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Tonghui TH2822 LCR meter review, teardown and possible hacking
<< < (3/3)
reagle:
Just bought a TH2822A on ebay shipped form China and am pretty happy with it. $175+$35 shipping, got it in less than a week and it's a solid instrument.
A full review is coming up in a bit, but it's already pretty obvious there is no way it's a copy of a BK Precision's design. They are definitely made from the same design or even made by the same people (probably Tonghui if I had to guess) and then just tweaked for branding purposed. Other than a position of a few buttons, the BK and TH are the same thing. Every software feature, behavior etc are identical. Definitely not a copy, more like variants :)
 I even watched Dave's review of the BK Precision one again to compare notes- everything was the same minus the Min/Max/Avg bug that TH did not have. And of course the PC software and its inability to handle more than 5 or 6 com ports is the same either ;)
So- a solid meter and a good buy
crisr:
Hi, got here after I saw a local ad for a TH2822A for a reasonable price. The thing that seems to differentiate it from the BK879B and others it's that, according to the ad and the datasheets I found, it does 4-wire measurement (the 5-terminal test slot, if you count guard), and even comes with Kelvin test leads. It also has a backlight, according to the pictures. Nowhere in the BK879B datasheet or on the other threads about this model I saw anything about 4-wire measurement...

Anyway, I am between this model and a Agilent U1733C (which would cost about 40% more). One of the things I would like to do is measure low resistance and capacitance, so I think 4-wire is very convenient for me, but in the past I have been disappointed with "no brand" test equipment (a local-rebranded multimeter) which turned out to be unusable in the uA / low mA range and since then I have always tried to buy brand-name (read Fluke / Agilent) gear. What would be your opinion about it's quality and accuracy compared to the brand name ones?
vtl:
As requested, I got my hot air station and sucked off the I2C eeprom and read the contents with a universal programmer
carloscuev:
Here I posted my findings by sniffing my TH2822A. By looking at your EEPROM dump I noticed that the data accessed by my meter when going into 10kHz mode are empty in yours, they are:

Switching Frequency from 1k to 10k:
Read @0x0326: A5 5A
Read @0x0328: 3A 9D BF 49 9C 23 3A CB
Read @0x073A: A5 5A
Read @0x073C: C8 9A 21 3D 02 39 35 3C
Read @0x0B26: A5 5A
Read @0x0B28: 03 AB 7E 3F 8E 2B 33 3D

In your EEPROM they appear empty, I'm not so sure about what does that data is, but I think it may be calibration data because in that other forum topic, reagle's (user) meter sniffing threw out different data in those same adresses. If this is not calibration data it could be some encrypted code, the funny thing is that for each frecuency selection the meter reads a new set of data, please read my entire post on the other topic.

I do not have a unviersal programmer, but I got a bus pirate, just need to learn how to use it to make a backup of my eeprom and start playing around.

But by now you could try changing the address 0x0054 to the value 0x0B (instead of 0x07 as it is in your memory dump) which means to power-up in 10kHz mode, this might not work because this is just the power-up frecuency value, there might be other bits to flip in order to make it possible, but it would be a nice try, if possible, reading the probes with an oscilloscope would confirm if it's generating 10kHz.

From address 0x0052 to 0x0063 there's the power-up data and this are my updated findings:

Write @0x0052: 01 (Mode, 00: L, 01: C, 02: R, 03: Z)
Write @0x0053: 0E (Sec. Display, 11: Frequency, 0E: ESR, etc. )
Write @0x0054: 0B (Frecuency, 0B: 10kHz, 07: 1kHz, 03: 120Hz, 02: 100Hz)
Write @0x0055: 00
Write @0x0056: 00
Write @0x0057: 00
Write @0x0058: 00
Write @0x0059: 00
Write @0x005A: 00
Write @0x005B: 00
Write @0x005C: 00 (??, 00: L, 01: CRZ)
Write @0x005D: 00
Write @0x005E: 00
Write @0x005F: 00
Write @0x0060: 00
Write @0x0061: 00
Write @0x0062: 00 (Rate, 00: slow, 01, fast)
Write @0x0063: 00 (01: AUTO, 00: otherwise)

I can tell your meter powers-up in AUTO mode, starting in capacitance measurement (this is normal because the leads are usually open reading some pF of capacitance), showing on the secondary display the ESR, in 1kHz frequency and in FAST update rate.

Nothing different between your 1kHz meter and my 10kHz meter so far. They could be different firmwares, or the presumable calibration data is not really that, but some encrypted code which tells the MCU which version it is, or maybe even both. But if this is true, the encryption key would be different for each meter, as we 3 (reagle, you and me) got different data for the 100Hz, 120Hz and 1kHz ranges.
trobbins:
Checking if anyone has attempted to fix a 2822 LCR meter, or has come across service info or a schematic for any similar LCR variants.

I have the MCP BR2822 handheld version, and accidentally sparked the input with a charged capacitor.  The meter still powers up and functions, but displayed measurements are not correct.  The 4 kelvin input terminals are protected by back-to-back diodes in a few configurations (although no identifiable fusing), but are directly connected to opamp input (U19 TL071C pin 2) and capacitor coupled (under pcb) to other opamp inputs (U15 TL064C pins 3, 5 and 12), and through analog switch to U15 pin 10.  So hopefully just one or both opamps, but there are a few 74HCT4052 analog switches that may have suffered.

The BK 879 seems to be significantly different - certainly in layout, and it has an smt fuse (https://www.eevblog.com/forum/testgear/bk-precision-lcr-meter-879b-with-adc-failure/), so perhaps not too helpful.

I'll try and do some sleuthing of voltages through the opamps and analog switches to see if there is anything obvious.
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