Jump to content

IBM 5120 Maintenance Guide: Difference between revisions

From RetroTechCollection

Josh (talk | contribs)

5 user groups
10,244 edits
undo
VisualWikitext
Deep technical pre-PC IBM page with verified sources (Bitsavers MIM/MAP, Wikipedia, IBM Archives) โ€” honest gap disclosure where IBM did not publish per-board cap values
ย 
Add machine photograph
ย 
(One intermediate revision by the same user not shown)
Line 1: Line 1:
This guide documents preventive maintenance for the '''[[IBM 5120]] Computing System''' (announced February 1980, also designated IBM 5110 Model 3). The 5120 shares its PALM processor with the [[IBM 5100]] and [[IBM 5110]] but is in a substantially redesigned desktop chassis with a built-in 9-inch CRT, detached keyboard, two built-in 8-inch floppy drives, and a larger linear power supply. The machine is approaching 45 years old at the time of writing; all preventive maintenance is restoration-class work.
[[File:IBM 5120 view 1.jpg|thumb|320px|IBM 5120 Computing System. Both 8-inch diskette drives, the display and the logic run from one supply enclosure containing two independent supplies. (Image: Wikimedia Commons, CC BY 3.0)]]
This guide documents preventive maintenance for the '''[[IBM 5120]] Computing System''' &mdash; the machine IBM's own service documentation calls the '''5110 Model X3X''', or "5110-3". It shares its processor and card set with the [[IBM 5100]] and [[IBM 5110]], but puts them in a desktop case with a detached keyboard, a built-in display and '''two 8-inch diskette drives''', and it has the most unusual power supply in the family.<ref name="mim5120">IBM, ''IBM 5120 Maintenance Information Manual'', SY34-0192-0, December 1979 &mdash; hosted on this wiki as [[IBM 5120 Maintenance Information Manual]]. Section 2 items 207 (AC box) and 208 (power supply locations, including PC boards A1 and A2, resonant capacitor C4, transformer T1, circuit breaker CB1 and fuses F2, F3 and F4); item 272 (Power &mdash; Y1 socket pin assignments and the DC rail tolerance table); items 273, 274 and 284 (AC, DC and internal diskette DC voltage distribution); and the "5110-3 Power Supply", "DC Power Distribution", "DC Voltage Tolerance" and "Reference Voltage" pages of the Theory section. The scan carries no text layer, so it has to be read by eye.</ref>


== Safety Warning ==
== Safety Warning ==


The 5120 PSU is a '''linear''' supply with large bulk filter capacitors after the bridge rectifier. The 9-inch CRT carries lethal high voltage. Before any work inside the chassis:
'''The 5120 does not have a linear power supply, and it does not have one power supply.''' It has '''two supplies in one enclosure''':<ref name="mim5120" />


# Power off and unplug the mains lead.
* '''PC board A2''' &mdash; a small, high power, high frequency '''transistor switching regulator''' (TSR) developing five DC voltages for the computer itself. The logic manual labels this board "5110 Power Supply Board A2": it is the [[IBM 5110]]'s regulator board, reused unchanged.<ref name="logic5120">IBM, ''IBM 5120 Computing System Logic Manual'', SY34-0193-0, December 1979 &mdash; hosted on this wiki as [[IBM 5120 Computing System Logic Manual]]. The DC power distribution sheets, which show the supply as "5110 Power Supply Board A2" and carry the rail distribution and cable net lists.</ref>
# Wait at least 30 seconds.
* '''PC board A1''' &mdash; a '''ferroresonant''' supply developing three DC voltages for the internal diskette units.<ref name="mim5120" />
# Discharge each PSU bulk filter capacitor through a 1 kฮฉ / 5 W resistor.
# '''Discharge the CRT anode to chassis ground via a high-voltage probe.'''
# Verify both with a multimeter.


The 5120 carries a 9-inch CRT (vs the 5-inch tube on the 5100 / 5110), so the anode voltage and stored energy are correspondingly higher.
Both have rectified-mains sections. The ferroresonant board additionally carries '''resonant capacitor C4''', which is under mains-level AC stress in normal operation and stores real energy. Before any work inside the enclosure:


== Documentation Set ==
# Power off and unplug at the wall. The AC box carries live mains to the line filter module, terminal blocks '''TB1''' and '''TB2''', circuit breaker '''CB1''' and the power switch, and on to both diskette drives through connectors '''P4''' and '''P5'''.<ref name="mim5120" />
# Discharge the reservoir capacitors on both boards through a resistor, and confirm with a meter on a DC range.
# Treat C4 as a mains-side component, not a signal-level part.


