-11 V at D1 Positive

[Slartibartfast]

Hello.

I only have a flat line showing as the trace on my screen.

In an attempt to diagnose the issue I followed the steps in DSO138 troubleshooting - No AV- (no trace) (even though I did have a good 'AV-' reading) and all was well till step 4. My negative D1 voltage is correct but instead of having -8V at the positive I have -11V. The guide then says:

If these voltages are incorrrect check Q1, L2, D1, and C24

How do I "check" these components? I've kind of eye-balled them and they look fine but how do I do a proper check?
(note the Q1 values checked out fine for me in Step 3).

I am also getting -11V at TP25 (instead of -8V). Not sure it that's related or not.
Also I am unable to fine TP26. Could someone describe where to find it?

Thanks for your help,
Slarti.

[MNTech]

If the voltage at AV- is OK don't worry about the slightly high voltage at TP25. That voltage is regulated down to -5V by U3. Anything over 8V at TP25 will work. TP26 is the same point as AV-.

V2 is the last test point before the signal enters the microprocessor. The voltage at V2 with no input should be approximately 1.6V. My unit measures 1.7V.
When you do the VPos alignment the microprocessor reads it and adjusts the Zero point accordingly.

Set the switches to DC, 1V, X5.
Touch the red test lead to end of D2 (battery voltage) near the edge of the board.
The voltage at V1 should be 1/100th of the input with the switches set as above. The 9V battery drops with load so expect about 0.08V at that point.

The voltage at V2 should increase about 0.13V depending on your battery voltage when you touch the red lead to D2.

If the voltage at V1 doesn't change check the circuit between the input and V1
If V1 is OK and V2 is not check the circuit between V1 and V2.

Look for solder shorts or bad connections.
This PCB is not easy to solder, the pads are small so it's possible to that you have joints where the solder is on the lead but not connected to the pads. If you look at the opposite side of the board on a good joint you will see a little solder has flowed through. Reheat questionable joints, add solder if needed.
If you can read the schematic and relate it to the physical board you can follow the signal from the input and see where you lose it.
Good Luck.

[Slartibartfast]

Thank you for your reply.

V2 is the last test point before the signal enters the microprocessor. The voltage at V2 with no input should be approximately 1.6V. My unit measures 1.7V.

Cool, I'm getting 1.66V at V2

Set the switches to DC, 1V, X5.
Touch the red test lead to end of D2 (battery voltage) near the edge of the board.
The voltage at V1 should be 1/100th of the input with the switches set as above. The 9V battery drops with load so expect about 0.08V at that point.

Hmm, with the switches set as you describe touching the red lead to D2 seems to make no difference. I am getting 0V at "V1" and 1.66V at "V2" regardless of whether I touch the red lead to D2 or not.

I then checked the continuity of the lead and was surprised to discover the red end was not connected to the central connection of the BNC plug at all. The center of the BNC socket in the board is connected to the wire coming out the back but the center of the plug on the lead is not connected to the red alligator clip. The black alligator clip is connected to the outside of the BNC socket (and ultimately all the way to "DGND") but the red is not connected to the center. I assume it should be?

Anyway, if I directly connect a lead from the wire coming out the back of the BNC socket to D2 I do get 92mV at "V1" and 1.8V at "V2" (0.14 higher than before). This certainly makes it look like I have a faulty lead. As it is I had actually already ordered a proper oscilloscope lead (with an attenuator) so will try that when it arrives.

Thank you for your help,
(: