Audio amp fault finding - Page 1
Forum section: electronics / repair

author: Mkus
author_rank: Newbie
timestamp: September 29, 2018, 11:08:22 pm
content: Hello,I have some basic understanding of circuits but I'm in the deep end on this. I'm trying to determine if it's feasible or even possible to repair an old amp. I noticed that the right speaker in my car didn't work suddenly, and I traced the fault to the amp. After measuring the faulty output, it seems it has reversed it's polarity, the marked positive terminal is actually negative. The voltage is roughly the same on both channels. Faulty channel does output sound but at a very low level. No other fault indications or blown fuses, however it did start popping a bit on startup for a few days before that channel failed.It's a 2-channel amp with bridged output possibility, CLS CA21 if it matters. I took it apart but no clear signs of destruction, just some flux residue that I cleaned up. I attached few pictures of the board and the channel output transformers/transistors/mosfets on it, as I thought some of those might be faulty. I didn't desolder them yet for testing since I wanted to ask for advice first. Maybe something wrong on the bridging circuit?I've identified these components per channel:4xFQP50N06 Mosfet2x2SA1694 & 2SC4467 Power transistorsFMG22S + 22R recovery rectifier diodes(?) - why would you need these with dc input power? Related to bridging?Top:Bottom:

author: oPossum
author_rank: Super Contributor
timestamp: September 30, 2018, 12:48:30 am
content: A DC voltage of a few tens of millivolts on the output is not unusual. Weak audio or loss of audio on one channel can be caused by dirty pots or switches. So work the pots back and forth and jiggle the switches to see if that changes anything. The next step would be to check for audio on the output of the op-amps in those SIL packages.The fast rectifiers are for the 'high' frequency switching power supply.FQP50N06 are the power supply MOSFETS.2SA1694 & 2SC4467 are the bipolar audio output transistors.

author: drussell
author_rank: Super Contributor
timestamp: September 30, 2018, 02:11:21 am
content: Quote from: Mkus on September 29, 2018, 11:08:22 pmSounds like a bad coupling capacitor.DoNOTpower it up while all those transistors are dismounted from the heatsink. You would likely blow up some of the semiconductors, even at idle.

author: PKTKS
author_rank: Super Contributor
timestamp: October 03, 2018, 01:50:55 pm
content: I will risk a LOOONG SHOT of 2 cents opinion...... based *ONLY* on your words and photos.AS it looks (by 25V on a 12V car battery) the FAB. managed toassemble a BOOST regulator to improve power output.By the number of MOSFETs and diodes I will guess a synchronous boost topologyThe wrong voltage readings may be due to the faulty boost regulationTake care as boost regulators can spike really high voltages on the coil.I would follow a regular boost SMPS troubleshoot routineby testing each and every CAP for faulty ESRs and the MOSFETsSecond step the audio output itself it is still ok due to any SMPS surge...long shot 2 cent opinionPaul

author: Shock
author_rank: Super Contributor
timestamp: October 03, 2018, 03:27:15 pm
content: Here is a decent youtube channel I recommend.He goes through a few amps breaking it down step by step what to look for.https://www.youtube.com/user/barevids/videos