Saturday 27 May 2017

Timex Sinclair TS1500 Repair

This is an old post, preserved for reference.
The products and services mentioned within are no longer available.

What do you get if you cross a ZX81 with a ZX Spectrum? One of these:
This is a Timex Sinclair TS1500, a follow up to the TS1000. That was just a rebadged ZX81, the same issue 3 ZX81 board in a ZX81 case (with a bit of extra RF shielding). This is much more than that. It was only released in the United States, so is a but of an unusual sign here in the UK.
They have started with the BASIC ZX81 circuitry, Z80 CPU, 8K BASIC ROM, but have updated the ULA with a proper crystal oscillator, and built in wait state control (which seems to be an afterthought on the ZX81). They have taken the Spectrum case and keyboard, and also 8x4116 DRAM giving 16 K of RAM. The date code shows late 1983, the Sinclair ZX Spectrum was onto issue 3 boards by that point, seems odd that Timex were redesigning and pushing the ZX81 at this stage.
To power the DRAM, they have used something very similar to the Spectrum's non-too-reliable DC-DC converter. The chip count is about the same as the unexpanded issues 1 ZX Spectrum, with which this has a lot in common.
The 5V power is still provided by a 7805 regulator, mounted on the right hand side of the case. If this was going to get heavy use, I would substitute a switching regulator, but this particular unit is fine to stick with the 7805 and heatsink.
Although that power connector / edge connector arrangement looks very Spectrum like, they have stuck with the ZX81 style 3.5mm mono jack for the power connector, rather than the Spectrum's 2.1mm DC jack.
The Mic, Ear and TV options have been moved to the back like a Spectrum.
This one has been sent to me by a friend in the states. It appears to power up, but there is no response from the keyboard. Taking it apart shows it has the same fault as many ZX Spectrums do, one of the membrane tails has cracked.
Before I can tackle that, I need to check the TV output. This is NSTC RF, and I do not have a suitable tuner here. It needs a composite video conversion anyway, so that's the first task. What appears to be a modulator box is actually just a screening can around an area of the PCB. Within is an MC1374 based TV modulator circuit. There is also a bit of rust in there, seems to be contained within, presumably from moisture within the can.
This makes the conversion a bit more complicated that usual. The typical approach on a later ZX81 is to disconnect power, input and output from the modulator and install a small buffer amplifier board.
I thought about it for a while and decided the best option was to remove the MC1374. That would disable the rest of the circuitry in there, as well as providing convenient points to pick up the 5V supply and video inputs. It also leaves the option to revert to RF at a later date, should anyone want to.
I used one of the video buffer boards from The Future Was 8 Bit (these are currently on offer until the end of the month). I started with the bare board version and added wires for the required connections. The power, input and output with thin wire and some solid tinned copper wire for the three ground pads. The remaining two pads were for audio passthrough, which aren't needed here. The TS1500 doesn't have audio, although it does have the Spectrum's speaker cutout in the base!
The board fitted nicely onto the plate inside the modulator case, stuck on with some double sided tape. The top two ground connectors give a bit of extra support and hang over the side of the case, held down by the lid.
The two green wires on the right connect the 5V and video signal into the board. The green wire on the left threads under the plate and connects to the output phono connector (the original wire having been disconnected). With the lid back on the modulator, you wouldn't know it had been modified.
Powering on, it looks like we have a picture. The TS1500 is by default in 60Hz NTSC mode. It should be possible to switch to 50Hz PAL mode by grounding a pin on the ULA, but for the moment NTSC is fine.
To test it, I connected a spare keyboard. I couldn't use a ZX81 keyboard for two reasons, one the connectors are spaced apart like a Spectrum, and secondly it appears they are arranged like a Spectrum, with the columns reversed and the rows rearranged. Since it appears to be wired like a Spectrum I used a spare Spectrum+ lid.
It just felt wrong using the Spectrum keyboard to type into what was effectively a ZX81, but most of the keys are the same, so even the extended 'press two keys at once' Spectrum+ keys worked. Only things like the " key which presses the symbol shift and P keys is no good, as on the ZX81 (and TS1500) it needs shift+ P, however things like the arrow and delete keys work correctly.
Looks like we have a functional machine, so it is wired like a Spectrum, and it wasn't much of a surprise when I confirmed it was pretty much a Spectrum.
The metal faceplate was attached with double sided tape, so just a case of carefully prying off the cover without bending it. Under that was a familiar looking keyboard membrane.
Comparing this to an early ZX Spectrum membrane, they appear identical, and indeed both have suffered the same fate.
The metal faceplate is also interchangeable with it's Spectrum counterpart.
As is the keymat, but obviously the symbols on the mats and faceplate are different.
So do not do this, I was just testing the membrane before sticking down the cover. This is probably what the TS2000 / TS2048 would have looked like, but it was dropped in favour of the TS2068.
Finally, the cases are also it would seem very similar.
Given all this, it seems reasonable to assume a ZX Spectrum replacement membrane should be suitable. This one of from RWAP.
That fits in place perfectly.
The keymap and faceplate fit back on, and it's time to plug into the board.
No problems there, the tails were the standard length, and don't seem to fowl anything. Time to test it out.
All keys working, so in answer to the question, 'can you use a ZX Spectrum replacement membrane to repair a TS1500?', yes, you can.
I am assuming it is software compatible with the ZX81, but so far I have not been able to load anything from tape. The TS1500 does look to have a transistor buffer on the input, so I may need to fiddle more with the levels.

