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Deep technical pre-PC IBM page with verified sources (Bitsavers MIM/MAP, Wikipedia, IBM Archives) — honest gap disclosure where IBM did not publish per-board cap values
 
Rebuilt against IBM's own MIM SY31-0405-3 (hosted here). Removes the false 'linear PSU with transformer, bridge and series-pass regulators' - IBM calls it a transistor switching regulator - and with it the invented rail table, the invented 9-11 kV anode figure and the unsourced '13 bipolar gate arrays' PALM board; replaces them with IBM's five rails (+5/-5/+8.5/+12/-12), the loaded and unloaded tolerance table, the protection trip points, the 5-second restart rule, the +6 V meter-reference ch...
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This guide documents preventive maintenance for the '''[[IBM 5100]] Portable Computer''' (announced September 1975). The 5100 is the first commercial implementation of the [[SCAMP]] project and uses the IBM '''PALM''' (Put All Logic in Microcode) board-level processor, an integrated 5-inch CRT, an integrated keyboard, and a built-in DC300 cartridge tape drive. The machine is approaching 50 years old at the time of writing; all preventive maintenance is now restoration-class work.
This guide documents preventive maintenance for the '''[[IBM 5100]] Portable Computer''' (announced September 1975). All of it is restoration-class work: the machine is fifty years old, and every figure below comes from IBM's own ''Maintenance Information Manual'', which is hosted on this wiki.<ref name="mim5100">IBM, ''IBM 5100 Maintenance Information Manual'', SY31-0405-3, October 1979 &mdash; hosted on this wiki as [[IBM 5100 Maintenance Information Manual]]. Section 2 "Locations": 200 (functional units), 206 (power supply), 207 (AC box, old and new style), 209 (A1 board card locations and jumpers), 211 (covers), 212 (service position). Tape: 221 to 233, including the jackshaft housing (223), cartridge stops (224) and motor pulley (226) service checks and adjustments quoted here. Display: 240 (removal and raster adjustment), 241 (Z3 socket pin assignments). Keyboard: 250 to 255, including key module removal and flyplate replacement. Power: 270 (CE meter calibration check), 271 (power supply removal), 272 (Y1 socket pin assignments and the DC rail tolerance table), 273 (AC voltage distribution). Maintenance sections 501 to 595 name the individual cards. Section 4 "Theory" describes the supply as a transistor switching regulator and gives the protection trip points, the processor's 15.1 MHz oscillator and 66.2 ns clock, the four program levels and the 204K-byte tape cartridge capacity.</ref>


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


The IBM 5100 PSU is a '''linear''' (50/60 Hz transformer + bridge + linear regulator) supply with large bulk filter capacitors after the bridge rectifier. These capacitors hold a charge after power-off. Before any work inside the chassis:
'''The 5100's power supply is not a linear supply.''' IBM's maintenance manual describes the DC supply board as "a small, high power, high frequency '''transistor switching regulator''' (TSR) supply".<ref name="mim5100" /> That matters before anything comes apart: a switching supply has 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.


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


The internal 5-inch CRT carries '''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 deflection / flyback board.
# '''Mains in the AC box.''' The AC box carries live mains to the line filter, the terminal block, the fuse holder, the power on/off switch, the fan and the tape unit motor.<ref name="mim5100" /> Unplug at the wall &mdash; the machine's own switch does not isolate everything in that box.
# '''Stored charge in the DC supply.''' As above.
# '''The display.''' The display and control panel are fed '''+5 V and +12 V''' over the Z3 cable and have no mains connection of their own,<ref name="mim5100" /> but the tube's extra-high-tension supply is generated inside the display assembly. IBM's own manual carries the warning "the display unit contains high voltages; therefore, use extreme caution when making internal adjustments."<ref name="mim5100" /> Follow the [[CRT Discharge Procedure]] before working near the tube, and discharge a second time a few minutes later.


== Documentation Set ==
See also [[Battery Explosion, Capacitor or Corrosion Damage]] and [[Recommended Tools]].


