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Safety precautions when working on amp?!

Erskine Ears

pfm Member
Hi.

I'm about to begin my first attempts at upgrading some of the components of my amp.

I believe there are parts in there that hold a lot of charge.

How do I ensure I don't kill myself while doing there mods?
 
(Caveat - If you have little or no experience in electrical safety and electronics in general, then please seriously consider taking it to someone who does).
Ensure it's unplugged from the mains at for least 24hrs before starting any work.
Short all big capacitors to flatten them - be careful, don't weld your best screwdriver to the chassis here. (I use a big wire wound resistor with a couple of flying leads to do this).
Double check its unplugged from the mains.
Remove all rings, wristwatches and intimate piercings.
Check a third time you've removed the mains cable.
Wear an anti static wristband that is grounded to earth to prevent ESD discharge into sensitive components.
If you have any doubt whatsoever on dabbling when the cover is off, then put the lid back on and take it to someone who knows.
 
Short all big capacitors to flatten them - be careful, don't weld your best screwdriver to the chassis here. (I use a big wire wound resistor with a couple of flying leads to do this).
For explicitness - roughly what value (order of magnitude) resistor?

BugBear
 
'Check a third time you've removed the mains cable.'

+1 to that. Sounds silly, but easily done. Esp when in the testing phase of fixing an amp.

Another practical 'gotcha' on general testing: if you are going to measure voltage and current, buy two DVMs and get into the habit of using one for one measurement kind only -pref a different colour or with a post-it note or similar reminder! Sooner or later you will find yourself picking up a meter with the leads still set for measuring current (ie a very-low impedance between them) switch the dial to volts, probe the circuit - and wonder where the smoke came from... until you realise you've just shorted a live circuit / & do not pass Go, do not collect £200...
 
...although quite a lot of modern meters do tend to have separate probe sockets for measuring voltage and current, so it's impossible to do the above.

24 hours for discharging caps sounds rather overkill. I test voltages across big caps with a multimeter, and if it's more than a volt or so I will wait for it to discharge, or use a discharging resistor if I don't want to wait that long.

... and do check for a fourth time that you have unplugged the mains cable! Also anything else (input cables, speaker cables) if you're in the middle of testing it as well as playing with the internals.
 
its possible because, even with the separate sockets (which all my meters have) the current-connected probes remain a dead short regardless of measurement selected on the DVM - that's what gets you into trouble - switching the measurement range, forgetting also to switch the probe connection.
 
Unplug the mains from the wall and the iec...


So speaks the voice of many snaked cables, oww f÷ck...ztttt.
 
'Check a third time you've removed the mains cable.'

+1 to that. Sounds silly, but easily done. Esp when in the testing phase of fixing an amp.

Another practical 'gotcha' on general testing: if you are going to measure voltage and current, buy two DVMs and get into the habit of using one for one measurement kind only -pref a different colour or with a post-it note or similar reminder! Sooner or later you will find yourself picking up a meter with the leads still set for measuring current (ie a very-low impedance between them) switch the dial to volts, probe the circuit - and wonder where the smoke came from... until you realise you've just shorted a live circuit / & do not pass Go, do not collect £200...

YES!! This is a top tip. I always force myself to return the lead to the Volts/Ohms socket immediately and before doing anything else, even if I'm expecting to measure current again in 10 mins... Too easy to get distracted by something and then 3 mins later "oh I'll just check that 15V rail hasn't drifted"... you set the DVM to Volts and BANG!

With valve amps beware of coupling caps which are still charged to 450V.... they may only be 0.47uF but with 450V on them you know about it!
 
DITTOif any appliance has its own capitivated lead ensure that the plug end is doubled back to the appliance whilst you work on it with the lid off. allow time for the caps to discharge
 
With valve amps beware of coupling caps which are still charged to 450V.... they may only be 0.47uF but with 450V on them you know about it!

Yes indeed.

When I was about 13 my Dad brought home an old duff Cossor scope his firm was chucking out. "have a go at fixing that" he said so I did. The fault was quite simple, an O/C resistor in the tube bleeder chain but it meant the 0.5uf 2.5 kV EHT smoothing cap stayed charged up - for two days after the scope was switched off.

Guess how I discovered that.

Good caps those big grey TCC thingies.
 
24 hours for discharging caps sounds rather overkill
Ten minutes is enough for most solid state.
There are some crazy valve designs out there with high voltage capacitors with no intentional discharge path. The most extreme case is the Quad ESL speaker
 
Ten minutes is enough for most solid state.
There are some crazy valve designs out there with high voltage capacitors with no intentional discharge path. The most extreme case is the Quad ESL speaker

Remember we're talking about (from what I can gather) a bit of a greenhorn, who may not be au fait with the fun and joy that working inside equipment with crapacitors can bring. You and me both know what to do and how to handle them (he says whilst elbows deep into a URS6 valve amp measuring voltage to the EL34s) but I'd rather see someone with little experience continue posting on this group and not become a statistic ;-) We all make mish-steaks at some point, that's how we learn, through our own stupidity and occasionally cursing designers for their design (why the hell did they strap those heatsinks live in the Roksan CD player !!)
 
I always unplug and discharge through a resistor. Hint: If you're explaining to visitors how a NAP250 works, don't touch the two power transistor cans with two fingers whilst pointing at them. There's 100v between them on the PS board and 40v on the power amp board. That wasn't me by the way, it was my supervisor showing some guests but the amp was on my bench!

I never did work on valve amps - I went straight to CRT monitors which technically are valves. I would still be as cautious of 450V as 25kV.
 
I was about 15 when I poked around inside a tiny 3W amp and took mains up my right arm and the big bang blew the main house fuse as well. Dad not amused, even though I had been chucked onto the bed by the action.
 
If you're performing adjustments that require the power to be on (e.g. DC offset, bias, etc), a pair of small and slim crocodile clips with insulation sleeving is much more practical than the basic probes you get with cheap DVMs. Also note that even the smallest of turns on trim-pots can affect huge changes in offsets and biases, especially in aged gear, so be very subtle with the adjustments!
 
Never mistake a reel of tinned copper wire for solder. Unlikely to be lethal, but certainly painful if you try to solder holding a short length of tinned copper wire instead of solder!
 
After you have finished with upgrading, use a RCD circuit breaker for the mailns socket when you first turn the amp on. Just in case if there's any short circuit.
 


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