Brandnew HDMi 3 to 1 switch box behaving erratic from strangly behaving bad led port 2
Most of you probably may have read my previous article about fixing the only HDMi port on my heavily used 27 inch Philips monitor. It was successfully fixed after all ordered products were received. The excellent uv hardened epoxy coating and the copper rework sheets to repair all 19 broken off tracks and my good G600 LCD microscope made it possible! Although I recently replaced my tiny 4.7 inch G600 for the bigger 7 inch Andonstar LCD HDMi Microscope with decent led side lights and better lenses. Mainly because the G600 made for this task too dark snaphots. Those superclear Andonstar photos to show the fine HDMi repair result can be found on my FaceBook link :
Because those 19 tracks were so close to each other in only about 10 mm of space I made the repair much easier by only replacing all 12 signal lines and the +5V track (pins 1,3,4,6,7,9,10,12,13,15,16,19 plus the +5V track on pin18). And ground pins 5,8,11,17 and the track to pin 2 were bend up on the HDMi connector so they were not needed on the board. They were wired with 2 thin wires. And track 14 was nc so that connector pin was also bend up and left unconnected.
Constantly switching HDMi cables on the one HDMi port in the end broke the inside plastic, why I also mentioned ordering this HDMi switchbox to prevent this problem from ever happening again:
However, soon after my monitor was fixed this hardly been used switch box started to behave very erratic as this short Youtube video shows:
This problem suddenly started happening showing very bright green leds on port 1 and 3 and most of the time the no longer working led on port 2. Although the switchbox HDMi output selector still worked fine, superbright leds 1 and 3 also got a too high voltage of about 3V instead of the usual expected 2V.
And when measured with my recently repaired VC200 DMM all leds seemed to work although the resistance measured over led 2 was very low. It was an unsteady about 54 ohmic value. And also when this led was taken out of the circuitboard the resistance was there. But it tested fine when a 2V dc voltage was applied.
Showing practically no flowing current and a bright lit up led. As also next photo shows. And after this test was done the low ohmic resistance was strangely for a short period completely gone! And this resistance when measured was never the same value either! Very strange! But before I started suspecting this bad led, let me explain that I first had examined all chips that were used in this switchbox. Thinking they were internally damaged somehow never expecting that this led was the culprit.
Mainchip U4 in the circuit is a 64 pins LK391 HDMi switcher chip that also drives each of the port leds. A sot23-5 JW5250a chip U2(smd code JW30) provides the about 1,8V DC voltage for the mainchip out of the by each HDMI port provided 5V. Why I at first suspected the mainchip because smd sot23-5 chip U2 worked. A 10 page datasheet of U2 can be found here:
https://datasheet.lcsc.com/lcsc/2012171205_JoulWatt-Tech-JW5250SSOTA-TRPBF_C729472.pdf (Other datasheet links only showed one page!)
The function of chip U8(can also be an U7 chip) with smd code _G2009 is still unknown.
All diodes and resistors measured fine.
So in next photo I located the track that went from mainchip U4 to led 2 and cut it open (U4 pin 16). Which I did with my new USB-C chargeable Smart Electric Sharpening Pen I also used in my HDMi repair. And it is that track that now is repaired and afterwards was coated with the great working uv epoxy coating in next photo.
With the no longer connected led 2 I concluded that the low ohmic resistance in led 2 was still there. So this ruled out that the mainchip U4 was causing this.
And since no other tracks connected to this port 2 led it only could be the led itself! So to summarize this unexpected culprit, it only was this erratic behaving led that was the reason for the erratic functioning of this switchbox as the Youtube video clip showed. Following photo shows mainchip U4 with all port leds D5,D6 and D8. And also U2 and U8 with all 3 HDMi input ports.
Below photo shows on the left my old G600 LCD microscope and my new Andonstar 246S LCD Microscope.
Naturally I previously also had contacted the Aliexpress seller that his product already had a very annoying failure even after hardly been used. But since my 15 day buyer protection just no longer was valid I was left with a brandnew but bad product. Which is certainly not the first time something similar on Aliexpress happened! Previously it was a useless 4TB SSD that never worked trustworthy. Sadly again money wasted because of a corrupt seller! And before that a never working XBOX360 dongle, that I, even in protection time did proof with excellent video that the Wifi dongle never worked. However the seller never accepted my claim on his stupid definition that the Gamepad online test website is a joypad safe test. Because as I noticed only on a real XBOX360 it is a definite safe test of proof!
Happily both Switchbox and my trusted 27 inch monitor are repaired. All I had to do to fix my brandnew 3to1 HDMi switchbox was replacing erratic behaving led 2/D6.
Above photo on the left shows unknown chip _G2009, and photo on the right chip JW5250S. This repair shows that even if leds do work they still can behave very erratic in a circuit causing a mainchip going berzerk by flashing and showing too bright working leds.
In such cases we always may count ourselves lucky that we mostly are able to fix or improve any bad working product. Which makes it all our worth while.
Albert van Bemmelen, Weert, The Netherlands.
Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.
P.S- If you enjoyed reading this, click here to subscribe to my blog (free subscription). That way, you’ll never miss a post. You can also forward this website link to your friends and colleagues-thanks!
Note: You can read his previous article on Green VC200 DMM Lithium Upgrade