Thursday, 22 June 2017

Microace repair

This is a Microace. A 1980's clone of the Sinclair ZX80 (and nothing to do with the Jupiter Ace).
The colour scheme seems to be a photo negative of the actual ZX80. Can you tell which is which? Looks a bit Andy Warhol, but I think he was more of an Amiga man.
Inside is a PCB which looks very similar to the ZX80 board, with a few minor circuit changes and a slightly different layout.
The dimensions, mounting points, connector locations and keyboard are all the same as the ZX80. The board has a few differences. This one seems to date to late 1980, the Z80 has a later date code, so was presumably replaced at some point.
This is how the board looked when it arrived here. The owner has started the work on the board, replacing the missing 7805 regulator and UM1233 modulator, fixing a faulty ceramic resonator. He has also replaced the capacitors and removed various aftermarket modifications that had been made to the board (including various switches and wiring for an external keyboard). He also removed D9 which put it into PAL mode as it was presumably designed to run in NTSC mode.
The circuit is pretty much the standard ZX80, apart from the addition on two extra RAM chips and one logic chip to address them, making this a 2K machine. This was an optional extra, and has been removed to aid diagnosis.
The other things the owner changed was the ROM chips which was getting hot and was presumably faulty. This had been swapped for a ZX80 ROM, usually a 2532, although I didn't remove the label to check.
There were some modifications around the ROM chip. Due to a problem with was apparently 'out of their control', they ordered the wrong chips or the manufacturer made a mistake. Either way, the A12 line needed to be disconnected from the ROM chip, and that pin held high. The line passes through the pad for the chip, so the track was cut either side and a link made to reinstate the connection for the rest of the board. That all tested out OK.
The original ROM and all four of the RAM chips did indeed test as faulty. The ZX80 ROM I tested on a ZX80, and that was fine. So why wasn't it working. Everything else looked OK as far as I could see?
Now comes the folklore segment. When first advertised, apparently no mention or credit was given to Sinclair or the ZX80. Later adverts (this is from Byte Magazine, November 1980) state this is 'licensed by Sinclair Research Ltd.'.
The same magazine does have an advert for the ZX80, so yes, contrary to general opinion, Sinclair did market the ZX80 in the US, but at $200, compared to $149 for the Microace ($169 for the 2K model).
A later edition of Byte (April 1984) included the Microace advert and a review, but no more adverts from Sinclair themselves. This licensing deal was apparently the result of a court case where Microace claimed their machine was different because all the code in the ROM was different.
But it wasn't different, it was the standard ZX80 ROM, what they had done is wired D3 and D4 the wrong way around and swapped the bits in the ROM. This made it look binary different to the Sinclair ROM, even though it ran identically. I verified this on the board, D3 and D4 were indeed reversed.
To test this, I built up a 2364 adapter board, which allows you to use a standard 28 pin ROM in the 24 pin ROM socket. I cut the tracks on the adapter and wired D3 and D4 backwards.
You might just be able to see the green wires on the left side of the socket. I fitted this and powered on.
Success, the Microace was running again. The options at this point would be to make up a ROM image with D3 and D4 bits reversed and burn to a 2532, or cut the tracks on the board and reverse them so a normal ZX80 ROM could be used, or leave the adapter in place, which is what I have gone for as it seems to offer the most options.
That was using the standard ZX80 4K BASIC ROM image. The hardware should also be capable of running the ZX81 style 8K BASIC ROM, however A12 had been disconnected from the socket.
I reinstated the cut tracks and removed the bypass wire to reattach the A12 line to the ROM socket. The owner then has a choice of the original 4K BASIC or the later 8K BASIC. The jumpers on the 2364 ROM board select one of 8 ROM images in a 27C512 EPROM, 4 in a 27C256, 2 in a 27C128 or a single ROM image in a 27C64.
With that working, I refitted the additional chips for the 2K upgraded, with another two new 2114 chips, and the tried that out. The usual test of RAMTOP is as follows.
This returned 18432 as expected, the top of available RAM is 2K above the 16K base address of the RAM, so the full 2K had been detected.
To protect them from damage, the owner hadn't sent the case or keyboard. A good idea since the plastics are very brittle after all these years. But it did leave me without a keyboard.
The Microace uses the same principle as the ZX80, there are interwoven spiral tracks for the row and column for each switch, and a keyboard overlay with silvered conductive pads on the back to make these connections. I didn't have the original piece, so I made one from the front half of a faulty ZX81 membrane.
The ZX81 actually has the rows connecting the keys, they would have just been circles or squares for each key originally, but it doesn't matter as the rows are connected on the back of the PCB anyway.
The ZX81 membrane will probably go well with the black case, if the owner wants to use 8K BASIC.
Or one of my ZX80 style overlays could be used, as unsurprisingly, the spacing is identical.
So that's it finished, 4 faulty RAM chips and a faulty ROM and some interesting rewiring to avoid copyright (which didn't seem to help).
With that all in place, a bit more testing and it's ready to go back to be reunited with it's case.
If you can't find a Microace or a ZX80, you could always get one of my Minstrel ZX80 clone kits from my Tindie Store.

