Josh: New page from DEC K-MN-SD00000-21-JG00.A service manual, CPU booklets and David Hall's zx.net.nz notes

2026-10-01T23:29:45Z

New page from DEC K-MN-SD00000-21-JG00.A service manual, CPU booklets and David Hall's zx.net.nz notes

New page

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[[File:DECpc LP interior (zx.net.nz).jpg|thumb|right|340px|Inside a DECpc LP: drives at the top, power supply at lower left, main logic board, riser and CPU module on the right (David Hall, zx.net.nz)]]
Opening, battery, memory and configuration for the '''[[DECpc LP]]''' range (333sxLP to 466d2LP), from DEC's ''DECpc LP PC Service Maintenance Manual'' (K-MN-SD00000-21-JG00.A), its CPU module booklets, and David Hall's notes on surviving machines.<ref name="smm" /><ref name="cpu743" /><ref name="zx" />

== Safety ==
DEC's service manual requires an antistatic wrist strap when handling boards, and warns that removing the cover before unplugging the mains and monitor cords can injure you or damage the computer.<ref name="smm23" /><ref name="smm26" /> Its tool list is a 4½-digit multimeter, a Phillips screwdriver and the wrist strap.<ref name="smm23" />

== Opening the case ==
# Switch off all attached devices and the computer.
# Unplug the mains cord from the wall, then the mains and monitor cords from the computer.
# Unlock the outside cover.
# Remove the four retaining screws at the rear.
# Slide the cover back until it clears the lip of the front bezel, then lift it off.
<ref name="smm26" />

== Clock battery ==
DECpc LP machines shipped with a Rayovac 840 4.5 V alkaline battery fixed with Velcro next to the board's power connector. Hall reports that it leaks and causes serious damage, and advises removing it as soon as the case is open.<ref name="zx" /> Clean any leakage before fitting a new battery; see [[Battery Explosion, Capacitor or Corrosion Damage]].

DEC's replacement procedure:<ref name="smm40" />
# Record the configuration settings from BIOS Setup.
# Switch off, unlock and remove the cover.
# Unplug the battery cable from the header marked BATTERY CONN on the main logic board and take the battery off its mounting pad.
# Plug in the new battery, refit and lock the cover.
# Run BIOS Setup and restore the settings.

Hall's notes on the circuit: the battery plugs into J12, positive pin nearest the floppy connector. It feeds the VCC pin of the 82C206 peripheral controller through a diode on the back of the board that drops about 0.56 V, and the 82C206 accepts up to 6.7 V. Because of the diode a reversed battery does no harm but does not work. Hall replaced the original with three lithium AA cells, about 5.4 V open circuit, giving about 4.8 V after the diode.<ref name="zx" /> For building a pack, see [[Battery Refurbishment]].

== Clearing CMOS and other jumpers ==
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''DECpc 3xx/4xx LP main logic board jumpers (DEC service manual)'''
! Jumper !! Function !! Settings
|-
| J1 || On-board VGA IRQ9 || Fitted: enabled. Open: disabled
|-
| J2 || On-board VGA || Fitted: enabled. Open: disabled
|-
| J3 || CMOS memory || 1–2: normal. 2–3: clear
|-
| J4 || Reset switch || Fitted: enabled. Open: disabled
|-
| J5 || Parallel port || 1–2: printer. 2–3: bidirectional
|-
| J10 || Factory test || Open: normal. Fitted: manufacturing test mode
|-
| J11 || Video display type || Fitted: colour monitor. Open: mono
|-
| J28 || VRAM size || 1–2: 512 KB. 2–3: 1 MB
|-
| J29, J30, J31 || Reserved || Open
|-
| J35 || VGA I/O write || 1–2: local bus. 2–3: local and ISA
|}
<ref name="smm28" />
Pin 1 of each jumper is the square pin.<ref name="smm28" />

The 486 CPU modules carry their own jumpers for the vacancy socket, cache size, CPU clock and CPU type. DEC's service manual table for the 450d2LP and 466d2LP lists J103 under both the vacancy socket and the cache size, and gives the same clock settings for 25 MHz and 33 MHz. The 433dxLP CPU booklet gives the vacancy socket jumpers as J101, J102 and J113, the cache size as J103 and J104, and a separate clock jumper for each speed (J109 25 MHz, J110 33 MHz, J111 40 MHz, J112 50 MHz). On the 433dxLP module the factory setting is J110 fitted for 33 MHz. Set the jumpers from the booklet that came with your module; the service manual table contains errors.<ref name="smm30" /><ref name="cpu743" />

== Memory ==
DEC's rules: 36-bit SIMMs of 70 ns or faster, 4 MB or 16 MB only, bank 0 filled before bank 1 starting from the leftmost free socket seen from the front, and only the combinations in DEC's table, up to 64 MB.<ref name="smm32" /> Hall adds that the machines are fussy and likely work only with fast-page-mode SIMMs.<ref name="zx" />

