TC1 lcr-meter transistor-tester fix

[cuistots]

the  3.3V regulator normaly works.

[indman]

U1- the 74HC573; 74HCT573 is an 8-bit D-type transparent latch with 3-state outputs
Try replacing this chip. Another option is to remove this chip and connect the ATMega outputs directly to the display lines through 10kΩ resistors.
They are already on the display board. But this option is only at your own peril and risk. 

[Krafte]

Yesterday, I got a shiny new TC-1 directly from China
I successfully got a 1.38m firmware running and testing components seems to work.
However, the LCD keeps flickering and the device turns randomly off. Also, the menu is not accessible by double pressing the button.
I think the reason is the stock firmware on U4. I tried to replace it with the custom one. But it seems the chip is not responding to the programmer.
I tried everything I could imagine , different PCs different uart adapter, all combinations of power on cycles for U4, double and triple checking the connections and pinouts, but nothing helped.
Anybody with same experiences? Any advice apart from replacing U4 by the proposed circuit of Markus?
The only thing that's a little strange are markings on the chip. It is "STC 16L104W".

UPDATE:

I got the chip to respond to me using my RPi 3 and stcgal (I strongly recommend this tool). It reported as STC15L104W.
However, flashing was not possible -> it always stopped while switching baud rates.
Finally with baud rate 1200 it was flashing, slowly but successfully

[Kamlakkannan]

Hey, I just got the lcr t7 from China yesterday(after almost 3 months). It worked for sometime then it just displayed a white blank screen. After sometime it worked, but then again it displayed just white blank screen. This process repeated and now I am stuck with something that just displays a white screen. I checked the capacitors they aren’t short. The battery is good and I don’t know what to do next, so can you please help me.

[Kamlakkannan]

And again after sometime it started working.

[Kamlakkannan]

And now again just a white screen

[alfagulf]

I would like to add my experience with the TC1 that I bought from Banggood as a replacement for another unit which I zapped by connecting a charged high voltage capacitor by mistake  .
The new unit arrived faulty. 
The unit did not power up, so I assumed the battery is dead and need to be recharged, which I did, but still pressing the start button did nothing.

I contacted them and they offered a refund which I accepted.

However I suspected the problem is around U4 (STC15W104) since it manages the start button and sends a signal to the boost converter U5 to start.
I noticed that pin 3 (P3.5) is not going HIGH when I press the button which of course the reason why it is not powering up.
I first reprogrammed U4 with the alternative  firmware https://github.com/atar-axis/tc1-u4 , but that did not change any thing.
I then replaced U4 with the one I had in my first faulty unit.  I started to have some intermittent success where the unit will power up sometimes and shuts down unexpectedly.

I then did more testing around the push button when I noticed C13 a debounce capacitor that had its resistance rather low (around 30K).
I removed it and the tester worked like a charm.
However I then replaced it with a good 100nF capacitor to keep the debounce function.

The U4 which I remove (I suspected to be faulty) turned out not faulty, I found out that by reprogramming it to output a 1Hz pulse on all ports which it did.
So I guess the problem for me was the faulty ceramic C13 capacitor. 

I hope this information might help some of you.

Thanks

[dvd4me]

Hello,

I just got today the TC1 from Bangood and it was defective, not turning on.
I found this post and after measuring the caps on the battery or the +3.7V rail, nothing wrong.
Plugging the battery made the screen to briefly turn on.
At the power on button, I noticed there was no voltage on the button contacts, just a very low 0.3V.
Putting 5V at the output of the U3 regulator made the screen to turn on but the boot stops at the daniu logo.

I just saw the post before me and noticed the C13 warning it might be leaky.
Removed the C13 and now it powers up at the insertion of the battery, yey!
Measuring the voltage at the power button is 3.8V ! So that's the normal voltage it should be on a normal tester.
I just put the cover back and that's it for now.

Now it works, see the second picture.

[shved]

A friend gave me TC1 with bulged battery and some damage.
I've cleaned it up and swapped in used samsung mobile phone battery. Changed c13 as a precaution. Everything works, as far as i can check. But stand by draw is around 20mA. Is that normal? Do i need to add a a battery cutoff switch? I have several LC, RLC, ESR, ... meters, but with external power(no battery only USB or 12v PSU), so i plan to use it for away jobs witch are quite infrequent.
My point is: the tester will be always "flat" when i grab it.
Any advise?
 

[madires]

That stand-by current is much too high. A classic power switch would be a simple fix. If you want to run one of the OSHW firmwares (k or m-firmware) you can replace U4 with a small circuit with two BJTs and the stand-by current will be about 5µA. With U4 the current should be around 0.2mA.

[shved]

Thanks madires.
I think I've fixed it. Now the stand-by current is around 0.2mA.
The problem was in the caps C6 & C7. The caps turned in to resistors and loaded the battery and 5V rail through the diodes. I don't quite know where the energy went, the caps didn't heat-up. I've tracked them down because 5V rail read too low(~4,5V) and battery check on boot showed the wrong voltage. To isolate the issue I've disconnected the diodes in turns, when 5V came back clean, I knew immediately to check the caps!
Also, the factory where they assemble the bloody things uses some reactive flux and doesn't clean it off. I strongly suspect that it's a factor, seen that before.

Anyway, anyone who has excessive stand-by current(around 20mA) check the caps C6 and C7! And I advise to wash the flux off the boards.

