To TRS-80 COCO 2 Or Not To COCO

I've had a TRS-80 64K PAL Colour (not Color, it's an Australian version) Computer 2 laying around for a little while now. Purchased as an untested item and as such I presumed would have some issues to deal with. Thankfully I've finally I found some time (in amongst everything else)  to look into what issues if any there might be, and decide if it's worth going further and conducting a full restoration.

Plugging the computer in and turning it on revealed that there was indeed some life inside; but it's bit of a half life with a screen full of garbage characters. On the plus side the garbage is over a green background, green being the colour it's supposed to be. I was suspecting this was going to be a DRAM issue, a little bit of googling seemed to confirm suspicions. Time to open the COCO up.

First switch on of the COCO and a load of Garbage Characters on the screen.

From the awful amount of rust on the modulator it appears the computer has had a hard end of life.  It looks as if the COCO's spent some time in damp back sheds and garages. What we're looking for at this stage though is the DRAM. This is to be found on a plug in board hovering above the motherboard, with some cardboard shielding over it.

TRS-80 64K PAL Colour Computer 2 Motherboard

Arranged in a row on the plugin board, are eight 8k MB8264A DRAM chips, these provide the systems total of 64k. Unfortunately the DRAM is soldered onto the plugin board, these needed to be removed to find the problem ones. Rather than solder replacement DRAM back in directly I also took the opportunity to solder IC sockets.

Plugin DRAM Board, eight 8k MB8264A chips.

After some time consuming un-soldering a bit of snipping, re-soldering and finally mounting new DRAM, four ICs in total the Colour Computer came back from the dead. I now have the expected boot screen on a green background, this is a good start.

I'm not sure if it's the LCD TV, the computers modulator or something else, but the quality of the display is abysmal. Still now that I know the computer is essentially working I'm going to give the whole system a going over and full restoration.

Plenty more updates and some proper investigations into the world of TRS-80 Colour Computers to come.

And We have a Working Computer, Sort Of.

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Amstrad NC 100, Don't Blow a Fuse

In the last few days I took delivery of an Amstrad NC 100 notepad computer that had been listed on everybodys favourite auction site as not working. The purchase was an educated gamble as all my prior research on the NC 100, along with helpful advice from the Twitter Sphere, pointed to the major cause of failure for these devices being an easy fix. A simple blown fuse that would probably require replacing or circumventing.

After taking the NC 100 apart, locating the fuse where it should be, in the bottom left corner of the main board (power socket facing towards you), I ran a continuity test, the resulting in our prospective blown fuse scenario proving correct.

Amstrad NC 100 Main Board, Left: A Dusty and Blown Fuse. Right: A Temporary Jumper Wire Fix

The non-working Fuse is a  PCB Leaded, 500 mA, 250 V component, which I didn't have to hand. So in order to power up the computer I simply removed the fuse and soldered in a copper jumper wire as a temporary fix. (Next time I place a general component order I'll obtain a new fuse.) This is fine, just as long as I insure the polarity of any power supply is correct, Tip / Outer positive, inner negative.

Interestingly, based on the type of fuse I found in my NC 100 , there seem to be a couple of revisions of the NC 100 board. A number of online references indicate that the fuse should be an SMD component, not a though hole type one. 

A Now Fully Functional Amstrad NC 100

To test, I reconnected the main board to the LCD panel, and screen came to life with a Friendly 'Lithium battery is low' warning, not unexpected as there was no backup battery in the device at the time. Fix completed, and case reassembled, and I'm now the proud owner of a rock bottom priced Amstrad NC 100.

Now to find something for this little notepad computer do, maybe I'll take it to the office and garner some curious and possibly concerning looks at meetings.

Further Viewing

For an entertaining and informative overview of the NC 100, including the replacement of (in this case an SMD) fuse, head over to YouTube and the EEVblog

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RetroChallenge 2017/10: Part 5

NEC PC-8401s' Capacitors & NiCd Sorted

Since discovering the leaky capacitors in the PC-8401s LCD module (last post, Part 4), I've replaced the problem components and the screen is now working without further issues. So, while had the main case opened I also decided to replace all the other potentially problem components and give the unit a good service.

I replaced all the capacitors with like for like values. The one exception being a 1200uf 25v, which is now a 1200uf 35v variety. The extra voltage level is not an issue, though the replacement capacitor is twice the length of the one it replaced, luckily there was space to lay it on it side.

The other remaining problem component was the 30 year old NiCd battery, which amazingly hadn't vomited it's guts all over the motherboard. The NiCd battery inside the PC-8401s is there to keep the DRAM and system clock ticking over should there be no main battery or indeed mains power. (No compact flash for 1985 user convenience).

