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NEC PC8201a
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Yarvik TAB250

Apple iPhone 5G clone



Other electronic gadgets
Battery Manager Ultra
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NBOX Media Player
LED-2 Inexpensive Video Projector


Also see:
Cycling Holidays
DutchBikeBits.com


Yarvik TAB250 Android Tablet Info and Review

After being away from tablet computers for a very very long time, in February 2012 I decided to buy a tablet to play around with.

Because there are a wide range of tablets available and the advantages and disadvantages were not too obvious to me without using one, I decided that I would purchase a fairly inexpensive tablet second hand to start playing with. I settled on a Yarvik TAB250 which I bought from a seller on marktplaats.nl for €65.

My tablet was supposedly unused, but the flash disk contained photos of the previous owner, evidence of several games which had recently been uninstalled and a copy of a (completely forgetable) Hollywood movie. I didn't complain as the tablet was in as-new condition.

Why Android ?

When there is an open source system available, I'd always rather use that than a closed source alternative. Android is open-source. You can download it and build your own version (like these people). The device is yours. You can do anything with it, including writing your own applications and downloading those which others have written without having to pay the manufacturer of the device.

Compared with buying an iPad, which is closed, an Android device offers freedom. You never have to "jailbreak" an Android device in order to do what you want to do with the device that you paid for.

What's more, Apple's insistance on suing people for things which were mostly invented 20 years ago is enough to ensure that I wouldn't ever consider buying their products, so I'm happy to see that there many other options available. However, for balance, here's an iPad review.

TAB 250 specifications

The Yarvik TAB250 has a multi-touch capacitive touch screen of 800 x 480 resolution, 1 GHz processor (Telechips TCC8902), 4 Gb of Flash memory and 256 Mb of RAM. It runs Android 2.3.3 (Gingerbread). This specification is OK but not especially good by todays' standards. Plug an inexpensive micro-SD card into the side of the tablet for a bit more memory and you have a similar specification to the first generation iPad in a smaller form factor with an open platform and a much lower price even when new.

The TAB250 has a multitouch capacitive screen, which means that gestures such as pinch zoom are possible.

Memory can be expanded not only with micro-SD, but also with USB disks. In addition, the USB port supports USB keyboards and mice. Headphones can be plugged into a 3.5 mm socket, and there is a HDMI port allowing the picture to be displayed on a TV or projector if you wish. These are nice features to have and very practical. I have used the tablet in conjunction with a video projector to display photos and videos in presentations.

However, there's unfortunately a degree of "marketing" involved. According to the spec writers this is a "1GHz" tablet. However, actually, the processor clock is 720 MHz. There is also a graphics processor which runs at about 300 MHz, so they quote the two clock speeds added together. This is not honest. Unfortunately, it's also common in the specs for tablets. However, my idea with buying this tablet was to get some experience with Android at a low price, and it certainly offers that.

The TAB250 is quite similar to the TAB211, TAB220 and other models from Yarvik. It's also similar to some models from other manufacturers. Some of the pre-installed wallpaper graphics include the word "roverpad" in the middle. Googling for this word revealed that the RoverPad 3WT71 has a similar specification and looks identical. This is probably also very similar in other regards. Dealextreme also have a very similar tablet. I suspect that many other tablets based on the Telechips TCC8902 chipset are very similar indeed.

Problems with the Yarvik TAB250

As with many cheap Chinese tablets, the build of Android installed, and the setup of the build, are not particularly well customized to the hardware.

This results in it being difficult to find useful apps for the device, and in many of them not running even if you do find them. Fixing these faults makes the device much more useful - but why is it necessary ?

Problems that I found, and the solutions if I've found them, are here:

  • No Android Market (Google Play) installed as standard. This is a big problem because it's where most apps can be found. (fixed)
  • A Getjar app is installed but has a limited selection of apps and is a little buggy. It also keeps running in the background, consuming memory and making other apps slow down. This is fixed by making Android Market work and by removing the GetJar app.
  • The screen size is set incorrectly. This means that many apps simply can't be installed. (fixed)
  • Camera orientation is wrong in portrait mode, making results of some photo apps unpredictable. In any case, the camera is poor.
  • Accelerometer orientation wrong in most (all?) apps which use accelerometer
  • While it is supposed to work with a USB keyboard, attaching one causes a crash

If you follow my instructions for installing Android market / Google Play and fixing the screen size then almost all useful apps available in the Android App market will install and run on the TAB250.

Finally, the really bad problem. THE POWER SUPPLY IS DANGEROUS.

