I purchased a RETRO.care MAXduino kit to build up for used with my ZX Nuvo 128 to use with .txz files. I found it was well packaged and the provided instructions were good. It is a quick and easy build. The kit came with all the required parts to do the assembly. I found a case on Printables to fit it. I printed up the case in white, but did a blue print in place inlay on the buttons. This case was a bit bland, so after building and testing, I made a few minor changes to the case and reprinted it.
It was a quick and easy build. Put the capacitors in the right place as there are 2 values, put the LED in in the correct direction. That is about it. The microcontroller ic was already programmed with the latest firmware. I guess I started putting it together before taking the pictures.
Everything is nicely labeled on the pcb design.
It didn’t take much time at all to assemble with so few parts. To get the pcb in the 3d printed case, I had to shorten the Reset button a bit. It seems someone managed to wedge the button in, I guess by putting the button stem through the hole and flexing the other side of the case out enough to get the audio jacks to fit in. I preferred to shorten the reset button, as I don’t want it to stick out too far and accidentally press it anyways. It is basically flush with the outside of the case, but easy enough to press when I want to.
After that it was down to plugging in the remaining parts and putting it into the case.
I tested it out and it worked great. I was able to load .tap and .tzx files without any issues on my Nuvo 128.
The case was a bit boring. I decided to make it a bit more colorful and fit the style of my Nuvo 128 a bit more.
To make the print in place inlays. I created an inlay model for each of the 4 colors in Tinkercad, plus the primary case model with the hollows in it. I make a skirt like ring around each set of models that I export with every part. I choose my first color to print, then once it is finished, I set the printer to maintain the bed temp, and swap and purge the filament for the next color. I have it print with zhop on, and the inlays are either 1 or 2 layers in height. I remove any skirt, and purge line, then print the next color model. Then repeat that process for each color. Finally I print the main model over top. You can see the Green line was kicked loose a bit, but it was good enough that it wasn’t worth starting over.
I made two other minor changes to the case. I created an angled fill in around the USB Port to close the hole up a bit. I also found that the back plate set deeper into the back of the case than I liked, so I shortened the lip a bit so that the back was more flush. It was minor, but I like it better that way.
This device is similar to a Tapduino for the Commodore 64, but is not compatible with it. I purchased it for use with the ZX Nuvo 128 for .tzx files, as it can already use .tap files. I can use it with my Timex Sinclair 1000 though. I don’t know if I will get any of the other compatible computers in the future, but I don’t currently have plans to.
I like the old 8bit computers. Being in the US, the ZX Spectrums are not really around here. I while wouldn’t be against getting an original Spectrum 128, I like building things. They also make reproduction cases based on the original 16/48k Spectrum case. I found the DonSuperfo’s reproduction Spectrum board designs. He made a 48k reproduction as well as he has several designs for 128k Spectrums. I found the ZX Nuvo 128 Rev 4A design of his. This version is his 128k based design with a DivMMC built in, and it fits in the 48k Case.
The Spectrum 128 portion is mostly through hole components. The DivMMC portion has a CPLD. There are a few surface mount parts otherwise, and I didn’t have any trouble with anything except the CPLD. The CPLD is very fine pitch, and was not designed for manual soldering with a soldering iron. My smallest iron tip still touches a couple pins at a time, note the smallest tip wasn’t the best choice. While I got it in the end, I do not intend to be soldering any more of those CPLDs with an iron in the future if it can be avoided. Due to that I am very glad I didn’t attempt the Rev 5a with a second even larger CPLD on it.
I used Don’s documentation on Github. To get the parts, I started by ordering a HARLEQUIN 128K REV 2D kit from ByteDelight. It is a starting point for the build. I started there to get the bulk of the parts from it, there are a few parts that were not needed. Since I am in the US, I ordered the kit from ByteDelight’s Ebay listing, which was the cheaper way to go due to shipping etc. If I ordered from his website, I would have likely purchased the “Parts Only Set (NO PCB)” option.
