Speaking of Oscilloscopes

capnbob · 10th October 2010, 10:02 PM

I bought a new dual trace Tenma 30MHz Oscilloscope with an LCD Frequency counter (model number 72-6802) from MCM Electronics in December 2007. It worked fine until a few months ago when the LCD display started losing parts of the numbers. Now it is impossible to read the numbers. I contacted MCM who checked and even though it is a current model, they say that the LCD display is not available as a replacement part. So, I don't know what to do. Anyone have any ideas on how to repair it?

Michael Bean · 10th October 2010, 10:43 PM

Hi Capnbob

Most of the LCD units I've seen have electrical contacts made of dense conductive foam rubber that tend to compress and shrink over time. I had the same problem with my Fluke DVM. I fixed it by first cleaning the contact surfaces with denatured alcohol, then adding shims of thin plastic made from a thin, non-conductive plastic sheet to put more pressure on the contacts. I did that about 2 years ago, and it's been working fine ever since.

Hope that helps.

richiem · 10th October 2010, 10:57 PM

Michael's advice is spot on. Clean both of the rubber contact bar and the display's contact area with the denatured alcohol.

capnbob · 11th October 2010, 10:33 PM

Thanks for the advice. This display is molded into a small metal frame that has hard wired multi-pin connectors on the back. I can easily remove the display unit, but cannot separate the display from the frame with the pinouts on the back. It is a small part and I am surprised that I cannot find a replaement part. Anyone else have problems getting parts for Tenma products? I can't believe that MCM treats the Oscilloscope like a toaster.

richiem · 12th October 2010, 08:43 PM

Well, they just import the stuff, nothing more. There's probably a replacement display that will work or can be made to work, but the details need to be known -- what the pin connections are, etc.

atomkid · 11th April 2011, 05:52 AM

Hi Capnbob! I am Atomkid and new to this forum. I also same with you who brought this Oscilloscope and the LCD Display segment/numbering out until unreadable. So how is your solution and what's the current condition? Solve? Any replacement parts?
I have email to Farnell and bring up this issue for market claims matter. Now still waiting the reply.

cliffyk · 12th April 2011, 10:40 PM

Such displays of that vintage fall in to two broad functional categories, "dumb" raster driven components that display dots where they are told to by external circuitry--and "smart" displays that display, typically from internal bitmaps, what they are told to via commands delivered by a host device (typically via serial connections).

The first thing to do is determine into which of these categories the display falls, and then seek out potential replacements based on the functional category.

10th October 2010, 10:02 PM	#1
capnbob diyAudio Member Join Date: Jun 2010 Location: Wilmington, NC	Speaking of Oscilloscopes I bought a new dual trace Tenma 30MHz Oscilloscope with an LCD Frequency counter (model number 72-6802) from MCM Electronics in December 2007. It worked fine until a few months ago when the LCD display started losing parts of the numbers. Now it is impossible to read the numbers. I contacted MCM who checked and even though it is a current model, they say that the LCD display is not available as a replacement part. So, I don't know what to do. Anyone have any ideas on how to repair it?

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10th October 2010, 10:43 PM	#2
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	Hi Capnbob Most of the LCD units I've seen have electrical contacts made of dense conductive foam rubber that tend to compress and shrink over time. I had the same problem with my Fluke DVM. I fixed it by first cleaning the contact surfaces with denatured alcohol, then adding shims of thin plastic made from a thin, non-conductive plastic sheet to put more pressure on the contacts. I did that about 2 years ago, and it's been working fine ever since. Hope that helps.

10th October 2010, 10:57 PM	#3
richiem diyAudio Member Join Date: Sep 2010 Location: Grapeview, WA	Michael's advice is spot on. Clean both of the rubber contact bar and the display's contact area with the denatured alcohol.

11th October 2010, 10:33 PM	#4
capnbob diyAudio Member Join Date: Jun 2010 Location: Wilmington, NC	Thanks for the advice. This display is molded into a small metal frame that has hard wired multi-pin connectors on the back. I can easily remove the display unit, but cannot separate the display from the frame with the pinouts on the back. It is a small part and I am surprised that I cannot find a replaement part. Anyone else have problems getting parts for Tenma products? I can't believe that MCM treats the Oscilloscope like a toaster.

12th October 2010, 08:43 PM	#5
richiem diyAudio Member Join Date: Sep 2010 Location: Grapeview, WA	Well, they just import the stuff, nothing more. There's probably a replacement display that will work or can be made to work, but the details need to be known -- what the pin connections are, etc.

11th April 2011, 05:52 AM	#6
atomkid diyAudio Member Join Date: Apr 2011	Hi Capnbob! I am Atomkid and new to this forum. I also same with you who brought this Oscilloscope and the LCD Display segment/numbering out until unreadable. So how is your solution and what's the current condition? Solve? Any replacement parts? I have email to Farnell and bring up this issue for market claims matter. Now still waiting the reply.

12th April 2011, 10:40 PM	#7
cliffyk diyAudio Member Join Date: Nov 2009	Such displays of that vintage fall in to two broad functional categories, "dumb" raster driven components that display dots where they are told to by external circuitry--and "smart" displays that display, typically from internal bitmaps, what they are told to via commands delivered by a host device (typically via serial connections). The first thing to do is determine into which of these categories the display falls, and then seek out potential replacements based on the functional category.

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