DSO138 troubleshooting - No AV- (no trace)

vicshi
Posts: 1
Joined: Thu Mar 17, 2016 6:34 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by vicshi »

Want to share my experience in troubleshooting DSO 138. There was no (-5V), it was +0.67 V instead. Screen was white, and no LED blinking. After several hours troubleshooting I found a reason. Inductance L4 was faulty. When tested it showed very high resistance. After I replaced it with good inductance (60 uH) DSO started to work.
efifi
Posts: 1
Joined: Wed Apr 13, 2016 3:55 am

Re: DSO138 troubleshooting - No AV- (no trace)

Post by efifi »

Hi there - i need some help

I have power, i have a loading screen and a display - however no wave signal.

Here is what i got when i followed the instructions



It is assumed that power supply is good and V+ has a value near 9V. This is reasonable since the digital portion of scope has already been working. It is also expected that you have a good digital volt-meter for the measurements. Before you start remove the LCD module so that you have easy access to the check points.
9V POWER GOOD

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.
PERFECT

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.
PERFECT

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.
1 ) Vc2 is close to 0V but Vb1 is at about 8.5V. In this case Q2 is likely damaged and its collector is shorted inside.
2 ) Vc2 and Vb1 are not zeros but their values are almost the same. This case indicates Q2 is possibly damaged and its collector is open inside.
3 ) Vb1 is very close to V+. This case indicates that the emitter-base junction of Q1 is likely shorted inside.
4 ) The difference between Vb1 and V+ is significantly larger than 0.6V. Usually this case indicates that the emitter-base junction of Q1 is open.
PERFECT

THIS IS WHERE THINGS GO WRONG
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.
I GET A SMALL POSITIVE VOLTAGE AT THE NEGATIVE POLE BUT I GET -1V AT THE POSITIVE POLE
ON Q1 I GET
8.3 on BASE
8.5 on EMITTER
0.09 on COLLECTOR


Step 5: Check V- at testpoint TP25. This voltage should be around -8V. If you get 0V then L1 is possibly open.
At TP25 I GET -0.52V

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


Look like something wrong around - Q1, L2, D1, C24 - but they all look good to me. Unless one of the components is bad but i don't know what to do or how to test them? I check polarity of C24 and all good. Could i swap Q1 with 2N3906? but what is L2 and is it likely there is something wrong with it?


many thanks for your help

Clive
rafapratas
Posts: 5
Joined: Sun Jun 05, 2016 5:36 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by rafapratas »

Hi everyone !

Yesterday, I've been finishing my DSO138 oscilloscope. After finishing everything, I have the "no trace" problem too. I've been reading all replies on this forum, but nothing worked for me. I've tested a few points on the pcb, and the voltages that I have are:
-> AV- = -0.68V
-> AV+ = 4.93V
-> +3.3v = 3.3v
-> +5v = 4.98v
-> v1=0.19v
-> V2 = 1.15V
-> V3 = 4.25v
-> TLVL = 1.13V
-> V+ = 8.18V
-> TP 22 = 3.3V
-> V-/TP 25 = -2.34V

After following the procedure on the first page of this topic, I've measured the required points and components:

Step1: 0.8V
Step2. V Base Q2 = -1.41 V
Step3: V collector Q2 = 6.22V
V Base Q1 = 8.1V
Step4: V D1 - = +0.23V
V D1 + = -2.46V (problem here)
Step 5: V TP25= -2.35V (problem here)
Step 6: AV- = -0.7 V

GND to AV- voltage ~~ +2.3V

I think the problem isn't coming from Q1 and Q2 or resistors... But after days trying to find the problem I accept any try to solve this. I've not changed the transistors , regulators and the inductor yet.

Anyone knows what can I do to try to solve my problem?

Thank you very much in advance.

Greetings,

Rafael Pratas
jye1
Posts: 1226
Joined: Wed Feb 20, 2013 4:58 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by jye1 »

It looks D1 shorted. Or reversed?
rafapratas
Posts: 5
Joined: Sun Jun 05, 2016 5:36 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by rafapratas »

I've tested it, and for the direct current flow, 0,4 , and infinite impedance on the reverse. I think the problem isn't from the diode D1
Attachments
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jye1
Posts: 1226
Joined: Wed Feb 20, 2013 4:58 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by jye1 »

Please disconnect L1 and see if you can get higher negative voltage at D1 anode. It may be over loaded.
rafapratas
Posts: 5
Joined: Sun Jun 05, 2016 5:36 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by rafapratas »

Yes, I've removed L1, and on the D1's anode I have about -20V. What can I test now ?
jye1
Posts: 1226
Joined: Wed Feb 20, 2013 4:58 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by jye1 »

This means the negative voltage generating circuit is good. The low AV- was caused by overloading. From the schematic you can see AV- connects only to U2, R10, and C11. Please check these components, especially U2.
rafapratas
Posts: 5
Joined: Sun Jun 05, 2016 5:36 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by rafapratas »

Hi,

Now I can get a waveform similar to a square wave, but very rounded. And on the SEN1 switch, the 0.1V scale is not working, because when the switch is on that position the wave disappears. But we are on the good way
rafapratas
Posts: 5
Joined: Sun Jun 05, 2016 5:36 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by rafapratas »

Hi,

I have another problem: I don't have any signal. The signal line appears, but neither the square wave or a DC input signal is working. Basically Some problem in the signal acquistion. What should I test ?

Thank you
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