I would have went with the Rev 2d kit overall, except it doesn’t support a couple things, and I wanted the built in DivMMC. ByteDelight’s kit is great with the instructions that it comes with, but they are only available with the kit, and I could not get the BOM for the 2d at the time. Don has since posted the 2d BOM on the Superfo Harlequin 128 Facebook page files section.
I ordered my Nuvo 128 Rev 4a PCB from DonSuperfo by way of contacting him through the Facebook group. It can alternately be ordered from PCBWay’s Projects section. I didn’t need 5 of them, and I didn’t want to try to sell the spare PCBs somehow. Unfortunately for me the CPLD wasn’t included, which would have made the build so much easier. I think he does sell them with the CPLD, I just didn’t know to ask. The CPLD was $16.00 from Digikey, but it was quite difficult to install. It also took a bit of work to program, as I had no Jtag programmer around.
Since I couldn’t get the Harlequin 128 Rev 2d BOM at the time, I had to wait for the 2d Kit to arrive. Once I had the kit, I took the instructions from it which have the parts listing and the Nuvo 128 4a BOM from Github to find what parts I would still need.
Then I placed an order with Digikey to get the remaining parts, well mostly. I had to order the one obsolete ram chip from Ebay. I also had the various additional diodes and capacitors I needed in my inventory as well as a couple of the 74hc series ics. Below is a list of the parts I purchased, but a couple parts are missing, I will have to find the copy of the BOM I had edited to find the last few items to add to the list below.
1) UMC UM61512AK-15 a single one is needed as well, that is an out of production part. I ordered it from Ebay.
27) additional 1N4148 diodes are required.
Various capacitors are required that were not in the 2d kit. Some that were in the 2d kit had to be swapped due to available space.
I believe a couple parts are missing from the above list due to me having them in my inventory already. I will have to review the full BOMs. There were a couple IC Sockets that were required, and for some sockets I decided it was best to use machine pin header strips rather than cutting up regular sockets.
That is the parts to build up the Computer, minus the case and keyboard, which I sourced otherwise.
Building the Nuvo 128 4a:
I started with all the small low stuff. The AD724 NTSC/PAL Encoder was first. It came presoldered to the Harliquin 128 2D PCB, so I had to desolder it from there. I used my hotair gun for that. I then used my soldering iron to solder it onto the Nuvo PCB. I had thought of leaving it on the board and buying the AD724 from Digikey, but it was $26 just for one. Next the Diodes and Resistors. I think installed the Transistors and Resistor Arrays next. I did be sure to not build up parts around the CPLD though. I could have done the CPLD first, but I wasn’t up to it yet. I wanted to get in some soldering practice and work up to that.
On the board J9 is to switch between “Composite Sync” for RGB mode and “Composite Video” on the 9 Pin MiniDin. It is made with the pins shorted for “RGB” mode, making it so you can’t switch it. So I cut the trace on the bottom of the PCB. ”Composite Video” can be used as the Sync signal for RGB video depending on your display/upscaler. I was having issues with using the RGB “Sync only” signal with RGB, and I wanted to be able to use plain Composite Video as well, so I switched it to Composite Video. It my have been an issue with my upscaler’s connection at the time.
Next I put on the CPLD and after that the MicroSD Card Slot. Notice the missing Diodes by the MicroSD Card Slot, they were left off to provide room to solder on the Micro SD Card Slot. I then went back and finished the Diodes. The MicroSD Card Slot was easy to install, but that CPLD wasn’t. Even though the CPLD looks good in this picture, it did not work properly. I tested all the pins I could and used my microscope to view it, I couldn’t find any faults. I was able to program it with the JTAG J17 header there just below it, but it wouldn’t initialize once the Nuvo 128 was built. In the end, I had to desolder it (using hot air). I then cleaned up the pads and CPLD and reinstalled it, partially soldering with my soldering iron and partially with hotair. It then worked properly.
That is a MicroSD Card Slot. The CPLD is tiny and has a lot of pins. The header below it is standard 2.54mm pin spacing.
Nearly ready to install the ICs.
Note: the Keyboard Connectors are 1 sided and must be installed turned the correct direction (which is different for each).
Above about all that was left were the switches, and the 3 directly soldered on ICs.
