Spectrum Computing

Hi,

I've started building a new Spectrum from a PCB bought online. I've bought just the pcb with very few components and started to gather the other parts. I have the memory chips, cpu, ULA and some resistors.

Regarding the power coil, can anyone help me with the specs for that? Number of turns for both sides and diameter of the core.

I am trying to build one as I think these are pretty expensive considering an ULA replacement can be found for 19GBP vs 12GBP for this coil.

Also, for the LM1889, I've found on ebay some very cheap ones 1.89USD each and some at 12GBP each. Should I trust the chinese sellers? Are those ok?
Anyone tried those?

Some sellers based in China are good, but others are just box shifters / resellers.

The good sellers (who are specialist electronics sellers) are likely to supply working chips. The same may, or may not be true of the other sellers.

A lot of the cheap chips are cleaned up ‘pulls’. Some are old new stock. Some are remarked lower specification parts that have been relabelled as higher spec parts. And some are faulty, partly faulty or fake.

That’s the fun of buying on trading sites

Mark

I am also building the same one. but I recovered all the components from a damaged version.
I suggest you to weld the resistors and capacitors first and then the sockets for the integrated circuits.

I have a few boards that are not working properly but the PCB is not damaged and can be fixed.

So I am trying to build this one with less recovered components.
I am very curious about the results. I know a guy that built one of these but has some noise on the video signal.

So, anyone can help me with the coil specs?

payty wrote: ↑Fri Nov 15, 2019 3:41 pm So, anyone can help me with the coil specs?

I would, if I knew what they were... Sinclair never, as far as I know, detailed the specifications. So to work it out, it means testing and disassembling an existing ‘coil’. Unless I missed something in the service manual.

Mark

In fact it's a quite easy to recreate an original coil. Pear from speccy.pl made an excellent tutorial available here - https://www.speccy.pl/forum/index.php?topic=2236

I made a dozen of them - they all work without a problem.

PABB wrote: ↑Sun Jun 21, 2020 10:32 am In fact it's a quite easy to recreate an original coil. Pear from speccy.pl made an excellent tutorial available here - https://www.speccy.pl/forum/index.php?topic=2236

I made a dozen of them - they all work without a problem.

Does that site require a login? I get a form from your link. In polish.

Can you copy and paste here the coils specs? It sounds like information which needs wider circulation.

It does - but registration it's quite simple (I can help).

I'll ask Pear if I can copy/paste his guide here...

Until then:
primary winding - 14uH
secondary winding - 78uH

coil core - RSZ-8X10-V-4P (available here - https://sklep.feryster.pl/)
winding wire - DNE-0,315-L-GR2 (as above)

viewtopic.php?p=31658#p31658


you can find info in this post.

payty wrote: ↑Tue Nov 12, 2019 6:10 pm I have a few boards that are not working properly but the PCB is not damaged and can be fixed.

So I am trying to build this one with less recovered components.
I am very curious about the results. I know a guy that built one of these but has some noise on the video signal.

Have you completed the job? I also had problems with video signal disturbances ...

I've got most of the parts in place but haven't powered it up yet.

today I have received the nebULA module that goes in place of the original ULA.

For the lower ram I have ITT 4116-4N chips which are 250ns. The Sinclair repair manual states that the lower ram should be 150ns.

So far I have used the 250ns chips to repair some of the computers from my collection which had dead lower ram chips. And they appear to be working (tested them 30-60 min with the Retroleum test ROM). At least 5 computers have been repaired like this.

Should I expect to have any problems because of the different specs?
Are there any 'real world' tests that I should perform? I am thinking about certain games that could not work because of these, or other situations.

About the speed rating / access time of chips: typically a manufacturer will aim for a design speed when manufacturing the chips. However in production, some will work okay at the design speed, some will be slower and some may be faster.

Hence before putting the chips into their encapsulation, they are graded. The faster chips are then sold at a higher price and the slower chips are sold at a lower price (often there being far more slower chips than faster chips).

As time goes on, improvements in the production process normally result in more higher speed chips compared to lower speed chips but in order to continue selling the higher speed chips at a premium, some are not always marked up and sold at their actual real speed rating.

You should also understand that the testing done is fairly vigorous, but the resulting grading is very conservative.

Hence a lot of 1980s and 1990s chips can be ‘overclocked’ far more than people expected.

So if they work, then that’s fine. But do keep in mind that you are not the first person to have tried using slower DRAM chips. It does not always work for some makes / batches of 4116 (or equivalent) DRAM chips. The same applies to fitting slower DRAM chips in the ‘upper’ (extension) RAM positions.

In terms of ‘real world’ tests, in a ZX Spectrum (just like most of the microprocessor systems of this era), the Z80 and the memory always runs at the normal system speed. There are differences in timing between the Z80 fetching an instruction and reading / writing data, or reading from or writing to an I/O device. But it’s doing all of that anyway as a result of the ROM code.

The device in a ZX Spectrum that really sets the required speed of the ‘lower’ RAM is the ULA. As it needs the RAM to be fast enough to grab the screen data on time, every time.

So in practice if it runs a diagnostic / memory test okay, then try it with any machine code games you have. Preferably games that either use the floating bus or which have fancy multicoloured effects or draw in the border area.

Mark

Thanks a lot for detailed explanation!

Fortunately, the Retroleum rom has a floating bus test, but I will test the computer with some games as well.