Timex Sinclair 1000 Part 2: Recap and 16k Internal Ram

If you haven’t seen it, you may want to check the Part 1 Post about some other updates to this system: https://hobbytronics.home.blog/2019/12/15/timex-sinclair-1000-part-1-multi-region-composite-mod/

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.

To do the ram upgrade I used Tynemouth Software’s guide found here: http://blog.tynemouthsoftware.co.uk/2017/10/zx81-internal-16k-ram-reversible-version.html

GadgetUK164 did the upgrade on a video here: https://www.youtube.com/watch?v=14eL_oWhhXo

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.


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. The real trick though is doing it the way he had ensures the wire has good contact and won’t have a chance of becoming loose. So 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 solder to the diode lead. 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 retro 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.


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 out too as fully working. The problem I have is the PAL/NTSC switch (the old channel switch), is offset a bit to far to the edge of the board, and I can’t get it set in the NTSC setting while inside the case. I have to heat the solder and push it to the one side. The holes for the switch were very large, you can see that in my Part 1 post where I have the switch and the cleaned out holes. I should be able to shift it enough to get it where it needs to be due to the size of those holes. A little update, I did shift the switch position and it now lines up perfectly with the bottom case.

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 out with. They look kind of cute beside one another. You can find info about the TV repair in another post here.

I think the last thing I should need to do is making up some labels for the case. Where it says “TV” I want to put “AV” or Composite or something. On the bottom where it shows CH2 CH3 I want to make up a new label that shows PAL and NTSC as the options there.

Timex Sinclair 1000 Part 1: Multi Region Composite Mod

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.

I used Tynemouth Software instructions on the Composite Mod. Found here: http://blog.tynemouthsoftware.co.uk/2016/11/ts1000-multiregion-composite-video-mod.html

I used the stripboard circuit from TheFutureWas8bit Atari 2600 mod:


I also used info from GadgetUK164’s video: https://youtu.be/5OQuJ-GMwF4

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.

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.


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.

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.

Next I installed the switch and tested it. Be aware, I made a mistake installing the switch..

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. I won’t be reassembling the system though until the ram upgrade is finished. The computer did come with the 16k ram expansion, but the expansion doesn’t work. I had looked at repairing it, 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 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. I don’t have to damage the board or anything, the Timex came with the socket already installed with the 2k ram chip in it. I just have to pull that and do the few wires onto the new chip.