The authoritative IBM service documentation set for the 5100 is preserved at [http://bitsavers.org/pdf/ibm/5100/ Bitsavers]:
== Documentation ==


* '''SY31-0405-3''' — IBM 5100 Maintenance Information Manual (MIM), October 1979. Comprehensive CE document covering theory of operation, PALM microcode appendix, diagnostic procedures.
The 5100 service set is hosted here:
* '''1608314''' — IBM 5100 Maintenance Analysis Procedures (MAP), March 1976. Per-symptom troubleshooting decision tree.
* '''SY31-0429-2''' — IBM 5100 Communications / Serial I/O Maintenance Information Manual, October 1976.
* '''S131-0599-3''' — IBM 5100 Portable Computer Parts Catalog, November 1976.


Download all of these for any serious 5100 restoration; the MIM and MAP are the foundation documents.
* [[IBM 5100 Maintenance Information Manual]] &mdash; SY31-0405-3, October 1979. Locations, service checks, adjustments, theory of operation, and the MAPs. This is the foundation document.
* [[IBM 5100 Portable Computer Parts Catalog]] &mdash; S131-0599-3, November 1976. IBM part numbers for every assembly, including the AC box capacitors and the tape drive motor run capacitor &mdash; but '''no electrical values for any of them'''.<ref name="parts5100">IBM, ''IBM 5100 Portable Computer Parts Catalog'', S131-0599-3, November 1976 &mdash; hosted on this wiki as [[IBM 5100 Portable Computer Parts Catalog]]. Figures 8 (tape drive assembly), 10 and 11 (AC box assembly, old and new style) and the power supply assembly listing. IBM part numbers only; no electrical values.</ref>


== Opening the System Unit ==
== What Is Inside, and Where ==


Tools: Philips #2 screwdriver, T15 / T20 Torx (some fasteners), anti-static strap, high-voltage probe.
IBM's functional-unit drawing (section 200) names the major assemblies: the Step and Run switches, the '''AC box''' (under the display), the '''display''', the alphameric and numeric keyboards, the '''power supply''', the '''A1 board''', the '''tape drive''' and the '''control panel'''.<ref name="mim5100" />


# Power off and unplug all cables (mains, BNC video out, communications adapter, printer).
The A1 board is a card cage, and the maintenance manual treats each card as a separate FRU with its own section. The card complement, by position:<ref name="mim5100" />
# Locate the cover fasteners on the rear and bottom of the chassis. The 5100 uses a clamshell construction — the upper shell lifts off after removing the rear screws.
# Lift the upper shell carefully — '''the 5-inch CRT and deflection board are mounted in the upper shell''' and connected to the planar by cables. Do not yank.
# Inside (from rear to front):
:: '''Power supply''' (left rear) — linear PSU with transformer, bridge, bulk caps, series-pass regulators on a heatsink.
:: '''PALM processor board''' (centre) — the board with the 13 bipolar gate arrays.
:: '''Executable ROS''' and '''Language ROS''' boards.
:: '''RAM (RWS) cards''' — 16 KB modules.
:: '''DC300 tape drive''' at the front.
:: '''Keyboard''' on the front panel — separate assembly with capacitive sensing electronics.


== Inspecting the Chassis ==
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:80%;"
|+'''IBM 5100 &mdash; A1 board cards'''
! Position !! Card !! MIM section
|-
| A2 || I/O cable driver card || 501
|-
| C2, C4 || Nonexecutable ROS cards || 510
|-
| D2, D4 || Nonexecutable ROS cards || 515
|-
| E2 || ROS adapter card || 520
|-
| F2 || Base I/O card || 526, 528
|-
| G2 || '''Controller card''' &mdash; the processor || 531, 535
|-
| H2, H4 || Executable ROS cards || 545
|-
| L2, L4 || Read/write storage cards || 557
|-
| N2, N4 || Read/write storage cards || 561
|-
| &mdash; || Tape control card || 570
|-
| &mdash; || Keyboard PC board || 575
|-
| &mdash; || Auxiliary tape adapter / control cards (5106) || 582, 584
|}


