Slow blink in test mode - doesn't work and voltages wrong

Post Reply
daveNC
Posts: 3
Joined: Tue Jan 16, 2018 1:44 am

Slow blink in test mode - doesn't work and voltages wrong

Post by daveNC »

This unit is confirmed to be legitimate. It sort of worked for a while and then became flaky. Not sure how to diagnose this.
LCD works but the wave form is all wrong or nonexistant.
With the LCD removed.
JP 3 and JP 4 are closed.
Test mode LED blinks slowly.
I measure the following voltages:
V1: 2.81 when LED is off and 2.82 when on
V2: 1.26 LED off and 3.27 LED on
V3: .06 LED off and 1.93 LED on
AV-: .65 off and .70 on
AV+: 4.92 off and 4.94 on
+3.3V: 3.26
+5V: 4.98
V+: 8.44

Any suggestions are greatly appreciated.
MNTech
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: Slow blink in test mode - doesn't work and voltages wron

Post by MNTech »

The slow blink means the test mode did not find a short. To find opens you need to read the schematic and determine what points pins PB0 - 15 and PC13-15 of the MCU are connected to and look for the signal there. This is a long and laborious process so you may want to leave that for a last resort. Since the LCD displays the correct screen it's unlikely it's a digital line problem.
The -5V at AV- is generated by a circuit that uses pulses from the MCU. I suspect this is why it is missing in the test mode and would explain the changing voltages at the V test points. The test mode is for checking digital signals.
I have noticed many times problems presented on this forum turn out to be bad solder connections. When you solder you want to heat both the pad and the component lead. I found this board sucks up a lot a heat and with the small pads it's hard to do a proper soldering job. Reheating the joints and adding more flux or solder will often help.

You can check the analog path with a voltmeter and the battery.
Power up the DSO138 and set the switches for CPL=DC, SEN1=1V, and SEN2 = x5.
Measure the voltage at V2. It should be about 1.6 volts. Record this voltage.
Connect the red test lead clip to the end of D2 closest to the edge of the board. D2 is next to the black power connector.

The battery voltage should be at the center pin of J1 where it connects to the PCB.

The voltage at V1 will be 1/100th of the battery voltage, approximately 0.090V ( 90mV)

The voltage at V2 will be about 0.14 volts high than the voltage recorded earlier.
daveNC
Posts: 3
Joined: Tue Jan 16, 2018 1:44 am

Re: Slow blink in test mode - doesn't work and voltages wron

Post by daveNC »

Thanks. I'll solder back over everything as a first attempt.
daveNC
Posts: 3
Joined: Tue Jan 16, 2018 1:44 am

Re: Slow blink in test mode - doesn't work and voltages wron

Post by daveNC »

Thanks! That fixed it...

I went over most of the solder points on the board reheating and adding a bit of solder where the holes looked somewhat less than full. That fixed it! Now it runs fine. Very good advice.
Post Reply