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Amp repair safety precautions?

S

SuperBassSam

Feb 9, 2009
104
0
Dorset, UK
#1
Hi all & thanks for reading.

I am beginning to dabble in electronics and have repaired a few battery powered things such as pedals, drum machines etc without incident. However I would really like to start repairing amplifiers although I am a little nervous using mains voltages. What precautions could I take to minimise the risk of electrocution? We have an RCD fusebox in our house but I plan to buy an adaptor anyway.

Also I was interested in what sort of damage a broken or incorrectly repaired amplifier can do to my other equipment. I have heard that if DC voltage flows through the speaker cables it can destroy your cab, how easy would that be for a novice with a basic understanding of electricity & schematics to do? I would get all my gear PAT tested after I'd finished working with it to make sure it was safe for future use.

Thanks a lot

Sam
 
#2
Unplug the unit. Make sure it's unplugged. Unplug it again.
Take off all your jewelry.
Do not rest your hand on the chassis while using the other inside the amp.
Discharge large capacitors to ground before working in power supplies.
Insulate screwdriver shafts with tape.
Wear shoes.

Use an oscilloscope to test audio output. Whether DC will destroy a speaker depends on the voltage and current capacity of the source. You can use a 9 volt battery to test speaker polarity.
 
#4
A big isolation transformer with variable voltage. Very expensive.
But GFI outlet in the least.

The question is what kind of SS amp? Power SS amps, even the micro heads, have high rail voltages and current available that their output alone is a safety hazard unless you use speakons.

When you test, get a big cap to put in series with the speaker to block DC. If you had a shorted output device, you'd likely see the protect mode, or breaker trip before you could damage anything.

Do you really have any SS amps to work on? or are you thinking of hot-rodding some amps? If that's the case, just pre-amps are best to tweak.
 
#8
http://www.geofex.com/Article_Folders/SPO_Test.htm

Great info on this site. Every thing I learned at electronics school is here for free.

looks like lots of the educational links are dead :( but its still a great site. I built one of these boxes, using 2 40W bulbs, some metal rough-in boxes from lowes and a DPDT switch to go from "regulated" to full power. I mod with confidence now.

London power books are a great resource, however expensive

I wise man told me once though....the day I stop riding white-knuckle scared is the day I park my bike.
 
#10
SurferJoe46 said:
I grew my second set of teeth on fly-backs - and then color came out and those could REALLY bite!

What's a little voltage through your body anyway?
Click to expand...

Heh. I was about 12 years old when I learned that taking off the back and putting the screws on top of the TV before working on it was a really bad idea. (They roll off and fall into the HV circuitry). Fortunately, my parents weren't in the room at the time, else my career may have gone in a completely different direction.
 
#11
Munjibunga said:
Unplug the unit. Make sure it's unplugged. Unplug it again.
Take off all your jewelry.
Do not rest your hand on the chassis while using the other inside the amp.
Discharge large capacitors to ground before working in power supplies.
Insulate screwdriver shafts with tape.
Wear shoes.

Use an oscilloscope to test audio output. Whether DC will destroy a speaker depends on the voltage and current capacity of the source. You can use a 9 volt battery to test speaker polarity.
Click to expand...

1+ to all of this

Also join DIY audio it is a forum for us tinkerers.

A variable output isolation transformer (Variac is a common brand) is a must as sooner or later you will need to power it up to test and this allows you to control current so as to not let the smoke out during testing a faulty unit.
 
#12
this is a reasonable alternative to a variac, and can be built cheaply.

http://www.geofex.com/Article_Folders/SPO_Test.htm

Duplex power receptacles have a small tab that can be broken off with needle nose pliers (US receptacles) so that the two plugs can be wired independently (in this application in series). I see you are in the UK and the different power supply voltages and receptacle designs mean you will have to adapt the concept (shown in the link for US/ north America mains power). DO YOUR HOMEWORK and learn about this it works but testing is NOT forgiving of errors so think, learn, think, then test using common sense safety practices.
 
#13
Thanks a lot for everyones advice, this really helps. Also that geofex website seems great so thanks for that. I will definitely look into getting a Variac, am I right in thinking they need to be installed into a case of some kind? As all I can find seem to be the actual components.

Seamonkey, you said "unless you use Speakon cables" - I do use speakons for cab connections, are they safer than regular jacks? Really I'd just like to start out repairing and getting used to amplifier circuits, I've got a Hughes & Kettner bassforce that is currently in bits that I bought dirt cheap so to get that working first would be ideal.

Thanks a lot
 
#15
An uncluttered work area with good lighting is important.

Having the right tool for the right job helps you to perform your tasks safely and not damage the amp that you are working on. This includes a proper set of pliers, cutters, screwdrivers, and nut drivers. Don't skimp on a good variable heat soldering iron and accessories such as a solder sucker, picks, a fan with a filter for extracting soldering fumes, and solvents such as acetone and isopropyl alcohol for cleaning flux.

A Link Removed like this can be used to drain capacitors before you start working on the amp.

You can build an [Invalid or Expired Link Removed] to hold the chassis securely.

As others mentioned, a variable transformer is nice to have but there are less expensive alternatives such as the light bulb limiter. If you are in the market for a variac, be careful with some of the made in china models that are being sold today. Some of them are unsafe to use. You can sometimes find good used ones a surplus electronics supply houses for a fraction of the cost of a new one. Same goes for other test equipment.

There are places that sell amp tech tools such as Invalid Link Removed or Link Removed where you can get an idea as to the kind of stuff to buy.
 

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