Another Failure 20 with measurements and some test mode?


My new MK-312BT is also suffering from "Failure 20".

Because all the measurements and scope screenshots I decided to create a new topic - I hope it is OK.

To be honest I am lost. I cannot say I fully understand the signal generation and the test procedure. So debugging is quite a challenge for me.

I will be grateful for any suggestions or observations.

My PCB is 1.3B.
FW is mk312-bt/firmware/Custom Boot Message f005-MK312-BT/ElectrodesReady.bin.

I did few modifications to the MK-312BT but I don’t think any of them is “Failure 20” related.

  • R35 and R46 are 220k instead 200k (I guess 10% off is not the problem).
  • I didn’t use MAX232.
  • I didn’t use battery VREG – I am powering it from lab PSU directly to the battery input.
  • I added a reverse protection diode (inspiration from 1.3R).
  • I modified MIC input like it is in 1.3R.
  • I used 22pin IDC cable and used only 2GND pins (instead of 4).
  • I used 42TU200 transformers – some other user tested it and it is supposed to work just fine. Center tap output is not connected.
  • I used two 1R resistors in parallel for R30 – I also tried 3 and 4 in parallel to fight the 42TU200 lower DC resistance.

Sometimes the ATMEGA does not startup correctly. The LCD is not correctly initialized (I guess) the top row is full of rectangles, the backlight is ON. The crystal oscillator is ticking correctly at 8MHz. Fuse bytes are correct. I tried to add 100nF cap between RST and GND to slow down the startup time – It made it even worse. The power consumption is 55mA in this case. 5V and 9V rails are OK.

Question – with a “Failure 20” on the LCD the backlight is OFF – is this normal? Also every 10s or so the backlight and output LEDs flashes and nothing else happens.


Power consumption 42mA at 12V.
5V rail: 5.10V
9V1 rail: 8.92V (I might replace 9V VREGs for more precise ones in the future)
9V2 rail: 9.20V

**Signals after power up – Yellow = 5V rail, Red = signal

PB0: no signal

PB1: no signal



LTC1661 OUT A:

LTC1661 OUT B:

LTC1661 – Red = OUT A, Yellow = OUT B:

U10A output:

U10B output:

Red = OUT A, Yellow = U10A output:

Red = OUT A, Yellow = R30 current sensing resistor:

LTC1661– Red = DIN, Yellow = SCLK:

Red = Q1 gate, Yellow = Q2 gate (against ground):

Red = Q3 gate against 9V1 rail (floating scope):

Crystal when the ATMEGA does not startup correctly:

Crystal when the ATMEGA does startup correctly:

// OK, I found it is documented already
I also managed to enter some kind of a test mode. It happened when I tried to measure signal between ground and Q3 gate (floating scope). It also happens when the R30 value is too high – 1R instead of 0.5R. The MK-312BT then boots into this – see the photo. Buttons pressed change one number, Multi Adjust changes other number, and output intensity pots do nothing.


Test mode? When I disconnected one of two 1R in parallel (R30).

Can someone confirm these waveforms?

  • Is the voltage at R30 suppose to be so low - peaks are only 100mV?
  • Is the output coming from the DAC correct?
  • Is the output from U10A correct?

I am unable to measure impulses across transformer coil (floating scope) - it keeps getting into “test mode” - not sure how is this possible.

I will try to desolder and measure all FETs. But I measured each of them before soldering.

Any ideas, please?


[quote=“bumerang, post:1, topic:629”]
Question – with a “Failure 20” on the LCD the backlight is OFF – is this normal? [/quote]

I don’t know the unit well enough to answer the other questions (still fighting error 20 with one of my 2 units) but on my unit which is facing error 20 the backlight is on. So i guess it’s not normal that the backlight is off. It’s also constantly flashing the LEDs.


wow, that’s weird. Backlight is working and the ATMEGA is able to control it - it is on when in test mode.
What do you mean by “flashing the LEDs” - does it look intentional (I mean like for example 1Hz 50% duty cycle) or unintentional (like in my example ~0.1Hz and ~100ms flash)?

I will try to add few 1uF caps to 5V rail to each chip. It hurts when I see the super thin +5V copper trace. And the minimal copper pour clearance… why?!

I wonder if there is a way to identify the moment when ATMEGA restarts itself. When for example watchdog is activated - is the RST pin internally puled down so it is measurable?

