Search Posts:

Diagnosing PDP 11 h742a 15V failure

SHARE

Return to Threads

  Diagnosing PDP 11 h742a 15V failure by Bill Degnan - 01/23/2017 11:27
The backplane lost the +/- 15V signal while I was testing the drive controller.

The power supply is hard to get to in the rack. Pictured is the h742a power supply disconnected from the rack but not the computer. I positioned two cinder blocks on each side of the rack with a strong board between them to form a table. Unbolted from the rack I was able to move the h742a 90 degrees to sit it on the board with the power connections intact. Now the unit is much easier to access. Click image for larger view.


I removed the power control board from two h742s including the unit to be repaired. It's always good to be able to compare and contrast a working board with a non-working board. Pictured is the inbound 15V signal connector ("IN") coming into the board, the fuse, and the outbound port P9 (OUT) that is routed to the h745 voltage regulator. Click image for larger view including both control cards side by side.


I knew that the likely issue was a blown fuse, why? I had previously installed a simple PC power splitter cable into the 15V power circuit. With this I could measure the signals on both sides of the fuse. Basically it's like an extension cable with a "Y" output. The "to P9" end connects to the other end of the P9 Molex connector, and the "for test" connector (with black electrical tape to cover when not in use) can be used to measure the voltage coming from the h742a *before the fuse*. The connector at the bottom of the photo attaches to the Molex connector coming out of the control board. Click image for larger view.


The splitter is installed. Click image for larger view.


With the splitter in place I could check the h742a fuse without opening the power supply enclosure by simply probing the test connector hanging down from the power supply. Turns out, yes it was the fuse. I must have caused this when testing the 15V signal coming into the M7762 RL11 control board. I still needed to pull the control board and set up the cinderblocks, etc but if the fuse had not blown, then I would have first turned my attention to the h745 voltage regulator.

Reply

Resources:


Buy a Commodore Computer Poster

Popular Topics and FAQs


  • Commodore B Series Tips and Tricks
  • Aerocomp TRS 80 M 1 Expansion Unit DDC
  • Items Wanted
  • Lobo Max 80
  • Zenith Z-19-CN
  • Prototype PET 2001 photo
  • Using Toggle Switches to Analyze Memory
  • Commodore Disk Archive Project
  • PET 2001 Prototype at Gametronics 1977
  • Jim Butterfield Photo
  • IMSAI 8080 With Processor Tech. Cutter
  • Secrecy is the keystone of all tyranny
  • Cromemco System Three
  • Northstar Horizon - Boot Problem
  • Computer History and Restoration Links
  • Commodore BX-256-80 - 8088 Co-processor
  • S-100 board testing with Z-80 ICE
  • Donner 3500 - an early portable computer
  • Digital (DEC) PDP 11/05 NC Assembly
  • Univac 1219 rescue
  • IMSAI 1.4 BASIC vs. MITS 8K BASIC
  • Fido BBS listing node list 6-13-1986
  • PDP 8e
  • MITS 88-2 SIO (2SIO) for BASIC
  • Visual Technology Inc Model 1050
  • Amiga 2500 Restoration
  • The Evolution Of IBM Computers
  • Replacement teletype print hammer head
  • Archiving and Copying Software 101
  • Computers Built 1940 - 1950
  • CBM B-520 (a.k.a B256-80 or B500 256)
  • RCA COSMAC Microkit
  • Commodore 64K C-116 Mods
  • MITS 8800b Turnmon 9600 baud
  • Catweasel, 8in and 5 1/4
  • Raspberry Pi as Gateway to Internet
  • Digital PDP11 late 1969 early 1970
  • PDP 11/40 72 inch cabinet model
  • PDP 11/40 Industrial 11 model
  • Digitial MicroVAX 3100 30 System
  • Digital VAX 4000-200
  • Commodore 64 / 1541 DRIVEKNOCK
  • Booting the System Using RL02 drive
  • PACS: Reflections by Kathleen Mauchly
  • Tele-Graphic Computer Systems Inc.
  • Commodore B Series SID Jukebox?
  • Installing Core into PDP 11/40
  • Setting Up OpenVMS 7.1 DNS CLERK
  • Felt-Tarrant Comptometer Model J
  • NextStation Color
  • Digital Rainbow (PC100-B2)
  • 1970 Compusad Compulogical Tutor
  • Archiving Papertapes Using DSI NC 2400
  • 1976 P.C.C. Features the MAI JOLT 6502
  • 1961 Beckman DEXTIR Computer
  • UNIVAC 1 and UNIVAC File Computer 1
  • Past Issues:


    tcf 2010 TCF2010 Andy Crawford Bob Evan

    This image was selected at random from the archive. Click image for more photos and files from this set.