1 Frand Super-Capacitor Battery Replacement

I decided to replace the battery with a 1 frand 5.5v super capacitor. I've previously made similar modifications to my Tandy M100 and M102 machines to good results, with RAM storage remaining intact for over a month before (until I got board waiting) I reapplied mains power. Similarly to the previous m100 modification, I bent the pins of the capacitor over horizontally before attaching wires which are the soldered on to the PCB. The capacitor itself is attached to the circuit board with double sided tape, keeping very firmly in place.

So next time it's back to doing something else arguably useful with a 30 year old computer.


See RetroChallenge Intro, Part 1, Part_2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8

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RetroChallenge 2017/10: Part 4

My NEC PC-8401s' LCD Module Faces Capacitor Issues

I'd noticed, well couldn't help noticing really that the PC-8401s' LCD screen would flicker, and every now again turn off unexpectedly after extended use. Over the last week the occurrences became more frequent. Interestingly or comfortingly the screen flickering has no effect on the Main Computer, everything stays resident, you can even continue typing while the screen is off. So what ever the problem? It is appeared localised to the LCD screen. Time to pry open the computer and see what's happening.

A quick look over the main board revealed nothing of immediate concern, though at some point I'll want to replace the NICAD backup battery and the 30 year old capacitors, thankfully though none of these components have been leaking. Semi confident the main board is okay it was time to crack open the LCD bezel, not the most convenient thing to do.

NEC PC-8401 Main Board

Unlike the main case the bezel is mostly held together with molded plastic clips, taking it apart took some considerable carefully spent time. Once off however it was immediately clear where the problem (hopefully the only problem) lay.

NEC PC-8401s' LCD Panel

We've got 2 exploded leaky electrolytic capacitors that need to be replacing, and while we're at it the other 10 should be swapped out for good measure. I guess considering the age of the computer this is not unexpected, all this is really part of the excitement of playing around with old hardware.

Some Picturesque Leaky Electrolytic Capacitors

Well then, before continuing any further it's of the the electronics supplier to procure some of these tiny caps, of which I have exactly none of in my parts draw.

As a quick aside the panel was manufactured by Epson.

Back view of the NEC PC-8401s' LCD Panel

See RetroChallenge Intro, Part 1, Part_2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8

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Think Different, the TRS-80 Model 100

And now for something completely different, the first articles (along side the usual stuff) based around the portable computing powers of the TRS-80 Model 100. Yes I'm jumping to the right for a look into something not Sinclair related, though not I think not without some correlation. On first glance there is not a lot to connect the cheep yet stylish Sinclair home micros to what could best be described as an 'American Design' (baring that internals are all Japanese) beige business mans machine.

TRS-80 Model 100 With the Full Kit.

Yet, initial target audience aside, the Model 100 seems to be something outside the a strict business box, it feels more like a portable home computer, with its Micosoft Basic interpreter and built in rudimentary applications. This is not a computer designed to run CPM and Visicalc, not straight from the factory at least, this is a computer that if one ignores initial hefty sales price tag, would have appealed to any home enthusiast of the period. One just has to glance at the included manuals to see that, in particular the Technical Reference Manual.

Yes the Technical Reference Manual, a 127 page thesis on the entire design and construction of the Model-100, this is an 80's hardware hackers dream. The guide presents the full schematics, lists every single part used in construction (with catalogue numbers) and even goes right down to LCD screen design.

The hardware itself is by no means shabby, the Model 100 packs in a modem, full serial and parallel ports, a very clear LCD display and a fantastic full sized mechanical ALPS keyboard. All these features of course contributed to the high price point of the little portable back in 1983. Equally, these features make the model 100 a very interesting and usable(?) machine today. The Model 100 is the perfect kind of micro for some modern experimentation.

All good stuff, however I need to get down to some quickly required (non business) business.

A little Leaking Battery Work Required

I picked a Model 100 very recently, one in excellent condition, this machine seemed well loved (and used) by its former single owner, and included where all the original manuals and documentation. What it did need was a good clean and a new internal battery. Yes is the old leaky soldered-in NiCd battery curse of old computers. 

The Model 100 uses a NiCd battery to keep 32K of static RAM juiced up when the computer is powered down. The battery is rather important as the static RAM is the internal storage medium, there are no flash drives in thid machine. While the battery was still holding charge, it had began it's decay, and had started leaking ever so slightly. Time for a replacement.

