DSO138 troubleshooting - No AV- (no trace)

DSO138 troubleshooting - No AV- (no trace)

Postby jye1 » Fri Mar 20, 2015 2:41 am

You found AV- is bad (it is 0V or demonstrates a small positive value). All the other voltages are good and scope grids are displayed correctly. However, there is no trace. Because of the absence of AV- there is no negative power supply for the analog channel. As result signal can not be properly fed to ADC. The following are some general steps you can take to uncover the causes of this problem.

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.

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.

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.

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.

For all about cases please first make sure soldering is correct and good. Then depending on difference cases try replacing Q1 or Q2 for quick verification.

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.

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

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

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

Postby clondono » Wed May 27, 2015 12:00 pm

Hello Ive had my DSO 138 for about a week now and its been working better than i could have ever imagined. But all of a sudden yesterday the trace dropped out its at the very bottom and the offset and trigger can't be moved no matter in what setting. I've tried all different leads, I've checked the battery and i've gone though all the checks here but i still cant get a trace. My next idea is to flash it but im wondering what else i could do.
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Re: DSO138 troubleshooting - No AV- (no trace)

Postby jye1 » Wed May 27, 2015 1:17 pm

Did you check the voltages AV-, AV+, and V1, V2? Please check them if you haven't. Such issue is usually due to problem in analog circuit. Re-flash firmware won't help.
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Re: DSO138 troubleshooting - No AV- (no trace)

Postby clondono » Wed May 27, 2015 4:15 pm

yes i did, all my voltages are present however at vc2 and vc1 they were off by -1v not sure much that matters. i just find it really weird it dropped out of nowhere, i was working on a system that was less than 15v
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Re: DSO138 troubleshooting - No AV- (no trace)

Postby clondono » Wed May 27, 2015 5:20 pm

So i flashed the scope and everything is back to normal now, guess it might have been a bug or a glitch in it.
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Re: DSO138 troubleshooting - No AV- (no trace)

Postby loudspeaker » Fri Jun 05, 2015 1:32 pm

exist replacement for Q1 8550 and Q2 9014? type BCxxx?
I can not find original type here..
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Re: DSO138 troubleshooting - No AV- (no trace)

Postby loudspeaker » Sat Jun 06, 2015 1:13 pm

VGEN is ok with LCD...
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Re: DSO138 troubleshooting - No AV- (no trace)

Postby digdas » Sun Jun 07, 2015 9:20 am

I did:

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.

And I found Pin 46 on U1 itself equals to 0V. How to proceed?

I think the trace on the scope is at max voltage (green line at top of display.
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Re: DSO138 troubleshooting - No AV- (no trace)

Postby jye1 » Mon Jun 08, 2015 3:23 am

Did you measure VGEN with LCD plugged in?
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Re: DSO138 troubleshooting - No AV- (no trace)

Postby loudspeaker » Mon Jun 08, 2015 7:42 am

exist replacement for Q1 8550 and Q2 9014? type BCxxx?
need buy spare parts.... pls help..
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