Inspection items (in order):
Knowing the card names matters because they are what the MAPs call out. '''IBM does not use the name "PALM" anywhere in the service documentation.''' Its glossary defines "controller" as "the microinstruction processor within the 5100 Portable Computer",<ref name="mim5100" /> and the keyboard section refers to "the processor card (G2)". The processor runs from a '''15.1 MHz oscillator''' producing '''66.2 ns''' clock pulses, and has four program levels, each with sixteen two-byte registers that live on the processor card and are addressable as the lowest 128 bytes of read/write storage.<ref name="mim5100" />


# '''PSU bulk filter capacitors''' — visual inspection for bulging tops, leaked electrolyte, or discolouration. Bulk caps on a 45+ year old linear supply are the highest-likelihood failure point.
There are jumpers on some A1 cards that select 5100 or 5110 operation &mdash; check them before condemning a card pulled from a donor machine.<ref name="mim5100" />
# '''Series-pass regulator transistors''' — heat-sink-mounted; check thermal paste; replace if dried out.
# '''Flyback / deflection board''' — inspect for cracked solder joints around the flyback transformer (vibration over 45 years).
# '''DC300 tape drive belt''' — urethane perishes; if the belt is gummy, it must be replaced before any tape work.
# '''Keyboard''' — IBM beam-spring or capacitive sensing depending on revision; foam pads may have degraded.
# '''CRT''' — inspect for cracked phosphor, focus drift, weak emission (a fully warmed-up screen that's still dim indicates cathode emission loss).


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


* Soft brush and low-pressure compressed air for the planar, PALM board, ROS / RWS cards, tape drive bay, and PSU vents.
The supply produces '''five''' rails: '''+5 V, &minus;5 V, +8.5 V, +12 V and &minus;12 V'''. The '''+8.5 V rail is the family signature''' &mdash; nothing in the later IBM PC family has it, and a supply that comes up on +5 V and +12 V but not +8.5 V leaves the machine dead with a healthy-looking logic rail.<ref name="mim5100" />
* '''Do not''' use compressed air on a leaking electrolytic — it spreads electrolyte.
* Wipe the keyboard with a slightly damp microfibre; remove keycaps with the IBM tool (not pulled by hand) to avoid breaking the stems.
* Use IPA on the DC300 head; '''no cotton-tipped swabs''' — cotton fibres in the head gap cause read errors. Use foam swabs.


== PSU Voltage Checks ==
Measure at the '''Y1''' power connector on the A1 board. IBM's tolerances:<ref name="mim5100" />


The 5100 uses a multi-rail linear supply. The MIM SY31-0405 documents the rails and their tolerances. Rail values (per MIM):
{| class="wikitable styled-table" style="width:80%; text-align:center;"
 
|+'''IBM 5100 &mdash; DC rail tolerances (IBM figures)'''
{| class="wikitable styled-table" style="width:100%; text-align:center;"
! Voltage !! Loaded !! Unloaded (Y1 disconnected)
|+'''IBM 5100 PSU rail tolerances (typical linear PSU)'''
|-
! Rail !! Approximate target !! Notes
| +5 Vdc || 4.6 to 5.5 || 5.5 to 6.5
|-
|-
| +5 V (logic) || +4.75 V to +5.25 V || PALM logic and ROS / RWS
| +8.5 Vdc || 7.9 to 9.35 || 7.4 to 9.0
|-
|-
| +12 V || +11.4 V to +12.6 V || Tape drive motor, CRT deflection
| +12 Vdc || 11.0 to 13.2 || 9.8 to 12.2
|-
|-
| &minus;5 V / &minus;12 V || As marked || Bias rails for PALM and CRT
| &minus;5 Vdc || &minus;4.6 to &minus;5.5 || &minus;3.7 to &minus;4.7
|-
|-
| CRT high-voltage anode || ~9–11 kV (5-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 exact per-rail values are documented in the MIM. If rails are out of tolerance after recapping the bulk filter caps, suspect a series-pass regulator transistor (germanium or early silicon) — replace with modern silicon equivalent.
The unloaded column is not simply "a bit high". With Y1 disconnected, +5 V rises out of its loaded band while +12 V, &minus;5 V and &minus;12 V all '''fall'''. Bench-testing the supply off-load and condemning it because +12 V reads 10.5 V is a mistake this table prevents. The Y1 pin assignments are reproduced on the [[IBM 5100 Capacitor Replacement Guide#Y1 socket pin assignments|capacitor guide]].


== DC300 Tape Drive Maintenance ==
=== Check the meter first ===


The DC300 1/4-inch cartridge tape drive is the 5100's primary storage. Each cartridge holds 204 KB. Maintenance items:
The 5100 carries its own voltage reference so a service meter can be checked against it. Ground at '''J2-D08''' and measure at '''J2-S02''': a zener provides '''+6 Vdc'''. If the meter does not read exactly +6 V, correct every subsequent reading by


* '''Drive belt''' — urethane perishes after 30+ years. Replace with a modern QIC-class belt (the QIC community still supplies these).
: actual voltage = 6 &times; measured voltage &divide; reference voltage
* '''Capstan rubber''' — replace if glassy or cracked. Use isopropyl alcohol to clean glassy surfaces as a first attempt; replace if cleaning does not restore grip.
* '''Read / write head''' — clean with IPA on a foam swab.
* '''Head alignment''' — verify against a known-good tape; the MIM documents the alignment procedure with test cartridges.
* '''Erase head''' — clean as for read / write head.


== ROS and RWS Module Care ==
IBM's worked example: with the reference reading 5.8 V and the +5 V rail reading 4.8 V, the rail is actually 4.97 V.<ref name="mim5100" /> On a fifty-year-old machine being assessed with a cheap meter, do this before anything else.


The Executable ROS and Language ROS are large MOS ROM modules. They are static-sensitive and pin-fragile:
=== Protection, and what a shutdown means ===


* Always wear an anti-static strap when handling ROS or RWS cards.
The supply protects itself rather than blowing fuses on its outputs:<ref name="mim5100" />
* Do not flex the ROS module — they crack along the bond lines.
* Reseat ROS cards in their sockets if intermittent — oxidised socket pins are a known cause of garbled language banners.


== PALM Board Care ==
* '''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 PALM board carries '''13 bipolar gate arrays in metal cans'''. These devices are '''unobtainium''' — there are no aftermarket or NOS replacements outside donor 5100s. Care points:
'''After any automatic shutdown, leave the machine off for at least five seconds before switching on again.'''<ref name="mim5100" /> That is IBM's instruction, not a rule of thumb; a faster cycle leaves the supply latched off and sends you chasing a fault that is not there. A shorted decoupling capacitor on a card presents as an overcurrent trip, not a blown fuse, so pull cards one at a time with a five-second wait between attempts.


* Never apply mechanical force to the gate arrays.
== Opening the Machine ==
* Reseat the PALM board in its connector if the system fails to POST — oxidised pins on the PALM board edge connector are a common cause.
* If a PALM gate array has failed (diagnostic ROS reports specific check-stop), the only repair is a donor board.


== CRT Maintenance ==
Tools: a Philips screwdriver, an anti-static strap, a meter and a high-voltage probe. The manual's covers section (211) and service position (212) give the sequence; the machine has a defined service position, and using it is easier than balancing the assembly on a bench.<ref name="mim5100" />


The 5-inch internal CRT and its deflection / flyback board are mounted in the upper shell:
Power off and unplug at the wall before starting, and remember that the AC box sits '''under the display''' &mdash; mains wiring is in the part of the machine you lift first.<ref name="mim5100" />


* '''Anode discharge''' — always discharge to chassis ground via HV probe before any work.
== Regular Cleaning ==
* '''Flyback transformer''' — visible cracking in the insulation indicates impending arc-over; flyback replacement requires a donor.
 
