Speaker/PA Amplifier Repair Help.

[William Henderson]

Hello,

I am looking to see if there is someone in the Rlab group that has experience with audio amplifier repairs that would be happy to teach me the ways.  I enjoy electronics and tinkering and would like to learn to be able to fix my own amplifiers and speakers with built in amplifiers. I used to do some electronic repairs but it has been a while since regular electronics repairs.

I have 5 broken items that I would like to learn to be able to fix so I can continue to use them to support my local community events and performances.

The broken items that I would like help with fixing;

Alto TS212S - Amp Module - Powers on, signal present to the input LED, there is no sound coming from the output.  I wonder if it could be one of the transistors but my guesses are just guesses, there was no significant event to stop it from working. I have opened the unit up and put a multi meter to multiple components.

Crown 2 Channel Amp - One channel has an intermittent fault.

Mackie Thump 12 Active - 2 x Amp Modules - No power - these units were plugged in when a generator surged.

Active Home Cinema Sub Amp module. - I can’t remember the issue, I have not unboxed it in a while. 

Thank you in advance to anyone that can help and offer advice. I look forward to learning with you and having more usable speakers around.

Best wishes

William

[Andy Bennett]

Hi,

I am looking to see if there is someone in the Rlab group that has experience with audio amplifier repairs that would be happy to teach me the ways.  I enjoy electronics and tinkering and would like to learn to be able to fix my own amplifiers and speakers with built in amplifiers. I used to do some electronic repairs but it has been a while since regular electronics repairs.

 

That's quite the list of things to fix. Good luck!

I've been enjoying https://www.youtube.com/@MendItMark/videos recently.

I'm not sure if I can put anything I've learnt into practice but I feel that I've picked up on a few good techniques after watching a handful of videos.

Knowing how to measure transistors for shorts and having the schematics seem to be the most obvious starting points for most of the stuff he does.



Best wishes,
@ndy

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and...@ashurst.eu.org
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[Tom Allen]

Hey Will!

I too have been binging the MendItMark videos and trying to learn this dark art myself as i also have many failed amps and modules here (some of which came from you!). but despite spending many many hours on it so far, i still haven't managed to fix any amps at all :(  He makes it look so easy! Wish i had more understanding of it myself. I feel like i get it, then when i get to a real world repair, i find results that i just don't understand. i guess i need to be more confident in removing components and testing them off board one by one, but the fear of tens of hours of wasted time puts me off i guess. I've also found blown transistors, ordered more, fit them, only for them to blow instantly which really put me off the process as that is so frustrating as i still don't know why that happened (i thought i had done all the relevant tests)

amp and power supply repair seems to be a far more complex job than it would at first seem. 

If anyone has anymore resources on diagnosis to recommend that would be very helpful

Tom

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[pad...@padski.co.uk]

Hi Folks,

 

I used to fix amps for a living, and I’ve been a little reticent to dive in here because yes, it’s very rarely simple, and often the case that familiarity with particular things and their common issues is the only way to get a quick fix.

There are however a few essentials worth knowing (these are not exhaustive, nor in any procedural order…..!!!!):

1. You need a variac, with an ammeter – ideally an old-school not digital ammeter, 5A or so... As Tom notes – ‘replace some faulty parts, switch it on and they’re dead again’. Winding it up with a variac can help prevent this, unless the parts are particularly low current.. Get used to the initial surges due to mains transformers and large caps. It is invaluable to be able to spot high current, or to see (oh yeah you want an oscilloscope too!) the output going offset before attaching a load..
2. You need the schematic, but also need to remember that it’s not necessarily a lot of use for pinpointing the faults. While the amp is faulty, the schematic is incorrect. It is very useful for making educated guesses as to what else might be dead based on what you have already found to be faulty. There may also be ‘sophisticated’ feedback mechanisms and protection circuitry shutting stuff down.. more on this later. Note that this circuitry may also be faulty… :/
3. You need to remove and test (properly) all the power transistors, and their drivers, and depending on what you fine – maybe more. ‘Not short’ is not good enough, bipolars can become 2 diodes and it’s really annoying.  Mosfets have their own whole world of annoying foibles. Plus of course they are expensive and you don’t want to replace them multiple times (see #1)
4. Check the power supply rails in isolation if possible
5. Try not to assume ‘random failure’ of individual parts. It tends to be pretty rare, and there is more often some kind of cascaded fault, or user error, or……..
6. Some amps are just ridiculously overcomplicated (feedback/protection/unforgiving/whatever), and just not worth bothering with unless you are very familiar with them, or if they are very expensive. Often the only guarantee here is to thoroughly test every semiconductor out of circuit, as well as checking as best you can everything else. This was what we did with Yamaha/Crown/Carver PA amps back in the day. If you’re quick, and they are amenable to disassembly, this is not too painful.

 

The last amp I was given to fix (C-Audio SR707 iirc), a few weeks ago went like this:

Symptom – blows the mains fuse a few seconds after switch-on.

The ‘few seconds’ was due to a delay circuit – resistors + PTCs in series with live until a relay kicked in, then presenting an apparent dead short to the mains.

With that bypassed, it showed the short when being brought up slowly on the variac.

The power supply had a plug allowing it to be disconnected from the main board (excellent). I unplugged it and now all is well, voltages are good.

I checked for obvious shorts – between every permutation of: 0v, outputs, DC rails, every power transistor pin etc. Nothing is short. So now bits have to start being removed.

Everything is soldered on the main board which is an absolute bitch to get out – requiring almost total disassembly of everything including desoldering loads of wiring, so I made my apologies and binned it.

This was for a good friend of mine to whom I owe some favours. Mostly I’ll only bother with nice valve amps nowadays.

 

I hope this helps

 

Cheers

Paddy

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[Tom Allen]

great input paddy

I really struggle with the effort reward ratio of amp repairs too. I got 20+ hours into trying to repair a crown amp (had schematic) before I gave up.

Even professional repairers struggle. I took an old QSC amp to a repair guy in Tilehurst ( who has now retired (bob noyze)) and gave him £150 and when i got it back it worked for just one event before dieing again :( again replacement of that model can be had for around that money :(

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