Programming the CPLD:
I didn’t have a JTAG programmer. With some direction from people on Don’s Facebook Group page, I looked around and found how to setup a Raspberry Pi 3b as a JTAG Programmer. The guide was old, and the one required part for it to work is no longer part of the default pi os, I had to find an old clone of the Github repo to get it to install properly. After that I found they had an “updated” guide that was to work, but I couldn’t even get that to install properly. So at this point I programmed the CPLD. Directions I used: with the work around of finding the other repo: https://linuxjedi.co.uk/2020/12/01/programming-xilinx-jtag-from-a-raspberry-pi/ The “newer” directions, which are also linked in the above that I couldn’t get working at the time. https://linuxjedi.co.uk/2021/11/25/revisiting-xilinx-jtag-programming-from-a-raspberry-pi/
See the Programming Warning below..
Warning: As seen in the picture below I programed the CPLD “before” putting on any of the other ICs (except the video encoder). This worked fine.. But when I found the CPLD wasn’t working properly, I tried to program it again after installing all the ICs, and nearly fried the Pi. It seems it was trying to power the Nuvo 128 5V components by the 3.3V VCC pin on the JTAG programmer. I expected the VCC pin on the JTAG Header was going to be the 3.3V which only goes to the CPLD, as that is what we are programming with the header. It appears that the regulator doesn’t prevent reverse voltage back feeding from the “Output” back through it and out the “Input” pin, and nothing in the design otherwise is setup to prevent it. It melted the casing on my dupont connectors and melted the VCC and Ground pins loose in a matter of seconds. As I was watching the Pi’s Video output it listed a Low Power warning, which warned me of what was going on. I tried to pull the jtag header, and the VCC and Ground wires came right out of the dupont crimps. Thankfully it didn’t do any damage beyond the connectors. To verify the programming, I then reconnected the JTAG header, but “Except” the VCC pin. I powered the Nuvo 128 by it’s main power jack, which powered up the CPLD. Then the JTAG Pi programmer was able to read the CPLD and program it, although it was already programmed properly. That did verify it was likely working and that it was soldered on as well as before at least.
Finishing Up the PCB:
Once the CPLD was programmed, I soldered on the 3 ICs and installed the other ICs in the sockets.
I missed installing the top IC before testing the board, it was not critical though. It is for the Joystick port.
The buttons were not installed yet, and I was waiting on the 5 position dip switch. I rigged up a minimum Composite Video connection for testing. With the dip switch not installed, Don’s ROM defaults to the Test ROM and the DivMMC disabled.
Composite Bodged in
Further testing revealed that the DivMMC was not working. That took me a good while to sort. I examined the soldering, checked for shorts and found pinouts for the Rom and CPLD. I traced the pins for the DivMMC ROM, Ram, and CPLD unable to find an issue. The CPLD pins are so small that it was difficult to verify I was on the right pin though even with test leads with very sharp tips.
I eventually decided to remove the CPLD and try putting it on again. I removed the JTAG programming header, as it was in the way and was going to be melted with the Hot Air gun. I covered up the nearby parts and sockets with Kapton tape to help prevent melting them. After I used hot air to desolder the CPLD, I cleaned up the pads and the CPLD pins and resoldered it on. I did then connect up the Pi3 to the JTAG header… see the warning above, I nearly fried it due to how VCC seems to be wired on the PCB. After verifying the CPLD was programmed ( I downloaded the programmed data to the pi, then programmed it again, and downloaded it again then compared the downloaded files.). I then put the properly prepared Micro SD Card (must be formatted properly, and must have the esxdos files of the matched version on it https://www.esxdos.org/index.html), and it worked!
Running a demo from the DivMMC
Preparing the Case:
Now that it was fully working with the DivMMC and the 5position Dip Switch had arrived, it was time to make the PCB fit into the 48k case. The opening where the 9 Pin Mini din ( old rf opening) needs enlarged along the bottom edge a bit. The opening needs made for the DB9 Kempston Joystick port. The opening for the Micro SD Card Slot needs made. The two openings for the buttons on the side need made. Since I wanted to use the Multi ROM, I wanted the Dip Switch to be accessible as well, so it needed an opening. I have since seen another of Don’s boards done by someone that put the Dip Switch on the bottom of the PCB, and cut out an opening in the bottom. That looked very neat, I don’t know where that would align on the Nuvo though, and it was well after I finished this build and had my opening in the side of the case for the DIP Switches.
