DSO138 troubleshooting - No AV- (no trace)

[jye1]

Please check resistors R9, R10, and R11.

[miketheb]

Hi All

Update.........

I replaced Q1 which had failed with 2N2907 but any PNP switching transistor will probably be OK

Then I replaced L2 with a 470uH 400mA inductor (was 1mH at 1A) as a suspected the original one was causing Q1 t take too much current

Now it works fine ~ except the trace to 2 lines up from where zero should be ~ that's the next job

Hope this helps..................and why oh why oh why don't JYE come clean as they must have known what the problem was with "no -Ve issue" from day 1

Regards

Mike

[myscope]

Hi people,

My scope has no correct AV- (+ 0.68V) and has no triggering, also if switch 1 (GND/AC/DC) is on position AC, on dispay is shown GND in stead of AC.
when display is mounted AV- is correct, but still no AC.

For AV- is not -5V I think the problem is the power inductor L2, resistance is 4.5 Ohm.

What is the correct replacement for L2, user manual, step 1 (assemby main board) number 13, the power inductor is 1mH/0.5A. But in the schematics page 4 (power supply) L2 = 1mH/1A?

What could cause there is no AC?

Thanks four your reply!

[centurion]

Good day! When you start the device, there is smoke and burn the transistor Q1. Then the screen is loaded, but there is no trace. Change the transistor - again burns! What could be wrong? What settings should pay attention to?I do not have time to be measured before the transistor burns.

[jye1]

@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.

@centurion:

Please refer to the start of this post and check by the steps provided.

[centurion]

Thanks for the quick response! The problem I think in the inductor L2. She came to the wrong denomination, as indicated in the scheme.It had to be wound on a ferrite ring homemade. I think it got a little resistance. Do not tell the resistance L2?

[palo0406]

Hello guys very easy way to prevent heating Q1 and get on TP25 lower voltage around 9 volts is just change C23 and R40. I have combination c23 330pF and R40 2.2 kOhm and get on TP25 7,9 volts is enough for U4 and heating of Q1 is gone . power consumption of all circuit (completed oscilloscope) is 90mA.

[centurion]

Hello guys very easy way to prevent heating Q1 and get on TP25 lower voltage around 9 volts is just change C23 and R40. I have combination c23 330pF and R40 2.2 kOhm and get on TP25 7,9 volts is enough for U4 and heating of Q1 is gone . power consumption of all circuit (completed oscilloscope) is 90mA.

Hi! C23 and R40 did not refer to the transistor Q2? A transistor Q1 is lit because of the large collector current. I think the case in low impedance inductor L2 ...

[palo0406]

Hello guys very easy way to prevent heating Q1 and get on TP25 lower voltage around 9 volts is just change C23 and R40. I have combination c23 330pF and R40 2.2 kOhm and get on TP25 7,9 volts is enough for U4 and heating of Q1 is gone . power consumption of all circuit (completed oscilloscope) is 90mA.

Hi! C23 and R40 did not refer to the transistor Q2? A transistor Q1 is lit because of the large collector current. I think the case in low impedance inductor L2 ...

hello yes but Q2 is driving Q1 in case using lower capacitance on base of Q2 the current of Q1 is lower because is open shorter or something like that just try for my is working perfectly

[centurion]

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!