DECpc 325 Troubleshooting Guide

Corrosion from a leaking CMOS battery on a DECpc 325sxLP system board (David Hall, zx.net.nz)

Fault-finding on the DECpc 325 (DECpc 325sxLP) using chapter 7 and appendix C of DEC's DECpc 320sxLP/325sxLP User's Guide (EK-DECSX-UG.003), with repair notes from surviving machines.

DEC's troubleshooting sequence

1. Try a reset: Ctrl+Alt+Del, then the reset button on the front panel.
2. If the fault remains, switch off, wait 5 seconds and switch on again.
3. Check for loose cables and connections.
4. Check the system and monitor indicator lights.
5. Read any power-up test message and correct the cause, then reset to run the test again.
6. Work through the fault tables below.

Setup error messages

Setup error messages (DEC Table 7.1)

Message	Meaning	Action
Clock chip lost power	The clock chip is not working	Replace the battery, then check all settings in Setup
CMOS checksum invalid	Configuration error	Check all settings in Setup
Incorrect configuration data in CMOS	Setup does not match the fitted components	Check all settings; if the error persists, call service
Memory size in CMOS invalid	Wrong base memory setting	Correct the base memory field in Setup
Disk C failed initialization	Incorrect fixed disk type	Correct the disk setting in Setup
Time or date in CMOS is invalid	Clock does not match Setup	Correct the date or time in Setup

"Clock chip lost power" on every start means the CMOS battery is flat or disconnected. Check the board around it for corrosion before fitting a new one; see DECpc 325 General Maintenance.

System unit faults

System unit troubleshooting (from DEC Table 7.2)

Problem	Possible cause	Action
No response at power-on	Not plugged in	Switch off, plug in, switch on
Power on but no picture	Monitor off, brightness and contrast turned down, or cable loose	Switch the monitor on, adjust the controls, reseat the cable
Hangs or shows a RAM error with an EtherWORKS adapter fitted	Two adapters at the same memory address	Move one adapter to another address
Will not start from the hard disk	No operating system, partition missing or unformatted, or drive jumpers wrong	Install the system, partition and format, check the drive jumpers and terminators
Will not start from a diskette	Drive switch set wrongly, no system files, empty drive, worn diskette	Check the switch and the diskette
Will not restart after power-off	Not off long enough	Leave it off for at least 5 seconds
No response to the keyboard	Keyboard locked by password, not connected, or plugged into the mouse port	Enter the keyboard password; plug the keyboard into the keyboard port

If the front power button does nothing, check the rubber sleeve between the button linkage and the switch inside the power supply. Hall found it perished on every machine he had seen, leaving the linkage out of line with the switch.

Locked out by the security settings

The User Disk readme warns never to disable booting from both drives and the serial port, because the machine can then only be recovered by DEC field service. Jumper J7 on the system board discharges the CMOS RAM: pins 2 and 3 to discharge, pins 1 and 2 for normal running.

POST beep codes

DEC's BIOS reports errors found before the screen works as beep codes: groups of beeps separated by pauses, so 1-2-1 is one beep, two beeps, one beep. Fatal system board errors beep and halt; non-fatal ones show a message and ask for F1. The code written to I/O port 80h identifies the failing area, including tests that run without a beep code.

Beep codes (DEC Table C.6)

Beep code	Port 80h	Fault
1-1-3	02h	CMOS write/read failure
1-1-4	03h	ROM BIOS checksum failure
1-2-1	04h	Programmable interval timer failure
1-2-2	05h	DMA initialisation failure
1-2-3	06h	DMA page register write/read failure
1-3-1	08h	RAM refresh verification failure
1-3-3	0Ah	First 64K RAM chip or data line failure, multi-bit
1-3-4	0Bh	First 64K RAM odd/even logic failure
1-4-1	0Ch	Address line failure, first 64K RAM
1-4-2	0Dh	Parity failure, first 64K RAM
2-1-1 to 2-4-4	10h to 1Fh	Bits 0 to 15 of the first 64K RAM, in order
3-1-1	20h	Slave DMA register failure
3-1-2	21h	Master DMA register failure
3-1-3	22h	Master interrupt mask register failure
3-1-4	23h	Slave interrupt mask register failure
3-2-4	27h	Keyboard controller test failure
3-3-4	2Bh	Screen initialisation failure
3-4-1	2Ch	Screen retrace test failure
4-2-1	34h	Timer tick interrupt
4-2-2	35h	Shutdown test
4-2-3	36h	Gate A20 failure
4-2-4	37h	Unexpected interrupt in protected mode
4-3-1	38h	RAM test or address failure above FFFFh
4-3-3	3Ah	Interval timer channel 2
4-3-4	3Bh	Time-of-day clock
4-4-1	3Ch	Serial port
4-4-2	3Dh	Parallel port
4-4-3	3Eh	Math coprocessor
Low-1-1-2	41h	System board select failure
Low-1-1-3	42h	Extended CMOS RAM failure

"Low" means a lower-pitched beep before the three bursts. In the 2-x-x group, 2-1-1 is bit 0 and 2-4-4 is bit 15; for example 2-1-4 is bit 3.

Riser card

The expansion slots are on a riser card. Hall found the tantalum capacitor C3 on one riser blown and replaced the riser with one from a second machine.

Diagnostics

The DECpc 325 User Disk carries DEC's configuration utilities and a set of diagnostic programs named for each subsystem: CPU_DIA, CPR_DIA, MEM_DIA, KBD_DIA, MOU_DIA, FDU_DIA, HDU_DIA, VID_DIA, SER_DIA and PAR_DIA. An image of the disk is on archive.org.

Related pages

• DECpc 325
• DECpc 325 General Maintenance
• Battery Explosion, Capacitor or Corrosion Damage
• Hard Drive Maintenance and Repair
• Floppy Drive Repair