TDS-210 Oscilloscope LCD help needed

[DTJ]

I turned on my 21 year old Tek TDS-210 for the first time in probably a year and the display was blank!

I opened it up and the power rails are all good and the LCD BL is working fine. The caps look ok physically and there's no ripple on the rails.


I noticed the display was very weakly observable from an angle above horizontal, it appears to be alive.

I think it may be a contrast issue. Digging through old posts here is seems that pin 1 of the display connector is the contrast adjustment.

Normal contrast voltage range (from an old eevBlog post) is:
-13.1V white on black display
-16.7V black on white normal display
-19.0V completely washed out display.

I measured on my unit and the contrast voltage varies from -20V to -23V when the contrast is adjusted. I'm guessing its way to low/high!

There was mention in the thread of a service menu that allowed gross shifting of the contrast voltage. My TDS-210 does not seem to have this menu option.

Any ideas guys?

Thanks

[george.b]

Got any pics? Could be just a toast polarizer.
The service option has to be enabled by pushing a button in the rear. Check the service manual, page 5-2.

[DTJ]

Got any pics? Could be just a toast polarizer.
The service option has to be enabled by pushing a button in the rear. Check the service manual, page 5-2.

Hi George - what sort of pic would show the polarizer was buggered? I think the polariser is a plastic film on the LCD face? Is that correct?

I'm more that happy to take some photos if I knew what sort of pic would help. The displayed characters are pretty weak and would be hard to show in a photo.


Thanks for the tip on the service button on the rear. I'll check that option out now.

[george.b]

Got any pics? Could be just a toast polarizer.
The service option has to be enabled by pushing a button in the rear. Check the service manual, page 5-2.

Hi George - what sort of pic would show the polarizer was buggered? I think the polariser is a plastic film on the LCD face? Is that correct?

I'm more that happy to take some photos if I knew what sort of pic would help. The displayed characters are pretty weak and would be hard to show in a photo.


Thanks for the tip on the service button on the rear. I'll check that option out now.

Yes, that's correct. There are actually two polarizer films, one in the front, one in the back of the LCD panel. Both can fail.
A toast polarizer will usually have a telltale discoloration, normally radiating from the center of the display, unless it's so far gone that the discoloration has taken over the whole panel, but by that point you would probably see cracks from the degraded adhesive as well. It often also smells unpleasant. A head-on picture of the display with the scope powered on should be sufficient to confirm, but if it seems normal to you (i.e. the same as it looked before you left it alone a year ago), then the polarizer is probably okay.

[DTJ]

Yes, that's correct. There are actually two polarizer films, one in the front, one in the back of the LCD panel. Both can fail.
A toast polarizer will usually have a telltale discoloration, normally radiating from the center of the display, unless it's so far gone that the discoloration has taken over the whole panel, but by that point you would probably see cracks from the degraded adhesive as well. It often also smells unpleasant. A head-on picture of the display with the scope powered on should be sufficient to confirm, but if it seems normal to you (i.e. the same as it looked before you left it alone a year ago), then the polarizer is probably okay.

Thanks George.

The display looks the same as normal to me.  It seems uniform across the face. A couple of photos are attached.

I accessed the service menu and found the contrast adjustments - it made no difference to the display. I'll have to hook up a test connection to the LCD contrast pin and see if the voltage varies when the contrast is stepped in the service mode.

Cheers

[george.b]

You're right, the polarizer is just fine.
Things that occur to me that you could try/check, if you find the contrast voltage is changing accordingly to the contrast setting:

• LCD board capacitors - I've seen those fail in two units I've serviced before, and then the display would misbehave. If there are any bad caps connected to the Vbias line, you could have problems there.
• LCD flat cable connector - at least a couple of users here have had problems with a bad connection there. You could check whether Vbias and Vcontrast are actually getting to the LCD board.

[DTJ]

I appreciate your advice George.

I'll check the capacitors tomorrow.

I think the flat ribbon connector is ok. The first thing I did was remove it and re-seat it  couple of times. I have not checked Vbias but I've been measuring Vcontrast @ the LCD board itself.

Cheers!