* '''Deflection coils''' — inspect for cracked solder joints at the yoke connector.
* Soft brush and low-pressure compressed air on the A1 board, the cards, the tape drive bay and the supply vents.
* '''Phosphor''' — if the phosphor is burned-in with a static image, the CRT has been operated for too long with a fixed display; replacement requires a donor CRT.
* '''Do not''' use compressed air on a leaking electrolytic &mdash; it spreads electrolyte.
* Clean the tape head with isopropyl alcohol on a '''foam''' swab. Never cotton: stray fibres in the head gap cause read errors.
* Reseat cards rather than scrubbing edge connectors. Oxidised card edges are a common cause of intermittents on this machine.
 
== Tape Drive ==
 
The 5100's tape drive is its primary storage, and it is a belt-driven mechanism with a jackshaft and a capstan &mdash; so it has consumable parts. IBM's own figures:<ref name="mim5100" />
 
* '''Cartridge capacity: 204K bytes.''' Every cartridge holds the same amount, and each new file needs 0.5K bytes for its header. A customer complaint that "tapes have different capacity" is a file-header arithmetic problem, not a drive fault.
* '''Motor pulley position''' &mdash; 0.063 inch &plusmn; 0.010 inch (1.57 mm &plusmn; 0.25 mm) from the motor mounting bracket as an initial setting, then adjusted with power on until the '''belt tracks in the centre of the pulley''' (section 226).
* '''Jackshaft housing service check''' &mdash; measure the gap between the spindle and the right jackshaft roll; the difference between the two measurements must be '''less than 0.003 inch (0.076 mm)''' or the housing needs adjusting (section 223).
* '''Cartridge stops''' &mdash; adjust if there is significant rocking action, or if the gap between the gauge and the spindle is '''greater than 0.005 inch (0.127 mm)''' (section 224).
* '''Noisy drive''' &mdash; IBM's own diagnosis is the spindle or jackshaft bearings. Replace as required.<ref name="mim5100" />
 
Other service checks in the same block: the cartridge-in-place and file-protect switch assembly (221), the forward and reverse select magnets (222), the locking wheel assembly (225), tape path and positioning including manual tape positioning and loose-tape rewind (229), tape rethreading (230), read/write head removal, replacement and cleaning (231), the LED-PTX assembly (232) and spindle roll cleaning (233).<ref name="mim5100" />
 
A perished belt is the single most likely reason a mechanically intact 5100 will not read tape. Replace it before spending time on the electronics.
 
== Keyboard ==
 
The 5100 keyboard is a '''capacitive''' design using '''flyplates''', not a contact keyboard, and the maintenance manual gives removal, disassembly and cleaning procedures for it along with spacebar, key module and flyplate replacement (sections 251 to 255).<ref name="mim5100" /> A dead or unreliable key is usually the flyplate, and IBM documents replacing it as a field operation.
 
Remove keycaps with the proper tool rather than pulling by hand, and wipe the assembly with a slightly damp lint-free cloth rather than flooding it.
 
== Display ==
 
Two different display units were fitted, and '''both carry the same IBM part number'''. One has a horizontal linearity sleeve and a horizontal linearity adjustment potentiometer; the other has neither.<ref name="mim5100" /> Do not conclude that a set is missing a part because there is no linearity pot.
 
The raster adjustments are on the display PC board and are labelled there. IBM's brightness procedure:<ref name="mim5100" />
 
# Centre the BRIGHTNESS control on the control panel.
# Press the top of the '''REVERSE DISPLAY''' switch to select a dark background.
# Adjust the brightness potentiometer on the display PC board until the white retrace lines '''just''' disappear.
 
Focus is a separate potentiometer on the same board. Centring, horizontal and vertical adjustments are covered in section 240. The Z3 socket pin assignments (241) tell you what should be arriving at the display before you open it.
 
