ZX Spectrum 48k issue 3b power problem?

edited March 2012 in Hardware
Hi :) I was lucky enough to find a couple of ZX Spectrums at a car boot sale yesterday. I spent the evening fiddling about with one (issue 2) and thats now upstairs entertaining my kids with Pippo as I write.

The second is an issue 3b. I am willing to fiddle about with the electronics and have a bunch of tools handy to do so, multimeter etc, but I have absolutely zero experience or understanding of the job. I have google, and I have this site and its documentation, but I dont really understand some (most) of it, so I wondered if I could please ask for some advice.

So when I turn on this spectrum, the copyright screen comes on in black and white. The colour could be a tuning issue but its tuned in fine for the working system.

Seconds later, the copyright screen is replaced by the black box in the middle, filled with thin vertical red lines. This flashes up for a second or so, then disappears - (no input to tv).

There is a small buzzing from the coil near where the power lead goes in during this time, but none of the chips heat up at all, including the ULA. Im wondering if this is a power issue? Its like it is deliberately cutting off its own power.

Thanks in advance for any advice offered!
Post edited by vinnivee on

Comments

  • edited April 2011
    the black screen with red lines is the ram test. This should be the first thing that happens when you plug in power (or reset. Is it a rubber key or a + case?)
    My rubbish website including the redrawn Amstrad schematics and the new home of the Sinclair FAQ wiki.
  • edited April 2011
    rubber keys. the lines stay on for a second or two before it shuts off. so do you think it is a ram problem then? would this also explain why the copyright screen is in black and white?
  • edited April 2011
    vinnivee wrote: »
    rubber keys. the lines stay on for a second or two before it shuts off. so do you think it is a ram problem then? would this also explain why the copyright screen is in black and white?

    the copyright screen will be black and white, there is no colour on the copyright screen on the 48K.

    when you say it "shuts off" do you mean it just generates a black screen?
    My rubbish website including the redrawn Amstrad schematics and the new home of the Sinclair FAQ wiki.
  • edited April 2011
    it is green on the other one?

    it shuts off - like ive unplugged it. one second there is a black box with the vertical lines, the next second my tv is telling me there is nothing plugged into it. if i take out the power lead then plug it back in it does the same thing again. it wont stay on for more than a few seconds.

    when i first plugged it in (when we got it home) the screen flickered until i realised it wasnt a tuning problem, it was where the power cord was going in. you know like when headphones dont work unless you hold the cord a certain way? like that. but it isnt the cord because it works fine in the other system.

    then it stopped doing that and now just goes through the on - redlines - off cycle. im wondering if its something really simple like a part needing a clean or a wiggle.. maybe im being too hopeful :D
  • edited April 2011
    It would be interesting if you could take a screenshot to see more clearly your problem.
    In a Spanish website:

    http://www.speccy.org/hardware/reparar.html

    you have several examples of what might be your problem.
  • edited April 2011
    Thank you for that link. It seems that the one that I thought was okay has the wrong colour copyright screen, im wondering if that is a problem worth looking into seeing as it seems to work okay other than that.

    Ive just filmed what happens when I turn the issue 3b on. You can watch here. After the copyright screen it reacts as if it has had some keyboard input but it hasnt. What appears is:
    2K
    2??

    Then it does its black square then off routine.

    Have you seen this before?
  • edited April 2011
    vinnivee wrote: »
    it is green on the other one?

    did you say the other was an issue 2? they have poor colour circuitry which needs adjusting to get the correct hue. There should be variable capacitors on the board that you can tweak to get the screen grey.

    vinnivee wrote: »
    the next second my tv is telling me there is nothing plugged into it. if i take out the power lead then plug it back in it does the same thing again. it wont stay on for more than a few seconds.

    do you get clicks when you press keys? it sounds to me like it could be the telly misbehaving. Have you got an older telly to try it on?
    My rubbish website including the redrawn Amstrad schematics and the new home of the Sinclair FAQ wiki.
  • edited April 2011
    Hi
    Mobutu wrote: »
    It would be interesting if you could take a screenshot to see more clearly your problem.
    In a Spanish website:

    http://www.speccy.org/hardware/reparar.html

    you have several examples of what might be your problem.

