Schematics of ET4401, ET4402, ET4410 and ET4510 LCR meter series reversed. - Page 1
Forum section: electronics / testgear

author: morris6
author_rank: Regular Contributor
timestamp: June 27, 2025, 07:13:11 pm
content: Don’t turn it on, draw the schematics.So here they are, the complete schematics of the Mustool ET4410 LCR meter with main board version 1.08. As discussed in another thread this instrument has the same hardware as the East Tester series of LCR meters.https://www.eevblog.com/forum/testgear/east-tester-et4410-desktop-lcr-meter/The PCB’s are just two-layer designs, reversal wasn’t too difficult. However, the values of the unmarked capacitors remain unknown.The front panel PCB and the main PCB are done as two separate schematics consisting of several pages each. These schematics are posted here as multiple page PDF’s.One page, main board measure section, is attached here in PNG picture style as a teaser.Enjoy!Edit 27-06-'25: page 4 of ET44Main.pdf corrected circuit around Q203 and Q204.Edit 03-08-'25: page 4 of ET44Main.pdf corrected position R210. Dates added.

author: morris6
author_rank: Regular Contributor
timestamp: June 27, 2025, 07:17:55 pm
content: Some “interesting” details of the Front panel design:There is an unpopulated footprint for most probably a CH376T USB and SD card File Management Controller. This chip exactly fits the signal traces of the footprint. The wiring suggests that the CH376T is used in SPI mode and forms a bridge between the MCU and a USB memory stick at the empty space left of the On/Off switch at the front panel.The isolated section handles the signals to the Handler connector on the back of the instrument. The DB-9 interface is described in the paper manual that came with it.Another not populated area for a 20 pin connector gives access to 10 IO signals of the MCU, another interface?

author: morris6
author_rank: Regular Contributor
timestamp: June 27, 2025, 07:21:27 pm
content: And the Main PCB also has some “features”:The reversal of base and collector of Q203 in the amplifier part of the signal source, it looks like it’s intended that way. (Main PCB Sheet 4)The way the null detector is arranged with U301, two amplifiers in series. There’s even a cooling pad with via’s to the back side of the PCB.The measurement section has two differential channels. One differential channel measures voltage over the “DUT”, VMEA. The other measures voltage over the current-sense resistor R316 to R322. Both differential channels can be switched to isolate DC by connecting C100 and C103 in the circuit.It looks to me that the adjustment process to attain the published accuracy must be quite elaborate. I think that the two trimmers R436 and R437 are only used to correct for DC offset in both measurement channels.Communication between Main and Front board is done with an isolated UART pair and with a separate optically isolated signal from Main to Front MCU, an interrupt? Member jheissjr in Reply #645 shows some details of this communication.

author: temperance
author_rank: Super Contributor
timestamp: June 27, 2025, 08:06:06 pm
content: I have the idea that the circuit around Q202, Q204,Q203 and Q201... is wrong. Q204 is always on as long as signal out is not close to -Vin and Q2020 never conducts. Q203 and Q201 also never conduct but for another reason.

author: morris6
author_rank: Regular Contributor
timestamp: June 27, 2025, 08:49:42 pm
content: Q203 and Q204 are functioning as diodes here, Base and Collector are actually connected together, I missed that. Will have to make a correction here, sorry for causing this confusion.Page 4 of ET44 Main corrected.

author: temperance
author_rank: Super Contributor
timestamp: June 27, 2025, 09:38:04 pm
content: Thank you.

author: anchung.chen
author_rank: Newbie
timestamp: August 03, 2025, 07:30:41 am
content: This should take a lot of time, thanks for your GREAT effort!

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2025, 08:22:52 am
content: The connection of the 2 amplifiers in U301 is not that unusual and I would not call it series connection it is more using the 2nd OP-amp as a parallel driver for more power. The only unusual point is the 1 ohm resistor value. Normally this should be more like 10-50 ohms to get better current sharing (depends on the OP-amp offset).The power amplifier still looks wrong, like the R210 at the wrong position. The resistor should be more between the input and output. So Q203 and Q204 emiters connected to the other side of R210.

author: morris6
author_rank: Regular Contributor
timestamp: August 03, 2025, 10:02:53 am
content: @Kleinstein : Thank you for spending some time to clear this up, seems I didn’t have a clear head at the time I did this part of the schematic and I also missed it during earlier issue with Q203 / 204. So Page 4 of ET44 Main had to be corrected once again, I am sorry for any inconvenience caused.

author: Martin72
author_rank: Super Contributor
timestamp: August 03, 2025, 10:58:55 am
content: Hi,First of all, thank you very much for your efforts. I had started it myself, but kept getting confused, so I postponed completing the circuit diagram indefinitely....A little tip:QuoteI would add a date to the edit, then you'll have something like a timeline of when changes were made.

author: morris6
author_rank: Regular Contributor
timestamp: August 03, 2025, 03:00:07 pm
content: Quote from: Martin72 on August 03, 2025, 10:58:55 amNice suggestion, will do.