For the Buttons, I kind of messed up and put the openings lower than they should have been, then I decided to “slot” them “upward” toward where they actually should have been. Since I had those odd openings I need to come up with a way to make them look a bit better. I made up a model for a button cap in Tinkercad. I printed it out of TPU. It fit on the button stems perfectly, and after the first test print, I made an adjustment and they worked great. They stick out just enough to press, they close the holes up well, and the TPU has just a tiny bit of give. The MicroSD Card Slot came out great. The DIP Switches are accessible even if it is not quite a perfect opening.
On to the Keyboard:
It was on to the Keyboard. I had found there is a replacement “Velesoft” anti-ghosting microswitch keyboard for the 48k case. I wanted that rather than the membrane keyboard. They are available from ZXRenew.co.uk who does the Reproduction cases such as I am using as the “Zelux”. AmericanRetroShop on Ebay in the US sells the Velesoft keyboard labeled as the MechBoardZX. Bytedelight also sells the keyboards, again not in in the US. I purchased my case and MechBoardZX from AmericanRetroShop due to shipping etc to the US. From what I see they look to be identical PCBs, all labeled up with the Velesoft design and just different branding otherwise on the silkscreen.
I did order the baseline keyboard without the LEDs. When I went to put it together though, I had decided to install the LEDs. If I decide I don’t want them on, I can always unplug the power to them. I had the LEDs and required resistors in my inventory so it was just a bit more time to install them. I couldn’t find what resistors came with the kit, I used 1k resistors I believe. In the end I think the Blue LEDs could have had a slightly higher value resistor, while the Green LEDs would have been better with a slightly lower value resistor. With the Nuvo 128 4a, it has a 5V Power header by the left Keyboard connector labeled as J15. I expect that header is intended for this type of use. I based my resistors on using 5V for the power, so there is no need for a Buck Converter Regulator as is generally included in the LED kits. I don’t see why they do that, and don’t setup for 5V power.. Yes the power losses are a little higher, and for an original ZX Spectrum, that may be an issue.
The PCB is very thin, and flexes. To keep it in place while I worked, I taped it to my silicone work mat. That way I could move it on my bench, but it wouldn’t move on its own.
Baseline KitConnected up, including 5V Power J15
Finished Setup:
Now that the Nuvo is finished and I built a new GBS Control with a RGB SCART Input on it, I can run the Nuvo 128 in RGB to any of my VGA, or HDMI Displays with audio. I don’t plan to primarily use it with my bench monitor. I can use this over on my desk that I keep setup for use with my old computers. I also built up a MAXduino kit, which I customized the case to be a nice match for the system. It is a quick and easy build from the Retro.care kit. It lets me load some games that I only have in the .tzx format which is not compatible with the DivMMC.
Some Notes.
C6 is a very tight fit, I had to find a capacitor small enough to fit there. It wedges in between U46 (the Regulator), J4 (the power jack), and SW1 (the 5 Position Dip Switch). It seems that could be adjusted with a bit of work, as the other versions don’t have it quite so tight. The value the capacitor is makes it a bit tougher to find that is small enough in diameter.
C28 and C29 need to be fairly small as well. In my case I had some rather large diameter ones that are very short, so they are raised off the board to clear the components around them. They can be found though, and thankfully I could use the ones I had due to them being so short.
U57 and U53 are a bit too close, they rub each other when installed. It just feels awkward.
As mentioned above the JTAG header J17 for programming the CPLD will backfeed power through the 3.3V regulator to the rest of the Nuvo board. This caused the JTAG programmer to try to power the whole ZX128 after the ICs were installed. I did find that something such as a 7805 Regulator will have that problem if power it applied to the “Output” while no power is on the “Input” side if there is a load on that “Input” side that can draw power. With the Nuvo 128 there is a lot of stuff on the 5V Input side of the 3.3V Regulator. I think a Diode just before the Regulator Input would prevent that, and as the DivMMC CPLD doesn’t draw much power it shouldn’t be a big deal if the 3.3V regulator can handle the slightly lower input voltage that would cause.