The power supply problem


The power supply pins have become loose and are now very easy to push inside

On 26 November 2012, after a few months of use, I had a problem with plugging the supplied power supply into the wall. First it simply fell out, which was surprising. I then found I cut plug it in, but that it felt a bit uncertain. It turned out that the pins on the plug were no longer firmly attached to the power supply.

This is dangerous because the back end of pins like this is usually live, and it was not possible to tell what they could make contact with inside the power supply. I posted a question on the Yarvik TAB250 forum including these two photos, but they initially chose not to publish what I had written or to respond to me privately. Days later, after I had already pointed out what the problem was through this web site, I was instructed to "state your full issue which you are having". I have sent a reply suggesting that Yarvik needs to recall these supplies and replace them with a safe model.

Further investigation

I've seen worse switched mode power supplies with Chinese products. However, almost good enough isn't enough when it comes to the safety of high voltage equipment. in my view, this design isn't robust and it isn't entirely safe. These are things which I observed inside the power supply:

  1. The single screw which was supposed to keep the pins of the plug in place within the housing is clearly inadequate. What you can't see from the photo is that this isn't the only problem. The "v" signs point to where there is fatigue in the plastic and the pins have very nearly broken off in these places too.
  2. This problem had obviously been spotted by the company. However, instead of addressing it properly they tried to solve the problem with a bit of hot melt glue during assembly. Unfortunately, the glue wasn't nearly enough to solve the problem. Actually from what I can see, the glue wasn't holding at all in my power supply, and I suspect that is true of many after they have been used for a short while.
  3. It's not all bad news. There's an inductor (at least I think it's an inductor) on the input which presumably is intended to help with power factor and reject noise.
  4. The power supply uses four 1N4007 diodes to make a full wave rectifier. These diodes are a good choice, as they are rated for 1000 V. Some cheap Chinese power supplies use a single diode for a half wave rectifier, which is less than ideal.
  5. There are two ZSF brand 400 V 10 uF capacitors rated for 105 C. A one ohm resistor separates the two. This presumably is for noise rejection.
  6. The main IC is an RM6203 from the Chinese semiconductor company Reactor Micro. This appears to be a well specified component and it offers a reasonably low standby power consumption of less than 0.25 W.
  7. The blue ceramic capacitor which links between the high and low voltage sides of the power supply is one of the most important safety critical parts of a power supply like this because if it were to fail short circuit it could connect high voltages to the output. This capacitor appears to be a proper X1 / Y1 rated part which should fail open circuit. Some Chinese power supplies skimp on this safety critical part.

The underside of the board shows more issues, but again it's not entirely bad news:

  1. Another view of the hopelessly inadequate single screw which was supposed to hold the pins in place.
  2. Spacing between 400 V tracks on the printed circuit board is only about 1 mm. This is not adequate for safety.
  3. The pins of the diodes of the full wave rectifier are not so neatly soldered as they could be, further reducing the spacing.
  4. On the positive side, the spacing between low and high voltage parts of the power supply is of the order of 6 mm, whch should be enough to make the thing safe.

While I have not taken the transformer apart to check how it is constructued, in my view it appears that the electrical design of this power supply is adequate. Issues of spacing do need to be looked at to make it better than adequate. However, the mechanical design is not up to the job. Hot melt glue between two glossy surfaces will never work well enough to keep something like this in one piece when it is subject to quite considerable stress each time it is plugged in. In my view the company should replace these chargers.

I'm not an expert on power supply design but I sent this analysis to a friend of mine who has designed SMPSUs for large multinational companies (there may well be one of his designs inside your television). His comments were as follows:

  • The clearance between the mains tracks should be 2.5mm, this is not so much an electric shock issue, but it could possibly cause a fire if there's no small fuse protecting the PSU should it short out
  • There looks to be an almost complete absence of EMC filters, so I wonder how they got it through the mains emissions tests. Possibly they didn't. I've yet to work on a PSU that could pass the emissions test without some form of common-mode choke - they may have achieved it with the two elcos and a resistor between, but I doubt it and the EMC test results were probably fudged.
  • Overall this PSU is unlikely to kill you, but the loose pins are a bad thing of course.

After reading this, Yarvik sent me a free replacement power supply. That's good service from Yarvik and currently I'm using their replacement because I don't think it's dangerous to use. However, so far as I can tell the new one is identical to the first one so I expect it to eventually break in the same way as the original.

