Envision TFT1780

[PlainBill]

Re: Envision TFT1780

On autorange:

'B to 7th pin' was .438 K ohms

'C to 7th pin' was .437 K ohms

OK, the secondary resistance matches. That indicates a transformer secondary isn't shorted. At this point I'll need a picture of the area between the two yellow lines. The bigger the picture, the better (within the limits allowed for .jpgs here).

PlainBill

[alegro]

Re: Envision TFT1780

Here's some closeups.

[PlainBill]

Re: Envision TFT1780

Here's some closeups.

Whooeee!!! This is getting miserable! It looks like this design monitors CCFL current, but not output voltage.

I've marked up the pictures and have a few points to check. On the bottom left and bottom right pictures I've identified the return lines from the CCFLs. Try to measure the voltage on them when the CCFLs are on. It may be easier to do this from the top; try to identify where the line comes through the board.
I've also identified the sense line on both pictures. Again, try to identify where this comes through the board at the electronics section in the middle of the board.

PlainBill

[alegro]

Re: Envision TFT1780

I've never done this before, and I don't want to fry my meter or do it wrong. Can you tell me where to put the + & - leads?

[PlainBill]

Re: Envision TFT1780

I've never done this before, and I don't want to fry my meter or do it wrong. Can you tell me where to put the + & - leads?

Well, I'll try. I've indicated the return, sense and ground lines on the back side pictures. The first way involves making measurements on the back side of the inverter. What you would do is hook everything up, including the wires from the inverter to the CCLFs. Set the meter to the 20 VAC scale. Arrange the inverter so you can touch the black probe to 'Ground' and the red probe to 'Return'. Plug in the monitor and have someone cycle the power button on the monitor (press it once, then a second or two later press it again) while you are holding the probes in place. The backlights should come on momentarily. While they are on, note the meter reading. Cycle the power button several times, seeing if you can get a consistent reading. Write it down, then repeat the process for the other end. If the readings are below 2 volts, reduce the range to 2 VAC. next, change the range to the 20 VDC scale and repeat the process on the sense lines.

If it is not possible to position the inverter properly, look at the traces. Notice that there are one or two holes at the end of each trace. These pass the signal to the top side of the board. Identify where each one comes through the board and do the measurements from the top side. If this is too difficult, take pictures of the areas on the top of the card that I marked off in red in my earlier post (make sure you indicate which is left and which is right). and I'll identify where to hold the probes.

PlainBill

[alegro]

Re: Envision TFT1780

I really appreciate your help on this PlainBill.

Left side:

return: 10.5 V AC

sense: 8.3 V DC

Right side:

return: 19.5 V AC

sense: 22.2 V DC

The right side sense took longer to return to zero (7 or 8 seconds).

[PlainBill]

Re: Envision TFT1780

I really appreciate your help on this PlainBill.

Left side:

return: 10.5 V AC

sense: 8.3 V DC

Right side:

return: 19.5 V AC

sense: 22.2 V DC

The right side sense took longer to return to zero (7 or 8 seconds).

This is a synergistic exercise. The more experience we get at troubleshooting these the sooner I can say "Look at this post and follow the steps." I don't think 'Two Seconds To Black' problems will ever get to be as easy as saying "replace all caps and let us know if there is a problem", but we are developing at least a technique to identify the cause.

Two things stand out. First, there is truly a massive difference between the two sides. The slow drop off would indicate a resistor that is no longer in circuit. HOWEVER, the next step is to determine if the problem is one of the CCFLs or on the board. To that effort, you are going to have to go through a few contortions. Rearrange the boards so you can hook the CCFLs that normally hook to the right side of the inverter to the left side, and the left side CCFLs to the right side of the inverter. Then repeat the tests. If the readings stay the same, the problem is on the board. If the readings swap ends, the problem is the CCFLs.

If the readings stay the same, I will want you to trace the sense lines and indicate what components they are connected to on the top of the board.

PlainBill

[alegro]

Re: Envision TFT1780

Thanks for the info. I like to learn the theory instead of just what to do.

I have something interesting to report. I did the readings for the left and right returns. Then the second time I pressed the power button to meaure the DC voltage on the left sense, the monitor stayed on. I heard some very faint squealing noises, and the voltage was holding (within the range listed below), and so I looked at the screen and it was on. The "cable not connected" message was displayed.

Then I did the right sense reading, and then pressed the power button and turned the monitor off. I then repeated this (the monitor stayed on again during the second sense readings).

One other thing: during some of the readings the meter was indicating between 10-100 micro volts while the monitor was off. Is this just an artifact of the meter?

Here's the voltages:

Left side:

return: 10.0 V AC

sense: 8.5-9.1 -stayed on, sqealed

Right side:

return: 19.8 V AC

sense: 8.4 V DC -while still on

[PlainBill]

Re: Envision TFT1780

Thanks for the info. I like to learn the theory instead of just what to do.

I have something interesting to report. I did the readings for the left and right returns. Then the second time I pressed the power button to meaure the DC voltage on the left sense, the monitor stayed on. I heard some very faint squealing noises, and the voltage was holding (within the range listed below), and so I looked at the screen and it was on. The "cable not connected" message was displayed.

Then I did the right sense reading, and then pressed the power button and turned the monitor off. I then repeated this (the monitor stayed on again during the second sense readings).

One other thing: during some of the readings the meter was indicating between 10-100 micro volts while the monitor was off. Is this just an artifact of the meter?

Here's the voltages:

Left side:

return: 10.0 V AC

sense: 8.5-9.1 -stayed on, sqealed

Right side:

return: 19.8 V AC

sense: 8.4 V DC -while still on

Well, obviously the right side sense voltage dropped down to something approximately the same as the left side. I'd suggest resoldering the components associated with the sense lines. I'm not sure about the squealing. But I'm sure if you were in contact with 700VAC you would squeal too!

PlainBill

[alegro]

Re: Envision TFT1780

The sense lines go to these tiny surface mounted resistors. I wouldn't be able to solder those. I can barely manage capacitors and other large components. Are there any other options?

[PlainBill]

Re: Envision TFT1780

The sense lines go to these tiny surface mounted resistors. I wouldn't be able to solder those. I can barely manage capacitors and other large components. Are there any other options?

A very limited number of options. If you have steady hands, a fine tip soldering iron and small diameter solder. If you have a tremor like I do, use a soldering iron with a small tip, small diameter solder, and rest your hand on something to steady it. If you are nearsighted like I am, taking the bifocals off and using a magnifing lamp (or squinting) works.

PlainBill

[kc0cdg]

Re: Envision TFT1780

The service manual here has a schematic and other info on that inverter board that might be of interest to anyone troubleshooting this part:

http://ftp.qrz.ru/pub/hamradio/schemes/monitors/aoc/AOC-LM700/aoc-lm700-service-manual.pdf