$20 LCR ESR Transistor checker project

[py-bb]

So I had one of these (a T7) but it just broke and now thinks everything's a zener diode. Any idea where one can get a "good" one - I love the feature set and speed - it was a great tool but yeah I'm annoyed now. Would appreciate any advice.

[Feliciano]

If the one you have has a protection IC, try removing it. Else, it looks you will have to find a replacement unit. The "good ones" are "old stock", so try thinking where to find that for your country.

[elecdonia]

So I had one of these (a T7) but it just broke and now thinks everything's a zener diode. Any idea where one can get a "good" one - I love the feature set and speed - it was a great tool but yeah I'm annoyed now. Would appreciate any advice.

Try this: Connect DC voltmeter to “A” and “K” zener test ports. Minus to “A,” plus to “K.” Then press LCR-T7 “START” button and observe voltmeter. It should read >25V. If <24V then there is a fault in the DC-DC converter circuit which steps up from the battery voltage to the expected 25-30V zener test voltage.

I believe the software checks this “A” to “K” voltage first. If >25V then it concludes “no zener is connected” and moves on to performing the standard transistor tester tests at ports 1,2,3. But if open-circuit voltage from “A” to “K” is <24V then it stays in zener test mode and won’t do anything else.

Also please test several zener diodes, for example 5V, 12V, 15V zeners. Are the results within +/- 1V of the zener’s rated voltage? Also while testing a zener diode: Does the zener get hot after few seconds? (It shouldn’t). If zener gets hot there is definitely a fault in the zener test hardware circuit.

I have three different LCR-x transistor testers with totally different internal circuit designs. Starting in 2022 some Chinese manufacturers substituted <$1 USD MCU chips for the traditional Atmel ATmega328, -324, or -644 MCU ( >$5 USD each and rising ). One of my LCR units with non-Atmel MCU developed the “zener gets hot” fault but otherwise still works OK otherwise.

My units have the following open-circuit voltages from K to A:
LCR-TC2 with  Atmel ATmega324 MCU:   25.4V
LCR-TC1         APT32F172K8T6 MCU:      30.2V
LCR-TC2         LGT8F328P MCU:             25.2V

Note: The Chinese manufacturers interchangeably label their models as LCR-T7, LCR-TC1, or LCR-TC2.The only way to identify “what is inside the plastic box” is from a photo of the PC board. However the vendors don’t provide photos of the PC boards. 

[py-bb]

So I had one of these (a T7) but it just broke and now thinks everything's a zener diode. Any idea where one can get a "good" one - I love the feature set and speed - it was a great tool but yeah I'm annoyed now. Would appreciate any advice.

Try this: Connect DC voltmeter to “A” and “K” zener test ports. Minus to “A,” plus to “K.” Then press LCR-T7 “START” button and observe voltmeter. It should read >25V. If <24V then there is a fault in the DC-DC converter circuit which steps up from the battery voltage to the expected 25-30V zener test voltage.

I believe the software checks this “A” to “K” voltage first. If >25V then it concludes “no zener is connected” and moves on to performing the standard transistor tester tests at ports 1,2,3. But if open-circuit voltage from “A” to “K” is <24V then it stays in zener test mode and won’t do anything else.

Also please test several zener diodes, for example 5V, 12V, 15V zeners. Are the results within +/- 1V of the zener’s rated voltage? Also while testing a zener diode: Does the zener get hot after few seconds? (It shouldn’t). If zener gets hot there is definitely a fault in the zener test hardware circuit.

I have three different LCR-x transistor testers with totally different internal circuit designs. Starting in 2022 some Chinese manufacturers substituted <$1 USD MCU chips for the traditional Atmel ATmega328, -324, or -644 MCU ( >$5 USD each and rising ). One of my LCR units with non-Atmel MCU developed the “zener gets hot” fault but otherwise still works OK otherwise.

My units have the following open-circuit voltages from K to A:
LCR-TC2 with  Atmel ATmega324 MCU:   25.4V
LCR-TC1         APT32F172K8T6 MCU:      30.2V
LCR-TC2         LGT8F328P MCU:             25.2V

Note: The Chinese manufacturers interchangeably label their models as LCR-T7, LCR-TC1, or LCR-TC2.The only way to identify “what is inside the plastic box” is from a photo of the PC board. However the vendors don’t provide photos of the PC boards.

While I appreciate the thought a diode sort of burned itself in two and it's next to some very unhealthy looking components, I accidentally scraped the diode off trying to clean it up a bit and see what it was and yeah it's bad...