[a_kiler]

Hi, everyone.

Like most of us here I unfortunately damaged my brand new T7-H. Capacitor dead screen... sounds familiar?   

Thx to advice:
@zappenduster: I bought 10uF capacitor; (didn't use it)
@indman: I removed DZ1 and DZ2 and... success, the screen is on. But sth else strange (for me) happend.
Tester works "like new".

Calibration: pass
Capacitor: pass
LED: pass
IR: pass
Even removed DZ2 - zener diode: pass

So if DZ2 is ok, DZ1 must be failure.

Oh, BTW - my is tester is a little bit different (pictures included) and DZ2 is 6V8C, DZ1 is V5L. I hope datasheet ic correct?

How can I examine DZ1?

@zappenduster said earlier:

ZD1 = zener-diode 6V8 ; DZ2 6-pin marked V05. Some pcb miss those two elements - maybe earlier layouts...

Is it possible the tester will work normally without DZ1 DZ2 or should I expect the unexpected.

[madires]

Yep, the tester works also fine without the input protection (DZ1 and DZ2). DZ1 should be a 6.8V TVS (not shorted = OK).

[belzrebuth]

I did the charged capacitor trick on mine so now it's dead..
DZ2 is certainly faulty and I've also found some shorted ceramics in there as well.
In total I've removed capacitors C14,C15,C24,C21,C23.
I haven't yet measured DZ1 but I've removed it for good measure.

I've also tested the 5V regulator out of circuit and it's working.

The problem is that the meter only boots when I'm applying 5V externally in the place of the 5V reg output.
When it boots it shows there's a DC source between terminal 2 and 3.
When connected to a USB plug only the red charging LED lights up but the meter is dead.

I think I'm probably having a problem with other stuff as well like the DC-DC section or else.

Since the MCU is probably working what might be going on with the power stuff?
How did I manage to save the MCU and I have a problem in the PSU section.
There is no input voltage to the 5V regulator when I'm connecting the USB plug so no wonder the meter is dead.

What could be the culprit here?
Would it be fixable?

[madires]

Presumably the ATmega's pins used for terminals #2 and #3 are shot and maybe also the control MCU U4. You can check the DC/DC converter by enabling it manually.

[belzrebuth]

Thanks for your reply madires.Really appreciate your help.
I get about 21.5V where I should be getting about 37.. (D2)
How should I enable the DC converter manually?!

I noticed that if I connect my bench PSU to the battery terminals providing 3.7V if I press the button I get about 0.35A current draw and then after a short period of time almost no current draw(0.9mA) or so.
Forgot to mention that I get about 3.4V at the 5V regulator input and 1.7V at it's output.
The regulator input and the 35V rail which is 21.5 maybe tell that the DC-DC converter might be faulty?!
I also get about 25ohms between the 5V rail and ground which seems low but then again the meter boots when I give it 5V and it's current draw is not excessive.

[madires]

If the boost controller is an AL693 pull up pin 4 (to the battery's voltage). That pin is connected to a pull-down resistor and pin 3 of U4.

[belzrebuth]

The marking of U5 is AL715..

[madires]

No idea about that one, but I'd guess it's quite similar.

[belzrebuth]

Nothing changed when I pulled the pin4 to 3.7V.
Same input to the regulator.
If nothing else down the line is preventing the 9 or so volts to appear maybe the DC/DC is bad.

Maybe I should just buy this IC and replace it, as long as it exists somewhere which I'm not sure of.
Seems like a non-standard part available in China only.

If I power the thing with 5V directly shouldn't it measure components though?
I mean if my MCU is also bad (which btw is an ATMEGA 644 not 324) I could just buy a new meter and discard this..
Are the capacitors I've removed critical to the operation when powered directly from 5V?

[indman]

I noticed that if I connect my bench PSU to the battery terminals providing 3.7V if I press the button I get about 0.35A current draw and then after a short period of time almost no current draw(0.9mA) or so.
Forgot to mention that I get about 3.4V at the 5V regulator input and 1.7V at it's output.

It is very likely that your MCU is damaged. Take a multimeter and check the resistance of the PA0-PA3 test ports against + 5V and the ground. You can also check the resistance of pin 4 (VCC) to ground.
AL619 (AL715, AL718) this is the marking of the SD6271 chip 

[belzrebuth]

Vcc to Ground is around 25ohms.
I mentioned it before and it seems weird that it's too low but when I power the thing directly with 5V its current consumption is within spec at around 350mA.
 PA0 to Vcc 560k
 PA1 to Vcc around 20οhms
 PA2 to Vcc around 112ohms
 PA3 to Vcc 2.2k

I wonder why they used a 644 in my revision instead of a 324 which is almost triple the price. PCB version is 2.0.

[indman]

belzrebuth From + 5V, the serviceable tester consumes from 24.6mA to 44mA. You have a defective processor, you need to replace it. It is not difficult if you have the skills to solder such things. Better to buy the same ATMega644. Chinese friends very often install those parts that are currently in stock. In terms of cost, the difference between chinese 324 and 644 is very small. They earn more due to the large number of devices sold. 

[belzrebuth]

Ah okay..thanks:)
I thought the current draw was normal all this time
I'll replace the MCU and post back.

[shved]

belzrebuth, I've also had excessive power draw. Double check the caps, esp. C6 & C7. Also check for corrosive flux. Give a quick read to the rest of the thread.