IBM 3270 PC Maintenance Guide

This guide documents preventive maintenance for the IBM 3270 PC (machine type 5271, 1983). Because the 3270 PC chassis is mechanically an IBM PC XT (5160), chassis-level maintenance is identical to the XT; the 3270-specific procedures cover the specialist cards (Keyboard Adapter, 3270 PC Display Adapter, APA, PSS, 3278/79 Emulation Adapter), the 5271 Keyboard Element, the 5272 colour display, and the 3270 PC Control Program software environment.

IBM 3270 PC (machine type 5271). IBM's own parts catalogue states that all base system parts for the 3270 XT products are the same as 5160 XT parts — only the 3270 cards and the displays are unique. (Image: Wikimedia Commons)

Safety

The system unit's supply is the 5160 XT supply — IBM's parts catalogue states that "all base system parts for the 3270 XT products are the same as 5160 XT base system parts", and that includes the power supply. It is a switching supply with a rectified-mains reservoir on its primary side, so treat every large capacitor on that board as charged until a meter says otherwise, and discharge through a resistor rather than a screwdriver.

1. Power off and unplug at the wall.
2. Discharge each bulk capacitor through a resistor.
3. Verify with a meter before touching anything.

The 5272 and 5151 displays carry extra-high tension on the flyback and the anode. Follow the CRT Discharge Procedure and discharge a second time a few minutes later.

The coaxial cable is a separate hazard, and not the one you expect. IBM warns twice in the maintenance manual: do not connect or disconnect the coaxial cable during an electrical storm. The run to the control unit is often long and sometimes between buildings.

See also Recommended Tools.

Documentation

Two of these are hosted here, and they are the two to start from:

• IBM 3270 PC Maintenance Information — SY27-2567-2, third edition, October 1984. The CE manual for the 5271: safety inspection, problem isolation charts, locations, removal and replacement, and the illustrated parts catalogue.
• IBM Personal Computer Family Service Information Manual — SA38-0037-00, July 1989. Chapter 10 covers the 3270 PC products; chapter 18 has the parts catalogue.

The rest:

• IBM G320-9539-0 — IBM 3270 Personal Computer Hints and Tips (June 1986).
• IBM 1502336 — PC 3278/79 Emulation Adapter Technical Reference (October 1983).
• IBM SC23-0103-2 — 3270 PC Control Program User's Guide and Reference (July 1984).
• IBM 84X0982 — 3270 Workstation Program v1.0 User's Guide and Reference (April 1987) — the Control Program's software-only successor.
• IBM 59X8423 — PC 3270 Emulation Program v2.00 User's Guide (March 1986).
• IBM 83X9705 — IBM 3270 Connection Technical Reference (April 1987).

Opening the System Unit

Identical to the IBM PC XT Maintenance Guide:

1. Power off and unplug all cables (mains, keyboard, video, printer, BNC twinax to controller).
2. Five Philips #2 screws on the rear panel.
3. Slide cover forward to release.

The 3270 PC interior is densely populated. A typical full configuration occupies:

• Hard Disk Adapter (Models 006, 026, 070, 071 only — not 030 or 050, which are diskette-only).
• Diskette Drive Adapter.
• 3270 PC Display Adapter, or the 3295 Plasma Display Adapter on Models 024 and 026.
• All-Points-Addressable card, where fitted.
• Programmed Symbols card, where fitted.
• 3278/79 Emulation Adapter — IBM calls it the DFCA, the Distributed Function Communications Adapter.
• Keyboard Adapter, or the Plasma Keyboard Adapter on Models 024 and 026.
• Printer/Memory Adapter, or the XMA with 1 MB on Models 031, 051 and 071.

IBM does not publish a fixed slot order for the 5271, so record the as-built configuration before you pull anything. One documented constraint: the XMA jumpered as LPT2 on a 5271 with a fixed disk is not supported — there is an interrupt-level conflict.

Card Inventory at Service Time

For each service visit, verify the card complement and check the as-built ROMs:

1. Keyboard Adapter — two ROM variants: 6323581 (simple) or 6323582 (complicated, with NMI button and serial port). This card holds the video BIOS for the 3270 PC Display Adapter; without it the video cards do nothing, which is the single most confusing failure mode on this machine. Models 024 and 026 use a Plasma Keyboard Adapter instead.
2. 3270 PC Display Adapter — SCN2672 PVTC at 0180h–019Bh. Inspect the SCN2672 socket for oxidation.
3. APA card (where fitted) — adds CGA modes + 3270-PC graphics modes.
4. PSS card (where fitted) — Programmed Symbol Set, RAM-loaded fonts.
5. 3278/79 Emulation Adapter — I/O at 2D0h–2D6h (older) or 2D0h–2D7h (newer). BNC twinax to controller.

What IBM's Error Codes Mean

The 5271 posts codes that a plain XT does not, and IBM maps each range to a field-replaceable unit:

• 28XX — the DFCA (3278/79 Emulation Adapter).
• 32XX — Display Adapter, Programmed Symbols Adapter, All-Points-Addressable Adapter, or the Top Card Connector.
• 44XX — Display Attachment Unit (on the /G and /GX, not the base 5271).
• 45XX — IEEE Interface Adapter card.

Two of these lie to you:

• A false 2801. If a long DFCA has been replaced with the newer short card, the Keyboard Timer Adapter may need changing too. Press F1 to continue after POST; if the host session then establishes normally, the 2801 is false. ECA001 directs the engineer to order P/N 1497249 (one EPROM) and replace the pluggable EPROM on the Keyboard Timer Adapter.
• A false 8505. The XMA diagnostic tests D000–DFFF; any other adapter with RAM or ROS in that range produces an 8505 that is not a real fault.

A plain 83-key XT keyboard in place of the 5271 Keyboard Element produces POST error 302 — press F1 to continue.