    I also have this page in english :-)

    http://www.speccy.org/hardware/reparar-i.html

    Other thing is if my english is enough for english people.Sorry :-P

    The problem with issue 3.

    I don't see this fault before. Maybe it's a rom problem ( very strange) maybe a lower ram problem. The last spectrum I repaired has a "similar problem". It started well but when I press a key, it hangs. The problem was a faultly ram chip 4116.

    Bye

    Jos? Leandro
  • edited April 2011
    i tried it yesterday with the working keyboard that is now on the issue 2, and there was no response at all. i dont think it is the telly because the issue 2 system works with it. however i do have two more old tellys (all my tellys are old ones!) i can try it on and i will do so :)

    i can look at the documentation for the whereabouts of the relevant transistors, but how do I go about tweaking them please?

    sorry for being so green. like i say, im willing to get stuck in but the terminology is like another language at this point and im finding that a real problem when trying to understand the faultfinding/servicing/repair guides.
  • edited April 2011
    HI
    vinnivee wrote: »
    There is a small buzzing from the coil near where the power lead goes in during this time, but none of the chips heat up at all, including the ULA. Im wondering if this is a power issue? Its like it is deliberately cutting off its own power.

    Thanks in advance for any advice offered!

    The buzzing is normal. The problem come when you don't hear it ( normally when there isn't -5v or -12v).

    In a issue 3 , only ULA heat up (and depends on the ULA type). Ram chips don't heat up

    The only chip that sometimes get very hot when they break are 41464 RAM chips ( spectrum +2A /-3).

    Bye

    Jos? Leandro
  • edited April 2011
    Hi



    I also have this page in english :-)

    http://www.speccy.org/hardware/reparar-i.html

    Other thing is if my english is enough for english people.Sorry :-P

    The problem with issue 3.

    I don't see this fault before. Maybe it's a rom problem ( very strange) maybe a lower ram problem. The last spectrum I repaired has a "similar problem". It started well but when I press a key, it hangs. The problem was a faultly ram chip 4116.

    Bye

    Jos? Leandro

    Thank you Jose :)

    I have a borrowed multimeter but Im not familiar with using it. It is a digital one from maplins. How do I check whether the ram 4116 chips are working okay? I can see them, they all look intact externally. I can follow instructions in plain language, but technical slang is over my head im afraid.
  • edited April 2011
    Hi
    vinnivee wrote: »
    Thank you Jose :)

    I have a borrowed multimeter but Im not familiar with using it. It is a digital one from maplins. How do I check whether the ram 4116 chips are working okay? I can see them, they all look intact externally. I can follow instructions in plain language, but technical slang is over my head im afraid.


    Sorry but I don't know any method to test RAM only with a multimeter. Even the oscilloscope does not help much (sometimes).

    I use a diagnostic board ( Winston) that it makes some RAM test and help me in my spectrum repair. I also use a board for IF2 interface (I made it) that has a EPROM with Sinclair rom test and fruitcake 128K RAM test.

    Bye

    Jos? Leandro
  • edited April 2011
    I have access to none of these things :/

    Does anyone know of a spectrum enthusiast/repairist in devon/cornwall who I might be able to contact about this, please?
  • edited April 2011
    Hi
    I’m no expert but I don’t think you have any faulty RAM chips.
    You can use the multimeter to check that all the voltages are correct. The PSU supplies 12 to 15 volt unregulated DC and the Spectrum regulates this to the +5,-5 and 12 volts.

    I think you are having a problem with the +5 volts which is generated by the 7805 reg. My thinking is that when you first switch on the +5 is Ok but then it starts to fail and voltage drops causing the malfunction you see, eventfully the voltage will drop below the level that the chips need causing the Spectrum to shut down.


    John
  • edited April 2011
    @John58

    As far as I am aware the spectrum 48k/48k+/128k and the Grey 128 +2 plug-packs are 9VDC - from 1 to 1.7 amps

    These are then internally regulated by a 7805 / 78S05 regulator for 5v, this is then internally transformed to -12v and -5v for the memory.


    @vinnivee

    The 7805 regulator is the crux of most power issues of a Spectrum after a ailing Power supply. once you have confirmed that is not the 9VDC pug pack then the next thing in the chain is the 7805 5v regulator.

    reading your description it sounds as thought the regulator is dropping out, but don't worry as there very cheap to get and easy to install even if you have little soldering experience

    You can buy the 78S05 from eBay on the cheap..

