Amplifier - negative rail had direct contact to amps ground (laptops HDMI KO) - Page 1
Forum section: electronics / repair

author: Matthew98
author_rank: Regular Contributor
timestamp: April 02, 2018, 09:35:15 pm
content: Hi guys.I bought a TDA7293 board (thought its not assembled, well thought bad... next time) made a quick ghetto amplifier for a bass speaker to have a 2.1 sound system. (Btw. connected from stereo to mono with just 8ohm resistor at each end conected together for the speaker. Sorry, will be changing them for 100ohm. Im still learning.)Well, once I connected a VGA display through HDMI to VGI connector, I heard strong 50Hz huss from the speaker and now I have HDMI free laptop.(Took me a while to get through that one... I probably wont have such expensive laptop ever again )I managed to get external display back with USB3 to VGA adapter and now.. Guess what. Did I change, repair, stop using the deadly amplifier ? Your correct. The USB3 to VGA adapter is dead as well.Now... I will have to take a look at the amp input with my osciloscope yet, weather I see anything dangerous anywhere but so far, can you tell me anything other than that Im stupid out of my stories ? Where to look for a fault, what to expect, maybe cheer me up by saying its probably just a fuse on main board near HDMI Chip and it can be repaired ?Hope you laughed a bit at my story. Its at my cost anyways.I have really no idea what could caused both HDMI and USB3 interferance. Of course, no other accessories such as Mouse or USB stick were interfiered. The amp is working just fine all the other time. Thank you all for understanding and help.You rock, guys !* When the amp is connected, but turned off, there was no interferance. Just remembered.

author: KhronX
author_rank: Frequent Contributor
timestamp: April 03, 2018, 12:03:03 am
content: It might help if you described (or drew a diagram of) how the things were connected (power & signal).Does your laptop's charger have a 3-pin (earthed) mains connection? And what about the external display? What's supplying power to the amplifier?Any photos of that amplifier? And any chance you can maybe draw or photograph what you were trying to say with "made a quick ghetto amplifier for a bass speaker to have a 2.1 sound system. (Btw. connected from stereo to mono with just 8ohm resistor at each end conected together for the speaker."?

author: Matthew98
author_rank: Regular Contributor
timestamp: April 03, 2018, 12:55:31 pm
content: Thanks for replying.So far i will answer just a few quick things. There is an exam coming :/ .There is a pic. of a board from ebay - It will be more or less the same, so you can see what components are used besides the amp itself.My laptops charger is connected by 3pin conector, but I bet the laptop is not earthed. Its Lenovo Y700 using their tipical rectangle power adapter. (+, - and sence wires if i had to guess ? ) The external display is an old Sony LCD conected again with 3 pin conector and earthed. I can confirm all of those statements later. i dont have any equipment on me right now.The amp uses split transformator +12V - 0 - -12V and is straight earthed (0 to earth). Its input is made from stereo to mono with 8 ohm resistors. It has a mono output, Its just a mono amp.I will be changing them to 100ohm, as it was suggested to me.Its gheto, because it was made in about a hour. The IC is coupled to the cooler with thermal duble sided tape. There is negative rail on the IC´s back, so thats a bit dodgy... Also, Its held together in an ATX power supply case mostly with hot glue. (Will be remaking it from scratch this summer with screws and all that...)I think is a ground problem. But I dont know why my HDMI is not working then ...

author: KhronX
author_rank: Frequent Contributor
timestamp: April 03, 2018, 01:05:23 pm
content: And what HDMI-VGA / USB3-VGA adapters were / are you using?The "stereo to mono" thing with resistors is coming from... where?

author: Matthew98
author_rank: Regular Contributor
timestamp: April 03, 2018, 02:31:04 pm
content: Oh, I see..Dont worry, HDMI is used only for video out. Audio is coming from 3.5mm jack.The HDMI to VGA was a cheap Ebay adapter as well as as the USB3 to VGA. The USB3 to VGA uses drivers in order to work and is based on FL2000 chip. The drivers are from Fresco logic.I belive they have not much to do with it, as USB and HDMI adapter did the same thing.

author: Matthew98
author_rank: Regular Contributor
timestamp: April 08, 2018, 05:56:59 pm
content: Took my osciloscope with me and I found the problem... There is a 16V Dc from amps. ground to real ground. Yaiks.What do you reckon guys ? Is the laptops HDMI out repairable with this all going through ground ? I dont know if ground fuses are a thing on laptops

author: KhronX
author_rank: Frequent Contributor
timestamp: April 08, 2018, 09:23:11 pm
content: Ground fuses are definitely NOT a thing, anywhere.Best case scenario, in case there are any TVS's on the HDMI data lines (which i kinda doubt), removing or replacing those might bring it back."Middle" scenario, in case there's some level-shifter / mux chip between the HDMI connector and the video card, "only" that might be toast.Worse scenario, in case the laptop's (only) using the in-CPU graphics, that secondary output might be "bye-bye".Worst scenario, if the graphics chip and/or the CPU is soldered to the mainboard... That's about that.What brand & model is that laptop, by the way?

author: SMdude
author_rank: Frequent Contributor
timestamp: April 09, 2018, 07:59:09 am
content: You need an audio transformer between your laptop and amplifier, that will stop the ground voltage problem.As for what is dead, I can't help you there!

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 09, 2018, 05:53:56 pm
content: Similar thing happened to a HDMI Capture-Card when I connected my Xbox One to it.It worked for about 2 or 3 months before crapping out. When disconnecting the HDMI-Cable there was significant arcing and a Multimeter showed about 40 V-AC and a little over 60 µAmps between PC-Chassis and Cable-Shield coming from the Xbox One.I'd suggest to ground everything to Mains Earth (the free contacts on your outlet) to make sure there's no voltage on what's supposed to be Ground. Cheap Power Supply-Bricks without Earth-Connection are a real plague in this regard!

author: Matthew98
author_rank: Regular Contributor
timestamp: April 15, 2018, 09:21:05 am
content: KhronX : i have a Lenovo Y700. Cpu and GPU are soldered on the main board. BTW.If I cant get tthe HDMI to live again, well, it could be worse...I still can output via USB3 adapters.SMdude : First time haring about this.Isnt better just solve the amplifiers problem ?But tell me more, I´d like to make a protection for my laptop.Update : Measerued everything. The Amp had poor contatct with mains earth. I discovered neagative ray is somehow connected to amps earth, as when I made proper ground connection, all current went to ground. I could measure direct ground to negative rail contact by multimeter, but I cant anymore. I dont think the amp would work with only + and ground, while ground being the negative rail... So thats mistery to me.Any idea what could cause the problem on PCB ? I doubt something shorted the circuit out in its case. The IC is isolated from grounded cooler guys btw, dont worry.