Spectrum Computing

It'd be really handy if I had a 16k Spectrum to assist in my hardware tinkerings, but they're much rarer than 48k Spectrums and perversely always seem to fetch a way higher price on ebay and other such sites...

Fellow Specchums, do you think the gods of retrocomputing would be displeased if I purchased another 48k Spectrum and gently downgraded it into a 16k?

Pervert.

Okay, admins, that’s an immediate ban.

You do know that a simple BASIC command can reconfigure BASIC to think it’s only a 16K machine?

This won’t affect machine code programs. To fool these, a simple circuit made up of wire and a low power SPST (on/off) switch is all that is needed for issue 2 to issue 4S boards.

I don’t recommend trying to downgrade an issue 5 or issue 6A board.

Mark

Well, the feeling in the room seems to be that it would be sacre.. sacril... wrong.

1024MAK wrote: ↑Fri Mar 27, 2020 10:22 am You do know that a simple BASIC command can reconfigure BASIC to think it’s only a 16K machine?

Oh, I'm talking about tinkering together projects that plug into the back. The upper RAM's a bit in the way if I want to memory-map stuff.

Well, I suppose I could just use the ROMCS pin to drop the built-in ROM out temporarily. That'll make a nice bit of space in the memory map. I don't particularly need BASIC for my tinkering...

Back to patiently watching ebay.

I have a boxed, almost mint 16K in my collection. I don't remember exactly how much did it cost me on ebay, but it was not more than a common 48K.

Are they really sought after so much?

Alessandro wrote: ↑Fri Mar 27, 2020 10:44 pm Are they really sought after so much?

I don't know... It could just be the ones I've seen happened to have an inflated price.

I think they're pretty rare, though - I imagine a lot of them would've been upgraded, and after the Spectrum prices were slashed a year after launch or whatever I think most people would've said to themselves, "Might as well pay a few notes more and have the 48k version."

Everyone including Sinclair were rather surprised that the 48K model was flying off the shelves far quicker than the 16K model even in the first year. So the 48K model is by far, the more common version.

Add in people upgrading, and although not rare, 16K models are not easily found. Hence they are relatively expensive on eBay.

@Firefox, there are over 128 free ‘8 bit addressed’ I/O port addresses to play with. More if you use 16 bit address decoding. Or are you intending to memory map something?

If yes, you can always use the ROM area as you suggested. Put +5V on the ROMCS and the ROM disappears from the memory map.

For issue 2 to 4S boards, to disable the "upper" 32k of RAM, take a piece of wire and connect pin 5 on IC23 (a 74LS32) to +5V. (link).

If you wire an SPST on/off switch in series you can enable it or disable it as you see fit. You can even wire in a LED to show the current status.

Mark

I had a 16K issue 2 from my childhood and in about 2006 it died as I tried to upgrade it to 48K. I sent it to some bloke on WoS who offered to fix it, and who then immediately stopped answering PMs and stole it basically. I had even sent him a donor 48K Plus I had bought on eBay as well. And I could see he was still active on the forum. Bit of a shame really.

Fortunately for me I kept the perfect condition case from the 16K, so last year I got a very reasonably priced refurbed 48K issue 2 motherboard from Retroleum and managed to replace the keyboard membrane without damaging the original plate.

1024MAK wrote: ↑Sat Mar 28, 2020 4:39 pm @Firefox, there are over 128 free ‘8 bit addressed’ I/O port addresses to play with. More if you use 16 bit address decoding. Or are you intending to memory map something?[mention]Firefox[/mention]

Yeah, I want to map actual memory...

1024MAK wrote: ↑Sat Mar 28, 2020 4:39 pm For issue 2 to 4S boards, to disable the "upper" 32k of RAM, take a piece of wire and connect pin 5 on IC23 (a 74LS32) to +5V. (link).

If you wire an SPST on/off switch in series you can enable it or disable it as you see fit. You can even wire in a LED to show the current status.

Ah! By "downgrading", I was meaning to desolder IC23 and replace it with a socket, so I could put the IC back (for a 48k Spectrum) or put a low-profile socket with pull-up (a) resistor(s) soldered across it in instead (for a 16k Spectrum). But that's even easier!

Thanks very much for the tip!

Glad I could help.

Do however double, nay, triple check that you have the correct pin on the correct chip, otherwise it could be carnage!

Mark

I don't see an issue with it, the downgrade can always be re-upgraded.

patters wrote: ↑Sat Mar 28, 2020 7:03 pm I had a 16K issue 2 from my childhood and in about 2006 it died as I tried to upgrade it to 48K. e.

It appears that it didn't die.
What happened is that you killed it !

I posted on WoS at the time that it would only load from tape for the first few minutes of being on then the ULA would get hot enough to burn your finger on. The upgrade stuff didn't cause that. Then it failed completely.

patters wrote: ↑Fri Apr 03, 2020 12:53 am I posted on WoS at the time that it would only load from tape for the first few minutes of being on then the ULA would get hot enough to burn your finger on. The upgrade stuff didn't cause that. Then it failed completely.

You didn't need to justify yourself, I was just joking.

But anyway, the process of upgrading it, since it usually requires soldering of at least the extra sockets, might leave you with the uncertainty of having left some tiny bit of solder, somewhere where it shouldn't be.
And I can assure you this can happen without much effort, specially, if you had to de-solder or re-solder any contact, where you used a manual solder pump for example.

That tiny bit of solder can create some sort of short or intermittent short, that can and will probably lead to something being burned by over heating, since a short usually implies a stronger current somewhere.

1024MAK wrote: ↑Sat Mar 28, 2020 4:39 pm Everyone ...

Well, maybe except those who owned an 81 with a wuhuhuoooOoobly 16k pack.