IBM 3270 PC/GX Capacitor Replacement Guide

The IBM 3270 PC/GX is an IBM 5371 System Unit — the same machine type as the IBM 3270 PC/G — driving an IBM 5379 19-inch display through an IBM 5378 Display Attachment Unit. IBM states that the 5371 System Unit is based on a 5160 System Unit for the IBM Personal Computer XT, that the 3270 PC, G and GX system board settings are the same as for the 5160, and that all base system parts for the 3270 XT products are the same as 5160 XT base system parts.

The system unit's capacitor work is therefore XT capacitor work, documented in full on the IBM PC XT Capacitor Replacement Guide: sixteen tantalums on the board, thirteen of them on +5 V, C55 a ceramic and C56 a tantalum as the only two components on +12 V, and 10 µF / 16 V as the replacement part throughout. The 3270-specific card set is covered on the IBM 3270 PC Capacitor Replacement Guide.

What makes the /GX worth its own page is the display subsystem. Unlike the /G's 5278 and 5279, the 5378 and 5379 are documented down to individual cards, because IBM's AT/GX maintenance manual survives and the AT/GX uses exactly the same two units.

An IBM 3270 PC graphics workstation. On a /GX the slim unit under the display is the 5378 Display Attachment Unit, and it is the unit that takes the mains. (Image: Wikimedia Commons)

Read this before you unplug anything

On a /GX the mains enters through the 5378, and the 5379 display is fed from it. That is the opposite way round to the /G, where the 5279 display takes the mains and feeds the 5278.

IBM: "the 5379 display receives power from the 5378 Display Attachment Unit", and "the 5378 has a 1.8-m (6-ft) power cable and requires its own power source." The on/off switch on the front panel of the 5378 controls the mainline power to the 5379 display.

IBM 3270 PC/GX — where mains actually enters

Unit	Mains	Fed from
5371 System Unit	Own mains cord	—
5378 Display Attachment Unit	Own mains cord, 1.8 m	—
5379 Display	None	The 5378

Two consequences. Switching the 5378 off kills the display, so a dark 5379 is not evidence of a display fault. And pulling the 5378's plug does not instantly de-energise the 5379 — whatever is stored in the 5378's supply is still feeding it.

Safety

• The 5379 carries an IBM hazard label reading "25000 V" (part 4421680), and has a rear safety shield as a separate part (4421919 mono, 4421918 colour). This is a 19-inch tube with a real EHT supply. Follow the CRT Discharge Procedure and take it seriously.
• Discharge the tube twice. IBM's own instruction: "Some CRTs contain a conductive coating on both the inside and outside surfaces to form a capacitor. Within some CRTs, a second capacitive charge builds up following the original discharge. It is therefore important to discharge each CRT a second time immediately before removal." That is not a folk remedy; it is in the manual.
• The 5378 carries an IBM hazard label reading "800 V" (part 4421640). Nothing about the slim, quiet box the monitor stands on suggests that, and it is the single most surprising fact on this page.
• Discharge the 5378's supply through a resistor and confirm with a meter before touching its boards. The 5378 also carries its own 6 A slow-blow 250 V fuse in the power supply unit (part 5214456).
• IBM's CRT handling rules for this display specify safety glasses (IBM part 5715010) and a long-sleeved garment when handling the tube, and warn against handling a CRT by the neck.

The 5378 Display Attachment Unit

The 5378 is a complete graphics computer in its own right: a Motorola 68000 graphics processor performing vector, character and image processing, with 128 KB of memory shared with the system unit's 8088 or 80286. It connects to the system unit by a 62-way bus extender cable and to the 5379 by IBM-supplied 2 m cables, and can be stacked over or under the 5371, 5373 or 5161.

Rails and how to check them without desoldering

The 5378's power supply provides +5 V, −5.2 V, +12 V and −12 V. IBM's own diagnostic tables give three checks that between them tell you whether the supply is healthy:

1. Look at the four power-supply LEDs. There is a location label on top of the power supply unit. If all four are on, the supply is up.
2. Measure +12 V at the fan connector. IBM's limits are +10.8 V to 13.2 V. Inside limits with a dead fan means a dead fan; outside limits means the supply or the interconnect card.
3. Check the interconnect card pinout at power connector P3:

IBM 5378 — interconnect card power connector P3

Voltage	P3 pins
+5 V	1, 2, 3, 4
+5 V sense	5
+12 V	6
−12 V	9
−5.2 V	10, 13
Return	7, 8, 11, 12, 14

Note the −5.2 V rail. It is not a typographical slip for −5 V; IBM's table says −5.2 V, and it is the ECL-style bias rail you would expect behind a 1024 × 1024 bit-plane display of this era.

