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Thread: IBM 5150 - Beep, disk activity - but blank screen

  1. #1

    Default IBM 5150 - Beep, disk activity - but blank screen

    Obtained a complete IBM 5150 today... 64 KB, dual FDD model. Computer is surprisingly clean and looks to be in good condition. However, I am having a boot problem. I get a single beep and hear the floppy drive try to read - but nothing comes up on the monitor.

    I performed the Minimum Diagnostic Configuration - and things seemed to work (got the 1 long/2 short beeps) http://minuszerodegrees.net/5150_516...iag_config.htm

    The monitor does seem to work (if I turn the brightness all the way up I see diagonal-ish lines). However I don't see a cursor or anything else.

    What are some other tests I can perform to determine what may be causing the problem? Thanks!

  2. #2
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    What video card do you have and what test equipment

    You could put an oscilloscope on pins 8 & 9 to see if your getting the horizontal and vertical sync signals, at least that would tell you if the video card is sending something out. If missing the card is probably dead, if they are there, then pin 7 for MDA video or pins 3,4 & 5 for CGA RGB TTL signals, if all there its probably the monitor. If its the IBM 51## (cant remember the number) I believe the contrast control can go dicky. Mine needs an occasional thump on the right hand side to get it to work (with brightness turned up, you can see the raster, but no text, with a quick thump it comes back, will need to fix that soon)

    Also some info here on no contrast https://www.vogons.org/viewtopic.php?t=68626

    BE CAREFUL opening up any CRT monitor, they can kill.
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  3. #3
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    What is the make and model of the video card? More than one video card?

    What is the make and model of the monitor?

    On the motherboard, what are the two video switches (5 and 6) in switch block SW1 set to?

    Quote Originally Posted by nullvalue View Post
    Obtained a complete IBM 5150 today... 64 KB, dual FDD model.
    By "64 KB", do you mean that the motherboard is the 16KB-64KB one? If so, what is the revision of motherboard BIOS? That information may become relevant later.

    Quote Originally Posted by nullvalue View Post
    I get a single beep and hear the floppy drive try to read - but nothing comes up on the monitor.
    Quote Originally Posted by nullvalue View Post
    I performed the Minimum Diagnostic Configuration - and things seemed to work (got the 1 long/2 short beeps) http://minuszerodegrees.net/5150_516...iag_config.htm
    All good signs.

    Let's see if your 5150, because it did not find something to boot from, jumped into Cassette BASIC. Type in the following, then press the ENTER key. If you heard a single beep, you are in Cassette BASIC, highly suggesting that the problem is restricted to video only.

    print chr$(7)

  4. #4

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    You say "try to read". If it is just for a few seconds, you can skip the rest of the text. But if it is longer, it can be that the PC is booting. Just type "DIR" and see if there is disk activity again. If so, it confirms that there is something wrong with the video. When having no beeps at start-up, it means there is something wrong some where in the range from the end circuit of the video card up to the monitor. The quickest test to check that is to exchange the video card and/or the monitor with another one. But I guess you don't spare parts.
    With kind regards / met vriendelijke groet, Ruud Baltissen

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  5. #5

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    Quote Originally Posted by Gary C View Post
    What video card do you have and what test equipment
    The video card is the MDA/Monochrome card (1501483 XM)

    Yes I do have a scope, and will test the pins you mentioned.


    Quote Originally Posted by modem7 View Post
    More than one video card? What is the make and model of the monitor?
    Only one video card. The monitor is the IBM 5151.

    Quote Originally Posted by modem7 View Post
    On the motherboard, what are the two video switches (5 and 6) in switch block SW1 set to?
    SW1 5/6 - both switches are set to OFF.

    Quote Originally Posted by modem7 View Post
    By "64 KB", do you mean that the motherboard is the 16KB-64KB one? If so, what is the revision of motherboard BIOS? That information may become relevant later.
    Well, I could be wrong but by 64KB I meant it has an ISA card installed that says "64 KB MEM. CO.". The motherboard says "64KB - 256KB CPU". I beleive this has the REV 3 BIOS - it has the 1501476 chip installed in U33.

    Quote Originally Posted by modem7 View Post
    Type in the following, print chr$(7) then press the ENTER key. If you heard a single beep, you are in Cassette BASIC, highly suggesting that the problem is restricted to video only.
    I typed in print chr$(7), pressed ENTER and yes - I did get a beep!