J12 for the ESP8266 module has the VCC Pin wired to the 5V Power rail. The ESP8266 Module that fits there is a 3.3V device. Some are “labeled” as “3.3V/5V” but I think they are “mislabeled” as none of them that I have been able to find have an onboard 3.3V regulator. The Nuvo128 has a 3.3V regulator for the CPLD, it could have been wired to J12. Provided it can handle the extra load. I have thought of modifying the board to get the 3.3V over there. I do have the module, the 3.3V powered one, but it doesn’t seem worth the trouble. It looks to be of limited use.
I don’t understand the choice to wire the 9Pin Minidin up with a Jumper J9 to switch Pin 4 between “Composite Sync” and “Composite Video” (which is defaulted to “Composite Sync” . The pinout is based on the Mega Drive 2 / Genesis 2 pinout. With the Mega Drive 2 / Genesis 2 9Pin Minidin port Pin 4 is “Composite Video” and Pin 5 is “Composite Sync”. On the Nuvo 128 Pin 5 is not connected to anything, and Pin 4 is by default “Sync” not “Video”. I expect there may be at least small quality improvement when using “Composite Sync” for “Sync” instead of “Composite Video” for “Sync”. It seems it must be small, as it looks that most of the RGB SCART Mega Drive 2 cables don’t use “Composite Sync”, they use “Composite Video”. I thought of wiring Composite Sync to Pin 5, but the output is great with using Composite Video for Sync so it doesn’t seem worthwhile. The little change of putting Composite Sync on Pin5 and removing J9 seems like a reasonable idea. I also find it a bit odd to use that type of cable, when the Resistors and Capacitors need to be removed from the cable to make it work properly with the Nuvo. Now if I use that cable by accident with at Genesis 2, it won’t do the upscaler, or display any good I am sure. It does make finding an “almost ready to use cable” for the Nuvo easier though.
Today my order from Jameco came in. Part of that order included replacement capacitors for the Electrolytic Capacitors in the Timex Sinclair 1000. There are only two in the computer a 22uF 16Volt and a 1uF 16Volt capacitor. I ordered the closest replacements that Jameco had, which was a 22uF 25Volt Aluminum Electrolytic, and a 1uF 25Volt Tantalum Capacitor (which btw are also a form of Electrolytic Capacitor, polarity does matter). They didn’t have a 1uF in an Aluminum Electrolytic, but they had the Tantalum which is a valid replacement for it. We specifically want to maintain the microfarad (uF) value, but a higher voltage rating is ok, try to keep it close as it does have an impact if it gets to far away from the original. The higher the voltage rating the larger the capacitor as well typically. With other suppliers you can find exact replacements, but I was placing an order with Jameco anyways, and I haven’t ordered from larger shops and also didn’t want the cheap china capacitors. I don’t know for sure Jameco’s are much better, but I hope so.
Replacement Capacitors
There is a difference with Tantalum capacitors in while they are Electrolytic capacitors and polarlized, the positive side is what is marked. Where with aluminum electrolytic capacitors the negative is marked. Keep that in mind if you go to swap them. They are more expensive, but I only needed 1 and it wasn’t a big deal.
Before I switched them out. You can see the two light blue caps there on the board.Here are the new capacitors installed.
So I fitted in the new capacitors. With the Tantalum I decided to insulate the one lead there, the positive one in this case. So I used some red wire insulation on it. If you look a these boards, the capacitor pads are doubled, as in there are a few closer points and wider points. The original 1uF was a radial capacitor, so it had long leads like the Tantalum as well, but you can see further to the right the “white” line there with a solder point and a “+” which is the solder point that would be used if it was an axial electrolytic like the 22uF is.