The IBM service documentation for the 5120 is preserved at [http://bitsavers.org/pdf/ibm/5120/ Bitsavers]:
The display generates its own extra-high tension internally. Follow the [[CRT Discharge Procedure]] before working near the tube, and discharge a second time after a few minutes &mdash; the coatings on a CRT behave as a capacitor and can recover a charge.


* '''SY34-0192-0''' โ€” IBM 5120 Maintenance Information Manual (MIM), December 1979 โ€” the CE service manual.
'''The 5120 has a circuit breaker, not a fuse holder, in its AC box''' &mdash; unlike the 5100 and 5110.<ref name="mim5120" /> A 5120 that trips CB1 is telling you something. Resetting it repeatedly without finding out what is a good way to turn a repairable fault into a fire.
* '''SY34-0193-0''' โ€” IBM 5120 Computing System Logic Manual, December 1979 โ€” gate-level logic diagrams. Essential for board-level fault isolation.
* '''GH30-0232-1''' โ€” Joint 5110 / 5120 Bibliography, September 1980.


The 5120 documentation set is thinner than the 5110's โ€” many user-facing manuals (APL, BASIC) for the 5120 are the 5110 manuals because the language ROMs are essentially the same. Cite the 5110 manuals where applicable.
See also [[Battery Explosion, Capacitor or Corrosion Damage]] and [[Recommended Tools]].


== Opening the System Unit ==
== Documentation ==


Tools: Philips #2 screwdriver, T15 / T20 Torx, anti-static strap, high-voltage probe.
* [[IBM 5120 Maintenance Information Manual]] &mdash; SY34-0192-0, December 1979. The CE service manual. '''The scan has no text layer''', so it cannot be searched; pages have to be read by eye.
* [[IBM 5120 Computing System Logic Manual]] &mdash; SY34-0193-0, December 1979. Gate-level logic and the DC power distribution sheets. Essential for board-level fault isolation.
* The 5120's user-facing language manuals are largely the [[IBM 5110]]'s, because the language ROS is essentially the same. Cite the 5110 manuals where a 5120 manual does not exist.


# Power off and unplug all cables (mains, keyboard, printer, communications).
== Inside the Power Supply Enclosure ==
# Remove the cover fasteners around the back of the chassis (typically 4โ€“6 screws).
# Lift the cover โ€” '''the 9-inch CRT and deflection sub-chassis remain attached to the lower chassis''', unlike the 5100 / 5110 where the CRT is in the upper shell.
# Inside layout (looking from the front):
:: '''9-inch CRT''' centred behind the front bezel; deflection board mounted to the side of the CRT cradle.
:: '''2 ร— 8-inch floppy drives''' below the CRT, mounted vertically in a drive cage.
:: '''Main PSU''' at the rear left.
:: '''PALM board''' and ROS / RWS cards on a backplane behind the drive cage.
:: '''Keyboard connector''' on the front edge.


== Detached Keyboard ==
Section 208 of the maintenance manual gives the physical layout. These are the parts IBM names:<ref name="mim5120" />


The 5120 detached keyboard is a separate unit:
<templatestyles src="Template:StyledTable/styles.css" />
ย 
{| class="wikitable styled-table" style="width:95%;"
* Cable connector at the rear connects to the system unit.
|+'''IBM 5120 power supply &mdash; identified components'''
* Keyboard has a palm rest and IBM Model F-class buckling-spring switches on some 5120s.
! Item !! Part
* Keycaps are removed with the IBM tool (not pulled by hand).
|-
| '''AC box''' || Power on/off switch, terminal blocks TB1 and TB2, a sealed line filter module, circuit breaker '''CB1''' and the AC line cord (item 207)
|-
| '''T1''' || Transformer
|-
| '''C4''' || '''Resonant capacitor''' &mdash; an AC-rated part, not an electrolytic, and not a general recap candidate
|-
| '''PC board A1''' || Ferroresonant supply for the diskette units. Carries fuses '''F2''' (+5 Vdc), '''F3''' (&minus;5 Vdc) and '''F4''' (+24 Vdc), connector J1 on the rear of the card, and connectors J2 and J3
|-
| '''PC board A2''' || TSR regulator board. Carries the AC1 and AC2 inputs and DC output terminals '''E1 to E14'''
|-
| '''P4, P5''' || Diskette AC connectors
|}