Tuesday 16 May 2017

BBC Micro Repair

This is an old post, preserved for reference.
The products and services mentioned within are no longer available.

Here we have quite a nice looking Issue 7 BBC Micro. It's nowhere near as yellow as it looks in this photo.
BBCs are a bit like Commodore PETs, they often contain all manner of upgrade boards and add ons. You never know what you are going to find until you open one up. This is one of the more boring ones, but still a nice machine. A tape only issue 7 model B with no expansion ROMs, disk controller, speech synth, ECONET or any other goodies.
Before I go any further, I've replaced the power supply mains filter capacitors, and finally got around to writing a blog post on BBC Micro Power Supply Repair.
With the power supply taken care of, time to turn it on and see what's wrong.
Not a bad start, a single beep, and a flashing cursor. This is a common fault, and is normally caused by a faulty system VIA (the 6522 to the left of the CPU). Handily, the user VIA (the 6522 below the CPU) can be borrowed to test this as it is only required for access to the user and printer ports, and the BBC will boot without it.
Swapping the VIA made a difference, so we can probably conclude the original was faulty. There is now a BASIC prompt, but the beep is now permanent, and the keyboard is not responding.
Fitting a known good 6522 gave the same results, so the orignal VIA was faulty, but there is something more. The VIA is necessary for boot because it controls a lot of the internal devices, and when it's not working, the boot sound isn't cleared and the keyboard won't respond and various other issues with screen mode etc. can occur. Tracing the control circuit out, the VIA drives a 74LS259 addressable latch. All the outputs of this were static, they should be moving around at boot, which was confirmed on a working beeb.
I removed and tested the chip, and it failed in the IC tester. I didn't have any new LS259 to hand, but I found a used chip which was a better match to the surrounding chips than the original was.
With that in place, I got the familiar double beep at startup, and the keyboard started responding.
The usual tests looked good, I'll leave that to soak test for the moment whilst I decide which disk interface (or rather disk interface replacement) I am going to fit.

Monday 15 May 2017

BBC Micro Power Supply Repair

This is an old post, preserved for reference.
The products and services mentioned within are no longer available.

