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DECmate I Capacitor Replacement Guide

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DECmate I (VT278)

The tables below list the electrolytic and tantalum capacitors in the DECmate I (VT278). Every value comes from DEC's parts lists in the VT278 field maintenance print set, MP-00900 (May 1981).[1] The aluminium electrolytics are on the monitor board, the keyboard and the optional DP278 communications board. The M8436 CPU module and the I/O connector board use solid tantalum capacitors only.

The VT278 contains a CRT and a mains switching power supply. Read CRT Discharge Procedure before removing the monitor board, and see Capacitor Failure Symptoms for what failed parts look like.

Monitor board (5414131)

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The VT278 uses the board DEC draws as the VT100 monitor board.[2] It holds ten aluminium electrolytics.

Monitor board electrolytic and tantalum capacitors
Reference Value Rating Type DEC part
C102, C103 22 µF 50 V Aluminium electrolytic 1016555-00
C106, C434, C440 470 µF 16 V Aluminium electrolytic 1016557-00
C312, C437 4.7 µF 25 V Aluminium electrolytic 1016553-00
C321 1000 µF 16 V Aluminium electrolytic 1016548-00
C439 75 µF 6 V Aluminium electrolytic 1016554-00
C441 10 µF 25 V Aluminium electrolytic 1016558-00
C318, C319 4.7 µF 35 V Solid tantalum 1016496-00
C315 1 µF 35 V Solid tantalum 1001776-00

[2]

C439 is the part most often named in VT100 monitor repairs. Randy Hick's notes describe it as a very common failure, usually with CR408 burnt as well, giving a blank white raster and a ballooned display. He fits a 75 µF 16 V part in place of DEC's 6 V one.[3] The same notes link C103 to white lines on the left and misshapen characters, C441 to thin letters on the right, and C437 to an overlapped, narrow display. See DECmate I Troubleshooting Guide.[3]

Keyboard (5413404)

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Keyboard electrolytic and tantalum capacitors
Reference Value Rating Type DEC part
C5 25 µF 25 V Aluminium electrolytic 1000075-00
C8 6.8 µF 35 V Solid tantalum 1005306-00

[4]

CPU module (M8436)

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M8436 tantalum capacitors
Reference Value Rating Type DEC part
C13, C14, C15, C16 6.8 µF 35 V Solid tantalum 1005306-00

[5]

DEC's parts list notes that this item may be replaced by part 1012084-01, an 8 µF 25 V aluminium electrolytic, so a board may carry either type in these positions.[5] The other M8436 capacitors are mica and ceramic.[5]

I/O connector board (5414322)

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C1, C3 and C5 are 6.8 µF 35 V solid tantalums (DEC 1005306-00). C2, C4 and C6 are 0.22 µF ceramics and C7 is a 120 pF mica.[6]

DP278 communications option (M8437)

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The DP278 board is an option and is not fitted to every machine.[7]

M8437 electrolytic and tantalum capacitors
Reference Value Rating Type DEC part
C56, C57, C58, C59 8 µF 25 V Aluminium electrolytic 1012084-01
C55 330 µF 6 V Solid tantalum 1002478-00

[8]

Power supply (H7836)

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The VT278 runs from an H7836 supply: H7836-A for 120 V and H7836-B for 220 V, rated +5 V at 2.5 A, +12 V at 2 A and −12 V at 0.25 A.[9] The print set's contents list a switching power supply schematic and parts list, but those sheets are missing from the bitsavers scan, which ends with the supply's assembly drawing. Read values and voltage ratings from the parts on the board.[9]

The H7836 is a mains switching supply with high voltage on its primary side. Unplug the machine and allow the input capacitors to discharge before working on it, and check them with a meter before touching the board.

Procedure

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  1. Unplug the VT278 and the RX02 drive unit. Discharge the CRT before removing the monitor board.
  2. Photograph each board and mark the polarity of every electrolytic before removing it. Solid tantalums are polarised too.
  3. Replace like for like in capacitance. Use a voltage rating equal to or higher than DEC's, and a 105 °C part where it fits.
  4. On the monitor board, inspect CR408 when replacing C439, since the two fail together.[3]
  5. After work on the supply, measure its outputs with the boards connected: +5 V, +12 V and −12 V.[9]
  6. Power up and watch the self-test. PWR OK should light, CPU OK should blink once a second, and SET-UP should appear with only ON LINE lit on the keyboard.[10]
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References

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  1. ↑ Digital Equipment Corporation, VT278 field maintenance print set, MP-00900, May 1981 (File:DECmate VT278 Field Maintenance Print Set (MP00900).pdf).
  2. ↑ 2.0 2.1 DEC, print set MP-00900, VT100 monitor parts list 5414131-0-DBP, sheets A1 and A2 (PDF pp. 70 to 71).
  3. ↑ 3.0 3.1 3.2 Randy Hick, "VT100 trouble analysis of video monitor board", text file hosted at bitsavers.org. Community repair notes.
  4. ↑ DEC, print set MP-00900, keyboard parts list 5413404-0-DBP, sheet A1 (PDF p. 64).
  5. ↑ 5.0 5.1 5.2 DEC, print set MP-00900, VT278 CPU module parts list M8436-0-DBP rev. D, sheets A1 and A4 (PDF pp. 27 and 30). Line 5 (C13 to C16) and note 141.
  6. ↑ DEC, print set MP-00900, I/O connector board parts list 5414322-0-DBP (PDF p. 57).
  7. ↑ Digital Equipment Corporation, DECmate Technical Description, EK-VT278-TD-001, March 1981, pp. 1-3 to 1-4 (File:DECmate Technical Description (EK-VT278-TD-001).pdf). DP278 and RL278 are single-board options.
  8. ↑ DEC, print set MP-00900, communications adapter parts list M8437-0-DBP, sheet A1 (PDF p. 45).
  9. ↑ 9.0 9.1 9.2 DEC, print set MP-00900: drawing contents (PDF p. 1), terminal sub-assembly parts list giving the H7836-A and H7836-B ratings (PDF p. 23), power supply assembly drawing (PDF p. 74).
  10. ↑ Digital Equipment Corporation, DECmate System Architecture Manual, EK-VT278-HR-001, June 1981, pp. 3-4 to 3-5 (File:DECmate System Architecture Manual (EK-VT278-HR-001).pdf). PWR OK, CPU OK and a passing self-test.
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