Replacing display in Tek THS700 scope


JOE
 

Hi all,

Has anyone had success replacing the display in a THS700 Series scope with a generic LM32P10 LCD module?

In my attempt there is some flashing of the display during power on but no useful display ever comes up.

I did swap back to the dead display to make sure I didn't damage the scope. I get a display, such as it is.

I did try a couple new displays so don't think I have a bad module out of the box.

Thanks much
Joe


Harvey White
 

Depending on the kind of display, you may or may not need a negative supply (older displays, STN, DSTN).  Those displays may have a contrast control, depending.  If the original display is a TFT display, you may have to consider the new display's vertical and horizontal drive polarity.   Many TFT displays have a built in power supply (some STN/DSTN displays do).  You may or may not have control lines to reverse the display in the X direction or Y direction.

Knowing what the old display is (electrically) can make a significant difference.

At the level of what you're doing (and I'm assuming that there is no controller on the display, but are the standard row and column drivers), you're looking at an LCD panel that mimics a CRT display with the modification that the data is R, G and B digital and that there's a pixel clock, typically about 25 Mhz.  Display panels have maximum timings that have to be adhered to, as well.


Harvey

On 3/7/2022 5:14 PM, JOE wrote:
Hi all,

Has anyone had success replacing the display in a THS700 Series scope with a generic LM32P10 LCD module?

In my attempt there is some flashing of the display during power on but no useful display ever comes up.

I did swap back to the dead display to make sure I didn't damage the scope. I get a display, such as it is.

I did try a couple new displays so don't think I have a bad module out of the box.

Thanks much
Joe





JOE
 

The original display is labeled LM32P10. This is a Sharp part number but I don't see any manufacturer marks. It's a non TFT dumb display.

The replacement units are sold as, and labeled LM32P10. This being the case I was expecting the replacement to be straightforward. So far, hasn't turned out to be so.


Harvey White
 

Ok, well, given that it's supposed to be a 4.7 inch, STN monochrome display, 4 bit interface parallel, CCFL, wants about -20 volts, QVGA; that's pretty generic.  Especially if you have the exact same model that you're replacing.

I'd ask the following questions, simply because they need to be asked in cases like this:

1) does the backlight work?  Stay working?

2) was the connection to the display on the right side (been there, had it reversed, generally doesn't make contact).

3) did you adjust the contrast?  Sometimes it's on the motherboard and not the display, and may need to be adjusted, especially in STN and DSTN displays.

Harvey

On 3/7/2022 6:15 PM, JOE wrote:
The original display is labeled LM32P10. This is a Sharp part number but I don't see any manufacturer marks. It's a non TFT dumb display.

The replacement units are sold as, and labeled LM32P10. This being the case I was expecting the replacement to be straightforward. So far, hasn't turned out to be so.





 

Joe,

I would add the following to Harvey's questions:

What were the symptoms that you saw with the original display in place?

What else did you check that led you to believe the display was the faulty component?

-- Jeff Dutky


Dwayne Reid
 

Hi there, Joe.

Question: is the only reason you want to replace the display because it has become too dim?

If so, there are people who have simply replaced the backlight CFL tube(s) in the display. I don't recall the details but a quick Google search should find who has done this in the past, along with details.

I'm probably going to want to do that for my trusty old THS-720P scope. That scope has been a real workhorse for me and gets constant usage.

dwayne

At 03:14 PM 3/7/2022, JOE wrote:

Hi all,

Has anyone had success replacing the display in a THS700 Series scope with a generic LM32P10 LCD module?

In my attempt there is some flashing of the display during power on but no useful display ever comes up.

I did swap back to the dead display to make sure I didn't damage the scope. I get a display, such as it is.

I did try a couple new displays so don't think I have a bad module out of the box.

Thanks much
Joe
--
Dwayne Reid <dwayner@...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
780-489-3199 voice 780-487-6397 fax 888-489-3199 Toll Free
www.trinity-electronics.com
Custom Electronics Design and Manufacturing


demianm_1
 

I replaced the CCFL in two THS720's with LED's. It makes an enormous improvement. Its not hard but it is a little involved. You will need a step up inverter and a backlight regulator. LED strips can be cut to fit and fit nicely where the CFL was and power can be had from the connector that powered the CFL inverter along with a display enable signal.


druid_noibn
 

Hi,

Love to hear more about what you did to replace the displays.

Many thanks,
John aka DBN


Bert Haskins
 

On 3/9/2022 10:12 AM, druid_noibn via groups.io wrote:
Hi,

Love to hear more about what you did to replace the displays.

Many thanks,
John aka DBN
I have repaired a whole bunch of displays in marine equipment using led strips.

These a usually segments of three leds in series with a small resister.

Then it's "rinse and re[eat" to the next segment, 3, 6, 9...

So the target voltage is 12 volts, and I have seen them working fine at 13.6 volts.

I usually use these with a buck-boost SMPS in current mode.

Please reply is this is not clear.

BH





Harvey White
 

I do something similar, but since I'm doing battery operated equipment where the highest desired voltage is 5.0 volts, I use an individual dropping resistor per LED.  Definitely not the most efficient way of doing things.  The traditional way would be to go with an inverter and series LEDS.

Depending on the dispersion of the LEDS, you can get hotspots near the LED itself, but that depends on the diffuser in the display.  A 4 inch strip contains 6 or 7 LEDS, they're designed to be connected with 0 ohm resistors (or wires) to parallel another strip.  An 8 inch VGA display takes about 4 strips (could take maybe another two).  At about 20 ma design max for each LED (rated to 30), the strips take 480 ma maximum, and are PWM controlled (P channel FET).  You'd want cool white LEDS to be able to get the best blues out of the panel.