    Quote Originally Posted by Ruud View Post
    You say "try to read". If it is just for a few seconds, you can skip the rest of the text. But if it is longer, it can be that the PC is booting. Just type "DIR" and see if there is disk activity again...
    After power on, about 25 seconds pass then I hear the beep and the drive light comes on/spins. It spins for about 10 seconds, then turns off.
    I tried entering "DIR" <ENTER> (like the blind test above), but nothing happens. However I do not have a boot disk currently (there's nothing in the drive) - so I'm guessing that'd only work if I ended up at a DOS prompt. You're right, I don't have any spare parts.

  6. #6

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    Quote Originally Posted by Gary C View Post
    You could put an oscilloscope on pins 8 & 9 to see if your getting the horizontal and vertical sync signals, at least that would tell you if the video card is sending something out. If missing the card is probably dead, if they are there, then pin 7 for MDA video, if all there its probably the monitor. If its the IBM 51## (cant remember the number) I believe the contrast control can go dicky.
    I do have signals on pins 7, 8 & 9. They all look identical on my scope. Followed your link to that vogons post and my screen looks just like that! I'll check out the rest of that post. Thanks!

  7. #7
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    So your video card seems good, however signals on 8 & 9 should be a bit different, ie different frequencies.

    mda_sync_hor.jpg mda_sync_ver.jpg

    From http://minuszerodegrees.net/mda_cga_ega/mda_cga_ega.htm

    and pin 7 should have a varying signal as thats the actual image signal, when combined with pin 6's intensity signal (worth checking pin 6 too)

    but its likely its working.

    If its the monitor, things can get more serious as voltages of ~15000V are on the tube.

    This has every detail on the monitor http://classiccomputers.info/down/mo...s_IBM_5151.pdf but please be careful. The tube can hold the high voltage for some time after the power has been switched off. There are some daft opinions about how long, and I have never had a problem after 30 minutes (but I am not guaranteeing this time is sufficient), but the thick wire to the rubber cap connected directly to the glass should be discharged before touching. An old ignition lead works well as a lead to discharge as its well insulated and high enough resistance to allow the static charge to dissipate without a high instantaneous current but needs to be modified sufficiently to make sure you get a connection.
    Last edited by Gary C; August 10th, 2020 at 08:20 AM.
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  8. #8

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    Quote Originally Posted by Gary C View Post
    If its the IBM 51## (cant remember the number) I believe the contrast control can go dicky. Also some info here on no contrast https://www.vogons.org/viewtopic.php?t=68626
    BE CAREFUL opening up any CRT monitor, they can kill.
    Both of the people in that thread reported that replacing their SN7406 chip fixed the problem. I put a scope on pin 9, seen the input, on pin 8 I also see some activity (though I'm not sure what I'm looking for)...

    So I'm trying to get a reading on those 2 pots.

    Brightness is 100K ohm. Across the pot its reading 80K. Reading the value, it seems to be working fine (as expected - brightness seemed to work).

    Contrast is 500 ohm. Across the pot is right on at ~500 ohms. Reading the value seems to work fine most of the time - seems a bit erratic (but could be my meter).

    Hit both pots with some DeOxIt - easier to move now and are both reading fine.

    Booted it back up, still nothing on the screen.

  9. #9
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    If you look at page 2 of the SAMs computerfacts, the output expected from pin 8 of the SN7406 is shown as a 3.2v peak to peak video signal and the contrast control provides a voltage into pin 4. Basically the chip takes the video signal from pin 9, inverts it and buffers it and outputs it on pin 8.

    5151.jpg

    Looks like the contrast is done by inverting the intensity signal, then inverting it again and applying it to the video subtractively.


    Be careful and check the signal path to page 2 http://classiccomputers.info/down/mo...s_IBM_5151.pdf
    Last edited by Gary C; August 10th, 2020 at 10:42 AM.
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  10. #10

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    Here's what the scope looks like for the 7406 pins 8/9 taken at 10us TIME/DIV. What do you think? It definitely looks inverted - voltages look right...

    SN7406 - Pin 8 @ 10us
    7406pin8-10us.jpg

    SN7406 - Pin 9 @ 10us
    7406pin9-10us.jpg

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