I tested the new and the original capacitors. The 1uF 16Volt came back with 1.114uF 1.1% voltage loss and 2.2 Ohm ESR. The replacement was 1.045uF .2% voltage loss and 2.5 Ohm ESR, so the old one was pretty close to the replacement values. The 22uF 16Volt was a bit different. The old one was 34.1uF 10% voltage loss and 15 Ohms ESR, while the replacement is 20.8uF .9% voltage loss and 0.86 Ohms ESR. I don’t know exactly what I should expect, but my thought is that the 22uF appears to be getting out of specification pretty well.
Once I had the new capacitors installed I did test that the computer booted, which it did. I then moved on to the 16k internal ram upgrade, as my 16k Ram expansion does not work, and it is an easy modification to do internally.
With the Timex Sinclair 1000 it is very easy, the computer came with a 2k ram chip already installed in a ram socket. That means I just needed to get a 62256 ram chip which is 32k, but I will only be using 16k of it, and run 4 wires.
2k Ram in the socket
You can see the original ram is in the socket, but it does not fill the socket, and is actually not even installed in Pin 1. I am sure many or even most ZX81 computers do not have any socket or may have the smaller footprint socket in place. GadgetUK164’s ZX81 had the two small ram sockets installed on his board, he used some header pins so that he could leave the original smaller socket under the larger ram chip. There may be some that have a smaller socket as well that is missing the top 4 pins, which could be removed and replaced with the full length one, or just clip the pins from another socket or use some header pins like GadgetUK had to extend the socket.
So that I didn’t have to damage the circuit board, I straitened the required pins on the new ram chip.
Pin 1, 21, 23, and 26 with the wires.
Next I pulled the 2k ram chip and installed the new chip. The 4 lifted pins are bent up so they won’t make contact with the socket pins.
installed
Above you can see the chip inserted and wired to the board. The wires are attached at D1, D2, D3 and D5 (yes we skip D4). Tynemouth mentions the order doesn’t matter, but they are in that order to be the easiest runs. I used some 30awg solid core wire for this, which also happened to come from Jameco today. I figured it was a pretty color and they are all datalines. I just tacked them to the diode leads. Tynemouth had desoldered the diodes and put the wires through the holes. Diodes don’t like a lot of heat, so soldering to them can risk breaking the diode, but desoldering then resoldering them can too.
Another view
The way I did it is not quite as pretty as Tynemouth’s example, but we are talking about a mod that includes straitening pins on an ic and having flying wires on them. He desoldered the diodes and put the wires through the holes with the diodes. Doing it the way he had ensures it won’t have a chance of the wires coming loose. I didn’t want to risk desoldering the diodes, I didn’t have a great desoldering iron, and these boards are delicate. I needed to be sure I had good contact, and not a bad solder joint that will fall off of the diode lead there and short to something else. I tinned the wires with solder before putting them to the diode, and added a dab of fresh solder and flux to the diode lead first. If it was corroded a bit the solder may not take, but it took very nicely and also adding that bit of solder does add some new flux to clean the point. I had my iron set on 300, which is what I typically use for board soldering. I also use leaded solder, which is what these old computers had to start with too.
The last bit I did to the Timex 1000 here was to add a heatsink to the ULA chip.
Finished
I have now finished the modifcations I had planed for this computer. I just need to wait for the thermal glue to dry on the heatsink before reassembly tomorrow night.
The next thing I need to do is reinstall it in the case. Then I will be testing the original keyboard to see if it is working properly as well as the computer itself. I hope to have this back together tomorrow night and see how it works out.
The Ram Upgrade works.
I got it put together and ran the check posted on the Tynemouth Software post. It shows 32k, but it is showing the 16k Rom plus the 16k Ram.
The keyboard tested good and is fully working. I had a problem with the PAL/NTSC switch (the old channel switch), was offset a bit to far to the edge of the board, and I could not get it set in the NTSC setting while inside the case. I had to heat the solder and push it to the one side to give it proper alignment in the case. The holes for the switch are very large, you can see that in my Part 1 post where I have the switch and the cleaned out holes. The switch now lines up perfectly and works as intended.
