Josh: Deep technical XT/370 + 3270 PC family page with verified sources (IBM SA38-0037-00 Service Info Manual, IBM GA33-3141-0, IBM 1502336, Kozuh/Livingston/Spillman IBM Systems Journal 1984, seasip.info, Wikipedia)

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Deep technical XT/370 + 3270 PC family page with verified sources (IBM SA38-0037-00 Service Info Manual, IBM GA33-3141-0, IBM 1502336, Kozuh/Livingston/Spillman IBM Systems Journal 1984, seasip.info, Wikipedia)

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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.

== Safety Warning ==

The 3270 PC uses the standard '''IBM 130 W switching PSU''' (Astec / Zenith). Mains-rectified bulk capacitors hold a lethal charge after power-off. Before any work inside the chassis:

# Power off and unplug the mains lead.
# Wait at least 30 seconds.
# Discharge each PSU bulk filter capacitor through a 1 kΩ / 5 W resistor.
# Verify with a multimeter.

The IBM 5272 / 5151 displays carry lethal high voltage on the flyback transformer and anode. '''Discharge the CRT anode to chassis ground via a high-voltage probe''' before any work on the display.

== Documentation Set ==

* '''IBM SA38-0037-00''' — ''Personal Computer Family Service Information Manual'' (July 1989), '''Chapter 10''' covers the 3270 PC family (5271 and 5371).
* '''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 (5160) Maintenance Guide]]:

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

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

* Slot 1 — IBM Hard Disk Adapter (Model 6 / 30 / 50 / 70).
* Slot 2 — Floppy Disk Adapter.
* Slot 3 — 3270 PC Display Adapter.
* Slot 4 — APA card (optional).
* Slot 5 — PSS card (optional).
* Slot 6 — 3278/79 Emulation Adapter.
* Slot 7 — Keyboard Adapter card.
* Slot 8 — XMA (1986 31/51/71 models) or other expansion.

All slot positions can vary; check the as-built configuration of your specific machine.

== Card Inventory at Service Time ==

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

# '''Keyboard Adapter''' — Two ROM variants: '''6323581''' (simple) or '''6323582''' (complicated with NMI + serial). '''Critical: this card holds the video BIOS''' for the 3270 PC Display Adapter. Without it, the video cards do nothing.<ref>http://www.seasip.info/VintagePC/5271.html</ref>
# '''3270 PC Display Adapter''' — SCN2672 PVTC at 0180h–019Bh. Inspect the SCN2672 socket for oxidation.
# '''APA card''' (where fitted) — adds CGA modes + 3270-PC graphics modes.
# '''PSS card''' (where fitted) — Programmed Symbol Set, RAM-loaded fonts.
# '''3278/79 Emulation Adapter''' — I/O at 2D0h–2D6h (older) or 2D0h–2D7h (newer). BNC twinax to controller.

== 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 [http://www.seasip.info/VintagePC/5271.html 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 (5160) Maintenance Guide]].

A '''fully-loaded 3270 PC''' (all card slots occupied, hard drive fitted) draws close to the PSU's 130 W capacity. Rail sag under load is more common than on a stock XT and is the first symptom of aged PSU electrolytics — see [[IBM 3270 PC Capacitor Replacement Guide]].

== 5271 Keyboard Element Maintenance ==

The 122-key Model F is a robust mechanism but has some specific service items:

* '''Foam pad degradation''' under the buckling-spring capacitive pads — common on Model F keyboards of this era. Symptom: keys stop registering or require excessive force. Replacement foam is available from the Model F restoration community.
* '''Cable strain relief''' — heavy cable on the keyboard side can damage the wire harness near the cable entry.
* '''Keycap removal''' — use the IBM tool. Do not pull keycaps by hand; the stems are easily damaged.
* '''Microcontroller''' (IBM part '''1503828''' / Motorola SC81155P) — described as "possibly a microcontroller" by community researchers; failure mode is uncertain because the part has never been publicly decapped. Replacement requires a donor keyboard with the same firmware.

== Display Maintenance ==

=== IBM 5272 Color Display ===

* CRT discharge: 12–15 kV at the anode.
* 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 ===

* Plasma displays of this era can develop pixel rot. Severely affected panels are not repairable; replacement requires a donor.
* Supports 62 × 160 or 46 × 106 text grids.

== Cable to 3174 / 3274 Controller ==

The 3270 PC connects to a host mainframe through an IBM 3174 or 3274 cluster controller via '''BNC twinax''' (or RG-62 coax with BNC terminators). Maintenance:

* Clean the BNC connector on both sides with deoxidising contact cleaner.
* Verify the cable is properly terminated at the controller end (75 Ω terminator).
* Verify the controller-side port is configured for a 3278 or 3279 emulator (the 3270 PC presents itself as a 3278 LU to the controller).

== 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 (5160) 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).
* Spare BNC terminators and RG-62 patch cables.
* SA38-0037-00 PDF (chapter 10).
* 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 (5160) Maintenance Guide]] — shared chassis procedures

== References ==

* [http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf IBM SA38-0037-00 (July 1989)], Chapter 10.
* [http://www.bitsavers.org/pdf/ibm/pc/3270/G320-9539-0_IBM_3270_Personal_Computer_Hints_and_Tips_198605.pdf IBM G320-9539-0 (June 1986)].
* [http://www.seasip.info/VintagePC/5271.html John Elliott, seasip.info — IBM 5271 (3270 PC)].

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[[Category:IBM]]
[[Category:Maintenance Guides]]

IBM 3270 PC Maintenance Guide - Revision history

Josh: Deep technical XT/370 + 3270 PC family page with verified sources (IBM SA38-0037-00 Service Info Manual, IBM GA33-3141-0, IBM 1502336, Kozuh/Livingston/Spillman IBM Systems Journal 1984, seasip.info, Wikipedia)