Spectrum Computing

So, the reason for those test values is because we really want four different binary test patterns. With the RAM in a Spectrum, it's divided into the 'lower' RAM and the 'extension' or 'upper' RAM. The 'lower' RAM runs from 16384 (0x4000) to 32767 (0x7FFF). The 'upper' RAM runs from 32768 (0x8000) t...

It's a hang over from the ZX80 (which only had a 4K byte ROM). The ZX81 ROM built on the ZX80 ROM (now 8K bytes in size). Then the ZX Spectrum ROM is a development of the ZX81 ROM... Sinclair wanted maximum features for lowest cost (read keep something that worked, and make everything fit in only 16...

Here's a circuit simulation. Because it does not include a potentiometer, I've used a fixed resistor. Also not included is a LDR, so I have used a clickable logic level input (swaps between L and H) a diode and a fixed resistor. This very crudly simulates day/night as if there was a LDR. https://www...

Let's play spot the difference! Front cover #1 https://i.postimg.cc/fJMrGVvq/IMG-7768.jpg Front cover #2 https://i.postimg.cc/KKrFcVRr/IMG-7769.jpg So which is for the +2 (grey) and which is for the +2A / +2B? Maybe a side view would help... https://i.postimg.cc/CBBNsykb/IMG-7775.jpg Okay, I'll open...

For the 128K, the +2 (grey) manual is useful. It includes a big, big chunk of the original ZX Spectrum rubber key manual. Not to be confused with the later manual for the +2A/+2B machines which are different again.

Mark

Don't confuse the diode like effects when testing a bipolar transistor like a BC108 with how it actually works when used in an amplifier or switching circuit. The transistors operation is far more complex. In this circuit, the transistor is used as a common emitter amplifier. The 100kΩ potentiometer...

Transistor lead outs are always viewed from below unless stated otherwise (this is opposite to IC/chip pin outs which are viewed from above). Yes, a BC108 is a NPN. And yes the emitter is the lead next to the tab. The middle lead is the base, leaving the remaining lead as the collector. The data-she...

If an admin or a mod is reading this, maybe update the title of the first post to better reflect the topic content?

Mark

So, reading through the topic, the remaining issues are the RAM fault and no sound from the speaker, but you do get sound from the jack socket, is this from the Ear, the Mic or both?

Mark

Got my hands on what I believe is a 16k zx , issue 4A As others have said, it's actually a 48K board (as manufactured) But the machine wont load any software. I get the usual blue to red background - when the signal starts i get an intermittent horizontal bar pattern , as if the sound jack was bein...

The RS232 port on the Interface 1 is also bit bashed. For both the 128K machines and the Interface 1, using the Z80 to carry out the parallel to serial (transmitting) or serial to parallel (receiving) means a relatively expensive UART chip is not needed. Sinclair wanting to keep costs low. I can't t...

Here is a web page that will help with the technical details. 128K BASIC has some commands, or changes in existing commands to access some of the extra hardware. The RS232, MIDI/Aux/Keypad ports are all " bit bashed ". The existing OUT command and IN function can access these. They are par...

Well done 8-) :D :dance Don't worry about the breadboard circuitry looking messy, more about that later... I presume you mean this I/O board from Tynemouth Software on the Sell My Retro site? Tynemouth Software made a blog post about this I/O board here . I also have one of these boards. I don't hav...

Don't know if these videos helps...





Mark

From BASIC you can have either: BEEP .1,23.2 REM which has a frequency of 999.3Hz or BEEP .1,23.21 REM which has a frequency of 1001.0Hz The choice is yours.... Anyway, this is what I typed into my Speccy (issue 6A plus): 10 FOR a=1 TO 5: BEEP .1,23.2: PAUSE 45: NEXT a: BEEP .5,23.2 Mark PS I have a...

Yeah just hold onto the barrel plug while running the hot taps and shower to get the room nice and steamy! ⚡ :lol: :lol: :lol: :lol: You'll forgive me (or maybe you won't), but I don't have any mains sockets (*) in the bathroom or shower room. And I get enough tingles from "modern" gadget...

That figure was later updated to 28,000 by Your Computer, which also revealed that the fault was down to the 240V input track on the power pack's circuit board running far too close to the 6V output. Thank you flange :D So out of my list, it's this one: PCB tracks for the mains not sufficiently cle...

Apparently the issue was this: So why the Recall? Some units were produced where the mains 240V could potentially become live on the outer metal positively charged part of the barrel connector – despite this supposedly being the low voltage DC connector. So, that narrows it down to these possibilit...

Keep in mind that Sinclair did not make the PSUs, they contracted the manufacture out to other companies. Hence there at least three or more different styles (they all look the same on the outside as they all used the same case). I don't know which were affected, or what the fix was (I suspect the r...

PeteProdge wrote: ↑Mon Feb 05, 2024 8:26 am Probably misattributed from its genuine case for the Commodore PET.

Some versions of the PET, not all.
Although NMOS 6502 (and the other microprocessors based on the same core) could be made to 'freeze' if it executed certain invalid instructions (12 exist).

Mark

Hi :D Yes, the MB8118-10 DRAM chips will work. Pin 1 (VBB, -5V) will need isolating. Pin 8 which is VDD +12V on a 4116 will need changing to +5V for a MB8118-10. For a HM4864P-2, again these DRAM chips should work. Again, isolate pin 1. And pin 8 now needs a +5V supply. You can leave the existing +5...

The colour and type/make/brand of the capacitors is not reliable indicators. Various were used during manufacturing and they are not matched to the board issue. The board colour (solder resist layer) is also not always helpful. Things to look for are: can you see a heatsink above the edge-connector?...

ZX Spectrum's palette is made of the primary colours of both systems, additive (Red, Green, Blue and White) and subtractive (Cyan, Magenta, Yellow and Black). It's all additive colour. Subtractive colour is for colour absorbing systems, like ink on paper. https://upload.wikimedia.org/wikipedia/comm...

Yello, 'cus it's the colour of the Sun and it brightens your day...

Reasons to be cheerful, part two.

Mark

First reason of course being that you got a Speccy

No worries

Mark