can the upper ram fail because of low amperage

[rleao]

Hi all,

i've got a zx spectrum 48k from ebay w/o a pówer supply. I had a 600mA 9v around and the computer started apparently fine (i removed the rf circuit). Then after failing to load some games I found that the computer had 16kb only available and after the poke xxx,85 xxx,170, it pointed to the ic20 memory IC. I ordered a few chips from retroleum and they will soon arrive. However, after reading an answer from another post, it crossed my mind if the weak power supply could be doing it? Would it be possible that a low current power supply is producing these failures? thanks heaps!

richardson

[1024MAK]

How much power is needed depends on which issue board you have. Some really do need 700mA of current at around 9V to 10V.

Lack of power will not harm the ‘upper’ RAM, but may result in a malfunction.

It is strongly recommended that you get a power supply rated at 1A with a DC voltage of between 8V and 10V. Do check the polarity before connecting and powering up.

If there is still a problem with IC20, then it could have been faulty before you got the ZX Spectrum.

Mark

[rleao]

thank you! i ordered a 9W 3A for that. Do you think changing the voltage regulator for a TSR 1-2450 would help?

[1024MAK]

Do you think changing the voltage regulator for a TSR 1-2450 would help?

Yes, that definitely helps. A modern DC-DC switching converter like a TSR 1-2450 will be better than 80% efficient. Hence the input current from the nominal +9V supply will be significantly less. And because these modern DC-DC switching converters are more efficient, they don’t generate around 2W of heat like the 7805 does.

Mark

[rleao]

awesome! thanks for all the help! I took a variable power supply from the lab and the memory was still failing (poke 32768,85 returning 117 with peek). One question though, after changing the voltage regulator, is the improvement of the -5 supply still necessary? I am in that age in which my eyesight is starting to deteriorate so i cannot be that avid reader of last year so help is more than ever welcome...

PS: I had a apple 2 clone as a kid (that costed way more than the sinclair clone - https://www.old-computers.com/museum/co ... t=1&c=1161) and i am thinking how much cooler it would be to have the sinclair clone... (https://www.old-computers.com/museum/co ... st=1&c=945)

[1024MAK]

One question though, after changing the voltage regulator, is the improvement of the -5 supply still necessary?

Ask as many questions as you like... I may not answer all of them though

Changing the 7805 and fitting a Traco Power, Recom or other good DC-DC converter will reduce the heat inside the case, helping the other components (membrane ‘tails’ or ribbons’ and the chips) to last longer. It may also solve other problems if the 7805 was misbehaving.

The 4116 (or equivalent) ‘lower’ DRAM chips need a supply voltage of +12V, a bias supply of -5V and an interface supply of +5V. Because the loss of the -5V supply can damage these DRAM chips, if you have a board that is earlier than an issue 4B and it has not had it’s TR4, TR5 and ‘coil’ based DC-DC converter circuitry upgraded, then I recommend upgrading it to the last Sinclair modification as shown here.

Mark

[rleao]

got a new chip from retroleum (the poke test in the manual pointed to ic20) swaped it (unflortunatelly without a socket as i didnt have one) and it is now working like a charm. thanks for all the help!

[1024MAK]

Good to hear that you got it working okay

Mark