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''Permitted memory configurations (DEC service manual)'''
! Bank 0 !! Bank 1 !! Bank 2 !! Bank 3 !! Total
|-
| 4 MB || – || – || – || 4 MB
|-
| 4 MB || 4 MB || – || – || 8 MB
|-
| 4 MB || 4 MB || 4 MB || – || 12 MB
|-
| 4 MB || 4 MB || 4 MB || 4 MB || 16 MB
|-
| 4 MB || 4 MB || 16 MB || – || 24 MB
|-
| 4 MB || 4 MB || 16 MB || 16 MB || 40 MB
|-
| 16 MB || – || – || – || 16 MB
|-
| 16 MB || 16 MB || – || – || 32 MB
|-
| 16 MB || 16 MB || 16 MB || – || 48 MB
|-
| 16 MB || 16 MB || 16 MB || 16 MB || 64 MB
|}
<ref name="smm32" />

== Drives ==
The machines ship with a 3.5-inch diskette drive as drive A. The diskette cable has four connectors: 1 for a 3.5-inch drive A, 2 for a 5.25-inch drive A, 3 for a 3.5-inch drive B and 4 for a 5.25-inch drive B.<ref name="smm41" /> The BIOS has no automatic detection or LBA support; user-defined types 48 and 49 take manual cylinder, head and sector values, and Hall suggests an XT-IDE ROM for larger disks or CompactFlash.<ref name="zx" /> Run BIOS Setup after adding or changing any diskette or IDE drive.<ref name="smm41" /> See [[Hard Drive Maintenance and Repair]] and [[Floppy Drive Repair]].

== Power switch ==
The front button works the switch inside the power supply through a linkage held to the switch by a short rubber tube, which disintegrates with age. See [[DECpc LP Power Supply Service]].<ref name="zx" />

== After service ==
DEC's acceptance test is a clean POST, one pass of the QAPlus/FE system board, memory, video, hard disk (non-destructive tests only), floppy, keyboard, serial, parallel and pointing device tests, and a successful boot of the installed operating system, followed by cleaning the outside of the case.<ref name="smm57" />

== Related pages ==
* [[DECpc LP]]
* [[DECpc LP Troubleshooting Guide]]
* [[DECpc LP Power Supply Service]]
* [[DECpc LP Documentation]]

== References ==
<references>
<ref name="smm">Digital Equipment Corporation, ''DECpc LP PC Service Maintenance Manual'', K-MN-SD00000-21-JG00.A, March 1996 ([[:File:DECpc LP Service Maintenance Manual (K-MN-SD00000-21-JG00.A).pdf]]).</ref>
<ref name="smm23">DEC, K-MN-SD00000-21-JG00.A, pp. 23–24. Safety requirements and recommended tools.</ref>
<ref name="smm26">DEC, K-MN-SD00000-21-JG00.A, p. 26. Unlocking and removing the cover.</ref>
<ref name="smm28">DEC, K-MN-SD00000-21-JG00.A, pp. 28–29. DECpc 3xx/4xx LP main logic board jumper settings and locations.</ref>
<ref name="smm30">DEC, K-MN-SD00000-21-JG00.A, pp. 30–31. DECpc 450D2LP/466D2LP CPU board jumper settings.</ref>
<ref name="smm32">DEC, K-MN-SD00000-21-JG00.A, pp. 32–33. Memory configurations and SIMM sockets.</ref>
<ref name="smm40">DEC, K-MN-SD00000-21-JG00.A, p. 40. Replacing the computer battery.</ref>
<ref name="smm41">DEC, K-MN-SD00000-21-JG00.A, p. 41. Connecting mass storage devices; diskette drive cabling.</ref>
<ref name="smm57">DEC, K-MN-SD00000-21-JG00.A, p. 57. Pass/fail criteria.</ref>
<ref name="cpu743">Digital Equipment Corporation, ''DECpc 433dxLP CPU Specifications'', ER-PC743-CA.A02, August 1992, p. 2, Table 1 ([[:File:DECpc 433dxLP CPU Specifications (ER-PC743-CA.A02).pdf]]).</ref>
<ref name="zx">David Hall, "Digital DECpc 340dx LP/425sx LP/433dx LP/450d2 LP/466d2 LP", ''zxnet'', last modified 2 February 2024, [https://www.zx.net.nz/computers/dec/decpc-lp/ zx.net.nz]. Battery, memory, hard disk and power switch notes.</ref>
</references>

[[Category:Digital Equipment Corporation]]
[[Category:Maintenance Guides]]

DECpc LP General Maintenance - Revision history

Josh: New page from DEC K-MN-SD00000-21-JG00.A service manual, CPU booklets and David Hall's zx.net.nz notes