Keyboard Adapter Dongle

A small external "dongle" cable splits the keyboard adapter card's DE-9 connector at the rear of the chassis into two 5-pin DIN connectors:

• One for the 122-key 5271 Keyboard Element.
• One that passes through to the standard XT keyboard port on the planar.

The dongle is a known service item — it can be lost, worn, or pin-damaged. Pinout is documented at seasip.info and is reproducible by a competent hobbyist with a soldering iron and the right connector shells.

PSU Voltage Checks

Same as the standard XT — see IBM PC XT Maintenance Guide.

A fully populated 3270 PC has almost every slot occupied and, on some models, a fixed disk as well, so it sits closer to the supply's rated output than a stock XT does. Rail sag under load that clears when cards are removed points at the supply rather than at any one card — see IBM 3270 PC Capacitor Replacement Guide.

5271 Keyboard Element Maintenance

IBM's parts catalogue lists the keyboard as part 6110344 in Figure 7 of the maintenance manual.

A correction worth making before you take one apart: the Model F has no foam pads. It is a capacitive buckling-spring design — the spring buckles and throws a flyplate against the sensing pads on the PCB. There is no foam-and-foil layer to degrade, and searching for one will cost you an afternoon. What does go wrong:

• Flyplate and barrel wear — a key that needs excessive force or stops registering is usually mechanical, at the barrel or the spring, not a dead sense pad.
• Corrosion on the sense PCB — the capacitive pads are exposed to whatever has been spilled on the keyboard over forty years.
• Cable strain relief — the cable is heavy and the harness fails at the entry point.
• Keycap removal — use the tool. The stems are easily damaged by pulling.

IBM's own note in the Service Information Manual gives P/N 1389098 as the replacement keyboard part, "for either the metal bottom or the plastic bottom keyboard". Both bottom types exist and both take the same replacement.

Display Maintenance

IBM 5272 Color Display

• Follow the CRT Discharge Procedure. This wiki has not found an IBM-published anode voltage for the 5272; the "12–15 kV" figure that used to appear here had no source and has been removed. Treat it as lethal and discharge it whatever the number is.
• Inspect flyback for cracked insulation.
• Pin 4 of the DE-9 selects mono vs colour. If the display does not show colour, verify pin 4 is being correctly asserted by the host.
• Color timings: hsync 24 kHz negative, vsync 63 Hz negative. Out-of-spec timings produce display drift / rolling.

IBM 5151 Monochrome Display

• Standard IBM 5151 maintenance — see standard IBM PC documentation.

IBM 3295 Plasma Monitor

• Fitted only to Models 024 and 026, which also carry the Plasma Keyboard Adapter rather than the standard one.
• Plasma panels of this era can develop cell failures. Severely affected panels are not field-repairable.

Coaxial Cable to the Control Unit

The 3270 PC attaches to an IBM 3274 (later 3174) control unit over coaxial cable. It is coax, not twinax — twinax is the midrange 5250 attachment and will not work here — and the link is point to point, one control-unit port per workstation, with no terminator at either end.

IBM's own isolation procedure, which is worth following in order because it costs nothing:

1. With the machine powered off, swap the coaxial cable with one from a known-working 3270 PC and repeat the connection test. If your machine now connects, the cable or the control-unit port is at fault, not the workstation.
2. If it still does not connect, reconnect both machines to their own cables and rerun the DFCA diagnostic. If the other machine now fails, the problem is external to your workstation.
3. If neither of those resolves it: power off, disconnect the coaxial cable at the system unit, and measure the resistance at the system unit's coaxial connector. It should be less than 1 ohm. If it is not, IBM's instruction is to replace the DFCA.

Clean both connectors with a contact cleaner, and verify the control-unit port is configured for a 3278 or 3279 — the workstation presents itself to the control unit as a 3278.

Do not connect or disconnect the coaxial cable during an electrical storm. IBM says this twice.

3270 PC Control Program Maintenance

The Control Program is on diskette (Models 2 / 4 / 24) or pre-installed on the hard drive (Models 6 / 26 / 30 / 50 / 70). Service items:

• Image the Control Program diskettes immediately if you have an original set — they are increasingly rare and the install procedure required mainframe sysprog assistance in period.
• Control Program v3.0 is required for Models 31/51/71 and all P-models. Earlier versions will not boot on these.
• If a Control Program install fails on a P-model, verify the keyboard is a 101-key Enhanced AT-style — the P-models do not work with the 122-key 5271 Keyboard Element.

Floppy and Hard Drive Maintenance

Standard IBM PC XT 5.25" 360 KB DSDD floppy and 10 MB ST-412 (Model 6) or 20 MB hard drive (Models 30 / 50 / 70). Maintenance is identical to the IBM PC XT Maintenance Guide.

Recommended Tools

• Philips #2 and T15 / T20 Torx screwdrivers.
• Anti-static strap.
• Digital multimeter with HV-rated probe.
• High-voltage probe for CRT discharge.
• IPA + foam swabs.
• Soldering iron (fine tip) + solder wick.
• Deoxidising contact cleaner.
• IBM keycap removal tool (for the 5271 Keyboard Element).
• A spare coaxial patch cable — the fastest way to prove the link is not the machine's fault.
• IBM 3270 PC Maintenance Information and IBM Personal Computer Family Service Information Manual, both hosted here.
• 1502336 PC 3278/79 Emulation Adapter Technical Reference PDF.

Related Pages

• IBM 3270 PC
• IBM 3270 PC Troubleshooting Guide
• IBM 3270 PC Capacitor Replacement Guide
• IBM 3270 PC/G — graphics variant
• IBM 3270 PC/GX — extended-graphics variant
• IBM PC XT Maintenance Guide — shared chassis procedures