Image Map Image Map
Page 2 of 5 FirstFirst 12345 LastLast
Results 11 to 20 of 43

Thread: AT&T PC6300 non-POST

  1. #11
    Join Date
    Aug 2017
    Location
    New York, Rome, London
    Posts
    108

    Default

    Sorry - I just realised you did test your parallel port dongle already. At this point I would suggest stripping the system to the bare minimum - just the motherboard and the power supply, nothing else, and see if you get any POST codes.

  2. #12
    Join Date
    Oct 2014
    Location
    near frankfurt/m, germany
    Posts
    981

    Default

    Quote Originally Posted by Trixter View Post
    I believe it's a volume control knob for the internal speaker.

    Yes. That wgs 6312 machine is a relabled version of the Olivetti M28 or M280, they use the same chassis. M28 is 8 Mhz, M280 is 12 MHz 80286. I have a M280 in very bad condition.

  3. #13
    Join Date
    Oct 2014
    Location
    near frankfurt/m, germany
    Posts
    981

    Default

    Quote Originally Posted by Valerio View Post
    Sorry - I just realised you did test your parallel port dongle already. At this point I would suggest stripping the system to the bare minimum - just the motherboard and the power supply, nothing else, and see if you get any POST codes.

    Yes, that's the way to go. But please keep harddisk and floppy on the power connector to provide some load to the power supply. Otherwise you may damage that as well.

    If possible, verify the EPROMs that they have valid content.

  4. #14
    Join Date
    Feb 2015
    Location
    Cleveland, OH, USA
    Posts
    980

    Default

    Sorry, I didn't realize these posts had been made until just now... I have disassembled everything and will begin by cleaning the case which has an accumulation of dirt and dust, though not the worst I have ever seen.

    I have been following the service manual procedures for diagnostics without POST. If I had seen 1ST1's post I would have known better than to test the power supply without a load, but I did so, briefly; i.e. long enough to take a measurement at the +5V / GND (+5.12V), at the terminals for the display adapter (+14.6V) and at the +12/-12 terminals (-10.8V / + 11.5V). These numbers may be insignificant however due to the lack of a load. I have a wire-wound resistor that I used for testing the IBM power supplies; I can add that to the 5V circuit for another test.

    I also followed the test for continuity & resistance on the motherboard, display adapter and expansion board. The SM only asks to verify that resistance is greater than 7 Ohms. Motherboard was 485 Ohms, expansion board was 172 Ohms, and display adapter was 1983 Ohms. No reference values were given in the manual but I have two spare display boards, both Rev 4 as this one is, and they measure 1900 and 2200 Ohms respectively. The spare expansion board measures 179 Ohms. I have no spare motherboard but I recall other motherboards in the IBM family coming back with similar numbers - mid hundreds.

    The fan is mounted on rubber isolators, I was surprised and pleased to find out. I don't have any but I found some on line and will be removing the fan before bathing the chassis.

    That's all I know for now; I will report more as I progress. Thanks to all for their suggestions.

    -CH-

  5. #15
    Join Date
    Feb 2015
    Location
    Cleveland, OH, USA
    Posts
    980

    Default

    Re-ran the PSU test with the two floppies attached to power and a 1.5 Ohm resistor (5W?) across the +5V and GND terminals. This time the numbers were all well within spec: 15V, +/- 12V, 5V. From which I think I can rule out the PSU for now. On to inspect the motherboard.

    -CH-

  6. #16
    Join Date
    Oct 2014
    Location
    near frankfurt/m, germany
    Posts
    981

    Default

    Check the BIOS eproms.

  7. #17
    Join Date
    Feb 2015
    Location
    Cleveland, OH, USA
    Posts
    980

    Default

    Quote Originally Posted by 1ST1 View Post
    Check the BIOS eproms.
    Returning to the service manual, I checked the jumpers; everything that was supposed to be jumpered was jumpered, nothing that wasn't. I checked and exercised each of the switches in DIP switch bank 0 and 1. All were set appropriately with the possible exception of Bank1, switch 3, which is set for External HDU ROM, and switch pair 5 and 6, which are set for 80 column / 25 line / color. Since this machine has 2 360K FDDs and the note says if the external HDU ROM is not found it will be ignored, I figure that's a non-issue. And I think the monochrome display can display differences of intensity to indicate different colors, so I think that's correct as well.

    On to the ROMs: I pulled the High ROM and read it as a D2764 (Intel); saved the results as BIN and HEX, and did the same for the Low ROM. I then compared them to the E and O ROM images that Jim Leonard contributed to the AT&T site https://sites.google.com/site/att630...Home/downloads and found they match, bit for bit.

    HOWEVER, I did find a BENT PIN on the High ROM ! I straightened it and reinserted the ROM carefully. I haven't put everything back together yet as I need to install a coin cell battery to replace the one I removed.
    Fingers crossed, etc...

    -CH-

  8. #18
    Join Date
    Aug 2017
    Location
    New York, Rome, London
    Posts
    108

    Default

    Good luck!

    PS The system will boot even without a battery, the only thing that will happen is that the real-time clock will forget the time when you switch off

  9. #19
    Join Date
    Aug 2006
    Location
    Chicagoland, Illinois, USA
    Posts
    6,229
    Blog Entries
    1

    Default

    Quote Originally Posted by clh333 View Post
    I then compared them to the E and O ROM images that Jim Leonard contributed to the AT&T site
    Cool, they helped someone

    Quote Originally Posted by Valerio View Post

    PS The system will boot even without a battery, the only thing that will happen is that the real-time clock will forget the time when you switch off
    Considering the RTC only tracks years between 1984 and 1992, this is no great loss When I use my 6300 for a while, I connect it to ethernet and use mTCP's SNTP program to auto-set the date and time in autoexec.bat.
    Offering a bounty for:
    - A working Sanyo MBC-775, Olivetti M24, or Logabax 1600
    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  10. #20
    Join Date
    Feb 2015
    Location
    Cleveland, OH, USA
    Posts
    980

    Default

    Reassembled, no screws left over, everything according to the manual, but same results: No display, keyboard lights flash, no "beep", no A: drive seek. Disconnected keyboard, same results. Disconnected display, same results.

    Assuming the power supply is good; there are only three boards inside the CPU. I have spares of two of them; I will have to try swapping them out.

    -CH-

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
  •