Leaking NiCd Battery

A little googling revealed an interesting replacement idea for the battery, suggesting the use of a super-capacitor. This seemed like a perfectly reasonable proposition, and so I took the advice and installed and 1 Frand super-capacitor. If doing this mod yourself, As noted in the original source, insure the 1 Frand super-capacitor used is rated 5 VDC or better.

1 Frand Super-Capacitor Battery Replacement

The replacement procedure it simple, remove the old NiCd battery, solder some wires to the circuit board and attach the capacitor. With Super-capacitors, polarity matters, so be careful to attach the negative anode to the negative wire / solder pad and positive anode to the positive pad. I bent the pins of the capacitor over horizontally before attaching wires, and hot glued (generously) the capacitor to the circuit board to keep it secure.

After putting the case back together I left the capacitor to change for a while, tested that if all external power was removed that yes the 32K static RAM would stay active. No more leaking battery and no more memory worries.

Now it's time to see what else we can do with this beast.

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Advancing along with the Retrofitting of a ZX81

At the end of the last article, 'Simple Start To Retrofitting A ZX81', a new power supply was sourced and a composite video mod was fitted. This entry sees the near completion of the internal retrofitting project; where some capacitors are replaced, a new voltage regulator is fitted, a lower Amperage CPU inserted and some heatsinks affixed.

Replace Those Old Capacitors

Over a number of years, particularly if exposed to continual high temperatures, electrolytic capacitors tend towards failure. While there is no obvious evidence that the capacitors in my ZX81 have bitten the dust, they are 35 old and probably due for replacement even if purely as a precautionary measure.

Depending on the Issue number of a ZX81, there will be either 2 or 3 electrolytic capacitors on the main board. I have an Issue 1 USA board, as such there are 2 capacitors I'd be replacing. The caps are numbered C3 and C5, are located up the back of the ZX81 PCB, to the right of the modulator and left of the expansion edge.

New Capacitors in Place

Visually the 35 year old capacitors seemed okay, there was no tell tale bulging, or leaking of electrolytic fluids. What wasn't so brilliant was the illegibility of the markings on C5, as the the outer plastic / paper casing of the Caps had shriveled over time (and heating). Handily the ZX81 Service and Assembly Manual is available from numerous online services, confirming the values required:

• C3 22uF Electrolytic 16V minimum
• C5 1uF Electrolytic 5V minimum

Both the existing capacitors had voltage levels specked well over the minimums at 50V. Higher voltage ratings are a good thing for capacitors, as a higher voltage rating will prolong their life expectancy. I had some 63V Electrolytic 22uF and 1uF caps at hand and so simply swapped those over for the existing ones.

Switch Mode Regulator

Heat is possibly one of greatest enemy of the aged microcomputer, and lurking inside a typical ZX81 are two major heat manufacturing components, the ULA and a linear voltage regulator. The ULA we can't do to much about, however the regulator can be replaced with a modern switch mode equivalent.

7805 Regulator and Replacement Recom R-78B5

Linear Regulators can get quite hot as all excess energy is converted to heat, the higher the voltage drop required the hotter the regulator becomes. As the case of the ZX81 has minimal ventilation this heat has very few places to go. Modern Switch mode regulators on the other hand produce very little in the way of heat, and no heat equals no ventilation problem.

There are a number of manufactures producing switching regulators, I'm using a Recom R-78B5.0-1.0L, which I sourced from Element14. If  you're conducting a similar mod, the main things to be sure of are that the switching regulator is a drop in replacement for 7805, is 5 volts and rated at 1A or 1.5A. (2A is overkill, unless your ZX81s controlling a nuclear power plant)

The voltage regulator is not hard to spot, it's attached to what looks like a tractor part, or aluminium heatsink if you prefer. If you ever wondered why your left hand got so toasty on a cold winters night programming (OK gaming)  on your ZX81, this thing is the most likely culprit.

Recom R-78B5 In Situ

I started the process by removing the bolt which secures the heatsink to the 7805  and circuit board, once unscrewed the heatsink slid out easily. The heatsink is not required after fitting the new regulator.

Removing the 7805 regulator itself proved slightly trickier due to the large amount of solder holing it in place, plus there being a channel selection switch (being a USA ZX81 variant) and some rubber like trim isolating the switch from the 7805 obscuring the pins at the base of the regulator. In the end I found it easiest to remove the channel selection switch before finally removing the 7805. Note that the process of converting the ZX81 to composite video out, had already made the channel selection switch redundant so decided not to reattach it latter.