If a 5378 supply will not come up at all, check the mains selector before anything else. IBM: "If a power supply fails (no power light on), look for a voltage switch at the rear of the 5378 power supply in the upper right-hand corner. The front switch will state the voltage it is presently set for (115 for U.S.A./Canada or 22[0] for world trade). Move the switch to the correct position and try the power supply again." A machine imported from the wrong continent will look exactly like a failed supply.

What IBM lists inside the 5378

IBM 5378 Display Attachment Unit — FRUs

Part	IBM number
Power supply assembly	4421613
Fuse, 6 A slow-blow, 250 V (in the PSU)	5214456
Fan, 12 V (two fitted)	4421586 in the family service manual, 4421916 in the AT/GX manual
Fan shockmount (four fitted)	4421589
Logic boards (two)	8223695 and 8223696
Logic board, support	4134800
Interconnect card	8233876
Logic gate	4421583, also listed as 4421585
Cable assembly, video	4421620
Cable assembly, signal flexi	8233832
Cable, 5371 attach	8529253
Label, hazard 800 V	4421640

The logic gate holds up to eight 4-wide by 3-high logic cards. IBM's card plugging chart:

IBM 5378 card plugging chart (IBM marks the board column A = Color, B = Mono)

Board	Position	Card	IBM number
A1	A2	Update Control Card	8223584
A1	B2	System Control Card	8223717
A1	C2	Graphics Processor Card	6243723
A1	D2	Display Bus Receiver Card	8233700
B1	A2	Clock and Video Driver Card	8223671
B1	B2, C2, D2	Bit Plane Card (up to three)	5147884

The power supply is a single assembly. There is no capacitor, no rectifier and no regulator listed separately anywhere in IBM's documentation for the 5378, and no schematic was published. This page publishes no capacitor values for it.

A self-inflicted fault worth knowing about

IBM: "5378 Display Attachment Unit boards can be wrongly inserted, which will damage cards."

The orientation rule: when installing a board, look for an angled corner to the land pattern and install the board with that corner farthest from the fan and to the top of the housing. The A1 board has this corner to the bottom of the housing.

For card pin lettering within a row — D, J, P, U, B, G, M, S — IBM offers the mnemonic "Dow Jones Prices Up Buy General Motors Stock".

The IBM 5379 Display

The 5379 is the best-documented single unit in the whole 3270 PC family, because IBM breaks it down to individual cards:

IBM 5379 Display — internal FRUs

Part	Mono (M01)	Colour (C01)
Card, power supply	4421761	4421621
Power supply assembly, primary	4421765	4421626
Card, video amplifier	4421763	4421623
Card, deflection amplifier	4421762	4421622
Card, CRT socket	4421766	4421707
Card, manual controls	4421764	4421625
CRT and bezel assembly	4421768	4421767
Degauss coil assembly	—	4421631
EHT cable	4421632	4421632
CRT ground wire	4421917	4421917
Fan	—	4421624
Fan, rubber mount	4421630	4421630
Cable, twin-ax assembly	4421629 (two)	4421629 (four)
Safety shield, rear	4421919	4421918
Label, hazard 25000 V	4421680	4421680

Two things in that list are worth acting on.

First, the power supply is split in two: a "primary power supply assembly" and a separate "power supply card". A 5379 with no life at all and a 5379 with one bad rail are therefore different repairs, and the parts are separately orderable — or, more realistically today, separately scavengeable from a donor.

Second, the CRT and bezel assembly comes in Northern and Southern Hemisphere variants for the colour tube. That is not a clerical distinction; colour purity on a shadow-mask tube depends on the Earth's magnetic field, and a hemisphere-mismatched tube will show purity errors that no amount of recapping will fix.

No capacitance values are published for any 5379 card. The manual stops at the card, and this page does not go past it.

The one capacitor fault IBM actually documents

This is the only capacitor-specific fault in the whole 3270 PC family that appears in IBM's own service documentation, and it is on the 5379:

5379 — "Display is out of focus because of arcing of capacitor connector. The old style plug and connector is green; the new one is brown. CRT/Adapter kit, P/N 59X6942, will correct this problem. Refer to ECA037."

