help with issue 3 48k speccy after recap and new tr4 and tr5

I have a 48K speccy that was not working.
After recapping and replacing TR4 and TR5 it came back to life
However after a short while running weird things start to happen.
It can produce some long beeps, then prints THEN a few times, followed by black squares on the screen.
It can continue doing this for awhile or might reset.

Any ideas what this could be please? and suggestions on what to do next?

Cheers
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Comments

  • Sounds like 1 or more of either upper or lower ram IC's have failed due to losing their voltages, when TR4/TR5 failed. Upper/lower ram IC's need -12v,+12v,and +5, otherwise they aren't happy. TR4/TR5 are part of the power supply circuit that supply the 3 voltages. Due to the simple design of these and to keep manufacturing cost low, there isn't any protection such as diodes or fuses to protect vital circuits from a daisy chain of failures. Wrong polarity from a power supply, or connecting/disconnecting interfaces whilst powered up is the easiest way to cause these failures.
  • Sounds like 1 or more of either upper or lower ram IC's have failed due to losing their voltages, when TR4/TR5 failed. Upper/lower ram IC's need -12v,+12v,and +5, otherwise they aren't happy. TR4/TR5 are part of the power supply circuit that supply the 3 voltages. Due to the simple design of these and to keep manufacturing cost low, there isn't any protection such as diodes or fuses to protect vital circuits from a daisy chain of failures. Wrong polarity from a power supply, or connecting/disconnecting interfaces whilst powered up is the easiest way to cause these failures.

    Thanks.How do I test each chip to determine which has failed?
  • haha found it.
    there was a very small cut off from a new capacitor leg that was tucked up against the leg of one of the upper ram chips. very well hidden. removed it with some tweezers and all is good in the world again.
  • That was lucky that it didn't cause any trouble to the ram IC. Pleased it's all working again. Celebrate a successful repair whilst having a brew and loading up some classic games.
  • bugger, after a short while running it still does it.
    I managed to run
    PRINT PEEK 23732 + PEEK 23733 * 256
    
    and got the correct result of 65535
    Changed the voltage reg as the left leg measures less than 100ohms (measured 91.4) as per zx.zigg.net initial checks suggest.
    now really not sure what to test next.
  • A clarification: the "upper" RAM chips only need a single +5V supply. They don't normally die when TR4 and/or TR5 die, as the 7805 voltage regulator suppliers the +5V. However if there was a spike on the +5V line, all bets are off.

    TR4, TR5, the coil (actually a transformer) and associated circuitry take the nominal +9V input and convert it to a +12V and a -5V supply. This circuit uses the +5V from the 7805 as a reference voltage. A failure of the +5V supply will then affect the +12V and the -5V.

    The 'lower' RAM chips (type 4116 or equivalent) require three supply voltages and GND/0V. These are +12V, +5V and -5V. The -5V is critical, as the loss of this with one or both of the the other supplies present can damage the 4116 RAM chips.

    The symptoms you describe are very likely caused by a faulty RAM chip.

    The quickest and easiest way of fault finding suspect RAM problems, is to run a test diagnostic program from ROM.

    Do you have any interfaces / expansions that can take a ROM chip or a ROM image file?

    Is the ROM chip on your Spectrum in a socket? Do you know anyone who can program an EPROM chip?

    Mark

  • edited July 2018
    The quickest and easiest way of fault finding suspect RAM problems, is to run a test diagnostic program from ROM.

    Do you have any interfaces / expansions that can take a ROM chip or a ROM image file?

    Is the ROM chip on your Spectrum in a socket? Do you know anyone who can program an EPROM chip?

    I have an interface 2!
    No other rom stuff unfortunately.
    I think one of my friends may have a eeprom programmer though.
    Post edited by redfiat on
  • SMART Card interface from Retroleum has a built in diagnostic rom. You can flash it with other test roms, upto 15 I think. I have one myself and wouldn't be without it now when fixing Spectrums. It also doubles as a SD-card game loader with built in Kempston joystick interface, so it's quite a handy bit of kit. Out of stock at the moment, but if you email the seller, he will let you know when they're available again. He makes them himself.

    https://retroleum.co.uk/spectrum-peripherals
  • Check the capacitors that you replaced. It sounds like one of them may have been installed the wrong way around.

    Bryce.
  • I'd suspect the lower RAM chips first - obviously check the work you have already done, especially the polarity of the capacitors. The machine seems to be acting like the machine stack has been corrupted - indicative of faulty RAM, and I'd say an early diagnosis would be the Z80 data lines are being messed up by a faulty RAM chip or two. Not sure if you can still get a lower RAM board, but that may be a good way forward if indeed lower RAM is to blame, those chips are expensive now and hard to find. Most offered on eBay are "pulls" (ie second hand) and it's a gamble on whether or not they can be trusted to work properly.
    Oh bugger!<br>
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