== Inspecting the Chassis ==
=== Fuses tell you which supply has failed ===


Inspection items in order:
IBM states that the ferroresonant board "is protected against excessive overcurrent by a fuse on each of the three dc voltage outputs".<ref name="mim5120" /> That makes diagnosis unusually clean for a machine of this age:


# '''Main PSU bulk filter capacitors''' โ€” visual inspection. These are the highest-likelihood failure point.
* '''F2, F3 or F4 blown''' &mdash; the fault is on the '''diskette''' side: a drive, the diskette control card, or a short on that rail. It does not implicate the computer's own supply.
# '''Series-pass regulator transistors''' โ€” heatsink-mounted; check thermal paste.
* '''No fuse blown, machine dead''' &mdash; look at board '''A2''', the TSR regulator, which has no output fuses and shuts down instead.
# '''9-inch CRT and deflection board''' โ€” inspect for cracked solder joints around the flyback transformer.
# '''Phosphor''' โ€” check for burn-in (static images from years of unattended operation).
# '''8-inch floppy drive belts''' โ€” perished urethane is the dominant failure on these drives.
# '''Spindle motor brushes''' โ€” wear is common after 40+ years.
# '''Head load solenoids''' โ€” fatigue.
# '''Drive cage carry handles''' โ€” plastic fatigue reported.
# '''ROS modules''' โ€” reseat in sockets (oxidised pins).
# '''PALM board edge connector''' โ€” reseat (most common cause of "dead PALM" symptoms).


== Regular Cleaning ==
== Expected Measurements ==


* Soft brush and low-pressure compressed air for the planar, PALM, ROS / RWS cards, drive bays, and PSU vents.
The TSR board produces the same five rails as the [[IBM 5100]]:<ref name="mim5120" />
* '''Do not''' use compressed air on a leaking electrolytic.
* Detach the keyboard for separate cleaning. Wipe with slightly damp microfibre. Remove keycaps with the IBM tool to avoid breaking stems.
* Clean 8-inch floppy heads with IPA on a '''foam swab''' (not cotton).
* Inspect every diskette before reading. If the oxide is shedding, do not load โ€” it will contaminate the head.


== PSU Voltage Checks ==
{| class="wikitable styled-table" style="width:95%;"
|+'''IBM 5120 &mdash; PC board A2 (TSR) rails and loads'''
! Rail !! What it feeds
|-
| '''+5 Vdc''' || Basic logic voltage
|-
| '''&minus;5 Vdc''' || Storage cards, BSCA cards, common ROS
|-
| '''+8.5 Vdc''' || Storage cards, keyboard, display adapter, printer adapter, BSCA cards, all ROS cards
|-
| '''+12 Vdc''' || BSCA cards, '''load resistor RL1''', and the 5114 R1 relay
|-
| '''&minus;12 Vdc''' || 5114 R1 relay, BSCA cards, asynchronous communications / serial I/O card
|}


The 5120 PSU is larger than the 5100 / 5110 supply because it must drive 2 ร— 8-inch floppy drives plus the 9-inch CRT. Probe each rail at the planar power connector.
Measure at the '''Y1''' power connector, or at terminals E1 to E14 on board A2. On A2, E1 to E3 are +5 V, E4 to E6 ground, E7 and E8 +8.5 V, E9 and E10 +12 V, E11 and E12 &minus;12 V, and E13 and E14 &minus;5 V &mdash; the same arrangement as the 5110, because it is the same board.<ref name="mim5120" /><ref name="logic5120" />


{| class="wikitable styled-table" style="width:100%; text-align:center;"
{| class="wikitable styled-table" style="width:80%; text-align:center;"
|+'''IBM 5120 PSU rail tolerances (typical linear PSU; verify against MIM SY34-0192)'''
|+'''IBM 5120 &mdash; DC rail tolerances (IBM figures)'''
! Rail !! Approximate target !! Notes
! Voltage !! Loaded !! Unloaded (Y1 disconnected)
|-
|-
| +5 V (logic) || +4.75 V to +5.25 V || PALM, ROS, RWS
| +5 Vdc || 4.6 to 5.5 || 5.5 to 6.5
|-
|-
| +12 V || +11.4 V to +12.6 V || Floppy drive logic, deflection
| +8.5 Vdc || 7.9 to 9.35 || 7.4 to 9.0
|-
|-
| +24 V (typical) || ยฑ5% || 8-inch floppy spindle motor and head load (may be 12 V on some drives)
| +12 Vdc || 11.0 to 13.2 || 9.8 to 12.2
|-
|-
| &minus;5 V / &minus;12 V || ยฑ5% || Bias rails
| &minus;5 Vdc || &minus;4.6 to &minus;5.5 || &minus;3.7 to &minus;4.7
|-
|-
| CRT high-voltage anode || ~12โ€“15 kV (9-inch CRT) || Measure with HV probe only
| &minus;12 Vdc || &minus;11.0 to &minus;13.2 || &minus;9.0 to &minus;11.5
|}
|}