My favourite apps

Apps that I've installed on the tablet include the following. The links to Google Play allow you to install them to your tablet directly from this website, even if you're using a different computer:

Bicycle Bell
Buienradar.nl - up to date weather information
Cyclestreets - cycle route planning software written by friends in Cambridge
Uitzending Gemist - watch TV programmes which we missed.
Gmail - Google's mail program.
Guardian and Independent - British newspapers
RTV Drenthe - local news.
Fokke en Sukke - funny cartoon.
Marktplaats - online trading.
Google Maps
Marvin - ZX Spectrum emulator. I can run old software that I wrote 30 years ago with this.
TV Gids written by an independent developer BenSoft is a really nice small app which displays the Dutch TV listings. Ziggo, our TV provider, also has an Android app but theirs is absolutely terrible.
TuneIn Radio - receive radio from stations all around the world
Amazon.com - Read eBooks using the FREE Kindle Reading App on Most Devices Amazon Kindle - a good e-reader.
Google Translate - sometimes I need a bit of help.

I've tried a few games, but I'm afraid none of them really grabbed my attention for long.

My First app

One of the advantages of an open platform is that you can write and distribute your own apps. I started doing this even before my tablet arrived.

I worked as a software engineer for 20 years, stopping at the end of 2003. Most of my work had been in C and C++ or in Assembler. I'd never written any Java before, nor of course ever written code for Android.

I downloaded the Android and Java SDKs and the Eclipse editor and built an app in just over a day. The development environment is excellent. Because at first I had no real hardware I had to run the app on an emulator, and then tried it on my childrens' phones. Quite productive for a first app on a new platform after a long rest, I thought. In keeping with our online webshop for bicycle parts, this first app is a bicycle bell with samples of the real bells that we sell. It's free to download.

If you've any software development experience at all, I think you'll find getting started with Android to be very easy if you try it.

SDK debugging with the TAB250 and Ubuntu Linux

At first I worked with the tablet emulator in the Android SDK. However, when the tablet arrived I wanted to be able to work on the real hardware as well

When I first connected up the USB lead and tried it, the Android SDK tools didn't recognise the new device. It didn't take long to fix this:

Plugging a USB cable into the TAB250 and running lsusb on my Ubuntu box revealed a new device:

Bus 001 Device 014: ID 18d1:deed

To enable the Android SDK to work with the TAB250 I had to create a new file for udev. I followed instructions here and added a new file /etc/udev/rules.d/91-android-rules with the following one line content:

SUBSYSTEM=="usb", ATTRS{idVendor}=="18d1", SYMLINK+="android_adb", MODE="0666", OWNER="david"

Replace "david" with your username.

After this, the debugger from the Android SDK works correctly with the table. You can download apps, install them and run them without the debugger.

ADB SHELL runs as root, meaning that you can potentially change everything from this point onwards. At least this bit was easy.

Google Play (Android Market) with the TAB250

One of the immediate problems with the TAB250 is its inability to access Google Play / Android Market. I solved this very quickly by following the instructions in this video. It is important to use the second set of files that he suggests, not the first. If you install the first then everything looks fine but Android Market fails to download anything.

Note that before doing this you must reboot the tablet by holding in the volume+ and power keys at the same time and go into the setup mode. Clear the cache and memory before booting and following the instructions in the video. It is easiest if you download the files that you need onto an SD card beforehand.

It is worth setting up the Android Market so that by default it stores applications on the flash drive rather than in internal memory. I found a tip about how to do this. Attach the debug cable and run the command:

adb shell pm setInstallLocation 2

This also enables the "Move to SD card" option for far more apps, meaning you can install more on your tablet before you receive messages about a lack of memory.

Removing the GetJar app

The Yarvik specific GetJar app delivered with the TAB250 consumes resources in the background, and isn't actually very useful for installing apps. It can be removed by using this command:

adb pull /system/app/GetJar_yarvik_getjar_com.apk
adb shell rm /system/app/GetJar_yarvik_getjar_com.apk

The first command retrieves the file from the tablet and places it on your PC. You can use this to restore the GetJar functionality at a later date should you wish to do so.

Modifying build.prop

Even though I'd now installed the Android Market, a lot of apps were not able to be downloaded. For instance, such common apps as that for Facebook could not be downloaded from Android Market. I decided to investigate.

A bit of reading revealed that the /system/build.prop file on Android holds a lot of possibilities for configuring the tablet. I have to advise caution on modifying this because it is possible to render the tablet incapable of booting. However, at your own risk...

