Sunday, 4 March 2018

Stag P301 EPROM Programmer upgrades

When working with old computers, one thing you find yourself doing a lot is burning EPROMs, and that tends to lead to picking up various EPROM programmers. This is my latest, kindly donated by The Future Was 8 bit, who found he had two of them.
I have several different programmers, which all have their strengths and weaknesses. The two on the right are my current USB programmers.
The smallest, cheapest, and easiest to use is the MiniPro TL866CS. This is the one I normally turn to. It is suitable for most modern chips up to 40 pin, and 12.5V programming voltage. It doesn't do so well with older chips which need higher programming voltages. It also has a few gaps in it's device support. For example  it can program the Lattice GALs 16V8 and 22V10, and the Atmel ATF16V8, but not the Atmel ATF22V10.
The one below that is the Wellon VP-390. This is larger, more expensive, and less easy to use. I tend to use this one only when the MiniPro can't help. It does support the ATF22V10 for example, and also chips up to 48 pin. It is also a bit flaky on older, higher programming voltage chips.
The Stag P301 is an older device, and communicates via RS232, but can be used standalone to copy chips. This does cope well with older EPROMs which need 21V even 25V programming voltages, it also supports some unusual chips such as the TMS2532, which is handy as it fits into Commodore PETs without needing an adapter like a 2732 does.
Unlike the USB programmers, this device relies on a memory buffer when programming. This can be read from another chip, or can be transmitted via RS232.
This does have the downside that it requires the device to have enough memory to hold the size of EPROM you are programming. The default device has 128K of RAM, enough for a 27C010 EPROM, but not enough for a 27C020, 040 or 080.
Stag did provide a way to upgrade this up to 1M (enough for a 27C080), using a plug in module, the P322. TFW8b had one of these. Only one of them, and that wasn't going in the programmer I was getting.
With that installed, the device is fully loaded with RAM. I put out a call out on twitter a couple of weeks ago, and located a few more of those, but none that were available.
I did think about trying to clone it, but it's quite a fiddly board with RAM chips on both sides, and small pitch connectors, so it would be a lot of effort. It may be possible to write some software to program the chip in 8 chunks, if it can offset the programming address. The current software doesn't support that, so it would mean working out the serial protocol and writing some new code.
At the end of the day, it is only necessary to support the largest chips, which tend to be modern 12.5V programming voltage devices, so both my USB programmers can handle them. It would still be nice to get one of those upgrade modules if anyone happens to have a spare P322?
The RAM expansion module hides under the battery pack. It is an interesting design. Actually no, let's be honest, it's an awful design. The battery pack is quite heavy, and isn't held that well by the plastic clip, so has a natural desire to fall out, or at least to slip out just enough to break the contact.
The battery pack is 8.4V, 650mAh, made up of 7 AA size rechargeable cells. These were originally 650mAh NiCd, but had failed in the past, and been upgraded to 2000 mAh NiMh cells. This wasn't holding charge anymore, and when I checked, four of the cells were measuring around 0.1V. I did try, but they were not willing to be recovered. The others seemed healthy, but couldn't work in series with the dead ones.
The Stag does have a built in charger, but that is designed for NiCd cells, so it's probably a simple trickle charger which will not be ideal for NiMh cells. This is likely to have led to the dead cells, if any of the cells are a bit lower capacity, they can put stress on the other cells. If one in the middle needs extra charge, that current has to pass through all the other cells, which just heats them up if they are charged.
Rather than order some more AA tagged NiMh cells, and go through the same cycle again, I looked at some alternative options. My thinking was to try to mount a battery pack inside the recess of the case, where the original battery pack fitted. I looked around, but there didn't seem to be many 8.4V packs around that would fit.
My next thought was to use some AA battery holders, and use AA eneloop cells. This would have worked, but didn't quite fit in the cutout, so it would have been resting on the cells, not ideal and they would be liable to fall out.
The next idea was to use AAA cells. Modern AAA rechargeables come in capacities greater than the original 650mAh NiCd cells, so it would be at least as good as the original, in fact about 50% better.
The AAA holders fitted nicely, and would allow me to use part of the original battery as a cover. There were some foam pads which previously held the cells in, I have left that in place, but I don't think it's actually touching the AA cells.
I stuck the battery holders in place with foam pads. I couldn't find any 7 way holders, so I used one 4 way and one 3 way, wired in series, and slightly reluctantly soldered to the battery contacts. I was going to try some kind of clip, but thought it was more important to get a good contact.
I dug out seven AAA eneloop cells, and topped them up in the charger. The plan would be to remove them when they needed charging, and charge them individually in a decent battery charger (such as this TechoLine BL700).
With those installed, the Stag powered up and showed the batteries as fully charged. I read and programmed several chips like this, and it seemed to be working nicely.
However, after I had finished programming various test chips, I checked the battery level again, and it was down to half. As a side note, the next job is going to have to be fitting a backlight or swapping the LCD for one with a backlight.
I checked the cells, and they were showing different voltages. I put this down to the fact they had led different lives, some had been used in my old DECT phones which had unsuitable trickle chargers that tends to damage the cells over time, others may have been in lower drain applications like remote controls. These were just some spare batteries I had lying around to test out the idea of running this from a pack of AAAs.
I put them in the charger to top up. They had all been full, but after recharging, it was clear they had been unevenly worn, one cell took only 81mAh to fill it up, another needed just over 1Ah. Just goes to show that even though those cells that had come out of the same batch, they had very different lives. Always remember, never mix batteries types, ages or chemistries as they are almost guaranteed to discharge at different rates, and the two cells that went flat quickly had compromised the whole battery pack.
As a temporary solution, I fitted a set of standard alkaline batteries. It goes without saying, don't plug in the mains adapter with non-rechargeables fitted. I don't actually have one of those for the Stag, so it is unlikely. This did work fine, and it is good to know that it is an option, but I would prefer to have rechargeables if I can.
A better solution is to get a new pack of cells and fit them all at the same time, and keep them as a set so they will hopefully age equally. I ordered a set of 10 Fujitsu 950 mAh NiHm cells, and installed 7 of those instead. They have been in use for a while now, and the battery is still showing full, so I think that is a result.
The cover fits back in place, and is more likely to stay there now there is no weight pulling it open.