It works.  Not elegant, but it does work.

Harvey

On 3/9/2022 10:49 AM, Bert Haskins wrote:

On 3/9/2022 10:12 AM, druid_noibn via groups.io wrote:
Hi,

Love to hear more about what you did to replace the displays.

Many thanks,
John aka DBN
I have repaired a whole bunch of displays in marine equipment using led strips.

These a usually segments of three leds in series with a small resister.

Then it's "rinse and re[eat" to the next segment, 3, 6, 9...

So the target voltage is 12 volts, and I have seen them working fine at 13.6 volts.

I usually use these with a buck-boost SMPS in current mode.

Please reply is this is not clear.

BH









JOE
 

The reason I'm replacing the display is the LCD panel has delaminated, or something of that sort.

I too have successfully replaced CCFLs with LCDs on some units.


JOE
 

I am still fighting with my replacement LCD display. My latest theory is per the Sharp spec the display expects a contrast pot between pin 11 and pin 12 of the interface. From the THS700 service manual it sounds like thise pins are unused. As soon as I can get back to this project I'll investigate this further.


Harvey White
 

That's an STN display.  All the ones that I've ever seen have a contrast adjustment, generally a variable voltage.  For graphics displays I think it was a negative voltage, for the little 7 segment displays/dot matrix on 5 volt only systems, I think it was between 5 volts and ground.

IIRC, if that's not connected somehow you have a problem.  I'd check the old display for a pot, or perhaps for a pair of resistors doing this function that gave you a fixed contrast. These are, as you know, rather old displays with rather old technology.

Harvey

On 3/9/2022 1:59 PM, JOE wrote:
I am still fighting with my replacement LCD display. My latest theory is per the Sharp spec the display expects a contrast pot between pin 11 and pin 12 of the interface. From the THS700 service manual it sounds like thise pins are unused. As soon as I can get back to this project I'll investigate this further.





JOE
 

The old display has no pot on it. Measurements I've made suggest the THS720P adjusts the contrast by varing the negative supply to the display. I do see some change in the display as I vary the contrast with the THS720P menu pick. However even at 100% information is barely visible. I feel the contrast range needs to be centered. I'm hoping a pot across the display contrast pins will do this. I hope to try it in a day or two.


Harvey White
 

Ok, so they automated it, at least some.  The negative supply to the display should not be adjusted from what I know, the contrast is a separate line.

Looking at the display's data sheet, the -VEE should be about -20 volts, varying between -19 and -21.  While it may change the contrast of the display, I personally would not want to change contrast that way.  There should be a 500K pot connected as a variable resistor between pins 11 and 12.  This indicates to me that the display was not intended to be remotely controlled for contrast.  It is possible that a voltage on either pin 12 or pin 11 may control the contrast.  I presume you have no schematics of either the panel or the scope.   I do agree that under normal circumstances, the contrast range ought to be centered.  VEE is pin 6.  I suspect that Tek did something non-standard here, or that the display is not quite the same.

Puzzling.

Harvey

On 3/9/2022 3:50 PM, JOE wrote:
The old display has no pot on it. Measurements I've made suggest the THS720P adjusts the contrast by varing the negative supply to the display. I do see some change in the display as I vary the contrast with the THS720P menu pick. However even at 100% information is barely visible. I feel the contrast range needs to be centered. I'm hoping a pot across the display contrast pins will do this. I hope to try it in a day or two.





JOE
 

Progress!

The new display does have a contrast pot on it. It's across pin 11 and 12 of the interface. The problem is the THS720P shorts these two pins together. Breaking the connection in the cable brings the display to life!

Thanks for all the info provided here that helped send me in the right direction.


JOE
 

Harvey,

I agree with each of your points. As you also see in the display spec Vee should be -20 volts. The THS700 service manual claims -10 to -20 volts on pin 6 of the interface. I measured around -14 to -16 depending on contrast setting in the THS700 display menu. Clearly non-standard.

Also interesting is the contrast pot on the new display sees to be about 15K not 500K as mentioned in the spec. I ended up with a pin 11 to pin 12 resistance of about 6K for a well centered contrast.

Finally, in case anyone tries to follow this path, the interface connector orientation described in the service manual is reversed. Go by the markings on the boards.


Harvey White
 

That's enough to confuse anyone on a non-standard implementation.  I have no idea what Tek was thinking of.  Hmmmm, I always thought that datasheets were important.  Now I wonder what the designers at Tek actually thought..... If they did.

Highly curious.  No question that it works, but I wonder why they decided to do that.

I sure wouldn't.

Wonder if it saved 5 bucks.

Harvey

On 3/9/2022 7:02 PM, JOE wrote:
Harvey,

I agree with each of your points. As you also see in the display spec Vee should be -20 volts. The THS700 service manual claims -10 to -20 volts on pin 6 of the interface. I measured around -14 to -16 depending on contrast setting in the THS700 display menu. Clearly non-standard.

Also interesting is the contrast pot on the new display sees to be about 15K not 500K as mentioned in the spec. I ended up with a pin 11 to pin 12 resistance of about 6K for a well centered contrast.

Finally, in case anyone tries to follow this path, the interface connector orientation described in the service manual is reversed. Go by the markings on the boards.