Sunday, 18 June 2017

Minstrel ZX80 Clone Issue 2.4 Boards

I am now shipping the V2.4 version of the Minstrel ZX80 Clone PCB.
These are now also available in blue, which goes well with the RC2014 keyboard. There is now a kit or fully built Minstrel with RC2014 keyboard available from my Tindie store.
The Minstrel ZX80 clone kit (also in my Tindie store) now comes with the choice of blue or green PCBs.
The blue wasn't actually planned, it was a mistake by the PCB manufacturers, but it's grown on me.
The v2.4 boards fix the issue identified on earlier boards with the pinout of the edge connector.
This means you can use things like the ZXpand (or here the new ZXpand+) directly with the Minstrel.
Note this is still an issue with the CAT command when you have more than a screen full of files in a directory (see previous article). I am currently working with the creator of the ZXpand to address this.
There are also a couple of minor mods such as larger spacing for the main input capacitor and through hole plating on the bolt hole for the regulator.
Also wider spacing on the pads on the video buffer transistor to make it easier to solder.
I've had a few queries about using larger size crystals on the Minstrel. The board was designed for a low profile crystal with an HC49S style case (e.g. this one from DigiKey), but some people have had difficulty locating those.
You can fit a normal HC49 size crystal, just bend the legs to rest the can on the components to the left or the IC to the right. Maybe add a drop of hotmelt glue to stop it moving around.
Regarding the frequency, I have been told that 6.5536MHz is close enough to work, but I have not verified that. You might need to change some of the values of the associated components to match the rated capacitance of your crystal, either the 47pF capacitor or the 1K resistor.
It is not possible to speed up the Minstrel by increasing the frequency of the crystal as this is tied to the TV signal generation, and a high frequency would not give a valid TV signal.
You can still fit these board inside a ZX81 case.
That build used a switching regulator, but you can still fit the original ZX81 heatsink and 7805 regulator.
The full range of PCBs, kits and fully assembled Minstrel ZX80 clones are available from my Tindie Store.
Finally, a video from Louis Scully building a V2.4 Minstrel clone in a white ZX81 case, like the one I built (full instructions here).

Monday, 5 June 2017

Reverse PC build

Most PCs are built the same way around.
On the front you get the power button, a few USB ports and LEDs, and some optical drives.
All the interesting stuff is hidden around the back where you can't get at it.
These pictures are from 2011 when I first built a PC in this case, a Fractal Arc Midi. It has since been rebuilt a couple of times, most recently in 2014 with a Haswell i5 which is still running nicely, so no plans to upgrade that.
The problem still stands that it's a bit of a pain to get around to the back of the PC to plug something new in. The answer is to turn it around. I modified a PC like that back in the 90s, when I needed access to lots of parallel and serial ports on the back of the case. That was a more traditional case with the power supply at the top, so I swapped that to where the optical drives went and fitted a sheet of metal bolted in where the power supply went with the power switches and LEDs, fan controls, 4mm terminal posts wired to the power supply etc. I can't find any pictures of that unfortunately. This one should be easier as the power supply is out of the way at the bottom of the case, so it can stay where it is. The optical drives don't get much use, so I can just use a USB DVD ROM drive whenever required
The front panel still needs to be sorted though. I don't use the audio jacks, so can safely ignore those. The USB ports are handy, but there are already lots of those on the back. However, I use lots of USB devices, so with a few extra brackets, I can redirect the front USB ports to the rear of the case.
This also gains an extra USB3 port as the original front panel wasted one with a single socket even though the internal USB3 connector is capable of providing two.
The power and reset switches and the power and hard drive LEDs are trickier. Then I noticed there were two 3/4 inch holes on the back designed for watercooling pipes. I have no plans to fit an external raditator, but I could use them to fit some switches.
After some searching I found some nice 19mm mounting hole 'vandal resistant' switches which were illuminated. I chose one with a green LED illuminated power logo and one with an illuminated red circle.
I couldn't get the exact ones I wanted, so these are designed for 24V operation of the LEDs, and the power button is latching. To get around that, I opened them up. Inside I found a 1K resistor in line with the LED.
I removed that and reattached the LED across the two connections so it could be driven direct from the PC motherboard as intended.
The latch mechanism was removed from the power switch, which turned it into a momentary push switch which is what I needed.
These were wired up with standard PC front panel colours. Two pin connectors for everything but the power LED which has two single pins (to cope with 2 pin or 3 pin motherboard headers).
Those fit nicely in the hole and will provide the new front panel for the PC.
Obviously, I need to keep the wiring inside neat.
The wires are passed through and run down the back plate.
These come out just next to the motherboard front panel connector block. I had fitted one of those Asus easy connect blocks, which may have been from a previous motherboard, but it makes these things easier to plug in right.
The extra USB ports just about fill the back panel. Four of them are from a PCI card, these are the ones I use for testing USB devices, worse comes to worse, they kill the PCI card, rather than taking out motherboard USB ports.
I put the USB 3 ports in a separate vertically aligned slot, which makes them easy to access for memory sticks etc. Time to put it into place on the corner of the office desk.
The LEDs are working nicely, as are the switches. I suspect this will be one of those things where I will feel a little smug each morning when I turn the PC on via the new switches. It is always nice when you do something useful that you use every day to make your life easier.
That's all wired up, I have better view of the power and activity LEDs, and clear access to all the cabling when I need it, and it's out of the way in the corner so I now have more floor space where it used to be.
Nothing for it now, I'm going to have to get some proper work done.

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