With the tablet connected to your PC with a USB cable, the build.prop file can be retried with the following command:

adb pull /system/build.prop /tmp/build.prop

After making any changes that you wish to try, you can put the new version back on the tablet with:

adb push /tmp/build.prop /system/build.prop

One of the things that I found annoying with the TAB250 as standard was that system text, e.g. the clock at the top of the screen, took up too much of the available screen space. It was due to the number of dots per inch for the display being set at 200. The actual number of dots per inch on the TAB250 display is 133. However, I found that 160 was a good compromise. Alter the following line, changing the dots per inch from 200 to 160:

ro.sf.lcd_density = 160

Other changes that I made were to change the brand and model in this file to represent the tablet in use:

ro.product.model=TAB250
ro.product.brand=Yarvik

I also changed the fingerprint so that the Android Market would notice that this was a "different" device and allow downloading of apps which are not allowed on the standard TAB250:

ro.build.fingerprint=telechips/full_tcc8900_evm/tcc8900:2.3.3/GRI40/20120301.092920:eng/test-keys

With the changes so far, Android Market now offers a vastly greater selection of apps, which download and run just fine. I also made a few other changes, but I'm not sure they really made much of a difference.

Increasing the heap size for the dalvik virtual machine is supposed to make things run a bit better:

dalvik.vm.heapsize=32m

This option either turns on more hardware acceleration, or perhaps achieved nothing at all:

debug.sf.hw=1

Attempting to get rid of the boot animation was recommended in some places but appears to achieve nothing of value, so I no longer recommend including this line:

debug.sf.nobootanimation=1

Becoming unreliable

A little after its first birthday, my tablet started to become unreliable. It required occasional reboots which were not required before, and then I had to restore to factory settings a few times.

I discovered that if it got to the point where the tablet would look up on the "Yarvik" screen and go no further this could be a problem with corruption of the Darvik cache. It was still possible to connect to the tablet using ADB and the following commands would resolve the problem and reboot the tablet:

rm /data/dalvik-cache/*
reboot

Installing Clockwork Recovery gave me boot time options to erase the dalvik cache and made it possible to get the machine running again wihout a complete restart.

However, the situation continued to get worse. I don't know where the fault lies, but I now think there is a hardware fault with my device. It could be that, for example, a ball underneath one of the RAM chips or the processor (both BGA) doesn't make good contact any longer, but this would not be economical to fix.

Teardown

I decided to take the tablet apart and at least look for any obvious faults:

The TAB250 comes apart very easily by use of a sharp object from behind the machine around the edge to dislodge plastic clips. Compared with many modern devices I have to say this tablet comes apart with ease and that's a good thing because if something had been obviously wrong, perhaps I could have fixed it.


Inside the Yarvik TAB250


There is a single large printed circuit board inside the TAB250 which houses the Telechips processor, Hynix flash and Samsung DRAM. A Realtek wifi chip has its own small PCB.


Unfortunately, there was nothing obviously wrong with this tablet so nothing I could obviously fix
The processor dates from the 25th week of 2011. RAM from the 31st week.

Because the tablet has become less and less reliable, it's also become less useful. To use it now I have to delete the dalvik cache several times a day. This is not convenient. It's difficult to say what the problem was. The tablet has not been handled roughly (at least not before it started to go wrong !) and the problems appeared quite slowly over time. It's no longer a computer that I can trust.

Is the problem due to the flash memory being worn out?

The problem which I'm now seeing looks very much like what I've seen in the past when hard disks have developed errors. It's quite possible that this is actually the same problem. i.e. the flash memory has been "worn out" due to excessive erase/program cycles.

According to the datasheet for the Hynix H27UBG8T2ATR NAND flash used in the Yarvik tablet, 3000 program/erase cycles are possible. Flash memory relies on error correction techniques being used by the software in order to achieve this level of reliability and as parts of the memory become unreliable they should be added to a bad block table and avoided. Has this happened with my tablet ? Has the table itself become full ? I don't yet know.

I'm looking for a tool to check this problem.

Other resources?

I'd have saved a bit of time if I'd found this page first.

Apparently the 5.2 version of the rom image here works well with the TAB250.

A good description of how to make a new boot animation.

Other tablets


A failed Difrnce DIT8070 below another disaster: the FujiFilm XF-1 camera.

Four years later, I've had two more Android tablets. I waited for a long while after the Yarvik went wrong and then bought a Difrnce DIT8070. This lasted for about 18 months and then went wrong in precisely the same way as the Yarvik, looking for all the world as if the flash memory has forgotten how to remember.

My current tablet is a Denver TAQ70202 Mark 2. This hasn't yet given up but I do wonder how much durability it has. Due to my bad experience with other Android tablets, this was selected solely on price - it was a free gift with a purchase of stationery for our business.

These things seem to have no durability at all. Certainly not comparable durability with my first laptop.



My first experience with tablet computers was way back in 1991.


These days I sell bicycle components and lead cycle tours for a living.