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

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). 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, but try to keep it close as it can have some impact if it gets to far away from the original. Also as the voltage increases typically the size does as well. 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. So 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 still did. Then I 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

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 side 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 look very good. You can see it beside the C64 Mini there which I was also testing the TV out with. The 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.

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