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Help with dead Spectrum +2a

TimppaTimppa
edited August 2005 in Sinclair Miscellaneous
Hello.

My Spectrum +2a died some time ago. It was powered on, I looked away and next time I looked at it, screen was black. I have replaced the 4 ram ICs, Z80 and ROM ICs from a working one, and that does not help. Also PSU seems to be ok, cause my second +2a is working.

The red power led is working and screen is steady black when I power on the machine.

Has anyone idead where to start replacing the components? I have fixed 48k by replacing a transistor, could this be as simple? There is much less documentation on fixing +2a/+3s compared to 48k.

Timppa
Post edited by Timppa on

Comments

  • FraserFraser
    edited July 2005
    Are you using the modulator output or the monitor socket? The TEA2000 IC is essential for the modulator. I had to replace one once. Is there sound at the headphones socket when pressing the up or down cursor buttons?
  • TimppaTimppa
    edited July 2005
    On 2005-07-14 12:16, Fraser wrote:
    Are you using the modulator output or the monitor socket? The TEA2000 IC is essential for the modulator. I had to replace one once.
    I have tested with both, no difference. I could try to change TEA2000 but problem is in the working +2a this IC is not socketed, so I need to solder it out first.
    Is there sound at the headphones socket when pressing the up or down cursor buttons?
    I need to try that one. I have tried to boot with BREAK key (should display test pattern with sound) but did not hear any sound.

    Thanks for you comments.

    Timppa
  • dastephdasteph
    edited July 2005
    If you don't want to replace the tea2000, or you would like a much enhanced picture then build this.

    http://www.worldofspectrum.org/forums/viewtopic.php?topic=5244&forum=4
  • FraserFraser
    edited July 2005
    I think pin 6 is the composite signal output. If there is a low voltage signal there then its dead. Its about 3.5V with the menu screen on.
  • SpiranthoSpirantho
    edited July 2005
    Hi,

    When you say the screen is steady black, do you mean the Spectrum screen (i.e. a white border around a black paper) or do you mean that when you turn it on the entire screen turns black?
    I presume you've tried retuning the telly slightly. :)

    Usually in my experience when the screen is black, it means there's a power sink somewhere, usually a RAM chip, which is lowering the +5V to less than it should be (about 1-2V commonly), thus preventing the ULA or anything else from producing any picture. Have you tried with a different ULA? Remember, don't put a good ULA into your dead Speccy as there may be other problems - put your suspect ULA into a good Speccy and see if it breaks (if it does, switch it back again!).
    Check your power lines +5V and ground - I don't recall you needing the other lines on a +2A. Are you sure the machine isn't working? Have you tried connecting a speaker to the sound output to see if anything happens when you press a key (it should click if it's in the menu).

    If you have access to an EPROM burner it may be worth your burning my ROM (see my other "anyone want a Speccy repaired?" thread for more info) and using that - I've not tested it on anything other than a 48K but it should work. Just remember to leech the CS and OE lines correctly.. they're in a different place on EPROMs.

    Either way, a completely black screen from a composite video source means your ULA isn't working or you've got a grounding problem on your video signal, almost guaranteed.

    Let us know what you try and how you get on!
  • TimppaTimppa
    edited July 2005
    On 2005-07-19 10:36, Spirantho wrote:
    Hi,

    When you say the screen is steady black, do you mean the Spectrum screen (i.e. a white border around a black paper) or do you mean that when you turn it on the entire screen turns black?
    It is steady black, no border.
    I presume you've tried retuning the telly slightly. :)
    Yes, I have a second working +2a: PSU and telly are just fine.
    Usually in my experience when the screen is black, it means there's a power sink somewhere, usually a RAM chip, which is lowering the +5V to less than it should be (about 1-2V commonly), thus preventing the ULA or anything else from producing any picture. Have you tried with a different ULA? Remember, don't put a good ULA into your dead Speccy as there may be other problems - put your suspect ULA into a good Speccy and see if it breaks (if it does, switch it back again!).
    Check your power lines +5V and ground - I don't recall you needing the other lines on a +2A. Are you sure the machine isn't working? Have you tried connecting a speaker to the sound output to see if anything happens when you press a key (it should click if it's in the menu).
    I have swapped bot roms, all 4 memory ICs, Z80A and keyboard. I measured PIN 6 on TEA2000 (as Fraser told to do) and on a working +2a there is voltage, but on dead one pin is 0V. This is at least some evidence. I have not dared to unsolder TEA2000 away from the working one to test with it.