No idea why this happened either, I've been treating it right!

[elecdonia]

While I appreciate the thought a diode sort of burned itself in two and it's next to some very unhealthy looking components, I accidentally scraped the diode off trying to clean it up a bit and see what it was and yeah it's bad...

No idea why this happened either, I've been treating it right!

I bet you have the same (very poorly designed) LCR-Txx model that I received a couple of months ago. Does the area where the PC board has bad (cooked) components look like the attached photo? And is the toasted diode labeled D2?

[GraemeG]

I have just received a "TC1" of this latest design elecdonia. Within a couple of days it failed while testing a few zeners and schottky diodes on the K-A terminals. After the failure it would always show a 4.1V zener across the K-A terminals and when I opened the box it was obvious that D2 was burned.

I was able to find a replacement SMD diode for D2 and it was soon working again but I noticed whenever I tested a zener diode both the zener and D2 would get very hot. Apparently there is nothing to limit the current through D2 and the tested device other than the ability of the DC-DC converter circuit to supply current!

I found a solution on another site (that I now can't find) that suggested cutting the track between C14 and the K terminal and inserting a 4K7 resistor. This limits the current to about 6-7mA even into a short circuit on K-A. This seems to work perfectly! Originally I tried a 10K resistor but the voltage drop was a bit too much and the tester identified it as a zener of about 27V. With the 4K7 resistor the open circuit K-A voltage is 29.7V and this is high enough that the detector assumes no zener is connected.

With an oscilloscope I also noticed that the test voltage was not very well filtered with about 1V peak-peak of 1kHz ripple. I added a 1uF 50V low ESR electrolytic from the K terminal to ground and this dramatically reduced the ripple but to be honest I'm not sure it changed the test results very much. I have tested several zeners now and there is no sign of over heating. The test results seem to be low by perhaps 0.3-0.6V but it's a bit difficult to be sure as there is some variation in diodes and the voltage does depend a little on the test current. In any case it is near enough to give a good/bad indication and if I open the box again some time I might tweak the voltage divider to give a slightly better result.

The bottom line is that every tester of this latest design is almost certain to fail when testing zeners, particularly low voltage zeners and or diodes.

[elecdonia]

I have just received a "TC1" of this latest design elecdonia. Within a couple of days it failed while testing a few zeners and schottky diodes on the K-A terminals. After the failure it would always show a 4.1V zener across the K-A terminals and when I opened the box it was obvious that D2 was burned.

This failure will occur sooner or later to every LCR-TXx unit having this PC board design where the 30V power source rail connects directly to the test socket K terminal.  This manufacturer really “went their own way” with both hardware and software. Unlike the original OSHW Transistor Tester zener test circuit, which has a “step-up switching voltage regulator” IC generating a steady 30V rail followed by a 10K series resistor, this manufacturer substituted a “step-up constant-current LED driver” IC. Their main mistake is they failed to place a proper series resistor >2K2 ohms between the 30V rail and pin K.

I was able to find a replacement SMD diode for D2 and it was soon working again but I noticed whenever I tested a zener diode both the zener and D2 would get very hot. Apparently there is nothing to limit the current through D2 and the tested device other than the ability of the DC-DC converter circuit to supply current!

Here’s what happened: When a zener or a diode (forward-biased) is connected between K-A while the tester is in a powered up state, this damages the current sensing input pin of the step-up IC (U7 on PC board). Actually any electronic component with low resistance <1K ohms or a capacitor with low ESR could cause this failure. This pin on the step-up IC (U7) has a maximum input voltage of 4V. This IC pin is connected to the A terminal of the test socket. Worst case is placing a direct short between K and A while the LCR tester is powered up with the LCD screen turned on. There is a filter capacitor (C14) on the 30V rail (K on test socket). C14 carries a 30V charge. Making a short circuit from K-A dumps all the energy stored inside C14 straight into the current sensing pin of the step-up IC. Although the duration may be brief, 30V is a lot more than 4V. In many cases only the current sensing portion of U7 gets destroyed. The rest of U7 continues to operate, generating 30V without any current limiting. This allows the peak current up to 500mA  through K-A. This is why a zener under test can get extremely hot almost instantaneously.

I found a solution on another site (that I now can't find) that suggested cutting the track between C14 and the K terminal and inserting a 4K7 resistor.

I used a 2K2 series resistor in my unit. If the series resistance is larger the tester may report that a zener is being tested while the K-A pins are open-circuit. A value of 2K2 is sufficient to protect U7 from damage.