The 9-inch CRT anode voltage is higher than the 5-inch CRT on the 5100 / 5110 โ€” handle with extra care.
IBM states the DC outputs may vary from '''+10% to &minus;9%''' of the rated voltage before they affect system operation.<ref name="mim5120" /> A '''+6 Vdc''' controlled reference is provided on the display card '''(G2)''' so a service meter can be checked before critical measurements.<ref name="mim5120" />
ย 
=== Protection, and the 60-second rule ===
ย 
Board A2 has the same protection as the 5100's and 5110's:<ref name="mim5120" />


== 8-Inch Floppy Drive Maintenance ==
* '''Overvoltage''' &mdash; shuts down when the +12 Vdc output exceeds '''+16 Vdc'''.
* '''Undervoltage''' &mdash; shuts down when the &minus;5 Vdc rail is less negative than '''&minus;3 Vdc'''.
* '''Overcurrent''' &mdash; shuts down when the current in the transformer primary is excessive.


The 5120 has 2 ร— 8-inch floppy drives built in (typically Shugart-class drives in IBM cosmetics). Maintenance items:
'''The restart rule is different on this machine.''' Where the 5100 and 5110 manuals say five seconds, the 5120 manual says that after an automatic shutdown the system "should be powered down with the on/off switch for at least '''60 seconds''' before it is powered up again".<ref name="mim5120" /> Cycling a 5120 faster than that makes a healthy supply look dead.


* '''Drive belt''' โ€” urethane perishes. Modern replacements available from the 8-inch floppy restoration community.
== Opening the System Unit ==
* '''Head load solenoid''' โ€” exercise; replace if fatigued.
* '''Spindle motor brushes''' โ€” wear is common. Brushes can be re-sourced from industrial motor suppliers.
* '''Read / write head''' โ€” clean with IPA on a foam swab. Never use cotton โ€” fibres in the head gap cause errors.
* '''Head alignment''' โ€” verify against a known-good diskette using the procedure in the MIM.
* '''Oxide shedding''' from old IBM Diskette 2D media โ€” inspect every diskette; do not load shedding diskettes.


== Optional 5114 Diskette Unit ==
Tools: Philips screwdriver, anti-static strap, meter, high-voltage probe.
ย 
# Power off and unplug at the wall, then disconnect the keyboard, printer and communications cables.
# Remove the cover fasteners at the back of the chassis.
# Lift the cover clear. Expect the display and the diskette drives to be heavy and awkward; have somewhere to put the cover down before you start.
ย 
The maintenance manual's locations section is the reference for what is where; item 208 in particular gives the supply enclosure layout described above.<ref name="mim5120" />
ย 
== Regular Cleaning ==


If the 5120 has an external 5114 attached:
* Soft brush and low-pressure compressed air on the boards, the cards, the drive bays and the supply vents.
* '''Do not''' use compressed air on a leaking electrolytic.
* Detach the keyboard and clean it separately. Remove keycaps with the proper tool rather than pulling by hand.
* Clean 8-inch diskette heads with isopropyl alcohol on a '''foam''' swab. Never cotton &mdash; fibres in the head gap cause read errors.
* Inspect every diskette before loading it. If the oxide is shedding, do not load it: it will contaminate the head and then every other diskette you try.


* Service the 5114 separately per the [[IBM 5110 Maintenance Guide]].
== 8-Inch Diskette Drives ==
* The 5114 has its own linear PSU with its own bulk filter capacitors.


== CRT Maintenance ==
The two internal 8-inch drives are fed AC directly from the AC box through connectors P4 and P5, and their DC rails come from the ferroresonant board A1 (fused F2, F3 and F4).<ref name="mim5120" /> That split is worth remembering: '''a drive that does not spin and a drive that does not read are different faults on different supplies'''.


