Troubleshooting help needed - voltages wrong

Troubleshooting help needed - voltages wrong

Postby csanii » Tue Feb 05, 2019 8:22 pm

Problem seems very similar to:
JP4 has not been shorted.

My voltages are almost identical with L3 connected.
Theirs: TP22: 0.99V, AV-: 0.59V, AV+: 1.29V, +3.3: 0.23V, +5: 1.78V, V1: 0.51, V2: 0.61, V3: 1.18, V4: 0.55
Mine: TP22: 1.00V, AV-: 0.64V, AV+: 1.30V, +3.3: 0.23V, +5: 1.80V, V1: 0.58, V2: 0.60, V3: 1.30, V4: 0.66 V+: 8.6V
Q1c=0.0V Q2c=6.21V
Q1b=6.3V Q2b=0V
Q1e=6.3V Q2e=0V
Voltage at bottom of R40 = 0V
L1, L2, L3, and L4 are not Open - all have resistance

With L3 disconnected:
TP22: 3.30V, AV-: 0.00V, AV+: 0.00V, +3.3: 0.00V, +5: 5.00V, V1: 0.00, V2: 0.00, V+: 8.6V
Resistance from DGND to:
AV-: 1.75k
AV+: 4.30K
+3.3: rising to 15.8k
+5: 4.09k

Pin5 on the switches are connected to AV+ and are not shorted to GND
8.17V at base of R38

Can you provide some assistance on what I should check for next ?
Posts: 2
Joined: Fri Jan 25, 2019 10:10 pm

Re: Troubleshooting help needed - voltages wrong

Postby jye1 » Thu Apr 04, 2019 1:03 pm

Was U5 hot when L3 was connected? If yes that means AV+ branch drawn too much current. Please check if U2 was installed correct. If it is reversed AV+ may not appear short when checked by ohm-meter.
Posts: 1209
Joined: Wed Feb 20, 2013 4:58 pm

Re: Troubleshooting help needed - voltages wrong

Postby csanii » Wed May 08, 2019 12:55 am

U2 was not installed correctly. I never would have checked for that as the board had U2 pre-soldered.
New board received with U2 in proper orientation.
I figured I'd give it a try to desolder U2 on the old board and reorient it.
I'll be honest I may have damaged U2 as it did not go anywhere as easy as the Youtube videos I watched on this.

Old board with all components in place and 9V applied, TP22 was @ 3.3V so JP4 was shorted.
AV-: -4.95V
AV+: -2.10V *
+3.3: 3.3V
+5: 5.02V
D1: 0.12V
Q2C: 5.54V
D2: 8.22V
V+: 7.39V
3.3V: 3.3V (this is TP22)
R38: 7.18V
R39: -1.41V
OSC: 2.09V
R40: 0.82V
V2: -1.18V *
V1: -1.25V *
V-: -5.62V
L1: -5.65V

Screen was connected and it boots with LED blinking twice.
Buttons alter screen display.
Switches do not make any changes to the screen.
With CPL set to GND and screen attached.

Most of the voltages are close but AV+, V1 and V2 are way off.
After doing more testing I found that L3 had a bad solder connection.
After that was fixed all the voltages were pretty close to what they were supposed to be.
I plugged in the screen again....and no display!

After more solder repairs and lots of time tracing voltages and resistance I plugged in all back together.
Switches now are working and I managed to get the test signal to show up.
Now to calibrate!!!

For all those that tried and gave up...keep trying and quadruple test your soldering....make sure the solder is filled on component side.
I suspect the holes are not plated through so if you only have a little solder on the back of board any traces on top of board may not be connected.
Also if you get a white screen re-solder the J1/J3 connections.
I don't know how many times I went over my soldering but finally it looks like it is working...well at least before I calibrate it....
Posts: 2
Joined: Fri Jan 25, 2019 10:10 pm

Return to DSO138

Who is online

Users browsing this forum: No registered users and 1 guest