The switches around the display &mdash; L32-64-R32 (242), Display Registers (243), BASIC-APL on Model C (244), Reverse Display (245), Restart and Lamp Test (246) and the Brightness control (247) &mdash; all have service checks of their own. A "faulty display" is often a faulty switch.<ref name="mim5100" />


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


* '''BNC video output''' (rear) — clean with deoxidising contact cleaner.
* '''Z1''' &mdash; communications / serial I/O adapter socket (section 280).
* '''Communications Adapter''' connector (if fitted) — DB-25 or proprietary; clean both sides.
* '''Z3''' &mdash; display and control panel cable (sections 210, 241).
* '''Serial I/O Adapter''' connector (if fitted) — RS-232 DB-25.
* '''Y1''' &mdash; power connector on the A1 board (section 272).
* '''Mains lead''' — replace if the insulation is cracked.
* '''J2''' &mdash; carries the +6 Vdc meter reference at S02 and ground at D08.
* Mains lead &mdash; replace if the insulation is cracked or hardened.
 
== Capacitor Health ==
 
IBM published '''no capacitance values''' for this machine: the maintenance manual treats the DC supply as one FRU and never draws its circuit, and the parts catalogue lists capacitors by IBM part number only.<ref name="mim5100" /><ref name="parts5100" /> The parts that '''are''' identified &mdash; the AC box capacitors and the tape drive motor run capacitor &mdash; are mains-rated components and are the ones worth replacing on sight. The full discussion, with the IBM part numbers and the method for recovering the values IBM never published, is on the [[IBM 5100 Capacitor Replacement Guide]].


== 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 AC RMS and 10 kV-rated probe for CRT anode.
* Digital multimeter &mdash; and use the +6 V reference above to check it.
* High-voltage probe for CRT discharge.
* High-voltage probe for the display.
* IPA + foam swabs.
* Feeler gauges (for the tape drive service checks).
* Soldering iron (fine tip) + solder wick.
* Isopropyl alcohol and foam swabs. No cotton buds.
* DC300 cartridge head-cleaning kit.
* Fine-tip soldering iron and solder wick.
* Modern QIC-class replacement belt for the DC300 drive.
* A replacement drive belt for the tape unit.
* Anti-static field service kit (for ROS / RWS handling).
* [[IBM 5100 Maintenance Information Manual]] and [[IBM 5100 Portable Computer Parts Catalog]].
* MIM SY31-0405 PDF.


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


* [[IBM 5100]]
<references />
* [[IBM 5100 Troubleshooting Guide]]
* [[IBM 5100 Capacitor Replacement Guide]]
* [[Recommended Tools]]


== References ==
== Related Pages ==


* [http://bitsavers.org/pdf/ibm/5100/ Bitsavers — IBM 5100 documents]. Authoritative IBM service documentation.
* [[IBM 5100]] &middot; [[IBM 5100 Troubleshooting Guide]] &middot; [[IBM 5100 Capacitor Replacement Guide]]
* [http://bitsavers.org/pdf/ibm/5100/SA21-9215-0_IBM_5100_Communications_Reference_Manual_Sep1975.pdf IBM 5100 Communications Reference Manual SA21-9215-0].
* [[IBM 5110 Maintenance Guide]] &middot; [[IBM 5120 Maintenance Guide]] &mdash; sibling machines, same processor and supply family
* [http://bitsavers.org/pdf/ibm/5100/SA21-9239-1_IBM_5100_Serial_IO_Adapter_Feature_Users_Manual_Jan1977.pdf IBM 5100 Serial I/O Adapter User's Manual SA21-9239-1].
* [[IBM 5100 Maintenance Information Manual]] &middot; [[IBM 5100 Portable Computer Parts Catalog]]
* [https://en.wikipedia.org/wiki/IBM_5100 IBM 5100 — Wikipedia].
* [[CRT Discharge Procedure]] &middot; [[Capacitor Failure Symptoms]] &middot; [[Recommended Tools]]
* [https://en.wikipedia.org/wiki/IBM_PALM_processor IBM PALM processor — Wikipedia].
* [http://dunfield.classiccmp.org/ibm5100/index.htm Dave Dunfield's IBM 5100 collection page]. Community restoration notes.


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