The 9-inch CRT and its deflection / flyback board are mounted in the lower chassis on a sub-chassis:
Maintenance points on drives of this era:


* Always discharge the CRT anode before any work on the deflection board.
* '''Drive belt''' &mdash; perished urethane is the dominant mechanical failure. Replace before chasing electronics.
* Inspect the flyback transformer for cracked insulation โ€” visible cracking indicates impending arc-over; flyback replacement requires a donor.
* '''Head load solenoid''' &mdash; exercise it; a sticky solenoid reads intermittently.
* Deflection coils โ€” inspect for cracked solder joints at the yoke connector.
* '''Read/write head''' &mdash; clean with IPA on a foam swab.
* Phosphor burn-in โ€” if a static image is visible on the inactive screen, the phosphor has been operated for too long with a fixed display; replacement requires a donor CRT.
* '''Head alignment''' &mdash; verify against a known-good diskette using the procedure in the maintenance manual rather than by trial and error.
* Focus drift โ€” adjust the focus pot on the flyback if accessible; if the CRT cannot focus, the cathode is end-of-life.
* '''Media''' &mdash; oxide shedding from forty-year-old diskettes is common and is contagious.


== ROS and RWS Module Care ==
== Optional 5114 Diskette Unit ==


Same as the [[IBM 5100]]:
If an external 5114 is attached, service it separately: it has its own supply and its own maintenance manual, [[IBM 5114 Diskette Unit Maintenance Information]]. The [[IBM 5110 Maintenance Guide]] covers it in more detail. The 5114's R1 relay is fed from the 5120's +12 V and &minus;12 V rails,<ref name="mim5120" /> so a 5114 fault can present at the host.


* Static-sensitive โ€” always wear an anti-static strap.
== Display ==
* Do not flex ROS modules.
* Reseat ROS cards if banner garbled.


== PALM Board Care ==
The display generates its own EHT and carries the '''+6 Vdc reference''' used for checking a service meter, on card G2.<ref name="mim5120" /> Always discharge the anode before working on the deflection board &mdash; see [[CRT Discharge Procedure]] &mdash; and inspect the flyback transformer for cracked insulation, which precedes arc-over.


Same as the [[IBM 5100]] โ€” 13 bipolar gate arrays are unobtainium. Reseat the edge connector before suspecting failure.
Phosphor burn-in from years of a static display is permanent; there is no repair short of a donor tube.


== Connector Care ==
== Connector Care ==


* '''Keyboard cable''' โ€” multi-pin proprietary connector; clean both sides with deoxidising contact cleaner.
* '''Keyboard cable''' &mdash; multi-pin proprietary connector; clean both halves and reseat.
* '''Mains lead''' โ€” replace if insulation cracked.
* '''P4, P5''' &mdash; diskette AC connectors. Mains-carrying: inspect, do not improvise.
* '''Printer cable''' (IBM 5103) โ€” IBM-specific connector; clean and reseat.
* '''Y1''' and '''E1 to E14''' &mdash; the DC measurement points above.
* '''Communications connectors''' โ€” RS-232 DB-25 and SDLC / BSC connectors clean per standard procedure.
* Mains lead &mdash; replace if the insulation is cracked or hardened.
ย 
== Capacitor Health ==
ย 
'''IBM published no capacitance values for the 5120.''' Neither the maintenance manual nor the logic manual draws the inside of either supply board, and there is no 5120 parts catalogue. The values have to be read from the parts in the machine in front of you &mdash; starting with '''C4''', whose markings are the single most valuable missing figure for this machine. The full discussion, including why a resonant capacitor must not be replaced with an electrolytic or with "something close", is on the [[IBM 5120 Capacitor Replacement Guide]].
ย 
== Things That Are Not Faults ==
ย 
* '''A 5120 that hums.''' Ferroresonant transformers are noisy by nature. A hum that has always been there is not evidence of anything. A hum that has '''changed''', or that comes with heat or low rails, is.
* '''+12 V reading low with the logic disconnected.''' See the tolerance table &mdash; and note that +12 V also feeds load resistor RL1, a deliberate minimum load.<ref name="mim5120" />
* '''A long delay before the machine responds after a protection shutdown.''' That is the 60-second rule.


== Recommended Tools ==
== Recommended Tools ==


* Philips #2 and T15 / T20 Torx screwdrivers.
* Philips screwdriver set and nut drivers.
* Anti-static strap.
* Anti-static strap.
* Digital multimeter with HV-rated probe (CRT anode is 12โ€“15 kV).
* Digital multimeter &mdash; checked against the +6 V reference on card G2.
* High-voltage probe for CRT discharge.
* High-voltage probe for the display.
* IPA + foam swabs (no cotton).
* Isopropyl alcohol and foam swabs. No cotton buds.
* Soldering iron (fine tip) + solder wick.
* Fine-tip soldering iron and solder wick.
* Modern 8-inch floppy drive belt replacement.
* Replacement belts for the 8-inch drives.
* IBM keycap removal tool.
* [[IBM 5120 Maintenance Information Manual]] and [[IBM 5120 Computing System Logic Manual]].
* Modern 8-inch head-cleaning supplies.
* MIM SY34-0192 and Logic Manual SY34-0193 PDFs.


