HP3478A backlight - my version - Page 1
Forum section: electronics / testgear

author: Miti
author_rank: Super Contributor
timestamp: April 29, 2018, 08:47:00 pm
content: One of the main complain when talking about the venerable HP3478A is the lack of back-light. Some are bothered and some are not, probably depending of individual lab lighting situation. I didn't find this to be a major issue. Nevertheless, I took the challenge to add the back-light to at least one of the two instruments that I have. From the very beginning the requirements were:- Simple- Everything must be reversible. No cuts, in the plastics, front panel only adding new stuff. The only accepted alteration is the removal of the reflective film from the back of the LCD.- Must look (almost)professional with even brightness.- Using real light guide, not sanded plastic or other improvised solution.The bigest challenge was to find a real light guide. I removed one together with all the diffusing sheets from a cracked LCD monitor but it was 4mm thick, no chance to squeeze it inside the HP LCD. One day, my daughter's tablet (an Acer Iconia W510) died and I decided not to try repairing it. It was too slow. Before I said bye bye to it, I opened it and removed the entire LCD back-light. The light guide in this case was only 0.53mm thick and the LEDs were on a flexible PCB in series groups of four. Even this one together with the diffuser sheets wouldn't fit behind my LCD so I had to change the zebra connectors with thicker ones. The donor of the thicker connectors was a scrapped LCD module from work. Now I needed something to hold together the thicker sandwich. The solution came from a hobby store, a rectangular brass tube that I filed and made it a U bracket and covered it with Kapton tape. Removing the reflective tape from the back of the LCD module was somewhat challenging, the glue was left behind but with a bit of IPA and some patience it came out nice.Materials:- Light guide, LED strip and difuser sheets from an old tablet.- Very thin double side adhesive tape.- K&S #8268 Rectangular Brass Tube.- Kapton tape.- Thicker zebra silicone connectors from an old LCD module.- Breadboard and components to make a current controller.- Aluminum adhesive tape.Process:- Remove the LCD from the meter and open it.- Remove the silver film from the back of the LCD and clean the glue with IPA.- Cut a piece from the LED strip with four LEDs.- Solder wires to the ends of each group of two LEDs and connect the LEDs inside the group.- Cut the light guide and the diffusing sheets to fit in the black support plastic behind the LCD.- Glue the LED strip to the edge of the light guide.- File two corners of the rectangular brass tube and remove one side. File and sand the edges.- Cover the resulting brackets with Kapton tape. Mine needed two layers of tape to create enough pressure on the zebra connectors.- Cut the zebra connectors to the same length as the originals.- Place a piece of aluminum tape on top of the LEDs to reduce light bleeding.- Assemble everything together and put the assembly back in the meter.- Make the current regulator and solder the wires.- Attach the current regulator to the side of the meter and solder the input wires.Enjoy...

author: Miti
author_rank: Super Contributor
timestamp: April 29, 2018, 08:50:38 pm
content: More pictures...

author: Miti
author_rank: Super Contributor
timestamp: April 29, 2018, 08:52:20 pm
content: More pictures...

author: Miti
author_rank: Super Contributor
timestamp: April 29, 2018, 08:54:42 pm
content: More pictures...

author: Miti
author_rank: Super Contributor
timestamp: April 29, 2018, 08:55:42 pm
content: Schematic of the current regulator to follow.

author: mtdoc
author_rank: Super Contributor
timestamp: April 29, 2018, 09:06:46 pm
content: Looks great!.I'm one of those who is not too bothered by the lack of backlight. But this looks so good I'm tempted to do it anyways...

author: Miti
author_rank: Super Contributor
timestamp: April 29, 2018, 09:17:44 pm
content: Thanks!The only thing that's bothering me is a bit of light bleeding from the left side where the LEDs are. If I only had 5 more mm to hide the LEDs it would be perfect... but hey...

author: bd139
author_rank: Super Contributor
timestamp: April 29, 2018, 09:34:28 pm
content: Nice job. Looks pretty good to me. Tempted to have a bash with my 3478A.

author: Miti
author_rank: Super Contributor
timestamp: April 29, 2018, 10:54:45 pm
content: And the current regulator for the LEDs. It gives me a current range from 3.5mA to about 25mA. Yes, I know, it is technically an adjustable voltage regulator but I wanted linear and two strings. The input voltage is not enough for one string. It works and there's no noise whatsoever.