If you want to support this blog, you can donate via Patreon or Paypal, or buy something from my store.

Sunday, 25 February 2018

Binatone TV Master 4 plus 2 (MK VI) Composite Video Conversion

Following on from the composite video conversion I did a few months ago on a Radofin tele-sports TV game, I have modified quite a few other TV games of a similar type.
I bought this one as it is the same model we had back in the seventies when we all looked just like the family on the box.
This one is a Binatone TV Master 4 plus 2, which is the same as the TV Master MK VI. This is similar to the original 4 game version (TV master MK IV), but with two additional games which use a light pen style gun.
The main unit on the 4 plus 2 is identical to the MK VI other than the label. The 4 plus 2 was a cheaper version where the light gun was an optional extra. The gun works in the same way as a light pen, and depends on the timing of the beam on a CRT screen to determine it's position. It will still work following the composite video conversion, but only on CRT televisions.
This is the black and white version, there was a colour version which looked almost identical but was marked 'Colour TV Game', and had white / grey box and buttons rather than the very 1970's day-glo orange. (date code on the chip is August 1978).
Inside is the usual General Instruments GIMINI AY-3-8500 chip, here as they often are in an unusual socket arrangement with no contacts on the four edge pins (they are used on later versions for 2 axis controls).
This was one of the few of these TV games I have found that didn't work. The sound was working, so it was playing, but I couldn't tune in the TV (even the old black and white portable I use for testing things with very poor video signals). The 'scope showed a valid composite signal entering the modulator section, so I just went ahead with the composite video out conversion, there seemed little point in repairing the modulator.
I tried something slightly different on this one, and fitted a header and a socket to the standard TFW8b composite video buffer board, that seemed to work quite well. The power was tapped at the bottom of the board, the video from the point it enters the modulator section, and the audio directly from the chip output.
The unit has a separate switch to turn off the internal speaker, so the audio is always fed to the TV set, but you can also turn on the speaker on the game itself if you want 'the full experience'.
Once powered up, there is a nice stable signal on the monitor, in glorious black and white.
The controls on this version are as simple as they get. One rotary control, and a mono 3.5mm jack.
An inside, one 1MΩ variable resistor. This one was was a bit flaky, but a squirt of contact cleaner and turn it back and forth a few times and it's now running fine.
Binatone like to put warnings on the back of these to get you to only buy their power supplies.
Is it regulated? Is it really? Looks like a standard unregulated linear power supply.
Well, I stand corrected. Yes that is a regulator. A bit rusty, but a regulator none the less.
A National Semiconductors LM342P-8.0, a bit like a 7808. This gives an 8V output which matches the case, although not the 9V the game specified, should be fine...
Time for some two player testing. The only time you will find football on my TV.

