Flat trace when calibrating probes

adr
Posts: 13
Joined: Thu Feb 01, 2018 12:33 am

Flat trace when calibrating probes

Post by adr »

(Working with a legitimate board; firmware -061)

See attached photo (CPL = AC; Sen1 = 0.1v; Sen2 = X5). The scope boots up fine and a trace appears just above the horizontal axis but I do not get the expected square wave with the red lead attached to the test point.

You can't really tell from the photo but there does appear to be a signal sweeping the screen but its amplitude seems really small. I have adjusted all the settings to see if I could get something resembling the square wave but to no avail.

Does anyone have any recommendations on how to resolve this?
Attachments
IMG_2320.jpg
jye1
Posts: 1227
Joined: Wed Feb 20, 2013 4:58 pm

Re: Flat trace when calibrating probes

Post by jye1 »

Have you done the "finger" test as in the manual? If the test is okay the problem could be the signal source. Check R26 and its related connections. Otherwise the problem could be with the probe or the analog portion of the scope. It is suggested to check soldering carefully.
adr
Posts: 13
Joined: Thu Feb 01, 2018 12:33 am

Re: Flat trace when calibrating probes

Post by adr »

R26 is spot on 1K. Solder points all visually look reasonable. Finger test doesn't produce the noise signal I would expect to see - the trace remains as it appeared with the probe calibration
MNTech
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: Flat trace when calibrating probes

Post by MNTech »

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 0.14volts high than the voltage recorded earlier.

If the scope is working the scope trace should go up about 1.8 divisions.
To see the test signal connect the red lead to the 1 KHZ test point and set the switches for CPL=DC, SEN1=1V, and SEN2 = x1.
adr
Posts: 13
Joined: Thu Feb 01, 2018 12:33 am

Re: Flat trace when calibrating probes

Post by adr »

Thanks for those test procedures. Here are my observations with the battery connected, LCD attached, CPL = DC, SEN1 = 1V, SEN2 = X5 - it appears I'm way off in certain areas:

Vbattery = 8.17v
Battery is center positive
D2 edge = 7.54v
V2 = 3.38v
V1 = 2.875v
AV- = 0.671v
AV+ = 4.94v
+3.3V = 3.31v
+5V = 4.98v
MNTech
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: Flat trace when calibrating probes

Post by MNTech »

AV- should measure -5 volts. You will have to remove the LCD to troubleshoot the negative supply. The negative power supply consists of Q1, Q2, U4 and associated components.
Pulses from the MCU turn on Q2 and Q1 for a few milliseconds causing a rising current in L1. When the pulse ends and Q2 no longer supplies current the falling magnetic field in L1 causes the voltage across it to reverse. That negative voltage is rectified by D1 and filtered by C24, L2 and C25 to about -9 volts. U4 regulates the voltage to -5.

First look for soldering problems and measure components. If you later start measuring voltages be careful because if you short something that holds Q2 on too long L2 will burn out.
This is a difficult circuit to troubleshoot, using an oscilloscope would be the easiest way. The voltages on the circuit diagram will give you an idea of what's going on.
Good Luck, I hope it's just a bad connection.
adr
Posts: 13
Joined: Thu Feb 01, 2018 12:33 am

Re: Flat trace when calibrating probes

Post by adr »

First, thanks a ton for your patient assistance - much appreciated.

I currently don't have access to an oscilloscope - not even the one I'm trying to get to work :-( of course - but I will try to borrow one next week.

In the meantime, I am rechecking solder connections and I'm using a voltmeter to compare to the schematic.

If I made the following measurements with my voltmeter, can I assume Q1 is shorted and/or there is an issue with D1 or L2?

Vbattery = 8.86v
Q1 E = 8.09v
Q1 B = 7.86v
Q1 C = 7.75v

Q2 E = 0.0v
Q2 B = minus 1.42v
Q2 C = 6.01v

D1 0.596 // 7.74v

L1 .595 // .595
MNTech
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: Flat trace when calibrating probes

Post by MNTech »

The voltage at Q1-C is a problem. Check if L2 is open. Measure it from D2 cathode to ground to check the connections are good too.
If it is open Q1 might be shorted too, if so replacing just L2 would result in L2 burning out again.

I noticed in my earlier description of the circuit operation I said L1 where it should be L2. L1 is just a filter.

The circuit doesn't work when the battery voltage gets much below 8 volts. I was examining it to see if it could be improved and managed to short something so I had to get replacements for Q2 and L2 from Digi_Key.
adr
Posts: 13
Joined: Thu Feb 01, 2018 12:33 am

Re: Flat trace when calibrating probes

Post by adr »

Rechecked all connections and they seem good. I have to admit though that when I did the initial install, I inadvertently put L2 in the wrong place and had to desolder it to relocate it to its correct spot. Perhaps something happened in that process.

Voltages:
(-) D2 to gnd 7.74v
(+) D2 to gnd 8.50v
(+) D1 to gnd 0.594
(-) D1 to gnd 7.28

If I was going to replace some components, e.g. Q1, Q2, L1, L2, D1, D2, it isn't clear what I would order. For example, Digikey lists quite a few Q1 8550 options - which would I order? Similar for other components.
MNTech
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: Flat trace when calibrating probes

Post by MNTech »

It's not clear if you measured the resistance of L2. i'm guessing it's open.

Should you need them, here are the Digi-Key parts I used. The leads on the coil are spaced a little wider so it takes a little bending to make it fit. It doesn't seat all the way down but works fine.
SS8550DTACT-ND
732-3783-ND

I replaced D2, the reverse polarity protection, with a MOSFET to reduce the voltage drop. This allows the DSO138 to get more out out the battery.
After you get the scope working you may want to try this so here is the part number.
NTD2955-1GOS-ND

How it works:
http://www.ti.com/lit/an/slva139/slva139.pdf
figure 3 shows the circuit I used, the diode shown is built into the MOSEFT.

One lead is soldered to the battery jack for ground and mechanical support.
DSO138 D2 replacement.PNG
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