To conclude, the fix is:
1) Cut the PC board foil leading to pin K and then connect 2K2 resistor from junction of D2 and C14 to pin K,
2) Replace U7
3) Replace D2 if faulty or burned
4) Check that open circuit voltage from K to A is approximately 30V
5) Place a 1K resistor across K-A and confirm that tester identifies this as a 5 to 7V zener.

[GraemeG]

this manufacturer substituted a “step-up constant-current LED driver” IC.

Thanks for this, I had not checked out the DC-DC converter circuit and did not realise the design change they had made. It is likely that the current sensing input is damaged on my device but it is still generating about 31.5V on C14 so with a new D2 and the extra resistor before the K terminal I still have a working circuit 

A 1K resistor between K and A reads as a 5.2V zener but this is with the 4K7 resistor that I used instead of the 2K2 you recommend. If I have it open again some time I might change the resistor as I am not sure how close I am to the threshold where the code decides a zener is connected to the K-A terminals. The built in battery is currently 3.87V and the zener circuit with 4K7 resistor is producing 29.7V on open circuit K-A terminals but this may drop as the battery discharges. Time will tell!

[alex_D]

Just bought a tester marked Multi-function Tester - TC1 from ebay.
It is never able to test any component but it does boot up ok and the display is ok.

It doesn't recognise any components in the 1,2,3 sockets, it just displays "unknown or damaged part" after
pressing the start button.

I have tried a lot of other simple components and I get the same result.
If I put a diode in the (K, A) part of the socket it does recognise it as a zener, so it seems to be stuck in zener mode.

It has a blue PCB with no transformer, uploaded a photo. On the clones table it looks like a TC1(2), but can't find
a schematic anywhere. cant see any damage on the pcb.

I just got the tester for resistors, capacitors and transistors and I don't need the zener function which seems
to be problematic in any case.

Anyone know how to completely remove and disable the zener testing function?

[madires]

If your TC1 doesn't detect any component connected the standard test pins then it's obviously broken. Have you notified the seller and asked for a replacement? In case of a genuine ATmega you can try an OSHW firmware and run the selft-test to check the test pins. The OSHW firmware can also be customized, e.g. disabling the Zener check.

[Messtechniker]

I think its time to put out a strong warning
about fake transistor testers.
The good times are definitely over.

[indman]

In case of a genuine ATmega you can try an OSHW firmware and run the selft-test to check the test pins.

Madires, this is not a genuine ATmega.This clone variant is built on LGT8F328.I also advise the clone owner to contact the seller with a complaint about a faulty device.In addition, I see a lot of traces of flux on the board, which can negatively affect the quality of work.

[alex_D]

Thanks for the info. I have already got a refund from the seller and they let me keep the fake box, so I will play with it later.
I have ordered a DIY kit tester instead so hopefully I will have more luck with this. 

[elecdonia]

Just bought a tester marked Multi-function Tester - TC1 from ebay. It is never able to test any component but it does boot up ok and the display is ok. It doesn't recognise any components in the 1,2,3 sockets, it just displays "unknown or damaged part" after pressing the start button.

Have you performed the “self-adjustment” procedure?
Do this by shorting test socket pins 1, 2, 3. The metallic strip of 3 crimp pins supplied with these LCR-Txx units can be used for this. Take care to place it so that it shorts pins labeled 1-2-3. Then press the button. It should display “self test.” After 10-30 seconds it should display “isolate probes.” Now pull out the short circuit and it should eventually display “test complete.”

If I put a diode in the (K, A) part of the socket it does recognise it as a zener, so it seems to be stuck in zener mode.

If your tester displays “unknown or damaged part” then it isn’t stuck in zener test mode.

It has a blue PCB with no transformer, uploaded a photo.

The unit in your photo is likely to be a clone using the LGT8F328P MCU.

Unfortunately the software is proprietary and cannot be upgraded. Eventually the open-source Transistor Tester software might get ported to the LGT8F328P MCU, but I’m not expecting this anytime soon. Note: The LGT8F328P claims to be an improved version of the Atmel ATmega328P. However it isn’t fully compatible at either the hardware (different pinout) or software level. This MCU is made by a Chinese semiconductor company named “LogicGreen.”

I have a similar unit with LGT8F328P MCU. It functions but has issues (inaccurate for some types of transistors). It does measure resistors, capacitors, and inductors reasonably well.