== Related Pages ==
See [[Recommended Tools]] for general workshop equipment.
ย 
== References ==


* [[IBM 5120]]
<references />
* [[IBM 5120 Troubleshooting Guide]]
* [[IBM 5120 Capacitor Replacement Guide]]
* [[IBM 5110 Maintenance Guide]] โ€” sibling machine
* [[IBM 5100 Maintenance Guide]] โ€” PALM / ROS procedures translate directly
* [[Recommended Tools]]


== References ==
== Related Pages ==


* [http://bitsavers.org/pdf/ibm/5120/ Bitsavers โ€” IBM 5120 documents]. MIM SY34-0192, Logic Manual SY34-0193.
* [[IBM 5120]] &middot; [[IBM 5120 Troubleshooting Guide]] &middot; [[IBM 5120 Capacitor Replacement Guide]]
* [https://en.wikipedia.org/wiki/IBM_5120 IBM 5120 โ€” Wikipedia].
* [[IBM 5110 Maintenance Guide]] &mdash; same regulator board; the 5114 is documented there
* [https://en.wikipedia.org/wiki/IBM_PALM_processor IBM PALM processor โ€” Wikipedia].
* [[IBM 5100 Maintenance Guide]] &mdash; the family reference for the processor cards and the DC supply
* [https://web.archive.org/web/20111110083525/http://www-03.ibm.com/ibm/history/exhibits/pc/pc_6.html IBM Archives โ€” IBM 5120].
* [[IBM 5120 Maintenance Information Manual]] &middot; [[IBM 5120 Computing System Logic Manual]] &middot; [[IBM 5114 Diskette Unit Maintenance Information]]
* [[CRT Discharge Procedure]] &middot; [[Capacitor Failure Symptoms]] &middot; [[Recommended Tools]]


{{Navbox-IBMComputers|state=collapsed}}
{{Navbox-IBMComputers|state=collapsed}}

Latest revision as of 03:40, 23 September 2026

IBM 5120 Computing System. Both 8-inch diskette drives, the display and the logic run from one supply enclosure containing two independent supplies. (Image: Wikimedia Commons, CC BY 3.0)

This guide documents preventive maintenance for the IBM 5120 Computing System — the machine IBM's own service documentation calls the 5110 Model X3X, or "5110-3". It shares its processor and card set with the IBM 5100 and IBM 5110, but puts them in a desktop case with a detached keyboard, a built-in display and two 8-inch diskette drives, and it has the most unusual power supply in the family.[1]

Safety Warning

[edit | edit source]

The 5120 does not have a linear power supply, and it does not have one power supply. It has two supplies in one enclosure:[1]

  • PC board A2 — a small, high power, high frequency transistor switching regulator (TSR) developing five DC voltages for the computer itself. The logic manual labels this board "5110 Power Supply Board A2": it is the IBM 5110's regulator board, reused unchanged.[2]
  • PC board A1 — a ferroresonant supply developing three DC voltages for the internal diskette units.[1]

Both have rectified-mains sections. The ferroresonant board additionally carries resonant capacitor C4, which is under mains-level AC stress in normal operation and stores real energy. Before any work inside the enclosure:

  1. Power off and unplug at the wall. The AC box carries live mains to the line filter module, terminal blocks TB1 and TB2, circuit breaker CB1 and the power switch, and on to both diskette drives through connectors P4 and P5.[1]
  2. Discharge the reservoir capacitors on both boards through a resistor, and confirm with a meter on a DC range.
  3. Treat C4 as a mains-side component, not a signal-level part.

The display generates its own extra-high tension internally. Follow the CRT Discharge Procedure before working near the tube, and discharge a second time after a few minutes — the coatings on a CRT behave as a capacitor and can recover a charge.

The 5120 has a circuit breaker, not a fuse holder, in its AC box — unlike the 5100 and 5110.[1] A 5120 that trips CB1 is telling you something. Resetting it repeatedly without finding out what is a good way to turn a repairable fault into a fire.

See also Battery Explosion, Capacitor or Corrosion Damage and Recommended Tools.