ECA037 (dated 1988-11-01, service code 33) gives the detail. Its stated purpose is to "provide a plug adapter kit that will correct possible connector incompatibility when installing replacement CRT/yoke assemblies and amplifier cards", and the machine affected is the 5379 Model C01 Display. The kit is used when a replacement CRT/yoke assembly or a replacement deflection amplifier card does not match the corresponding socket on the old deflection amplifier card. The procedure: select the correct adapter from the kit, cut the mismatching plug off the CRT/yoke assembly within 50 mm (2 in) of where the green and yellow wires enter the back of the plug housing, strip 6 mm (0.25 in) of conductor from both the existing and replacement wires, join green to green and yellow to yellow in the supplied in-line crimps with no bare conductor showing, compress with tool P/N 450898, and plug the XV connector into the XV socket on the deflection amplifier card. IBM's check before testing: on a male XV plug the upper wire is yellow and the lower wire is green; on a female XV plug the upper wire is green and the lower wire is yellow. Estimated installation time is half an hour.

Practical reading for a restorer: a soft or unstable focus on a 5379 is worth investigating at that connector before anyone reaches for a capacitor kit, and if a previous repairer has mixed old green and new brown connector parts the symptom is arcing rather than a dead display.

5379 figures to judge a display against

• 19-inch screen, viewing area 280 × 280 mm.
• 1024 × 1024 pels for the graphics layer, 93 pels per inch — equal to the IBM 5080 Graphics System Display.
• 52 frames per second, non-interlaced.
• 50 rows of 80 characters for the alphameric layer; characters in a 12 × 20 box.
• Colour: up to 16 colours for the graphics layer, up to eight for the alphameric layer. Mono: four intensities for graphics, two for alphameric.
• Weight 18.2 kg, or 25 kg with the lift/tilt/swivel stand. Tilt −5 to +20 degrees, swivel ±40 degrees.

The 5378 itself is 495 × 406 × 130 mm and 14 kg.

Things that are not faults

• A blank 5379 when the 5378 is powered up without the 5371. IBM: "When powering up only the 5378/5379 and not the 5371, the test pattern is not displayed. A blank screen exists; however, the customer application runs fine."
• A dark display when the 5378 is switched off. The 5378's front-panel switch controls the display's mains.
• Interference between a 5379 and a second 5151 display placed close together, which IBM warns about in two-display /GX configurations, and between the display and a printer or plotter placed closer than 305 mm.

IBM's error-to-FRU table for this path: a 44XX indication points at the 5378 Display Attachment Unit and the 5379 Display, or at the Display Adapter card on an XT-based machine.

When to recap

On the system unit, the triggers and the procedure are the XT's — see the IBM PC XT Capacitor Replacement Guide.

On the 5378 and 5379, with no published component data, judge on evidence:

• Visible capacitor failure, bulging, venting, leakage or corrosion on any board.
• A 5378 power-supply LED out, or a rail outside the figures above.
• Ripple visible on the 5378's rails under load.
• A 5379 that has lost focus — check the connector fault documented in ECA037 first.
• Geometry that drifts as the display warms, or a raster that shifts.
• Mains-side suppression capacitors in the 5378 that are cracked, discoloured or smell of fish in use. Replace immediately with correctly safety-rated parts of the same class; a general-purpose film capacitor is not a substitute.

Do not fit "typical" values into a 5379 deflection or power supply card. If a marking is unreadable, take the value from the identical part on a donor card, or measure it; guessing at values in a 25 kV deflection circuit is how tubes and people get hurt.

Related Pages

• IBM 3270 PC/GX · IBM 3270 PC/GX Maintenance Guide · IBM 3270 PC/GX Troubleshooting Guide
• IBM PC XT Capacitor Replacement Guide — the full system unit procedure
• IBM 3270 PC Capacitor Replacement Guide — the 3270-specific card set
• IBM 3270 PC/G Capacitor Replacement Guide — the smaller display subsystem, and the opposite power topology
• IBM 3270 PC AT/G and AT/GX Maintenance Information · IBM Personal Computer Family Service Information Manual
• Capacitor Failure Symptoms · CRT Discharge Procedure · Recommended Tools