I discovered a way to identify units with LGT8F328P MCU: Press and hold down the button for 30 or more seconds. The display language should switch from English to Chinese. Wait till tester times out and powers off, then press and hold the button for another 30+ seconds to switch back to English.

[alex_D]

Yes I tried the self test and just got the “unknown or damaged part” message. After pressing Start testing across the 1,2,3 pins gives 0 volts. testing across K,A gives 25.4v. Do you know what voltage I should get across the 1,2,3 pins after pressing the start button?

Thinking that there could be a bad connection to the test pins somewhere, might try working my way back re-soldering the joints. other than this thinking the 6 pin V05 (D2) might need swapping.

Yes, if I press the Start button for around 30 seconds the language does change to Chinese.

[Feliciano]

If you have an oscilloscope at hand, you could try to see whether there are pulses on the 123 positions when you press the test button.
With a DMM you could double check the continuity of the traces.
If the MCU is damaged, there's nothing more to do.

[elecdonia]

Yes I tried the self test and just got the “unknown or damaged part” message. After pressing Start testing across the 1,2,3 pins gives 0 volts. testing across K,A gives 25.4v. Do you know what voltage I should get across the 1,2,3 pins after pressing the start button?

OK, I will post the voltages from my LCR-TC1 which contains the LGT8F328P MCU. Its PC board is similar to yours.

Thinking that there could be a bad connection to the test pins somewhere, might try working my way back re-soldering the joints.

My experience is that shorts from solder bridges occur more often than open circuits from dry joints. I recommend a close visual inspection with a strong magnifier, microscope, or close-up photos taken by smartphone.

…other than this thinking the 6 pin V05 (D2) might need swapping.

When V05 failed in another of my Transistor Testers (different model) the display was “CELL” for one of the pins. I don’t recall which pin or the exact voltage reported by the “CELL” display message. FYI, “CELL” normally gets displayed when the tester software detects that a battery has been connected to the test pins. In my case V05 failed when I accidentally connected a charged capacitor. Replacing V05 fixed my tester.

Yes, if I press the Start button for around 30 seconds the language does change to Chinese.

This confirms you have the version with LogicGreen LGT8F328P MCU.

[alex_D]

Put the scope on the 1,2,3 pins, after pressing start I get a short series of pulses at 5 volts, except between pins 2 and 3 where I get 0 volts.

With the battery disconnected did a continuity test between 2 and 3 and have a short in both directions, can't see any obvious short, so I'll
need to trace the tracks and maybe try to draw a schematic, need to get a pinout for the MCU. At least PCB seems to have a 10k series resistor for
the K pin.

The short could be under the ZIF or the 6 pinV05, or a damaged MCU, at least the infra red remote test works ok.  lol.

[zzika]

To conclude, the fix is:
1) Cut the PC board foil leading to pin K and then connect 2K2 resistor from junction of D2 and C14 to pin K,
2) Replace U7
...

Could you please tell me which step-up IC (U7) it is?

The marking on the my chip is  HYEQG.

Where can I buy those step-up IC?

[GraemeG]

I don't know what chip U7 is but looking at your board D2 shows no sign of burning or heat. If you still measure 28-32V on the K-A pins with no component connected then U7 is still working and all you need to do is to cut the track between C14 and the K terminal and insert a 2K2 resistor. Once this change has been made this version of the tester seems to work well.

[zzika]

I don't know what chip U7 is but looking at your board D2 shows no sign of burning or heat. If you still measure 28-32V on the K-A pins with no component connected then U7 is still working and all you need to do is to cut the track between C14 and the K terminal and insert a 2K2 resistor. Once this change has been made this version of the tester seems to work well.

The voltage on the K-A pins is 4V - the voltage of the Ni-Mh battery.

I need to replace U7 but I can't find which circuit it is.

[indman]

zzika, it is advisable for you to read this topic carefully or use the search system for the necessary information on the forum. Information about the U7 chip has already been discussed in this topic:
U7 - this is SY7200A (marking HY6VE)

[zzika]

This topic has 328 pages. Before asking the question I tried reading the discussions to find an answer, but without success. The search system could not find a suitable post for me according to the parameters I had.

Thank you for your response. I already ordered the SY7200A so if anyone needs info I'll leave a message here if everything is working as it should.

[indman]

Before asking the question I tried reading the discussions to find an answer, but without success. The search system could not find a suitable post for me according to the parameters I had.

In the screenshots below, I show you that the search engine on this forum is working fine.

[RoGeorge]

LOL, thanks, I didn't know that button either. 
Was using Google with site:eevblog.com appended in the search terms.