My TODO list:

  • Add more decoupling C
  • Route +5V directly to ATMEGA with a wire
  • Try the other FW - HelloFriend.bin
  • Change R35 and R46 to 200k exactly
  • Desolder and meassure al FETs

  • Add more decoupling C
    - done - added 1uF ceramic + 220uF close to ATMEGA, added 4.7uF ceramic to the LCD board, added 4,7uF ceramic close to DAC

  • Route +5V directly to ATMEGA with a wire
    - done

  • Try the other FW - HelloFriend.bin
    - done

  • Change R35 and R46 to 200k exactly
    - done - one is 201.1k the other is 201.5k

  • Desolder and measure al FETs
    - done - all FETs are OK

I also measured resistance of all transformer windings - everything seems to be fine.

Nothing helped.

Any ideas what to do next?


According to this:$+1+0.000005+10.20027730826997+50+5+43 r+176+112+304+112+0+100000 v+448+352+448+80+0+0+40+5+0+0+0.5 r+160+320+160+160+0+200000 a+176+176+304+176+8+15+-15+1000000+4.499937501562461+4.5+100000 w+176+112+176+160+0 w+304+112+304+176+0 w+304+176+368+176+0 w+176+160+160+160+0 w+176+192+112+192+0 g+160+320+160+352+0 r+112+192+112+320+0+100000 r+112+80+112+192+0+100000 r+160+80+160+160+0+100000 w+112+320+160+320+0 R+160+80+160+48+0+0+40+5+0+0+0.5 R+112+80+112+48+0+0+40+9+0+0+0.5 o+10+64+0+4099+5+0.0001953125+0+2+10+3 o+4+64+0+4099+5+0.00009765625+1+2+4+3 38+0+0+1+101+Resistance

Voltages at U10A pin 2 and pin 3 both should be 4.5V when LTC1661 is disconnected.

Edit 2:
My voltages U10 are:

Pin 2 = 4.46V
Pin 3 = 4.44V
Pin 6 = 4.57V
Pin 7 = 4.60V

Pin 1 = 6.04V
Pin 7 = 6.39V

Outputs are suppose to be 6.25V I am few % off - I would say it is OK. Resistors around OPAMPs are OK, OPAMP is working.


Looks like I’m here alone :frowning:

Another try - adding 1R resistor to the center tap of the transformer to more closely mimic the DC resistance of the 42TL004. No luck.
I will try to find real 42TL004 somewhere.

@Cerb: would you be so kind and measure few points on the working unit with an oscilloscope?
all against ground

I am interested in test sequence so please use a single shot and trigger to the PB2 (rising, ~3V).

Also could you please upload the exact binary file you used for the working unit? Not just link to github, upload your file please.


How the f*ck is this possible?!

Red is Q1 drain, yellow is R30 current sensing - all against GND.

There is a voltage ramp which means DAC + U10A + Q3 is working.

There is a switching - the voltage is pulled down with each Q1 impulse to its gate - this means Q1 is working.

But there is no current flowing through the transformer - am I right? There is no significant voltage drop across R30 (check the yellow voltage scale it is 200mV/div!). I cannot explain why there is some voltage drop at the end of the signal. I don’t get it.

The only explanation is the transformer is broken somewhere inside - I measured it twice.
Or the 0.5R resistor (R30) is dead short (thus no voltage drop) which it should not be - there are two 1R in parallel - brown -. black - black - silver - brown… 1R 1%.


Not sure why you deleted your post but you are correct! They are suppose to alternate. I will investigate it further. Still it does not explain missing signal at R30.

Maybe (and this is big maybe based on absolutely nothing) because this is a test sequence the magnetic field canceling is desirable - you don’t want to shock anyone by this test.

Anyway big thanks for your (deleted) reply. I will investigate this.


I deleted the post because, just as you said it seems plausible that in a test sequence the cancelling of the magnetic field is desirable and aditionally it could not explain the low voltage drop at R30.

You could also check the current limit of Q3 (IRF9Z24). To my understanding this MOSFET is applied as current limiter (constant current source). If you can’t get enough current through R30 it may be too much limited, the voltage ramp (red) shows the open-circuit voltage, to my understanding there should always be around 9V at Q1, when not switched on. It seems that you may are at the very beginning of the characteristic curve when you not even have enough current to get the 9V on Q1.

This is also exactly what you see at the very end of the test sequence, where the red curve does stay at the 9V. That the current does increase slightly at the end could have some cause in any capacitance or inductance of the circuit.

Hope this helps more.