Image Map Image Map
Results 1 to 5 of 5

Thread: Debugging a model 4p ga

  1. #1

    Default Debugging a model 4p ga

    Hi,

    I have an ebay rescue 4p that I'm trying to restore. At first power up I get the power light to come on, but nothing else happens. The screen is dark, no activity from the drives. All I hear is a faint buzzing.

    Upon disassembly I checked the ac in on the power supply and it is good. However the output voltages v1-4 are all near zero (with every thing connected).

    When I disconnected the DC power output cable and powered it up I get a low frequency chirping. V1-4 are pulsing. I'm not sure if this supply requires a load, so am not sure if this is a valid test. Does anyone know if this is the case?

    I have yet to replace the rifa caps, but they don't appear blown although one is cracked. Nothing else looks bad on the supply visually.

    I have the service manual but am new to troubleshooting switching supplies (if that is indeed what is wrong).

    I do have a working model 4ga that probably has the same supply, so I suppose I could swap them. However I was hoping I could debug this separately.

    Any suggestions or tips?

    Thanks in advance!

  2. #2
    Join Date
    Jun 2015
    Location
    Sioux Falls SD
    Posts
    328

    Default

    Visually inspect the solder joints around the connectors on the PS board. They sometimes crack and create issues.

    You do need a load to check voltages. Pulsed outputs under no load may be okay. Study Section 5.4 of the Service manual, and take a look at troubleshooting and adjustments starting at page 158.

  3. #3

    Default

    Making progress... thanks for the suggestions!

    I replaced the rifa caps on the power supply as a precaution. The old caps hadn't blown yet, but were way out of spec. I reflowed the solder around the power connector as they were ugly. I cleaned all of the flux off the ps board and couldn't see any other evidence of a failure.

    After power up, there was still no change. At this point I went under the assumption that the power supply buzzing was it entering protection and tried removing loads one by one. Well the first was the floppies and that did it. The fan kicked in and the computer showed signs of life. I then narrowed it down to a single drive. A quick test of the power connector on that drive indicated that the 12v rail was shorted.

    Seems like I might be look next for a bad cap on that drive. I found a similar problem described here:

    http://www.vcfed.org/forum/showthrea...ndon-TM-100-2A

    I'll report back once I get the drive working.

    George

  4. #4
    Join Date
    Jul 2005
    Location
    State College, PA
    Posts
    159

    Default

    Quote Originally Posted by mrthreeplates View Post
    Making progress... thanks for the suggestions!

    After power up, there was still no change. At this point I went under the assumption that the power supply buzzing was it entering protection and tried removing loads one by one. Well the first was the floppies and that did it. The fan kicked in and the computer showed signs of life. I then narrowed it down to a single drive. A quick test of the power connector on that drive indicated that the 12v rail was shorted.

    Seems like I might be look next for a bad cap on that drive. I found a similar problem described here:

    http://www.vcfed.org/forum/showthrea...ndon-TM-100-2A

    I'll report back once I get the drive working.

    George
    Look for a shorted tantalum cap on the 12V rail. I had the same problem on a 8" Mitsubishi drive on the 24V rail.

  5. #5

    Default

    I isolated the floppy motor board and determined the short was on the main board.

    The first cap I pulled on the main board along the 12v rail (c6) was the one that failed short. That was lucky!

    Since I had the drives out I lubed the rails and cleaned the heads.

    Once I buttoned everything up, the system booted from floppy!

    I've got a bit more work with this model 4p, but I'm pretty excited. It is even fully loaded: 128k, modem board, and graphics.

    IMG_20191026_144504.jpgIMG_20191026_144021.jpgIMG_20191026_145142.jpgIMG_20191026_153608.jpg

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •