DSO138 troubleshooting - No AV- (no trace)

[jye1]

AV+ looks incorrect. Did you check the type of U5? Is it 78L5?

[redmi]

You are right. I put 78L05 in place of 79L05. I changed by new 78L05 and new 7905 in the correct place, but still I get the same values. But now neither 78L05, 79L05 and Q1 and Q2 gets hot.

What should I do next?
Thank you for your time.

[jye1]

Can you please post all the voltages with the replaced parts?

[redmi]

Of course, the new values are:
V+=8.14V
AV-=0.66V
AV+=4.91V
Step 1: Check voltage at the lead of R40 that connects to U1 pin 46 (the node marked VGEN in schematic). This voltage should read about 0.8V. If you get a value close to 0V or 3.3V then the pin 46 of U1 may be open, or the pin inside is damaged.
0V

Step 2: Check voltage at Q2 base. It should be near -1.4V. If this is not the case then check R39, R40, C23, and Q2. Particularly, check the polarity of Q2 and make sure it is installed correctly.
0V

Step 3: Check voltage at Q2 collector (Vc2) and voltage at Q1 base (Vb1). Vc2 should be around 6.4V and Vb1 around 8.3V. If they are incorrect then you need to check R37, R38, Q1 and Q2. Usually you may find you are in one of the following scenarios.
Both 8.13V

Step 4: Check voltage at the negative and positive poles of D1. Voltage at D1 negative pole should have a small positive value. Voltage at D1 positive pole should read around -8V. If these voltages are incorrrect check Q1, L2, D1, and C24. Commonly found issues with these parts include Q1 damaged, L2 short or open, C24 short or open, and D1 reversed.
D1+=0.13V
D1-=0V

Step 5: Check V- at testpoint TP25. This voltage should be around -8V. If you get 0V then L1 is possibly open.
TP25=0.13V

Step 6: Check AV- at TP26. If V- is good but no AV- then check U4 for correct type and polarity.
AV-=0.66V

There is one special case where AV- is shorted to ground at some other places. Your voltage measurements show AV- is 0V while V- (at TP25) and voltage at D1 positive pole are significantly lower than normal. You can identify this case by measuring the resistance between AV- and GND with an ohm-meter (with power being turned off). If you get the resistance close to 0 ohm you need to find out the short spot first.
2.7 kOhms

Thank you for your time.

[redmi]

Hello again:

¿there is anything I can do to get my DSO138 working?

Thank you.

[unordung]

Hello,
I have got the same problem as many others, the scope works fine with the only fault being the trace missing.... (Quite a big fault).
I have narrowed it down to the Q1 PNP transistor being blown.
+5= 4.87v
+3.3= 3.26
AV+= 4.84v
AV-= .0694v
v1= 1.59v
V2= 2v
V3= 2.43v
TlVl= .208v

R40= 0.818v
Q2 Base= 1.4v
Q2 collector= 4.06v ===Should be/ 6.4v
Q1 base= 4.38v ===/ Should be 8.3v
A visual inspection of Q1 shows a blister on package, given other peoples lack of success I am reluctant to waste time replacing transistor if the underlying issue is with the choke.
Does anyone have any reasonable solutions?

[redmi]

Hello unordung,

Since July 28th they have not answered again.

[jye1]

Of course, the new values are:
V+=8.14V
AV-=0.66V
AV+=4.91V
Step 1: Check voltage at the lead of R40 that connects to U1 pin 46 (the node marked VGEN in schematic). This voltage should read about 0.8V. If you get a value close to 0V or 3.3V then the pin 46 of U1 may be open, or the pin inside is damaged.
0V

Step 2: Check voltage at Q2 base. It should be near -1.4V. If this is not the case then check R39, R40, C23, and Q2. Particularly, check the polarity of Q2 and make sure it is installed correctly.
0V

Step 3: Check voltage at Q2 collector (Vc2) and voltage at Q1 base (Vb1). Vc2 should be around 6.4V and Vb1 around 8.3V. If they are incorrect then you need to check R37, R38, Q1 and Q2. Usually you may find you are in one of the following scenarios.
Both 8.13V

Step 4: Check voltage at the negative and positive poles of D1. Voltage at D1 negative pole should have a small positive value. Voltage at D1 positive pole should read around -8V. If these voltages are incorrrect check Q1, L2, D1, and C24. Commonly found issues with these parts include Q1 damaged, L2 short or open, C24 short or open, and D1 reversed.
D1+=0.13V
D1-=0V

Step 5: Check V- at testpoint TP25. This voltage should be around -8V. If you get 0V then L1 is possibly open.
TP25=0.13V

Step 6: Check AV- at TP26. If V- is good but no AV- then check U4 for correct type and polarity.
AV-=0.66V

There is one special case where AV- is shorted to ground at some other places. Your voltage measurements show AV- is 0V while V- (at TP25) and voltage at D1 positive pole are significantly lower than normal. You can identify this case by measuring the resistance between AV- and GND with an ohm-meter (with power being turned off). If you get the resistance close to 0 ohm you need to find out the short spot first.
2.7 kOhms

Thank you for your time.

Did you measure the AV- related voltages with LCD plugged in? For firmware version -045 and -046 VGEN will be 0V if LCD is not in place.

[redmi]

I will try again:

there is anything I can do to get the DSO138 I bought working?

Thank you.

[redmi]

Please, don't ignore your customers.
Can you please let me know what can I do in order to get the product I bought from you working?

Thank you for your time.