author: Miti
author_rank: Super Contributor
timestamp: April 29, 2018, 11:08:16 pm
content: Quote from: bd139 on April 29, 2018, 09:34:28 pmI hesitated a lot but you know what is like when you have that tingle in the tip of your fingers... I just had to do it. And I don't think the other one will escape. The only big danger is damaging something, I don't think calibration is affected much at all. They were not calibrated in years anyway but they seem to perform pretty good.

author: rhb
author_rank: Super Contributor
timestamp: April 29, 2018, 11:33:24 pm
content: If anyone finds a suitable source of parts please post it. Looks beautiful.

author: tautech
author_rank: Super Contributor
timestamp: April 30, 2018, 05:19:11 am
content: Nice job.The LH side does indeed look a little bright, have you experimented with a good bit lower voltage to reduce the glare ?

author: bd139
author_rank: Super Contributor
timestamp: April 30, 2018, 06:38:13 am
content: Quote from: Miti on April 29, 2018, 11:08:16 pmI shot the calibration on mine the other day and need to pay up now so that’s not a problem

author: Miti
author_rank: Super Contributor
timestamp: April 30, 2018, 10:47:10 am
content: Quote from: tautech on April 30, 2018, 05:19:11 amI did. It just reduces the overall brightness but the glare is still there. It is not as bad as it looks in the pictures, the camera exaggerates the effect a bit.

author: Miti
author_rank: Super Contributor
timestamp: April 30, 2018, 10:51:31 am
content: Quote from: bd139 on April 30, 2018, 06:38:13 amDid you short the battery? You can try to calibrate it yourself if you have a GPIB interface.https://www.eevblog.com/forum/repair/hp-3478a-how-to-readwrite-cal-sram/

author: bd139
author_rank: Super Contributor
timestamp: April 30, 2018, 11:57:46 am
content: I don't have a GB-IP interface. TBH I'll just get it cal'ed. There's a place 10 minutes from me who will do it for a reasonable fee (slightly more than I paid for the meter)

author: Miti
author_rank: Super Contributor
timestamp: May 28, 2018, 12:13:44 am
content: And the same treatment to my frequency counter.

author: bd139
author_rank: Super Contributor
timestamp: May 28, 2018, 12:32:44 am
content: Looking good

author: Miti
author_rank: Super Contributor
timestamp: December 18, 2018, 09:19:48 pm
content: I tried to do the same thing on my second HP3478A and I goofed badly.I broke three pins on the connector that goes into the back of the LCD board. FCK!!!I don't suppose anybody has a dead HP3478A that can harvest a cable, do you? Or a part number I can buy somewhere?

author: coromonadalix
author_rank: Super Contributor
timestamp: December 18, 2018, 10:06:53 pm
content: there is flat cable to dip adapters, you punch the dip one into your flat cable ....... maybe digikey, mouser, electrosonicIm gonna check atr my job, we used them 4 years agofound this onehttps://www.mouser.ca/ProductDetail/Kobiconn/151-2P16-E?qs=sGAEpiMZZMs%252bGHln7q6pmxC8cBNg%2fQb9js617XOcI%252bA%3d

author: Miti
author_rank: Super Contributor
timestamp: December 18, 2018, 11:08:12 pm
content: Thanks Coromonadalix,I soldered some wires and bent them to simulate the original pins and it seems to work.

author: xrunner
author_rank: Super Contributor
timestamp: December 18, 2018, 11:11:07 pm
content: Very nice! I love it.

author: gdnichols31
author_rank: Regular Contributor
timestamp: September 29, 2020, 12:35:28 am
content: Great Job!

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 29, 2020, 09:13:01 am
content: I really like this, I have several HP/Agilent Power Supplies that need this treatment. My big questions are:1. Where can I get some of the thin light guide material from if I don't have an old monitor?2. Any pointers/part numbers to/for suitable LED strips?

author: Miti
author_rank: Super Contributor
timestamp: September 29, 2020, 12:47:00 pm
content: Try some laptop/tablet repair shops. Thy may have some broken LCD modules.