Documentation

[edit | edit source]
  • IBM 5120 Maintenance Information Manual — SY34-0192-0, December 1979. The CE service manual. The scan has no text layer, so it cannot be searched; pages have to be read by eye.
  • IBM 5120 Computing System Logic Manual — SY34-0193-0, December 1979. Gate-level logic and the DC power distribution sheets. Essential for board-level fault isolation.
  • The 5120's user-facing language manuals are largely the IBM 5110's, because the language ROS is essentially the same. Cite the 5110 manuals where a 5120 manual does not exist.

Inside the Power Supply Enclosure

[edit | edit source]

Section 208 of the maintenance manual gives the physical layout. These are the parts IBM names:[1]

IBM 5120 power supply — identified components
Item Part
AC box Power on/off switch, terminal blocks TB1 and TB2, a sealed line filter module, circuit breaker CB1 and the AC line cord (item 207)
T1 Transformer
C4 Resonant capacitor — an AC-rated part, not an electrolytic, and not a general recap candidate
PC board A1 Ferroresonant supply for the diskette units. Carries fuses F2 (+5 Vdc), F3 (−5 Vdc) and F4 (+24 Vdc), connector J1 on the rear of the card, and connectors J2 and J3
PC board A2 TSR regulator board. Carries the AC1 and AC2 inputs and DC output terminals E1 to E14
P4, P5 Diskette AC connectors

Fuses tell you which supply has failed

[edit | edit source]

IBM states that the ferroresonant board "is protected against excessive overcurrent by a fuse on each of the three dc voltage outputs".[1] That makes diagnosis unusually clean for a machine of this age:

  • F2, F3 or F4 blown — the fault is on the diskette side: a drive, the diskette control card, or a short on that rail. It does not implicate the computer's own supply.
  • No fuse blown, machine dead — look at board A2, the TSR regulator, which has no output fuses and shuts down instead.

Expected Measurements

[edit | edit source]

The TSR board produces the same five rails as the IBM 5100:[1]

IBM 5120 — PC board A2 (TSR) rails and loads
Rail What it feeds
+5 Vdc Basic logic voltage
−5 Vdc Storage cards, BSCA cards, common ROS
+8.5 Vdc Storage cards, keyboard, display adapter, printer adapter, BSCA cards, all ROS cards
+12 Vdc BSCA cards, load resistor RL1, and the 5114 R1 relay
−12 Vdc 5114 R1 relay, BSCA cards, asynchronous communications / serial I/O card

Measure at the Y1 power connector, or at terminals E1 to E14 on board A2. On A2, E1 to E3 are +5 V, E4 to E6 ground, E7 and E8 +8.5 V, E9 and E10 +12 V, E11 and E12 −12 V, and E13 and E14 −5 V — the same arrangement as the 5110, because it is the same board.[1][2]

IBM 5120 — DC rail tolerances (IBM figures)
Voltage Loaded Unloaded (Y1 disconnected)
+5 Vdc 4.6 to 5.5 5.5 to 6.5
+8.5 Vdc 7.9 to 9.35 7.4 to 9.0
+12 Vdc 11.0 to 13.2 9.8 to 12.2
−5 Vdc −4.6 to −5.5 −3.7 to −4.7
−12 Vdc −11.0 to −13.2 −9.0 to −11.5

IBM states the DC outputs may vary from +10% to −9% of the rated voltage before they affect system operation.[1] A +6 Vdc controlled reference is provided on the display card (G2) so a service meter can be checked before critical measurements.[1]

Protection, and the 60-second rule

[edit | edit source]

Board A2 has the same protection as the 5100's and 5110's:[1]

  • Overvoltage — shuts down when the +12 Vdc output exceeds +16 Vdc.
  • Undervoltage — shuts down when the −5 Vdc rail is less negative than −3 Vdc.
  • Overcurrent — shuts down when the current in the transformer primary is excessive.

The restart rule is different on this machine. Where the 5100 and 5110 manuals say five seconds, the 5120 manual says that after an automatic shutdown the system "should be powered down with the on/off switch for at least 60 seconds before it is powered up again".[1] Cycling a 5120 faster than that makes a healthy supply look dead.

Opening the System Unit

[edit | edit source]

Tools: Philips screwdriver, anti-static strap, meter, high-voltage probe.

  1. Power off and unplug at the wall, then disconnect the keyboard, printer and communications cables.
  2. Remove the cover fasteners at the back of the chassis.
  3. Lift the cover clear. Expect the display and the diskette drives to be heavy and awkward; have somewhere to put the cover down before you start.

