Friday, March 9, 2018

UNI-T UT33C multimeter: review and teardown (part 1)

About UNI-T

Uni-Trend (UNI-T) is a relatively new Chinese company (founded in early 90's) that has gained quite a lot of market share, especially in Europe. They produce all kinds of test equipment, multimeters, oscilloscopes, thermometers, etc. which usually offer a good balance between cost and quality. Their products lack the toughness of their western counterparts but they do work and most importantly, they won't break your wallet. For the general electronics enthusiast, they are more then well suited, but only when working with low energy, otherwise from a safety standpoint they're far from my no.1 choice. All in all, many of their products are worth checking out, but do make sure to read any reviews if you want to buy one.

The UT33C

The battery from one of my first multimeters (UT33C) has gone flat so I decided to review the meter it since it has to be torn apart anyway in order to replace the battery. In the next post I'll show a couple of modifications that should (at least theoretically) improve the robustness of this meter.

General description

The UT33C is part of the UT33 family and from the first look at the dial it's somewhat similar to the DT830 which everyone knows. There are however some differences. There is a hold button, a backlight button (mechanical latching), a thermometer based on a K-type thermocouple and a continuity tester. The case is made of a thick red plastic and the meter has a protective rubbery (I don't think it's legit rubber, maybe some sort of soft plastic) holster around it. The case itself is solid and does not creak when twisted in the hands. A  good safety feature is that the cases overlap around the edges so any flames or fragments will be contained should a catastrophic failure occur while taking a measurement.

Front of the device. Note that the connectors are all the same color

Back of the device and its holster.

The LCD is the same size as the DT830 but it's well protected by a thick plastic piece. On the back of the unit there is a standing bale which looks very fragile. It could be used to angle the multimeter to see its screen, but one should refrain to press the buttons or rotate the range selector while the multimeter is standing on the bale because it will most likely break off.

To keep things short, I won't get into any details regarding the accuracy and functions, but you can find everything on UNI-Trend's website.

To take the unit apart, one must remove the protecting holster then undo the 2 self-tapping screws on the back of the case. Then gently pry apart the case starting from the bottom. The top is held in place by a plastic latch. In the back cover there is a small compartment where the battery sits and the buzzer used by the continuity function. The electrical contact between the PCB and the buzzer is made by springs.

Bottom case contains the battery compartment and the piezo buzzer

Front case with the sliding contacts

The PCB is held in place with the help of a screw located near the battery and 2 plastic clips (bottom left and right). Regarding the battery, it's the classic 9V (6F22) which sits on a piece of foam glued to the board. On the bottom there are the 3 connections for the probes which are of the split type, but that's understandable considering the cost of this thing.

Internals of UT33C. The small 8 pin chip is a LM358 used to amplify the thermocouple signal

Split connectors and PTC
On the other side of the PCB there is the LCD assembly. To get it out, first undo the screw located near the battery, desolder the backlight LED then gently bend the plastic latches on the sides of the screen assembly.

Front of the PCB

The screen assembly. LED is built in, green - yellow color
Regarding the IC that powers this multimeter, it is an AME7106 (ICL7106 clone?) which is used in most low-end multimeters (including the DT830). If the ADC gets damaged, there's no way to replace it because it's in the form of a blob.

PCB with the screen removed
Regarding the protection, here is where the UT33C lacks. It only has a fuse  and a PTC thermistor (WMZ75S, 1K, made by Sinochip). Based on this, I wouldn't trust it for anything more than 50 volts. The fuse is rated 315 mA at 250V and is the cheapest one they could find (glass, fast type). Although there is space on the PCB for a fuse for the 10A circuit, they omitted it.  Another 2 cents saved!

The probes have a length of about 1 m. The wires are covered in a rubbery plastic. The probes are made of hard plastic and have an ergonomic shape. The nice thing is that the connectors are shrouded for increased protection. The tips are not very sharp but get the job done. Rated for CAT I 1000V, CAT II 600V and 10 A although I highly doubt they can hold 10 A.

Everything plastic, only plus are the shrouded connectors

My opinion about UT33C

The good:
  • Low price
  • Thick plastic case, rubber holster
  • Has backlight, continuity test, K-type thermocouple thermometer, measures resistance up to 20 MO
  • Crisp LCD screen
  • Decent probes for the price

The bad:
  • No protection besides a PTC thermistor and a fuse (for mA range). Unfused (!) 10 A range
  • Shared Volt / Ohms and milliamp connector can lead to accidents if wrong range is selected
  • To replace the battery the case must be opened (2 self tapping screws). This can't be done too many times. 
  • Limited to 3 1/2 digits (max. display 1999) because of its outdated ADC
  • Tilting bale breaks off after a few uses

Sunday, December 24, 2017

Fixing an Eurocolor CTV2146-TXT CRT TV

There's been quite some time since posting on the blog, mainly because I didn't have too much time to fiddle with electronics, at least not as much as I'd like to. There are quite a lot of projects that will soon be finished though, so make sure to check around from time to time.

In this post I'd like to talk about a recent repair job on an old TV set which a friend brought me to fix. I am not an electronics engineer nor do I have too much experience with fixing TV sets but I love a challenge every now and then.

