DECpc 325 Capacitor Replacement Guide

DECpc 325sxLP system board in its case. The capacitors on it are small epoxy-dipped parts; there are no aluminium electrolytic cans (David Hall, zx.net.nz)

Capacitor notes for the DECpc 325 (DECpc 325sxLP). DEC published no schematic or parts list for its system board, riser card or 115 W power supply. The DECpc 320sxLP/325sxLP Technical Reference Manual (EK-DECSX-TM-001) gives the supply's input and output specifications and a block layout of the system board, without component values, so the values have to be read from the parts themselves. David Hall's notes on his two machines record one capacitor failure: a tantalum capacitor, C3, on the riser card.

DEC's power supply specification

Chapter 9 of the technical reference covers the supply in two pages. It gives no circuit, only the limits the outputs must meet:

115 W power supply output specification (DEC Table 9-1)

Output	Minimum	Typical	Maximum	Load range	Ripple and noise, maximum
+5 V	4.80 V	5.00 V	5.25 V	1.0 to 12.0 A	50 mV
+12 V	11.40 V	12.00 V	12.60 V	40 mA to 4.2 A	100 mV
−12 V	−11.40 V	−12.00 V	−12.60 V	0 to 0.3 A	150 mV
−5 V	−4.75 V	−5.00 V	−5.25 V	0 to 0.3 A	100 mV

The hold-up time is 20 ms minimum. DEC's notes to the table say to measure ripple and noise with a 20 MHz bandwidth oscilloscope and with 0.1 µF and 10 µF capacitors added across the output connector terminals, and that the +12 V output must withstand a 5.0 A surge for 15 seconds, during which its regulation may be +7% or −6%.

On the input side, DEC gives 88 to 132 V on the 100–120 V setting and 176 to 264 V on the 220–240 V setting, 47 to 63 Hz, an input current of 3 A or 1.5 A, and an inrush current of up to 45 A. The current drawn by the system board with 8 MB of memory is 1.68 A at +5 V, 0.1 A at +12 V and 0.1 A at −12 V; DEC allows each expansion slot 2 A at +5 V.

System board

The board in Hall's photograph is marked DEC-386SX-E, revision A01, © 1991 Digital Equipment Corporation. Every capacitor visible on it is a small epoxy-dipped radial part in yellow or orange, and none is an aluminium electrolytic can. The larger teardrop-shaped parts, such as C53 beside transistor Q1 next to the J9 to J11 video jumpers, have the shape of dipped tantalum capacitors; the rows of small flat parts beside the serial and parallel connectors look like ceramic capacitors. Their markings cannot be read in the photograph, so read each one on your own board. DEC's layout drawing, Figure 3-1, shows the SIMM sockets, chips, jumpers, battery, connectors and the expansion slot that carries the riser, but no capacitors.

KEMET's engineers give the usual failure of a solid tantalum capacitor as low insulation resistance or a short circuit. On parts that failed short they found black marks on the case where the epoxy had charred from the heat at the fault. Check the board's tantalums for cracks or charring, then measure across each one with the power off. A reading close to 0 Ω marks a shorted part, although another part on the same rail can give a low reading too, so lift one leg to confirm before replacing it.

Riser card

The system board has one 16-bit ISA expansion slot; the riser card in it carries the machine's three ISA slots. In September 2021 the tantalum capacitor C3 on the riser of Hall's working machine blew. In January 2024 he fitted the riser from his second machine in its place. Hall does not give C3's value. If the marking on a failed part has burnt off, read it from C3 on another riser before ordering a replacement, and fit the new part with its positive lead where the old one's was.

Power supply

The supply runs from the mains. Its power switch is inside it and is worked from the front button by a linkage. DEC's manuals give neither the supply's maker nor a parts list for it. Unplug the mains lead before opening it and check with a meter that the large primary-side capacitors have discharged before touching anything.

Record the capacitors as you find them. Replace electrolytics like for like in capacitance, at the same voltage rating or higher. See Capacitor Failure Symptoms for what failed electrolytics look like.

Reading and recording your own parts

1. Unplug all power cords and cables. Unlock the chassis lock, remove the four retaining screws, two on each side, and slide the cover towards the rear.
2. Remove any adapter cards from the riser and photograph both the system board and the riser at high resolution before removing anything.
3. For each capacitor, record the reference (C3, C53 and so on), the capacitance, the voltage rating, the type (aluminium electrolytic, tantalum or ceramic) and the polarity mark. For aluminium electrolytics in the supply, record the series and temperature rating too.
4. Measure each tantalum for a short as described above.
5. Refit the riser and any cards, then run the checks below before refitting the cover.

Checks after any capacitor work

• Measure each output with the system board connected. DEC specifies the outputs only within the load ranges in Table 9-1, which start at 1.0 A on +5 V and 40 mA on +12 V, so the supply should not be judged on its own with no load.
• Compare each rail with the minimum and maximum in Table 9-1.
• If you have an oscilloscope, measure ripple and noise DEC's way: 20 MHz bandwidth, 0.1 µF and 10 µF capacitors across the output terminals, and compare with the last column of the table.
• Start the machine and watch for the power-on self-test beep codes listed in the DECpc 325 Troubleshooting Guide.

Related pages

• DECpc 325
• DECpc 325 General Maintenance
• DECpc 325 Troubleshooting Guide
• DECpc 325 Documentation
• Capacitor Failure Symptoms
• Battery Explosion, Capacitor or Corrosion Damage