DSO138 troubleshooting - No AV- (no trace)

[palo0406]

Changing the RC you probably change the opening Q1. Possibly reduced current. But it is indirectly and do not know how durable. I think that it still with resistance induction L2, as in the period of the charge goes high current ... Can someone tell her resistance? But either way to leave for dessert Thanks!

Hello I test different coils for L" but without succes the problem with overheating problem was still here .Before change C23 and R40 i had on TP25 - 23 volts. After change parameters on Q2 tested 2 days of running a everything working perfectly no heating on any component current not higher than 90 mA for complete circuit with display and connection to test signal all the time reading values . High current on Q1 is caused by wrong control transistor is to much open or is not closing . This is just my experience and maybe can help to someone who dont know how fix this issue.

[centurion]

again please help ... C23 replacement and R40 to no avail ... you need to know the resistance of L2 ... When I disconnect L2 all parameters are normal. should include it as a burnt Q1 ... Now L2 is wound on a ferrite ring and has a 1.4 mH induction and resistance of 0.2 Ohm

[caramol]

again please help ... C23 replacement and R40 to no avail ... you need to know the resistance of L2 ... When I disconnect L2 all parameters are normal. should include it as a burnt Q1 ... Now L2 is wound on a ferrite ring and has a 1.4 mH induction and resistance of 0.2 Ohm

Hello, yes I did too.. My L2 died (open) and I had to replace with another. Tried different values but the only one that worked was one of similar shape and size like the original (don't kwow the value ol L, but its resistance is about 2 ohm). Placing a 9 volt battery all works fine and display is ok. Overheats Q1 and Q2 but don't burn. The more it swallows the battery level, the less it overheats but trigger fails I will try the C23 and R40 replacement. Thanks palo406!!!

[centurion]

I will use the power supply with a negative voltage or a DC-DC Inverter

[caramol]

Finally, c23= 470pF and -R40 under 100ohm is OK for me. Displays fine, triggers fine, measurements OK... Total current 140 mA, 1.12 Watt of power... (mainly divided between display and overheat). Finally a shadow of doubt... in some pictures Q2 seems to be 45 degree twisted (not accogding to the mounting diagrams). I mounted it on its 'natural' position EBC and works properly, but I needed the C23 substitution.

[myscope]

@myscope:

Your AV- was good. It was a firmware issue that AV- disappeared when LCD was not in place. You can ignore that.

Inductor L2 can be 0.5A or 1A. Usually 0.5A is enough.

For the "AC" issue please check SW1 and resistors R17, R18, R19.

I've checked the switch and resistors, I think I used to much solder, the scope is working fine now.

Thanks for the advise!

[clondono]

My trace slowly drifts from the center line as I'm doing measurements, and when i switch trigger mode from auto to normal and back to auto my trace drops way out of the boundaries of the screen
I then have to do the 0V alignment and bring the trace back down. But the trace is constantly dropping out. Does anyone else experience these issues?

[GM3RVL]

See previous posts.

Got AV- after replacing faulty Q2 with BC184. Worked ok for several days (apart from noise spikes on trace)and then cloud of smoke. Q2 and L2 were blown. Rewound L2 (200 turns) but Q2 (new 8550) overheating at 9V supply.

Made new AV- supply based on Texas TPS60400 charge pump inverter powered from +5V board supply. Removed existing AV- components from board and fitted new supply. Used rewound L2 (1mH) with 1uF ceramic to filter AV-. Now all working much as original but takes under 100mA cf 120mA originally.

GM3RVL

[kvv213]

Hello Everyone!

I explained my problem here http://www.jyetech.com/forum/viewtopic. ... 1162#p1162 the problem is that it seems that the device works but it shows the trace only when I bend the board a little bit. I also check the device with this instruction and can report the following:

Step 1. Passed. I have 0,8V.
Step 2. Passed. 1,83V
Step 3. Failed. Vc2 10.7V (if I bend the board then it is 7.5V). Vb1 10,58V (if bend 9.9V).
Step 3.1. Passed.
Step 3.2. Failed. The values of Vc and Vb1 are not zeros and they are pretty similar.
Step 3.3. Failed. Vb1 is very close to V+ (10.74V).
Step 3.4. Passed.
Step 4. Failed. D1 negative is 0.12V that is OK. But D1 positive has 0.5V (if bend then -12V).
Step 5. Failed. V- at TP25 is 0.15V (if bended -14V).
Step 6. Failed. AV- at TP26 0.66V (of bended -4.96).

What to do?

[redmi]

Hello, I have the No AV- (No Trace) problem:

These are the values:
V+=8.09V
AV-=0.69V
AV+=5.6V
U5 gets really hot.
Tried replacing Q1 and Q2. Similar behavior. L2 is not shorted or open.

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 8.08V. Tried replacing both, still the same problem.

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+=0.12V, 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=0.12V

Step 6: Check AV- at TP26. If V- is good but no AV- then check U4 for correct type and polarity.
-> AV-=0.69V

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.
-> 4.46 kOhms

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