Screen blank, bad voltage on TP13

Screen blank, bad voltage on TP13

Postby AstrumSystema » Fri Jul 11, 2014 11:51 pm

I just finished assembling the kit, and when I power it on, the screen's backlight goes on, but nothing appears on the screen. When I measure the voltage on TP13, which should be 5 volts according to http://www.jyetech.com/Products/LcdScope/Troubleshooting_062C.pdf, the voltage is actually around 2 or 2.5.
Anyone know what's going on?
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Re: Screen blank, bad voltage on TP13

Postby jye1 » Tue Jul 15, 2014 1:31 pm

Can you please post all other voltages measured against those shown in the troubleshooting guide?
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Re: Screen blank, bad voltage on TP13

Postby WaltFricke » Wed Feb 25, 2015 7:27 pm

I have a similar problem - wrong voltage at TP13, plus a few other places. My screen and controls all function, but the displayed square wave voltages are a bit high, and there seem to be other oddities. I'm using a FG085 as a source, set at 5V.

Sending a 13KHz signal from the FG085 Function Generator (verified with a separate frequency meter) I get 13,000 HZ, and switching to 100Hz produced that, so I guess that part is working.

By fiddling with the controls, at 2V/ segment I got readings (at 55 Hz). I see almost 12V square waves, when they should be 5. They aren't horizontally stable, but march from right to left. But on the DC switch setting I am not getting a baseline trace. Or, if I am, it is up at the very top of the display. Another setting at 50Hz shows an apparent 0.5V @0.5/segment.

Using the troubleshooting section, with 9.88VDC in, those which were clearly off (based on the photograph in the instructions concerning troubleshooting) were:
V REF TP13 - 2.49V, should be 5.0
On the port where 3d pin in should be -4.5 or -2.4, was -2.6
Over on right side, where should be 0V, was 1.82
Same, where should be 0.6, was 2.16
MCU pin 5, should be 1.5V, was 5.97

Others were OK

Going through some of the Section 3 no trace checking:
AV+ OK, AV- OK
C. Analog - pin 7 of U2 is 2.06V, and 2.07 at R5
Voltages on DN1 starting from the one pin on top counterclockwise: 1.84, -6.38, 9.30
Pin 1 of U1 is -1.92
Pin 7 of U1 and pin 1 or U2 are both 0.96V

Trigger level Section 4, A. Voltage at C16 varies from .72 to 2.39 at ground, dc, x1, or 0.1v setting, or AC setting. At Frequency setting, is 0.

Nice 5V output from the U3 VR.

Any suggestions on what is causing VREF to be negative and half what positive should be? And why displayed signal voltages are high? I have some pictures of this, but don't see how to attach them.

Walt Fricke
Boulder CO
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Re: Screen blank, bad voltage on TP13

Postby jye1 » Thu Feb 26, 2015 9:49 pm

Voltage at TP13 is not related to anything so it can be ignored. It has no impact on functionality.

Please check the following:
1 ) Set SW3 to GND. Verify voltages of U2 pin 7 and U1 pin 1. They should be very close to 0V.

2 ) Keep SW3 at GND position. Check voltages of U2 pin 1 and U1 pin 7 while changing VPOS. The latter should closely follows the former.

3 ) Voltages at U5 pin 22 and pin 16 should be around 0.6V and 2.6V, respectively. More importantly their difference should be very close to 2V. Otherwise you will get large amplitude display error.

4 ) Sometimes when SW1 or SW2 are not properly positioned there will be large amplitude error.
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Re: Screen blank, bad voltage on TP13

Postby WaltFricke » Fri Mar 06, 2015 9:56 pm

OK - for #1: 2.27V and 2.25V. Nowhere near 0V in terms of voltages in this system.

#2: Vpos cursor bottom screen:
0.73V and 0.79V
Cursor mid screen:
1.55V and 1.54V
Cursor top screen:
2.42V and 2.42V

So those at least follow.

#3: 2.35V and 2.70V This is a 24 pin DIP, right? Not as hard to count pins as U4. So U2, pin 1 is quite a bit off?

I assume all measurements are to ground? I took the voltage regulator heat sink as the most convenient place to clip a ground for my ohmmeter.

So where might this suggest I look next? I'll remember that this forum doesn't automatically send a reply, and look more frequently. Sorry

Walt
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Re: Screen blank, bad voltage on TP13

Postby WaltFricke » Fri Mar 06, 2015 9:59 pm

Sorry, posted twice

Walt
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Re: Screen blank, bad voltage on TP13

Postby WaltFricke » Sat Mar 07, 2015 6:07 am

Rechecking assembly instructions - found why the test frequency didn't work - the wire loop went from outside hole to outside hole. Resoldered to center hole and kept upper hole. Test came to life. Freq = 501Hz.

But voltage levels displayed are off. When VPOS is set at 0, bottom of square wave is about 1.8V (2V per segment), and the top is at 8.2V, so the wave P/P is 6.4V.

Moving VPOS up maintains this P/P for a while, and then the display gets somewhat unstable.

Is it possible that the wrong test signal connection damaged a component?
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Re: Screen blank, bad voltage on TP13

Postby jye1 » Sat Mar 07, 2015 1:36 pm

It's good you got waveform displayed so most parts of the scope are working. For unstable did mean the traces moving horizontally? Please try adjusting trigger level to make them stable. Keep trigger source 'internal' (a low letter 'i' shown).

6.4V p/p value is a little off. Should be close to 5V. This may be caused by slide switch not positioning properly (the top two switches). Please try pushing them harder when switching to end positions.
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Re: Screen blank, bad voltage on TP13

Postby WaltFricke » Tue Mar 10, 2015 5:16 am

Well, not for the first time since it started acting up, my DSO 062 isn't really showing the waveforms. The frequency counter is, as it has been, fine.

But the screen looks like the ground level (offset?) is way up toward the top, maybe one dot of a 4 dot/5 segment block down. A horizontal line there, with a sort of square wave above it. Appropriate positive width for 500 Hz - two segments or 1ms at 0.5ms setting, 2ms total pulse width. Whole area where positive part of wave would be is "solid" black, though of course not very tall before it hits the top of the screen.

VPOS doesn't change this. Level only changes this when it is set up just at or above this sort of base or ground level, at which point the display (such as it is) moves from right to left.

Trigger is AUT i, so that's not it.

This is most stable at the 2V setting. At 0.2V the solid part (positive part of wave) drops to about one segment wide per trace. At 0.1V the traces get even narrower. The total pulse width, positive and negative, stays at 4 segments (2ms).

How much or little I press on the switches doesn't seem to matter.

Changing the slope does what one would expect, so that works. HPOS also works. So all the controls work, but something in the circuit which sets the ground level on the scope isn't working. The ground switch position has the apparent ground as described - almost at the very top.
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Re: Screen blank, bad voltage on TP13

Postby jye1 » Thu Mar 12, 2015 8:48 pm

Please try this. Set couple switch to GND so that the input is 0 volt. Move cursor to VPos indicator so that it is highlighted. Hold down the VPOS button for about 3 seconds. This will perform a vertical position calibration to have the 0V trace aligned with the small triangle at left screen border.
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