Logitech z906 power supply board REPAIR - Page 1
Forum section: electronics / repair

author: Dinesh6252
author_rank: Contributor
timestamp: March 29, 2021, 02:15:26 pm
content: Hello folks, i have got a logitech z906 subwoofer which is not working.(1). I opened up it to see whats going on there and found a ISL8560IRZ (U8) burned out, a helping member here named @vidi told me that this ic is responsible for converting the high voltage (46V) toget 12v output, Then the 12V rails is used as an input to the 5V LDO (U7) that makes 5V.(2). After that I checked all the voltages by a multimeter and found that the 46v was present but 12v and 5v were not present.(3). I then searched the forums got idea of using a second power supply. I used a PC PSU to get 12v and 5v output and joined the ground of PC PSU and the inbuilt power supply board.+(4). I turned it on and everything was showing on console and controllable but there was no sound coming out of the Amp. board. I checked two compponent U19 and U18 heating but that was it. Ieven touch the input jacks with bare hands to get any hum noise but there is no sound coming out of it. Now i am out of ideas. If anyone can help me out , that would be really great.

author: RayRay
author_rank: Frequent Contributor
timestamp: March 29, 2021, 06:29:26 pm
content: Honestly, it looks very bad! the U8 IC is clearly shot (probably the components under it as well, in the black charr), it's possible the trace is damaged also. U19 could be damaged as well (or it's just covered in leakage, hard to tell from the pic) And on the 2nd pic (electrolyte capacitors) I don't think that's glue man, they're shot to hell (and would need to be replaced!). Overall, if you have good soldering skills, a decent soldering station, and a hot air station, it's likely fixable (provided you'd source the parts) that said, it'd definitely be a time consuming fix, so consider your options wisely. If you don't have a hot air station, then the repair is not cost-effective (and buying a new speaker set would be best)

author: vidi
author_rank: Contributor
timestamp: March 29, 2021, 10:39:06 pm
content: (a) Hello. Sorry, I had no idea that it is so bad. The four U8 VIN pins are gone. And also C31. It would require to create & glue new traces from copper sheet. If you are interested about the U8 connections it looks like this:https://drive.google.com/file/d/1r9mgVU9VyeU67tp-dY_87jyvJg9b54nT/viewWhole Inkscape .svg document is at:https://drive.google.com/file/d/1XWBmyK9vNP4qce2RwMOwIh7a55EPO18O/view(b) I did not analyze the audio board in details, but U18 and U19 are LDOs (or linear regulators in general). U18 takes 5V rail (from your new external power supply) on input and provides 3.3 V on output. U19 should take 3.3 V from U18 and makes 1.8V on the output. I think the U18 provides power rail for common logic. U19 ( 1.8V ) is perhaps used to power ADC/DAC, I'm not sure.(c) Yes, LDOs and any linear regulator heats. For instance, in my case the faulty U7 (5V LDOs) had cca 90C on the package. Means, the temperature in the core was cca 120C - which triggers the thermal protection that caused the oscillations. The new one had cca 70C on the package.(d) This is a very rough calculation, just to give you an idea why does it heat. For instance consider the U18. If it takes 5V on input and get 3.3V on the output, it means the internal voltage drop is 1.7 V. And if it provides 0.500 mA it means the U18 has to dissapate (1.7 * 0.5) = 0.85 Watts. Thermal coefficient might be cca 40C per Watt which should increase the temperature about 34C. If you consider a common room temperature 25C the total temperature (34 + 25) should be somewhere between 45-50C. (Please correct me if my assumption is wrong, I'm not EE)(e) I would measure the voltage on the input/output of the U18/U19 to check if it provides a stable 3.3 and 1.8V output. Also please try to measure the current that flows in 5V rail. I recalled that in my case it was cca 480 mA when the system was fully on.Hope it helps.

author: Dinesh6252
author_rank: Contributor
timestamp: March 30, 2021, 05:58:26 am
content: Hello, i have checked voltages @ U18/U19 and the output voltage is 3.2v for U18 and 1.7v for U19 which I think is OK.I dont know why still no sound is coming from speakers, everything is working as expected in the console also.Update : I am providing the "3.3v keep alive" voltage by PC PSU but when 3.3v is being provided by the inbuilt power board the console turns ON and then turns off when I press any key or turn the volumeimmediately.Update 2 : The main Amp. ICs arent getting warm at all, even after turning it ON for 30 minutes.Edit: I am out of ideas now, if anyone can, please tell what should I check for?

author: perieanuo
author_rank: Frequent Contributor
timestamp: April 01, 2021, 09:07:52 am
content: after voltages check, if you say those are ok, check Vcc for audio amplifierclearly, the voltage that died with your step-up dms chip is the one who caused something else to fail, trace that voltagei hope it's the audio final amp that is affected, it's the sipliest to diagnose/repairin fact, what voltage was u8 making?

author: Dinesh6252
author_rank: Contributor
timestamp: April 01, 2021, 02:08:09 pm
content: Quote from: perieanuo on April 01, 2021, 09:07:52 amThanks for the reply mate, U8 is an dc-dc regulator which takes 46v and provides 12v on the output which is then supplied to a LDO (U7) that further regulates it to 5V.

author: perieanuo
author_rank: Frequent Contributor
timestamp: April 01, 2021, 02:32:18 pm
content: thanks,so my bet is some component on 46V supply rail took the shock (46V it's almost sure for audio final amplifier, so i'll verify first if this one works, try to disconnect his input and 'attack' him with some signal source, it may be your finger together with some screwdriver rod, like in the old days)

author: Dinesh6252
author_rank: Contributor
timestamp: April 01, 2021, 03:39:47 pm
content: Quote from: perieanuo on April 01, 2021, 02:32:18 pmI am getting 46v on output but not getting 12v and 5v because the U8 IC is shot, but that's not a problem since i have connected a external power supply for 12v and 5v and connected all the grounds together. The main console is turning on with all LEDs. But its the last step that I am stuck of getting any sound out from the speakers. I have checked all of the caps on amp. Board and they are getting 46v as well on all of them.

author: Dinesh6252
author_rank: Contributor
timestamp: April 18, 2021, 02:43:26 am
content: Can anyone help me with repair the last thing in the board that is the HV+ voltage has no enough amps to run the speakers on full power. When being run on full volumes the output voltage starts fluctuating and down to 26v. What should I check for getting full amps out?

author: ambrosia heart
author_rank: Regular Contributor
timestamp: April 19, 2021, 02:44:28 am
content: " When being run on full volumes the output voltage starts fluctuating and down to 26v."See photo and pm. Check ALL yellow circled components. The working principle of this circuitis nearly the same as SMPS used in all electronic appliances, converting 220V to any desiredvoltages.Comparing pins in your circuit and find them out to troubleshoot any peripheral componentsfrom which their values are differ.i.e. Are there any discrepancies of their values ? If yes, find them out.