    1. 78S05 ?0.99 + ?1.75 postage (2amp draw)
    2. (10x) 7805 ?1.97 + ?1 postage (1.5amp draw)

    Either of these would be good, the 7805 when used with a standard 48k unit is more than ample to provide clean power as well as offer some to peripherals like a DivIDE quite comfortably.
  • edited April 2011
    Hi Zetr0

    My 48K PSU rated at 9VDC at 1.2A actually reads 15VDC at the power plug. Some time ago, as an experiment, I connected it to a grey +2, and checked the voltage while it was powering it. The voltage had dropped to about 9VDC. The +2 own PSU rated 9VDC at 2A, measured 12VDC at the plug but dropped to about 11VDC when powering the +2.

    The point I am trying to make is that, yes I know the pre +2ab/3 power packs have a 9VDC rating on them but if anyone checks their out put when not connected to a Spectrum the reading will be higher and in my experience that is between 12 and 15VDC but as this is unregulated the voltage will be lower when powering the Spectrum. It should not drop below 9VDC, in my experiment the 48K PSU is probably being over loaded.

    If the above sounds like gobbledegook then it probably is.


    John
  • edited March 2012
    Zetr0 wrote: »
    @John58

    As far as I am aware the spectrum 48k/48k+/128k and the Grey 128 +2 plug-packs are 9VDC - from 1 to 1.7 amps

    These are then internally regulated by a 7805 / 78S05 regulator for 5v, this is then internally transformed to -12v and -5v for the memory.


    @vinnivee

    The 7805 regulator is the crux of most power issues of a Spectrum after a ailing Power supply. once you have confirmed that is not the 9VDC pug pack then the next thing in the chain is the 7805 5v regulator.

    reading your description it sounds as thought the regulator is dropping out, but don't worry as there very cheap to get and easy to install even if you have little soldering experience

    You can buy the 78S05 from eBay on the cheap..

    Either of these would be good, the 7805 when used with a standard 48k unit is more than ample to provide clean power as well as offer some to peripherals like a DivIDE quite comfortably.

    Hi :) We've been able to play on our (working, issue 2) 48k spectrum until a couple of days ago when the keys at the bottom right stopped working when the spectrum was hot. Its a pain because my nine year old has been learning Basic on it so needs the keys, its not just a matter of quickly loading up a game before they fail.

    I cut the very ends off the keyboard cables as I had seen that that sometimes works for people, but it didnt make any difference. The keys did still work briefly while it is cool. I thought it could be the heat rather than the keyboard as it was getting really hot, really quickly after turning it on.

    I had a look here and decided to replace the 7805 regulators on both the issue 2 and issue 3b spectrums at the same time. We did this today. We also did the composite mod on both at the same time because we've been having issues tuning them in to the telly.

    Right, so, the issue 2 isnt heating up like before in the five or so minutes we've tried it for, but the bottom right keys and all corner keys arent working. So Im pretty sure that we need to replace the membrane.

    The issue 3b now doesnt buzz when we switch it on. What happens, in order, is:

    1 - unlike before, it now (since replacing the 7805) starts on the normal 'sinclair research' screen.
    2 - after about thirty seconds, a load of goobledegook appears, pretty much as if we had just pressed a dozen random buttons.
    3 - then it blinks off to a black square filled with thin vertical red lines.
    4 - unlike before, this doesnt disappear.

    Does anyone have any ideas what the problem is please? I dont have a whole lot of 'measuring' equipment, so am mainly wondering if the problem is obvious and fixable without spending too much.
  • edited March 2012
    Some of the electrolytic capacitors on the issue 3b may be failing and causing the power supply to fail after the machine has been on for a while. It would be worth replacing at least all the caps on the right hand side of the board, and the two to the right of the lower RAM. Axial caps are harder to find cheaply these days, but radial ones are fine, they just don't look as neat.

    For the issue 2, it sounds like the heatsink was getting hotter than normal with the faulty regulator and damaged the part of the membrane above it. Does it work reliably with the case top/membrane from the 3b? You could hold it clear of the board for a bit to minimise the risk of damage to that one.
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