The patient

This TV is a Eurocolor CTV2146-TXT, old CRT technology from early 2000s (I think it was manufactured around 2004). CRTs are still fairly popular in some parts of Eastern Europe, mainly because of their lower cost and relative serviceability. Even though major companies like Samsung or Sony which had a big market share in CRT TVs have since moved on to newer technologies like plasma, LCD or LED displays, there are some smaller firms like Vestel from Turkey (the OEM for lots and lots of cheap CRTs) which still sell them. And they sell well, especially since the people were disappointed with all the planned obsolescence that newer tech has. Another key factor that has discredited the new products in the eyes of some consumers is the capacitor plague which has rendered many of the early LCD TVs and monitors useless after only 1-2 years of use. Sadly, this trend seems to continue, see Samsung's LCD TV problem after they used the troublesome Samwha WB series electrolytics in the power supply.

So yeah, one can't blame the people for wanting to stick to technology that has been proven somewhat reliable, especially with all the latest economic problems and constantly increasing prices for bells and whistles.

The problem

Getting back to the patient, after the workout (getting the thing in the house), I turned on the power and the problem revealed itself right away. The colors looked all mixed up, especially around the edges of the screen. It's like a whole area was showing different colors than it should have, with a magenta / violet tint. Here are a few pictures that explain things better:

Note the color at the top, blue instead of green

Feeding the TV a blue picture via the A/V port showed the problem a little better:

Since CRT TVs are based on analog technology and there aren't many things inside that can fail in order to show symptoms like those, the first suspect I had in mind was the CRT picture tube itself (also because the problem was affecting only an area of the screen). Oddly enough though, when navigating to an empty channel, the black and white image appeared OK:

Had the CRT been the culprit, there should have been problems with a B&W picture as well
So the tube also seemed to be in good working order. And then I remembered I saw these kind of colors somewhere else: when I was little and I got a magnet close to the screen of our first color TV, thinking of what my parents would say. Luckily, the old unit fixed itself after turning it off and on again. Another clue was the fact that most CRTs I know made a loud thud when turned on, thing which didn't happen with this one. The sound is made by the degaussing circuit which involves creating a strong magnetic pulse along the edge of the picture tube with the help of a coil. This pulse resets the stray magnetic fields that are picked up by the shadow mask, a metal mesh that's built in the picture tube. If the shadow mask is magnetized (even slightly), the electron flow through it is disturbed, generating anomalies on the display.

So I proceeded to open the case and have a look inside. The PCB had some signs that suggest that the TV has been repaired before (an electrolytic cap and a resistor were replaced), but other than that it seemed OK, no burn marks or anything suspicious.

The 11AK30 A4 chassis
Exactly as I feared, the board was full of G-Luxon crapacitors (which were a big part of the cap plague). Vestel didn't even bother to use 105 degrees rated capacitors and went with 85 degrees. That's really a big no-no, especially since CRTs can get hot while working.

Not the best choice for caps, but a very cheap one

To make things even worse, some of the joints looked like the one below. I wonder how this TV was even turning on.

Some other details:

ST92195C microcontroller made by STMicroelectronics

STV2248C video processor made by STMicroelectronics

Tuner TECC2949VG28B made by Samsung

Studying the schematic (see the end of the article for download) revealed that the degaussing circuit is made from a 9 ohms PTC (TH800, MZ72AL 9RM) in series with a coil wound around the screen. When powering on the circuit, the sudden inrush of current creates a magnetic pulse and quickly heats up the PTC, which in turn decreases the current through the coil. As long as the PTC remains hot, the current through the coil is very small. The coil itself was fine, measuring around 2 ohms so I proceeded to remove the PTC from the board. Opening its case revealed the following mess:

MZ72AL-9RM PTC used in the degaussing circuit. Note the burn marks.

The PTC pill looked severely burned, and so did one of the springs. I bet this was the result of a power surge, possibly from a lightning strike in a storm. I couldn't find the exact part, so I ordered a generic 9 ohm PTC instead and also got replacement electrolytic capacitors for everything on the board.

Taking the garbage out
After the boring job of replacing each and every electrolytic, soldering in the new PTC and touching up the cracked solder joints, the problem disappeared completely. It's been around a week now and no magic smoke escaped, so I'll consider this repair a success.

Things look normal again

No more discoloration
For anyone that encounters problems with this TV, I have put together an archive with the schematic, the service manual and the EEPROM dump at the link below. They should be good for all TVs equipped with the 11AK30 A4 chassis.

Eurocolor CTV2146 TXT 11AK30 A4 schematic, service manual and EEPROM dump (Google Drive)

To conclude, if I were ever asked an opinion about this technology, I'd say the following:

The good:
  • Good viewing angles amp; contrast, bright colors
  • High lifespan when engineered correctly (I still have in storage a Samsung from 1997 in perfect working order)
  • Reliable (if quality parts are used), proven technology
  • No privacy concerns unlike smart TVs
  • Cheap!

The bad:
  • High power consumption compared to LCDs (but not even close to what plasmas burn)
  • Heavy... This thing weighs almost 20 kilograms!
  • Bulky because of the long neck of the CRT tube
  • Obsolete from a moral standpoint
  • They are becoming pretty rare

Thanks for reading and see you all in the next year!