Editing DECmate III Troubleshooting Guide

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[[File:DECmate III (Centre for Computing History).jpg|thumb|right|300px|DECmate III system unit (Centre for Computing History)]]
Fault-finding on the '''[[DECmate III]]''' works module by module: the H7845 power supply, the 54-16489 system board, the RX50 drive, and the VR201 monitor and keyboard.<ref name="mp7" /> DEC's ''DECmate III Owner's Manual'' (EK-DM3BU-OM) and ''DECmate III Pocket Service Guide'' (EK-DM3BU-PS-002) are catalogued on Manx, but no scan of either is online.<ref name="manx" /> The PDP-8 FAQ describes the DECmate III as using the same technology as the [[DECmate II]], whose self-test codes and checks are on [[DECmate II Troubleshooting Guide]]. No DEC document online confirms that the DECmate III reports the same codes.<ref name="faq" />

The VR201 monitor contains a CRT and the system unit has a mains switching supply. Read [[CRT Discharge Procedure]] before opening the monitor.

== Before you start ==
# Check which supply is fitted. DEC built the DECmate III with an H7845-AA for 120 V or an H7845-AB for 240 V, so a machine moved between countries may have the wrong one.<ref name="mp39" />
# Check the monitor and keyboard cables. On the DECmate II the keyboard plugs into the VR201 monitor, not the system unit.<ref name="dm2spec" />
# Listen for the fan and the RX50 at power-on.

== Power checks ==
The system board takes power through J2. Measure with the board connected and the supply running.<ref name="mp23" />

{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+ '''DECmate III system board power connector J2'''
! Pin !! Supply
|-
| 8 || +12 V
|-
| 7 || +5 V
|-
| 5 || −12 V
|-
| 1, 2, 3, 4 || Ground
|}
<ref name="mp23" />

The DECmate III print set gives no tolerances for these rails. C31 and C50, 10 µF 20 V tantalums, sit beside J2.<ref name="mp23" /><ref name="mp45" />

The H7845 is rated 78 W (85 W for the -BA and -BB alternatives).<ref name="mp39" /> Its schematic is not in the print set. For the general approach to a dead DEC switching supply of this period, including testing with a series lamp and a dummy load, see the repair record on [[DECmate II Power Supply Service]], which covers the H7842 used in the DECmate II.

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+ '''Symptom, probable cause and action'''
! Symptom !! Probable cause !! Action
|-
| No fan, no drive activity || No mains, wrong supply for the mains voltage, or a dead H7845 || Check the cord and outlet. Confirm the H7845 variant. Measure J2 on the system board.<ref name="mp39" /><ref name="mp23" />
|-
| Fan runs, no picture || Monitor, monitor cable, or a system board fault || Check the VR201 on another DECmate II, DECmate III or Rainbow if available. Check J2 voltages.
|-
| Picture shimmers or the background is too bright || Ageing VR201 electrolytics || See [[DECmate III Capacitor Replacement Guide]].<ref name="rob3" />
|-
| Cloudy patches round the screen edge || Deteriorated glue between the VR201's safety glass and tube ("cataracts") || The glass can be removed with gentle heat and refitted on spacers. Discharge the CRT first.<ref name="rob3" />
|-
| Drive will not read any diskette || Spindle not turning || Check the spindle turns freely by hand. On one RX50 a misaligned cone lift cover bound the spindle.<ref name="rob3" />
|}

== RX50 drive ==
The RX50 slides out of the front after the spring tab on the base chassis is pressed.<ref name="mp37" /> Notes from a Rainbow restoration apply to the same drive: lost door pins let a door come adrift, and the drive cable can be crushed against a corner of the power supply when parts are refitted, giving an intermittent fault.<ref name="rob3" /> See [[Floppy Drive Repair]].

== Related pages ==
* [[DECmate III]]
* [[DECmate III General Maintenance]]
* [[DECmate III Capacitor Replacement Guide]]
* [[DECmate III Documentation]]
* [[DECmate II Troubleshooting Guide]]
* [[CRT Discharge Procedure]]

== References ==
<references>
<ref name="mp7">Digital Equipment Corporation, PC238 DECmate III print set, MP-01968-01, unit assembly wiring interconnection (PDF p. 7) ([[:File:DECmate III PC238 Print Set (MP01968-01).pdf]]).</ref>
<ref name="mp23">DEC, print set MP-01968-01, system board schematic, power connector J2 (PDF p. 23).</ref>
<ref name="mp37">DEC, print set MP-01968-01, BA25-H small system box drawing, note 2 (PDF p. 37).</ref>
<ref name="mp39">DEC, print set MP-01968-01, BA25-H small system box parts list (PDF p. 39).</ref>
<ref name="mp45">DEC, print set MP-01968-01, system board parts lists 5416489-1-DBP and -2-DBP (PDF pp. 45 and 55).</ref>
<ref name="manx">Manx catalogue of online computer manuals, search for "DECmate", [https://manx-docs.org/search.php?q=decmate manx-docs.org]. Lists EK-DM3BU-OM and EK-DM3BU-PS-002 with no online copy.</ref>
<ref name="faq">Douglas W. Jones, "PDP-8 Summary of Models and Options", alt.sys.pdp8 FAQ, posted 8 June 2001, section "What is a DECmate III?", [http://www.faqs.org/faqs/dec-faq/pdp8-models/ faqs.org].</ref>
<ref name="dm2spec">Digital Equipment Corporation, ''DECmate II Specification'', revision 4, 18 January 1983, section 1, [https://bitsavers.org/pdf/dec/pdp8/cmos8/DECmate/Decmate_II_Specification_Jan83.txt bitsavers.org]. The keyboard connects to the monitor.</ref>
<ref name="rob3">Rob Jarratt, "Rainbow 100A Restoration", ''Rob's Old Computers'', 29 July 2025, [https://robs-old-computers.com/2025/07/29/rainbow-100a-restoration/ robs-old-computers.com]. VR201 and RX50 repairs on a Rainbow 100A.</ref>
</references>

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