If you want to support this blog, you can donate via Patreon or Paypal, or buy something from my store.

Sunday, 18 February 2018

Minstrel ZX80 Clone Keyboard

I have added a new option to the Minstrel ZX80 Clone kits and builds, it's own keyboard.
This has been designed to match up with the Minstrel board, so it makes it almost a single board computer.
The Minstrel board itself remains ZX81 sized, the keyboard plugs into the keyboard connector on the bottom right.
These are mounted under the board. I am using 0.1" pin headers and sockets, and these will be supplied in the kits. You could use wire links, or even add cables and run the keyboard a short distance from the main PCB, a Minstrel-SK if you like.
There are a couple of unusual things with this PCB, both of which were done for aesthetic reasons. Firstly, there are no tracks on the top layer of the PCB.
The traces are all on the bottom. The keyboard is a matrix, the whole point of which is a set of rows cut across a set of columns with switches where they cross, you can see that here in the underside of a ZX81 keyboard membrane.
The obvious choice then is to draw the rows on one side and the columns on the other. Instead, I used a trick that used to be very common on matrix keyboards, but you don't see very often these days.
The tactile switches used have 4 pins, two sets of contacts wired in parallel, so in the above photo, the top two pins are actually the two ends of the same bit of metal. This means you can pass the signal through that wire link, and run another track underneath.
The green trace is on the PCB, the red is via the switches. You can see the blue column traces pass beneath the red lines.
This means you can lay the matrix out quite neatly on a single layer. This was done a lot in the 1980s when for large boards such as a keyboard, they could use a cheaper single sided PCB.
These days that is less of an issue, with the availability of cheap PCB production in small quantities, with boards coming in double sided with plated through holes, solder mask and silkscreen printing. The only slight problem I have is due to the volume of boards they produce like this, there is often a small code added to the silkscreen by the PCB manufacturer.
Sometimes they are kind enough to add these under a socket or an IC. But sometimes they end up in a really annoying place in full view on the assembled product.
I had an idea a while ago, but I had not had the right board to try it out on. My theory was they always put the code on the top of the board, so why not design it upside down? So I did.
This is actually the top of the keyboard PCB in the design files, and there they have stuck their code number. But when I get the board, I simply flip it upside down and have no annoying code number.
All I need to do now is design all my PCBs upside down. That can be quite challenging as it is mirrored, but in this case it was quite easy. This was my view in the PCB software.
However with anything more complicated, the mirroring would be confusing. I haven't looking into the format of the gerber files, but I presume it should be possible to write a bit of code to flip an existing board over. There's a coding challenge if anyone fancies it.
The new keyboard is available separately as a PCB or kit from my Tindie Store.
It is now an option on the main Minstrel ZX80 Clone kit Tindie Page, and also available as the Minstrel ZX80 Clone with Keyboard as a complete kit or assembled unit.

If you want to support this blog, you can donate via Patreon or Paypal, or buy something from my store.