If the wonderful BBC micro computer has an Achilles heel, its the power supply.
And the Achilles heel of the power supply is the Rifa brand mains filter capacitors. This is a rare photo of one which hasn't failed. Yet.
These have a tendency to fail and go bang and create large clouds of noxious fumes. It's not pleasant when it happens. And it will happen. These are all failing. If it hasn't gone bang yet, it will do. 
There are similar problems with mains filters on Commodore PETs and the same capacitors on the monitor in an ICL One Per Desk. Here you can see the cracks in the side of the case where the magic smoke escaped from.
This one has been removed from the case, you can see where it has bulged and shattered.
My standard practice whenever a BBC comes in is to replace these capacitors before I even switch it on. The one time I deviated with 'just a quick check before I do the power supply caps' resulted in one going bang as soon as power was applied, so now I stick to the rule.
There are two X class capacitors filtering the mains input, a 0.01uF and a 0.1uF. X class capacitors are rated to be fitted across the mains supply (as opposed to Y class, which is rated to be fitted in series with the mains supply for capacitive droppers etc.). This means they have mains voltage across them, so when they fail, they really fail.
The set on the left are from one that must have been quite loud, the centre set just a minor bang, and the ones on the left, the modern X2 class replacements. Also shown is an electrolytic capacitor which sometimes gives issues with slow or intermittent power supply startup. I normally replace those at the same time.
You can see the gold band on the new electrolytic capacitor in the centre of the picture, and the grey boxes of the new X2 caps. The modern 100nF cap is physically smaller, but there are sets of holes in the board for either size.
These power supplies are a little tricky to work on as you need to remove several earth bolts, push out the floppy drive power connector and mains switch in order to remove the board.
A tip here is to push out the switch (by pushing flat the retaining tabs) before removing the spade connectors.
All refitted, ready to begin testing the rest of the system.

Thursday 11 May 2017

ZX80 Keyboard Overlays

This is an old post, preserved for reference.
The products and services mentioned within are no longer available.

When the ZX80 was released in 1980, it came with a version of BASIC in a 4K ROM chip, known as 4K BASIC. Note this has nothing to do with RAM (the ZX80 has 1K, upgradeable to 16K), it is the size of the ROM chip.
When the ZX81 was released in 1981, Sinclair offered ZX80 owners the option to upgrade to the same BASIC as the ZX81. The upgrade kit consisted of a new 8K ROM chip (so this version is known as 8K BASIC), a ZX81 user manual and a keyboard overlay.
The overlay is the same as the ZX81 keyboard, and contains many new keywords, and moves some of the existing keywords around. So if you want to use 8K BASIC, you really need the new style keyboard. Software for the 4K ZX80 was not compatible with the 8K  BASIC, but an 8K BASIC ZX80 could load some ZX81 software (anything which didn't use the 'slow' mode display).
To use these, you opened up your ZX80, ideally without snapping the plastic or losing the rivets, although that is tricky.
The new 8K ROM replaces the old 4K ROM, and the keyboard overlay sits on top of the original.
You then end up with a much better and more capable version of BASIC, but a ZX80 that just looks wrong.
This is wrong, do not do this to your ZX80! You don't see many ZX80's like this these days, as most people prefer the classic look with the blue keyboard.
With the Minstrel ZX80 clones, I have been using ZX81 replacement membranes.
These only come in the version with the grey keys and 8K BASIC keywords.
For the ZX80 builds, I got some overlays printed which stick over the ZX81 membrane to make it look like a ZX80.
You then get the same problem, if you want to use the 8K BASIC, you can't use the nice blue keys, you need to use the ZX81 grey keys, so when I have been building 8K BASIC versions, I used the ZX81 black and grey colour schemes.
I have now designed some new membrane overlays, with all the new 8K BASIC keywords, but in the colour scheme of the original 4K BASIC keyboard.
This now gives you the choice of 4K or 8K BASIC, but keeping the look of the original ZX80. This can be stuck over a ZX81 replacement membrane to make a ZX80 8K BASIC keyboard.
This contains all the keyboard markings of the ZX81, but with the yellow, blue and white colour scheme of the ZX80 keyboard.
You can even fit one to your original ZX80 so you can use 8K BASIC without the embarrassment of having a ZX81 style keyboard fitted.
However, I wouldn't recommend modifying an original ZX80, you should buy a Minstrel clone instead!
The clone kits are now available with a set of overlays for either 4K or 8K BASIC, see my Tindie Store for more options.
I have also added complete built units to the Tindie store, so you can buy a white ZX81 cased ZX80, now with a choice of keyboard overlays.

My thanks once again to Grant Searle, these were adapted from his ZX81 overlay artwork.

I have revised the design slightly, moving the top row of keywords above the buttons, as on the ZX80.

The Minstrel boards, overlays, kits and built units are available from my Tindie store.

You can also order PCBs and / or overlays below. Price includes postage.

2022 Update: At the time of writing, there are still a few Minstrel 2 (ZX80) and Minstrel 3 (ZX81) kits available from  The Future Was 8 bit but with keyboards rather than overlays. Those are the Final Edition kits, there are unlikely to be any more.