The maintenance manual's locations section is the reference for what is where; item 208 in particular gives the supply enclosure layout described above.[1]

Regular Cleaning

[edit | edit source]
  • Soft brush and low-pressure compressed air on the boards, the cards, the drive bays and the supply vents.
  • Do not use compressed air on a leaking electrolytic.
  • Detach the keyboard and clean it separately. Remove keycaps with the proper tool rather than pulling by hand.
  • Clean 8-inch diskette heads with isopropyl alcohol on a foam swab. Never cotton — fibres in the head gap cause read errors.
  • Inspect every diskette before loading it. If the oxide is shedding, do not load it: it will contaminate the head and then every other diskette you try.

8-Inch Diskette Drives

[edit | edit source]

The two internal 8-inch drives are fed AC directly from the AC box through connectors P4 and P5, and their DC rails come from the ferroresonant board A1 (fused F2, F3 and F4).[1] That split is worth remembering: a drive that does not spin and a drive that does not read are different faults on different supplies.

Maintenance points on drives of this era:

  • Drive belt — perished urethane is the dominant mechanical failure. Replace before chasing electronics.
  • Head load solenoid — exercise it; a sticky solenoid reads intermittently.
  • Read/write head — clean with IPA on a foam swab.
  • Head alignment — verify against a known-good diskette using the procedure in the maintenance manual rather than by trial and error.
  • Media — oxide shedding from forty-year-old diskettes is common and is contagious.

Optional 5114 Diskette Unit

[edit | edit source]

If an external 5114 is attached, service it separately: it has its own supply and its own maintenance manual, IBM 5114 Diskette Unit Maintenance Information. The IBM 5110 Maintenance Guide covers it in more detail. The 5114's R1 relay is fed from the 5120's +12 V and −12 V rails,[1] so a 5114 fault can present at the host.

Display

[edit | edit source]

The display generates its own EHT and carries the +6 Vdc reference used for checking a service meter, on card G2.[1] Always discharge the anode before working on the deflection board — see CRT Discharge Procedure — and inspect the flyback transformer for cracked insulation, which precedes arc-over.

Phosphor burn-in from years of a static display is permanent; there is no repair short of a donor tube.

Connector Care

[edit | edit source]
  • Keyboard cable — multi-pin proprietary connector; clean both halves and reseat.
  • P4, P5 — diskette AC connectors. Mains-carrying: inspect, do not improvise.
  • Y1 and E1 to E14 — the DC measurement points above.
  • Mains lead — replace if the insulation is cracked or hardened.

Capacitor Health

[edit | edit source]

IBM published no capacitance values for the 5120. Neither the maintenance manual nor the logic manual draws the inside of either supply board, and there is no 5120 parts catalogue. The values have to be read from the parts in the machine in front of you — starting with C4, whose markings are the single most valuable missing figure for this machine. The full discussion, including why a resonant capacitor must not be replaced with an electrolytic or with "something close", is on the IBM 5120 Capacitor Replacement Guide.

Things That Are Not Faults

[edit | edit source]
  • A 5120 that hums. Ferroresonant transformers are noisy by nature. A hum that has always been there is not evidence of anything. A hum that has changed, or that comes with heat or low rails, is.
  • +12 V reading low with the logic disconnected. See the tolerance table — and note that +12 V also feeds load resistor RL1, a deliberate minimum load.[1]
  • A long delay before the machine responds after a protection shutdown. That is the 60-second rule.
[edit | edit source]

See Recommended Tools for general workshop equipment.

References

[edit | edit source]
  1. โ†‘ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 IBM, IBM 5120 Maintenance Information Manual, SY34-0192-0, December 1979 — hosted on this wiki as IBM 5120 Maintenance Information Manual. Section 2 items 207 (AC box) and 208 (power supply locations, including PC boards A1 and A2, resonant capacitor C4, transformer T1, circuit breaker CB1 and fuses F2, F3 and F4); item 272 (Power — Y1 socket pin assignments and the DC rail tolerance table); items 273, 274 and 284 (AC, DC and internal diskette DC voltage distribution); and the "5110-3 Power Supply", "DC Power Distribution", "DC Voltage Tolerance" and "Reference Voltage" pages of the Theory section. The scan carries no text layer, so it has to be read by eye.
  2. โ†‘ 2.0 2.1 IBM, IBM 5120 Computing System Logic Manual, SY34-0193-0, December 1979 — hosted on this wiki as IBM 5120 Computing System Logic Manual. The DC power distribution sheets, which show the supply as "5110 Power Supply Board A2" and carry the rail distribution and cable net lists.
[edit | edit source]


Retrieved from "https://wiki.retrotechcollection.com/index.php?title=IBM_5120_Maintenance_Guide&oldid=12498"