    How do I change ULA on +2a?
    If you have access to an EPROM burner it may be worth your burning my ROM (see my other "anyone want a Speccy repaired?" thread for more info) and using that - I've not tested it on anything other than a 48K but it should work. Just remember to leech the CS and OE lines correctly.. they're in a different place on EPROMs.
    I will give it a try. I have just bought EPROM burner to change +2a to +3e and also to try modified ROMS on 48k.
    Either way, a completely black screen from a composite video source means your ULA isn't working or you've got a grounding problem on your video signal, almost guaranteed.

    Let us know what you try and how you get on!

    Thanks for your comments. If someone has extra TEA2000 chip, I would like to get one to go on.

    Timppa



    [ This Message was edited by: Timppa on 2005-07-19 13:32 ]

    [ This Message was edited by: Timppa on 2005-07-19 13:32 ]
  • SpiranthoSpirantho
    edited July 2005
    On 2005-07-19 13:31, Timppa wrote:
    I measured PIN 6 on TEA2000 (as Fraser told to do) and on a working +2a there is voltage, but on dead one pin is 0V. This is at least some evidence. I have not dared to unsolder TEA2000 away from the working one to test with it.

    How do I change ULA on +2a?

    To be honest I know very little about +2As - I don't have one. I assume the ULA is just a chip on the board but I could be wrong.

    Looking at the datasheet for this chip (www.secarica.ro/tea2000.pdf) it shows that your modulator etc. is probably fine, it's just that the TEA2000 is giving out no info. Hence the next thing to look at is the inputs and power lines on the chip. You should have +0V on pin 9, Vcc on pin 11 (I imagine +5V but I don't know), and pins 18 and 1-5 should all have some voltage I would imagine, as I should think the Spectrum's white screen would require voltage on each of the RGB pins. If you have an oscilloscope check with that too. An oscilloscope would be particularly useful for checking pins 12 and 13 which should be 8.86MHz.

    Do all those pins check out?
  • dastephdasteph
    edited July 2005
    On 2005-07-19 16:59, Spirantho wrote:
    To be honest I know very little about +2As - I don't have one. I assume the ULA is just a chip on the board but I could be wrong.

    The ULA on a +2a, is (going to sound odd to anybody who hasn't seen it) suspended in a cutout on PCB, with pins connected to both sides of the board. Kind of cool to look at, but certainly a bitch to replace.

    [ This Message was edited by: dasteph on 2005-07-19 22:26 ]
  • SpiranthoSpirantho
    edited July 2005
    On 2005-07-19 22:25, dasteph wrote:
    The ULA on a +2a, is (going to sound odd to anybody who hasn't seen it) suspended in a cutout on PCB, with pins connected to both sides of the board. Kind of cool to look at, but certainly a bitch to replace.

    Ah, I think I see what you mean. In which case, do everything you can to be sure it is the ULA before replacing it... and that's of course assuming you can find a replacement!

    Without seeing a +2A I can't really comment but I would imagine it's possible to solder in a replacement, but you'd have to hack the connections in somewhat. I had similar fun repairing an Interface 1 (fitting a low profile surface mount chip in a DIP 14 socket is an interesting experience, believe me!)

    Edit: Just seen a picture... looks like it has 6,000,000 pins on it! If the ULA is dead I suspect you have no way of resurrecting it... not practically speaking, anyway. Best just hope it's not your ULA!

    _________________
    Ian Gledhill
    www.retroreview.com
    [email="IanG@+nospam.+amiganet.org"]IanG@+nospam.+amiganet.org[/email] (remove text between plus'es!)

    [ This Message was edited by: Spirantho on 2005-07-20 10:14 ]
  • LloydLloyd
    edited August 2005
    it IS possible to replace the ULA chip - I have done it. Broke lots of rules (like using plumbing flux etc) but got it working.
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