Sinclair ZX80 / ZX81 / Z88 Forums

Hi,

I have a zx spectrum board versin 4B. Some time ago I plugged it in to a AC-DC power supply and the voltage regulator blew up.
Yesterday I replaced it with a new one (that doesnt need the metal plate for heat dissipation).
Now I powers on, but I get a white border and a black screen.

What could be the problem ?

Thanks,
Regards

Hello

From what you say, that sounds like the system is detecting a RAM problem.

Do you have a digital multimeter? If yes, test the supply voltages on pins 1, 8 and 9 of IC13. Test between each pin and a suitable 0V/GND point (such as the modulator case) or to pin 16 of IC13. IC13 is at the front left of the board.

Pin 1 is VBB, which is -5V (-5.5V to -4V)
Pin 8 is VDD, which is +12V (+10.8V to +13.2V) (although lower than 11.5V normally indicates a problem).
Pin 9 is VCC, which is +5V (+4.75V to +5.25V) (the output from the 7805 or DC-DC switching regulator is normally very close to +5V).

See the link in my sig (below) for ZX81 chip pin outs, as also included are the details of the 4116 DRAM chip (or equivalent chip) used for the ‘lower’ RAM.

If the voltages are within acceptable limits, then move to the next section. Otherwise transistors TR4 and/or TR5 may be damaged and should both be replaced with new parts.

Does it always power up with a white border? Try at least ten times (power off for at least 30 seconds before powering up again).

If yes, it’s likely one or more DRAM chips are faulty. Most likely IC6 to IC13 (the ‘lower’ RAM). But it is also possible that something else has died.

Mark

Hi,

Got the chance to plug a ZX Diag Cart to the speccy. It indicates all lower RAM chips defective.
I only have +5 and +9 Volts indicated by the cart.
Is it worth it to replace them, or should I just get another known good board ? Seems like the voltage regulator explosion fried the RAM.
Before that the machine worked. If I replace the RAM, I don't know what other components might be toast also.

Thanks,
Regards

The 4116 (or equivalent) ‘lower’ DRAM chips NEED a +12V AND a -5V AND a +5V supply to work.

So transistors TR4 and TR5 need replacing. Use either a ZTX650 (if you can find one) or a ZTX651 or a ZTX653 for TR4. Use a ZTX213 for TR5 if you can find one (ZTX213 type are obsolete). Otherwise use a ZTX751 transistor to replace TR5.

Buy at least three of each type, just in case one of the DRAM chips has died, as it may overload and kill replacement transistors.

After fitting the transistors, apply power, then test the voltages with a multimeter that I listed in my earlier post. Test the -5V supply FIRST. If they are out of specification, immediately disconnect the power.

TR4 and TR5 are part of the DC-DC converter power supply. This takes the nominal +9V DC input and converts it to -5V and +12V.

Please report back.

Mark

Hi,

Thanks for your input. I have ordered the transistors, and when I come back from vacation will replace them, and see how it goes from there.

Regards

Hi,

I was able to replace the transistors and the computer now works again.

Thanks
Regards

Well done and congratulations

I hope you enjoyed the feeling of fixing it and getting it to work.

Mark