DSO Shell 15001K

[tmdso]

I think I have several concurrent problems so let me start with a description of problems. I can power up and get a grid, then the Test Signal Amplitude display comes up without pushing the ADJ dial and is unstable in that it quickly changes values from 3.3 to 0.1v. I can adjust it with the ADJ dial but the Test Signal display disappears and then reappears randomly in an unstable state. It won't stay set.

The LM1117 only puts out 2.2v and gets very, very hot. I suspect this IC is fried or defective. I have new chips on the way.

Using a 9 volt power supply that indicates 9.2 volts when powering the unit, the unit boots into an unstable screen, the ADJ dial works intermittently as do the buttons. I find that the ADJ dial only works in increasing the values while pushing the ADJ dial decreases the values. This is also intermittent. After about 3 minutes the LCD goes blank with a lighted screen, not black. I've never gotten a trace of any kind except for powering the board without the analog portion attached. At that point there was a weak square wave displayed so at least I know there is the potential of having this thing work. Using a 7.5v power supply allows the unit to boot and maintain its unstable LCD screen but obviously all the voltages are incorrect, including the 3.3v signals.

All voltage values are within the tolerances on the schematic and very close to the numbers in Step 4 of the instructions. The only value that is NOT correct is the 3.3v signal anywhere on the boards.

Would I be correct in assuming that this 3.3v signal could possibly be doing all of these problems?

As soon as the parts get here I'll replace U3 and U5 ICs just for good measure. My troubleshooting expertise is somewhere between amateur and advanced amateur, soldering ability is excellent and I have other o-scopes to test with.

thanks,
tony

[jye1]

2.2V is marginal for the MCU to run stably. The reading at meter is average. So it is possible there were ripples that went down below the specified lowest working voltage 2.0V for the chip. Let's see how it works after you replace the regulator.

[tmdso]

Thanks. I should have the parts in a couple days and post the results here. I am quite sure it's the LM1117 that's defective. Using a separate scope I can get the .1v and 3.3v (which is only 2.0v) square waves. BUT, the trace at the 3.3v setting has almost .5v of noise across the top of the square wave and becomes perfect after using cold spray directly on the chip. At the .1v setting there is a 70Khz signal of about 1v in normal configuration and when the cold spray is again added to the LM1117 I then get the 1Khz signal at .1v, again with noise at the upper and lower voltages. This is interesting to me so I'll post some pics of this later added to this post.

0.1v test signal normal operation

0.1v-normal.png (7.56 KiB) Viewed 11087 times

0.1v test signal after LM1117 cold sprayed

0.1v-cold spray.png (6.8 KiB) Viewed 11087 times

3.3v test signal in normal operation

3.3V-NORMAL.png (9.58 KiB) Viewed 11087 times

3.3v test signal after cold spray

3.3V-with cold spray.png (8.79 KiB) Viewed 11087 times

tony

[tmdso]

Installed new LM1117. 3.3v output is better but the chip still overheats and becomes unstable, so, no real help. Using a 9v source the unit boots to a bright white screen. Using a 7.5v source it boots to the grid and numbers but the result is the same as before - unusable.

By the way, does the on-board LED give any coded information?

[jye1]

The LED blinks once as a trigger edge is detected, and that it's. There is no coded message.

[tmdso]

I think the problem is a short somewhere on the main board. I found I've done the same thing as Rob111 with the rotary controller and removed it from the main board. Caps C10 and C11 are OK and I've checked all the solder points under a microscope and there's no bridging on my work or the stock work and no damage to the printed circuits that I can see. Something is drawing way too much power from the LM1117 3.3v source. I see there are test points indicated on the schematic. Is there a possibility of getting the test values to help locate my problem.

If not, how can I get a replacement main board? I've already purchased another unit and it appears to work normally. I'll check the defective board by comparison to the good board and try to find the problem but tell me how to obtain or purchase another main board please.

thanks,
tm

[jye1]

Please contact JYE Tech at jyetech@163.com for a replacement.