I found the heatsink on the voltage regulator had no heatsink compound on it at all. I removed it and added some silver thermal compound and reinstalled it.
After starting to put the system back together it wasn’t working properly. It was an issue with the ULA, if I bumped it then it would get flaky or work. I reseated the chip and then it was working again. I hope it continues to work. I did find someone has made modern replacement ULA chips, hopefully this one will be fine an I won’t have to get one. I have a Samsung LCD TV which I had been working to repair. I finally finished it last night and here is the Timex Sinclair 1000 Composite output on it. It looks very good. You can see it beside the C64 Mini there which I was also testing the TV repair with. They look kind of cute beside one another. You can find info about the TV repair in another post here.
I recently picked up a Timex Sinclair 1000. I first tried connecting it to my LCD TV which does have an analog tuner, but I couldn’t get the signal on it. I then dug out my VCR from the storage area and connected to that. That worked just fine. I am not sure why the TV wouldn’t take it directly. The signal looked pretty good too. I really didn’t care to use the 1000 on the TV all the time though, and not with the VCR all the time either. I went on and looked into a Composite Video modification for it. The modification is done with the same little transistor circuit as the Atari 2600, which I have done before. This is only when the system has the later ULA 2C210E chip such as mine, the earlier ULA 2C184E does not generate quite a standard video signal, so that requires a different circuit. I don’t know if any of the Timex models have the early ULA chip or not. The Timex Sinclair 1000 is a rebranded Sinclair ZX81. The RF Modulator is different for the US Timex model (and the US ZX81 which they did sell some of), there are a few other differences. The Timex 1000 came with 2k of ram rather than 1k like the ZX81 came with and outputs to 60hz NTSC video while the UK model does 50hz PAL video output.
Although in his video he had the OLD ULA chip, and the simple Transistor circuit didn’t work properly for him, he went on to make the more complex circuit later on. His initial bit was using the transistor circuit such as I ended up with.
I found good schematics for the ZX81 here with other useful info. This is where I ended up getting information as to what the various extra Modulator connection labels were.
I would post the schematics and other bits from the sources, but that is their material, this is how I used their material, you would need to build the transistor circuit from TFW8bit, or you can buy the board they sell which would simplify matters.
I went with a little different approach than I initially intended. I was going to leave the RF Modulator in but disconnected, that didn’t work out. There wasn’t much room, and I couldn’t remove the capacitor and resistor(?) and wire from the RCA jack easily, and I couldn’t get the strip board to fit reasonably with the Modulator still in the box. In the end I removed the internals of the RF Modulator and installed the new circuit inside.
I want to note that with the ZX81 and the US Timex 1000 (and I guess the ZX81 US kit), the pins used for the RF Modulator are different. It uses the “USA” marked pins, it uses 3 of them.
For the US model, it uses 3 wires instead of 2 for the UK versions. USA3 (Not FR3) is the far left wire, that is apparently the Video in signal although I don’t know where it comes from exactly, as that is not the direct pin from the ULA video output. The next USA2 is actually +5Volts. Then USA1 is the last and it goes down to the Channel 2/3 switch. That switch actually switches between Ground and +5Volts ( I use that later as Tynemouth did).
For the Composite signals, I actually removed the RF Modulator board etc:
Modulator bits, the board, with the 3 wires, and the Capacitor and resistor, and the output wire on the right.
Here is the board installed, it is built as TFW8bit’s stripboard, although on a larger piece of board so it would stay solidly in the modulator box.
Composite Video circuit.DIGITAL CAMERADIGITAL CAMERAThe Black is Ground and goes to FR3, Orange is going to UK2 that goes to the ULA video. Red is USA2 which is +5Volts
Above you can see the wires used. Originally the modulator of course used USA3, USA2, and USA1. In this case I am only using USA2, which is now the third wire not the middle one going in. The first is going to FR3, which is actually Ground, yes the modulator chassis is ground as well, but I didn’t care to solder to it, and had the opening for the third wire. The middle wire goes to UK2 which is the video directly from the ULA chip pin, and not the round about way that USA3 apparently gets it.
First I tested this out and it worked great. Here it is via the Composite input on my little LCD monitor I keep on my bench.
