Double Newbie - Switch Mode Power Supply and battery shenanigans - Page 1
Forum section: electronics / beginners

author: Tommy banana
author_rank: Newbie
timestamp: February 22, 2021, 01:47:37 pm
content: Hi,I am a double newbie - new to electronics and new to this forum. So please excuse my lack of knowledge....I have been getting more and more interested in electronics since the start of lockdown and since I got lucky fixing a couple of electrical things that saved a bunch of money.But I didn’t really have a clue what I - or all these mysterious components - were doingThen our trusty Marshall Kilburn Bluetooth speaker broke and I thought I’ll have a go fixing that. Basically it had stopped showing any lights on the control panel for about 2 years but still worked via Bluetooth and input jack but then stopped making any sound via the input jack and then finally stopped making any sound via Bluetooth either despite still pairing.I decided that a) would be good to fix this and b) it may be a good project to build some electronics learning aroundWhat I have done so far:So first of all I made a mistake when disassembling and thought a battery connection to the PCB was a connector and found out it was through soldered and damaged the battery connection in the process. At one point when disassembling I also created a short on the battery leads which might have made things worse - there was a spark!After disassembling I found there was some water damage on the top plate control panel which had rusted out three led bulbs (which tell you your current input selection and if you are running off battery) and three adjacent resistors. I have replaced these in the circuit.In this picture you can see the newly replaced bulbs and resistors.I bought an ESR meter and checked all the electrolytic caps and they all seemed fine wrt ESR and I can’t see any signs of leakage.At one point since disassembly I had the machine starting up and lighting one of the led bulbs with AC power connected and also connecting to my phone via Bluetooth.Then I tried to reconnect the battery and everything stopped. Does this suggests that possibly the AC power supply and the bluetooth module should be ok?At that point I realised I needed to learn some actual electronics to fix something like this , bought a book, and have been studying and accumulating equipment ever since. My wife thinks I have gone mad...I now have an isolating transformer and an old tektronik analogue oscilloscope , and have built a kit signal generator - so I am about to get to work troubleshooting.I can now recognise the switched mode power supply (SMPS) on the device and even was able to visually pick out the bridge rectifier and opto-coupler visually - when a few weeks ago the whole board made no sense at all. So learning is occuring even if the Kilburn is still not working! I find it harder to spot any battery management circuit or where on the board the power supply outputs DC into the next stage,and how the battery connection feeds into that.Where I need help at the moment is two fold.1. the damaged battery connectorHere is a picture showing the before and after - aaaargh.I have reconnected it to the board but the pads got damaged along the way and I don’t know how to work out troubleshooting or testing this. There are two separate connections in the same loom, one goes to the battery and one goes to the control panel - and presumably the off/on switch and/or the battery light.Here are the connections on my board tested with a continuity meter. Hope this makes sense.There are loads of videos on understanding SMPS but very few I can find on how a rechargeable battery joins in the cold side of such a circuit. I presume there is a voltage smoothing circuit maybe a zener diode and negative feedback loop there , plus a different circuit for charging the battery when AC is present but I am not sure what to test and how to proceed with making sure this battery connection is ok - nb the battery is still providing 12 V..Here are two photos of the cold side of the power supply - can anyone point me on these photos a) where any battery management circuit is b) where to test the battery connection and c) where to test the output of the AC power supply (with my new oscilloscope) as it goes to the next stage?Wrt troubleshooting strategy:1) Should I disconnect the battery and check the SMPS first and get the thing working from the AC first or might a damaged or incorrectly connected battery interfere with that ? NB If if I disconnect the battery currently it is till dead and the fuse at the AC input is ok.2) I am aware something in the signal amplification stage may be faulty as well....3( Is the ESR check enough to exonerate the electrolytic caps?If you need any more photos or info no problem.Many thanks in advanceTommy Banana

author: Tommy banana
author_rank: Newbie
timestamp: February 22, 2021, 05:23:24 pm
content: If I have phrased this badly or if I am in the wrong place for this kind of question could someone let me know?