The Recom R-78B5 dropped in easily and after soldering it to the board I powered the ZX81 on with no issues or incidents. The only thing to notice is a comparative drop in heat production, the keyboard certainly won't be roasting slowly over an aluminium hot plate anymore.

Cooling the ULA

Self-adhesive Heatsinks mounted onto the ULA.

With the heat now taken out off voltage regulation, this leaves the ULA as the only other major heat source. While there is no easy way to extract radiant heat from the ZX81 case, the overall temperature level inside has vastly subsided.The heat generated by the ULA is now able to permeate the cool void sans 7805. Hopefully attaching some self adhesive heatsinks to the ULA will help keep this vital part a little cooler in general, prolonging its lifespan.

I used two self adhesive heatsinks sourced from Jaycar, laid end to end on the ULA, these are a little overhang and don't quite cover the whole chip. If you're a little more OCD, then it is possible to track down exactly fitting 40 Pin DIP IC heatsinks at specialists stores like Retroleum, or more general and extensive component suppliers such as Element14.

CPU for a CPU, a Fair Swap

Swapping out the existing CPU is more about power consumption than anything else. There are no gains to be made in speed or reliability (unless the old CPU is on the blink of course). The original CPU in my ZX81 was Z80A  made by NEC an NEC - D780C NMOS chip. NMOS chips use quite a lot more power than the more modern CMOS equivalents.

An NMOS Z80 CPU requires  200mA, where as a CMOS Z80 CPU requires only 20mA when running at 4MHz. Considering I dropped the theoretical output of the regulator by 500mA when replacing the the linear regulator for the switching regulator that's a saving worth making.

You can drop in any 40pin DIP 8bit Z80 processor, no matter what the MHz rating. The CPU will only work as fast as permitted by the ZX81s oscillator circuit set at 3.35MHz. My drop in replacement is a 10MHz Zilog Z80.

Z80 processors are still readily available, they come up on Ebay (though proceed there with caution), specialists stores such as Retroleum, and once again though I procured mine via Element14.

Next Time

I still haven't covered everything, I'll attempt a summing up of the (semi) final fixes and extras and addons (which I failed to get to this time), next entry.

See  Part 1 and Part 3

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Simple Start to Retrofitting a ZX81

My Sourced ZX81 was Modified for the American Market

Having played around with emulating the ZX81 as part of building a Raspberry PI / Arduino platform in the AZ15, I thought it might be fun to revisit the real device.

To start I'd be needing a ZX81, a process that proved trickier than anticipated. There should be no shortage of ZX81s in Australia, yet they seem sparse enough to command high-ish prices, high enough to send me searching further afield. European and UK prices again seemed rather expensive, particularly if including postage. So to the USA then, a Timex 1000 would be just as acceptable, in the end I found a reasonably priced American fitted out ZX81, obviously sent to the new world before the official deal between Timex and Sinclair.

So what exactly can be done to update and bring a ZX81 into the now? Well, there is a great deal of information out there concerning the upgrading, modifying and repairing of ZX81s. Should you decide to do everything from scratch the information and resources are available. However the really good news is that many of the parts required for a refurbishment and upgrade can be bought right off the shelf.

New or Replacement Power Supply

Before even turning on my ZX81 for the first time (in my possession at least) I needed a power supply, my unit came sans power brick. While I had no option but to source a new supply, it's recommended that even if you still have an original Sinclair unregulated power brick that you should still replace it with a switch mode equivalent.

An original Sinclair unit is rated at 9V, however as the supply is unregulated that can get as high as 12V, this leaves the ZX81s internal regulator doing all the heavy lifting. Governing the required 5v (down from 12V) leads to an excessive amount of heat being generated inside the case.

You can use a regulated power supply of anywhere between 7.5V and 9V with a minimum 1000mA (1 Amp ) current draw. The easiest or most available substitution seems to be regulated 9V supply. The power supply should also have 3.5mm phono plug jack plug at the business end. Power input of +7.5V to +9V should be at the tip of the plug, with the lower / outer ring being ground.

I picked up an appropriate Switchmode Plugpack 9VDC 1.66A from Jaycar (Australian Electronics Chain), the unit provided a variety of jacks that can be substituted as or if required. Similar Plugpacks should be available just about everywhere. Ebay often seems to have packs marketed as being specifically for the ZX81.

Optionally a power switching cable can be added between the supply and the ZX81. This will save some wear and tear on the ZX81s power socket These can be made easily enough, or purchased from specialist suppliers such as the Retro Sparse Shop, that's if you don't want the hassle of tracking down appropriate parts yourself and in all actuality might prove more cost effective.

