalum; no formatted list
- Reviving some 80's electronics (Forum) by PhilA and EEVblog members. Original Portable and 386 recovered by replacing failed tantalums and cleaning battery corrosion; mentions caps on video and floppy controller boards
- Compaq Portable II questions. (Forum) by VOGONS forum thread. Owner of two Portable IIs with burnt PSU tantalums asking for values; shows the symptom and the gap in published lists
- Compaq Portable III Motherboard / CPU Problems (Forum) by VOGONS forum thread. Has a capacitor list. Names failing tantalums by position (PSU, C47, C49, C2) and suggests higher-voltage replacements; page 2 of the thread
- Compaq Portable III (ret.rocks one-off works) (Write-up) by ret.rocks. Restoration where all PSU caps and some regulators were replaced for the 5V/12V/200V rails, plus floppy-drive caps; no values
- A Pop, a Puff of Smoke, and an Ominous Red Glow... Not Good (Write-up) by Andrew (darkerview.com). Portable III whose PSU tantalum exploded on power-up; honest about the machine failing again afterward
Original power supply (11 parts)
The list from oldcrap.org, who noted the ratings from the original parts he took off the boards. The four 100 uF 250 V cans are the large silver ones near the top corner of the supply. The same write-up replaced this supply with a modern unit, so also see the power-good notes if you do that.
| Part | Value | Volts | Type |
|---|---|---|---|
| C1 | 100 µF | 250 V | check your board |
| C2 | 100 µF | 250 V | check your board |
| C3 | 100 µF | 250 V | check your board |
| C4 | 100 µF | 250 V | check your board |
| C14 | 1500 µF | 35 V | check your board |
| C15 | 3300 µF | 10 V | check your board |
| C16 | 3300 µF | 10 V | check your board |
| C19 | 470 µF | 25 V | check your board |
| C27 | 470 µF | 25 V | check your board |
| C28 | 470 µF | 25 V | check your board |
| C29 | 470 µF | 25 V | check your board |
Analog (CRT) board (23 parts)
The list from oldcrap.org covers the analog board under its own refs. F4 and G4 are written that way on the list (not C-numbers), so read the silkscreen on yours. The motherboard and the ISA cards use tantalum capacitors that are prone to shorting the power lines, and oldcrap.org suggests replacing every polarised tantalum with a modern electrolytic. The list does not cover those.
| Part | Value | Volts | Type |
|---|---|---|---|
| F4 | 10 µF | 25 V | check your board |
| G4 | 33 µF | 25 V | check your board |
| C108 | 33 µF | 16 V | check your board |
| C113 | 47 µF | 25 V | check your board |
| C122 | 22 µF | 100 V | check your board |
| C123 | 220 µF | 25 V | check your board |
| C124 | 220 µF | 25 V | check your board |
| C131 | 220 µF | 10 V | check your board |
| C132 | 470 µF | 16 V | check your board |
| C133 | 100 µF | 16 V | check your board |
| C134 | 1 µF | 250 V | check your board |
| C304 | 47 µF | 16 V | check your board |
| C307 | 10 µF | 25 V | check your board |
| C308 | 1 µF | 50 V | check your board |
| C309 | 1 µF | 50 V | check your board |
| C310 | 4.7 µF | 25 V | check your board |
| C311 | 47 µF | 25 V | check your board |
| C312 | 47 µF | 25 V | check your board |
| C313 | 47 µF | 25 V | check your board |
| C316 | 470 µF | 16 V | check your board |
| C317 | 470 µF | 25 V | check your board |
| C401 | 22 µF | 100 V | check your board |
| C402 | 10 µF | 25 V | check your board |
Shopping list
34 capacitors in all, grouped by value. Buy a few spares.
- 5 x 470 µF, 25 V (check your board): Amazon · eBay
- 4 x 47 µF, 25 V (check your board): Amazon · eBay
- 4 x 100 µF, 250 V (check your board): Amazon · eBay
- 3 x 10 µF, 25 V (check your board): Amazon · eBay
- 2 x 1 µF, 50 V (check your board): Amazon · eBay
- 2 x 22 µF, 100 V (check your board): Amazon · eBay
- 2 x 220 µF, 25 V (check your board): Amazon · eBay
- 2 x 470 µF, 16 V (check your board): Amazon · eBay
- 2 x 3300 µF, 10 V (check your board): Amazon · eBay
- 1 x 1 µF, 250 V (check your board): Amazon · eBay
- 1 x 4.7 µF, 25 V (check your board): Amazon · eBay
- 1 x 33 µF, 25 V (check your board): Amazon · eBay
- 1 x 33 µF, 16 V (check your board): Amazon · eBay
- 1 x 47 µF, 16 V (check your board): Amazon · eBay
- 1 x 100 µF, 16 V (check your board): Amazon · eBay
- 1 x 220 µF, 10 V (check your board): Amazon · eBay
- 1 x 1500 µF, 35 V (check your board): Amazon · eBay
Skip the sourcing
While you are in there
Affiliate links. As an Amazon Associate I earn from qualifying purchases. These links only search by name. Disclosure
Step by step
- Photograph both sides of the board before you touch anything. Zoom in on every capacitor. The photo is your map when you are three hours in.