Success.
There is a second modification though. The channel switch is no longer needed, and it can be re-purposed for something else. Tynemouth used it as a NTSC/PAL switch, which is a simple change. This involves R30, which goes to the one pin on the ULA, and to ground. It is a 10 ohm resistor that when pulling that pin to ground causes the system to operate in NTSC 60hz video mode. When it is disconnected from ground the ULA sets the system to run in PAL 50hz mode. To do this I reused “USA1” which goes to the switch center pin already.
USA1 ready to use.The Grounded side of R30 lifted from the board. R30 wired back inUSA1 usedHeatshrink insulationR30 now going to USA1, the exposed lead is covered with clear heatshrink tubing.
Next the Switch needs a bit of a change. As I mentioned earlier the switch changes USA1 from Ground and +5Volts. We don’t want to wire R30 to +5Volts. To do this you could remove pad from the circuit board, or you could trim the wire from the switch and ensure it was insulated. I didn’t want to alter the board, so I altered the switch.
OriginalPin removedInsulated with liquid Electrical Tape
Next I installed the switch and tested it. Be aware, I made a mistake installing the switch..
Switch to the Right “PAL” (Switch installed on the wrong side of the board… see below)Video output, PALSet to PAL 50hz mode.Switch to the Left “NTSC” (Yes the switch is still on the wrong side of the PCB.. see below)Video output, NTSCSet to NTSC 60hz mode.
So, some may notice I installed the switch to the wrong side of the board. So this won’t work in the case now. I realized this shortly after putting all of the tools away, but before I was going to reinstall the board.. So I desoldered the switch again and installed it to the bottom of the board like it should be. When I put the switch on, I reinstalled the old paper label they used to insulate it, and to beef it up a bit, and cover the hole from the old pin that went into the +5Volt pad, I put a little piece of Kapton Tape, you can’t see the tape in the photos though as it is under the original paper label.
Switch installed properly..
I then retested it. I have another bit I am looking to do with this board and that is to put in a 16k ram upgrade on board based on Tynemouth’s post on it, and that again GadgetUK has done a video on. It really is not going to be anything new, while the Composite mod, I did a little differently.
I haven’t tested the keyboard on this unit, I did order in a replacement keyboard for it, and if I don’t need it I will keep the original on it for now.
To finalize the upgrades, I have used a little Brother Label maker to update the Labeling on the case. They are stickers so it can easily be removed. They stick very well, so they should hold up fine. For this one I purchased some White on Black Background labels. I use these labels for various projects, and have been using them for years with good success. (Note: be careful with leaving batteries in these Brother Labelers, they drain batteries when off apparently and then the batteries leak. I have had it happen to the labeler at work, and my personal one here at home.)
The computer did come with the 16k Ram Expansion, but the expansion doesn’t work. So there is a Part 2 Post where I install 16k Ram Internally on this computer. I had looked at repairing the 16k Ram Expansion, and the one capacitor was bad on it, but replacing that did not correct the problem. They are rather complex boards, and while I wouldn’t mind repairing it, I am was not sure where to start. I thought of stripping out the 74 logic chips to test and such, but that is about all I could do. The internal 16k ram upgrade is quite easy though with the right ram chip and a few wires. It is also cheaper to source that one (32k which we only use half of) ram chip than the very specific odd chips in the ram expansion. I don’t have to damage the mainboard or anything to do the ram upgrade it is fully reversable. I mostly have to swap the onboard ram socket and do the few wires onto the new chip.
Update on the 16k Ram Expansion: I did end up getting a RAM Tester in early 2023. I pulled all of the the Ram ICs from the 16k Expansion, half of the ram chips had failed, maybe when that cap failed it made a surge? Or it lost the 12V power and that burned those out running without it? The 74 Logic ics were all good. I will not be repairing the Ram Expansion as I don’t need it and can not use it. I did fully strip the PCBs of ics to test them, I don’t know if there would be some project I may reuse it for in the future. I did damage one trace but it is repairable and could easily be rebuilt if I wanted to.
Muldrf's Hobbytronic 