Composite Video Output

The second thing in need of attention is the video signal. as the combination of modern (not the best quality admittedly) LED TV and RF signal emitting from the ZX81 is terrible to say the least.

There are various video mod available that will provide composite video out, ranging from ones that you can build yourself to others ready to fit into the case with minimal fuss. I've gone with an off the shelf fix in the form of the ZX8-CCB, a video mod created by the venerable Pokemon. The ZX8-CCB is available from The Sell My Retro website. Before going into the fitting of the ZX8-CCB, lets quickly look at other composite video mods.

If you choose to build your own then it would be hard to do better than the mod by Joulesper Coulomb detailed in the YouTube video, 'ZX81 Video Conditioning'. A lot of effort has gone into the mod, and spectacular results have been achieved, which can clearly be seen in the video. Minimal parts are required to undertake the modification, it is easily assembled on strip board and once built, the entire project can be housed in the existing ZX81s modulator case.

There are slightly easier mods to build yourself, most of these use a single transistor and a some resistors, such as the simple mod by the ZX-TEAM. You can see the results of that video modification on Terry Stewart's (retro computer collector extraordinaire) web page. However, these simple mods may not work so well with earlier issues of the ZX81 due to a missing back porch signal, and possibly leaving the resulting picture a little on the dark side.

For off the shelf options, there is the Atari (yes you read that correctly) composite video mod for the ZX81, sold by The Future was 8 bit. Dave Curran on his Tynemouth Software Blog, details the fitting of this interface to great effect. Lastly (of course there are bound to be more), there is the Sinclair ZX81 ZXVid - Composite Video/ULA Fix, available from Sell My Retro. You can see the results of this mod on the YouTube video ZX81 - ZXVid board, Composite mod.

Fitting the ZX8-CCB

The process of fitting the ZX8-CCB is very well documented in the instruction sheet supplied with the kit. There is no assembly of the kit required, and it's down to a little soldering to the ZX81.

To start, it's simply a matter of attaching the input wires to the back of the ZX81 circuit board: red wire +5V to ULA pin 40, the black wire 0V to ULA pin 34 and the yellow wire to video In at ULA pin 16.

For the output wires you have a choice to make, either integrating a small video jack (3.5mm) between the two cover parts (as suggested in the instructions sheet), or re-purpose the modulator casing. The second option is what I decided to go with.

In order to fit the ZX8-CCB into the modulator case, the case needs to be removed from the ZX81 circuit board. There are 2 or 3 wires going into the side of modulator, these can be trimmed of at the base, or de-soldered. Also there are two large pins directly underneath the modulator, these hold the modulator unit in place. These also need to be de-soldered to remove the modulator from the ZX81s circuit board.

Once the modulator casing is free from the ZX81 you can then set about removing the existing contents (though a combination of cutting and de-soldering). What's inside the box will vary depending on the modulator, on PAL models there is a cross bar and the main circuit board to remove, as I have a USA model there was no crossbar.

ZX81-CCB Input Wires Soldered at the Back to the ZX81 Circuit Board

After the modulator has been gutted it can then be re-attached to the main circuit board, with the ZX8-CCB placed inside the casing affixed with a square of double sided tape. The remaining wires are then attached: brown wire for video out to the center of the phono socket, and the the black wire 0V to to the side of the modulator casing.

ZX81-CCB housed insdie the Modulator Case. (Wires latter moved to run through vaccant holes in the casing)

In the initial fitting I had the three wires which are soldered to the ZX81s circuit board running over the side of the modulator casing. With the modulator lid placed back over the top, this was perfectly serviceable but looked a little untidy. I latter decided to move these wires and have them running through the now vacant holes (of which there are three) on the side of the modulator.

Composite Results

So what of the results? In a word 'amazing'. It is unfortunate that I don't have a CRT TV laying around to give the proper picture, though even upscaled through a composite to VGA converter box the clarity is almost as clear as on many an emulators, it's certainly the best video I've even seen emanating from a ZX81.

After Composite Video Mod (with a little HRG graphic teaser)

Of course I'm not the only one to have retrofitted a ZX8-CCB to a ZX81, these things sell like hotcakes. And as is the way with these thing, half way through typing up this post I found another blog post detailing a similar install of a ZX8-CCB. I'd suggest checking out Kevin Phillip's entry 'Rejuvenating My Geriatric Childhood Friend' for extra information. The better you're informed before making any medications there better.

Next Time

There are a couple more modifications I'm wanting to make, along with a couple of interface addons to try out. (Hint one of the is in the final picture). I'll get around to detailing those and more soon.

See  Part 2 and Part 3

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