- Unplug it, take out the batteries and let it sit. Old power supplies and CRT parts can hold a charge. If your machine has a tube, leave the tube and its board to a professional.
- Open it up and keep the screws sorted. A muffin tin or an egg carton works. Different screws go in different holes, and the holes never forgive you.
- Look for leaks. Clean any sticky residue off with isopropyl alcohol and a soft brush. A leaked capacitor leaves brown crust and can eat copper traces. Look at the traces under it, not only the part itself.
- Mark the polarity of every capacitor on the photo before it comes off. On an electrolytic, the stripe marks the negative leg. The board usually shows a half-shaded circle or a bar for the same spot.
- Remove the old capacitors one at a time. Surface mount: add fresh solder to both ends and rock the part off, or use flux and a low-melt alloy. Through-hole: heat one leg at a time and clear the hole with wick or a pump.
- Clean the pads and check for lifted pads or damaged traces. Clean flux off with isopropyl. A bad pad needs a jumper wire, not hope.
- Fit and solder the new capacitor, matching polarity. Same capacity, same or higher voltage, and it has to physically fit. Work from the smallest parts up.
- Clean the flux off and look at every joint. A good joint is shiny and shaped like a tiny volcano. A dull blob or a ball is a cold joint: reflow it.
- With a multimeter, check for shorts across the power rails before you power it on. Use the continuity or resistance setting. A reading near zero means stop and look for a solder bridge.
- Power it up and test everything: video, sound, controls, the lot. If something is wrong, check polarity first. It is the most common mistake and the easiest fix.
Tools
- Temperature-controlled soldering station. Steady heat so you do not cook pads. Most recaps are done between 320 and 350 C. Top: Hakko FX-951 · Mid: Hakko FX-888D · Budget: Pinecil
- Good solder. Rosin-core 63/37 flows and looks right. It is cheap and it matters. Kester 44 63/37
- Flux. Flux is what makes old joints let go and new joints flow. MG Chemicals flux pen · Chip Quik flux syringe
- Low-melt alloy (for surface mount). Melts old solder on both ends of a surface mount part at once, so it lifts off cleanly. Chip Quik SMD1
- Desoldering tool. Clears the old solder out of through-hole pads. Top: Hakko FR-301 · Mid: Engineer SS-02 pump · Budget: Solder wick
- Fine tweezers. For holding small surface mount parts exactly where you want them. ESD-safe tweezers
- Board holder. A third hand for the board, so both of yours are free. PanaVise circuit board holder
- Magnification. You cannot fix what you cannot see. Surface mount parts are tiny. Top: Stereo microscope · Mid: Head visor · Budget: Lighted magnifier
- Multimeter. Continuity, shorts and rail voltages. The one test that saves boards. Top: Fluke 117 · Mid: Fluke 101 · Budget: AstroAI
- ESR and capacitor tester. Checks old capacitors and your new ones before they go in. Top: Peak Atlas ESR70 · Budget: LCR-T7
- Isopropyl alcohol (99%) and a brush. For cleaning leaked goo and flux. Use the high-percentage kind. MG Chemicals 99.9% isopropyl
- Brass tip cleaner. A clean tip transfers heat. A dirty one does not. Hakko 599B
- Fume extractor. Pulls the smoke away from your face. Hakko FA-400 · Budget: Small bench fan
- Precision screwdrivers. Most old gear uses a mix of Phillips, Torx and security bits. iFixit Pro Tech Toolkit
- Kapton tape. Heat-proof tape to protect parts that sit close to the one you are changing. Kapton tape
Safety
- Good airflow, or a fume fan. Solder smoke is not for breathing.
- Safety glasses on. Solder spits and clipped legs fly.
- Iron in its stand when you are not holding it. Know where the cord is.
- Touch bare metal first, or use a wrist strap. Static kills old chips.
- No tube, no mains power supply. Leave those to a professional.
- Wash your hands after, especially if you used leaded solder.
Values are shown as published in the guides above, and boards changed during production. Check the board in front of you. A recap involves hot tools and, on some machines, dangerous voltages. You do it at your own risk.