This is the Timex Sinclair TS1500 I repaired nearly a decade ago (how time flies).
It is a nice machine, basically a 16K ZX81 in a ZX Spectrum case for the American market.
I don't think it was a big seller over there and was never released over here.
That means it is NTSC, and as such slow mode is about half the speed of a PAL ZX81 (fast mode is identical on both).
OK, 54.8% if you want to be specific.
The ULA does have a pin for PAL/NTSC selection, but it would seem wrong to convert it, and there does not seem to be an easy way to do that, it looks like that pin goes under the ULA.
Internally, it is a minimal design with a type of ULA © Timex and marked NCR. It has 16K of RAM from eight 4116 DRAM chips, an 8K BASIC ROM and couple of buffer chips in place of a load of resistors.
The redesigned ULA takes on a bit more functionality than the Sinclair version, with it's 68 pins vs the original 40, mostly the DRAM multiplexing I expect.
It has also unfortunately got a ZX Spectrum style DC-DC converter for the RAM, but this one is still going strong.
Loading from Tape
I was interested to see how the cassette input was implemented, as it looked like there was a lot more going on than the ZX81.
When I was looking at tape loading on the Minstrel 4th, I did have a fourth part planned where I looked at ZX81 compatibles, like the Lambda 8300 etc., but I think everyone had had enough for those, including me, so I left it for a while so I can spring it on you unexpectedly, round about now.
TS1500 Tape Output Schematic
The TS1500 is one of the earliest and probably the only official ZX81 compatible machine.
In searching around, I found several schematics, all of which were unfortunately wrong in one way or another.
This one is almost unreadable, but does turn out to have the correct value the others have wrong.
This one has no values, but elsewhere has the part numbers are correct, where others are wrong.
This one is the clearest, but one of the values is wrong.
I have redrawn a combination of all of those which matches the actual board.
The value of R19 is shown as 1KΩ on the least visible schematic above, and that appears to match the ZX80 and ZX81 schematics. The actual board also has 1KΩ fitted, the relevant parts are highlighted below.
The ZX81 schematic has a larger value resistor on the signal from the ULA, 1MΩ rather than 510KΩ, but has the same 1KΩ to ground.
The TS1500 also has the 47nF capacitor in parallel with the 1KΩ resistor.
The three ZX81 boards I repaired recently all had slight differences around this area. The capacitor C11, is the large brown ceramic disc on this board, 47nF, and the 1KΩ resistor R27 is at the bottom.
One of the other boards looked to be missing C11 and R27 from factory, no evidence they were ever fitted, the pads are solder levelled like the other no-fit parts.
I then spotted they appear to have been fitted directly on the pins of the mic jack to the side.
A third board looked like R27 was not fitted from factory, but C11 looks to have been desoldered at some point and both are now by the jack socket.
Very odd. Would moving those really make that much difference?
Something else that is odd is the unusual ground arrangement on the TS1500. Rather than directly grounding the output jack, as in the ZX80, ZX81, ZX Spectrum etc., the TS1500 has a 100Ω resistor in the ground path.
Confirmed by the actual PCB, the ground plane at the top is clearly connected to the ear jack on the right, but not the mic jack on the left.
Tape Input
A big difference to the ZX81 is it is an active input.
The ZX81 had a simple RC filter.
The TS1500 has a different RC arrangement, and then a transistor buffer.
A common mistake on several of the schematics was labelling C14 as C19.
It is sort of understandable as C19 is in the right place, but that is part of the tape out circuit, as shown above (and confirmed by tracing out the board). The 47pF capacitor is actually C14, just below the modulator.
As before, I have redrawn a version which matches the board.
This seems to do a reasonable job.
This is an inverting amplifier, and I have confirmed there must be an inverter internally on the input as the signal will not work if fed into a Minstrel 3 unless it is inverted first.
But as with any of these circuits, if the input signal is not very good, it will miss bits.
Practical Issues
I had an unexpected problem when trying this out.
Normally for loading, I use my TZXduino style unit to play .p files from an SD card.
As part of this testing, I also wanted to test loading from tape, but I couldn't find my usual tape leads.
No problem, I have any number of stereo jack to jack leads, that should be fine, I was using that on a Minstrel 4th last week.
But nothing loaded?
Odd, probing around on the board, there was nothing, no signals.
It was then I noticed the jack sockets.
These are the same style as used on the ZX81 and ZX Spectrum, and is designed for a mono jack plug.
On the Minstrel 3, I have the sockets wired to only the tip and the sleeve, with no connection to the ring.
A mono TS (Tip, Sleeve) jack has two connections.
A stereo TRS (Tip, Ring, Sleeve) jack has three connections.
With a stereo jack plugged in, I get the left channel on the top and the ground signal from the sleeve.
The right channel on the ring is not connected. You might think I should connect that in parallel with the left channel, but I that is a deliberate choice.
The reason is it also allows you to plug in a mono jack.
By connecting to only T and S, that will allow a mono jack to be connected with signal on the tip and ground on the sleeve.
If I had wired the ring in parallel, it would be shorted to ground on a mono jack,
Why Are You Telling Me This?
That all works fine with Minstrels and mono and stereo cables, so I am used to using them interchangeably.
I normally have a mono plug to plug lead for loading from tape, but I have lost that somewhere.
Not a problem as I have loads of stereo jack to jack leads.
When I offered up the stereo jack plug, the issue became obvious.
The problem is where the mono jacks are connected. On most tape decks, like the Sony TCM-818 I am using, the green jack connects to the tip and sleeve and does not connect to the ring. That seems the best option.
The TS1500, like the ZX81 and ZX Spectrum, use a different style of jack plug which I hadn't really considered before, but when you look at how the connections line up with stereo and mono jacks, you can see the problem.
There are only two connections. The signal input is on the tip, which is fine, but the position of the ground connection is where the ring is normally connected.
That means the tape is connecting ground to the sleeve wire and leaving the ring disconnected, but the TS1500 is ground referenced to the ring and does not connect to the sleeve.
I must have loads of those standard black and grey mic and ear leads for the Sinclair machines, but could not see any of them, nor any mono jack plugs.
In the end, I made up this lead using a couple of mono jack to phono socket adapters and a phono to phono lead.
Not ideal, but it worked fine as the sleeve of the mono jack extends to where the TS1500 connects to, and where the TCM-818 connects to and we can load games.
Yay, success.
Minstrel ZXpand II
Of course, there is another option.
The TS1500 has a standard ZX81 expansion edge connector.
I also recently released the Minstrel ZXpand II, which has onboard ROM, so no longer needs to connect a wire to the ROM select block, allowing it to be used on a ZX81 or TS1000 (with internal RAM upgrades) or the TS1500 (which already has 16K).
That means you can use the Minstrel ZXpand II with the TS1500 in several ways.
Edge Connector
The easiest option is the edge connector. This fits with the TS1500 and leaves access for the power and TV connectors.
However, it blocks the cassette connectors, which are lower than the RF jack.
The joystick is partly blocked by the power cable, so not ideal.
Minstrel Expansion Bus
The other option is to use one of the unpowered Minstrel Expansion Bus backplanes. These give four slots and a passthrough edge connector.
The Minstrel ZXpand II will then sit at the back and leave access to the RF, ear, mic and power sockets on the TS1500, and the joystick port on the Minstrel ZXpand II.
You also have four slots for various other expansion cards.
The Overkill Mega Bus
You could also combine a powered and unpowered Minstrel Expansion Bus and have 8 available slots, but also be able to power the TS1500 by an external power supply, if you prefer.
(you can't use the powered board directly as it clashed with the video and cassette jacks)
The Minstrel ZXpand II works fine with the TS1500, and you can load your usual favourite games.
Just remember that it is NTSC, so runs a lot slower than a PAL ZX81 which affects some BASIC games, most C or assembler ones are fine.
It also lacks support for some of the high resolution graphics modes that the Minstrel 3 supports.
All items are still available, and I can ship worldwide.
Including the Minstrel 2, 3 and Minstrel 4th (Jupiter Ace / ZX80 / ZX81 / Lambda 8300 / RC2014,) as available with built in keyboards.
Of to fit into a ZX81 case as a replacement mainboard.
All versions are available in kit form, or pre-built, and optionally the new Minstrel ZXpand II SD card and joystick interface.
Also the Minstrel Expansion Bus, powered and passive. If you want to add more cards, or do something like that shown in this post with the TS1500 and the Minstrel ZXpand II.
International shipping is a bit complicated at the moment, so if you want to order from outside the UK, please contact me using the contact link at the top of this page.
See here for more information:
Sorry to have to do this, and I know this is going to affect sales, but please understand it is the only option I have right now.
Stop Press!
I am going to do another batch of Mini PET 40/80s with keyboard, with a few updates.
These are pre-order now, shipping worldwide in November. £450 kit / £500 built.
Available exclusively from me, contact me using the link at the top of the page to get a price for shipping and any duties, based on your address.
The Mini PET 40/80 internal PET replacement boards are also available now, £350 kit / £375 built.
Again contact me if interested.
Patreon
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