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 Post subject: Electronics question
PostPosted: Sun Jun 21, 2015 3:24 pm 
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Location: Burblandia
I need to discharge the 2 large 10,700 Mf capacitors in my power amp. I have read several variations on creating a tool for discharging. They vary mostly on the size of resistor to use. Essentially these tools just incorporate a metal probe (nail) a wire and a resistor with an alligator clip at the other end. It is apparently a more controlled way than just shorting with a screw driver (many have promoted that method too ...but I remain more cautious). Can anyone weigh in on an appropriate resistor to use? I have 3 good tutorials, one guy uses a 470 OHM, 5 watt, another uses a 10 OHM 50 watt, while iFixit suggests a 20K OHM 5 watt resistor for large caps.

Maybe they all work....I'm hoping someone here can elucidate this for me a bit.


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 Post subject: Re: Electronics question
PostPosted: Sun Jun 21, 2015 5:50 pm 
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There has to be a formula for this!

Many decades since I got a "C-" in Electronics, but I'd guess more wattage is safer & more Ohms slower.


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 Post subject: Re: Electronics question
PostPosted: Sun Jun 21, 2015 6:27 pm 
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Won't those discharge on their own once the power is turned off? It should.

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 Post subject: Re: Electronics question
PostPosted: Sun Jun 21, 2015 6:32 pm 
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Squishy Tia wrote:
Won't those discharge on their own once the power is turned off? It should.

Well, I'm not sure how long it takes. I have read warnings that they retain lethal voltage for "a long time". I can't quantify that.


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 Post subject: Re: Electronics question
PostPosted: Sun Jun 21, 2015 7:00 pm 
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As I recall they do hold it for some time… got a multi-meter?


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 Post subject: Re: Electronics question
PostPosted: Sun Jun 21, 2015 7:28 pm 
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BDAqua wrote:
As I recall they do hold it for some time… got a multi-meter?

Yep.


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 Post subject: Re: Electronics question
PostPosted: Sun Jun 21, 2015 7:32 pm 
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Does it have DC voltage that high?


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 Post subject: Re: Electronics question
PostPosted: Mon Jun 22, 2015 11:24 am 
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Technically the voltage will be the same, the capacitance is just high (so voltage will flow for a long time).

I dunno how to discharge capacitors. I guess I never cared since the capacitors I'm used to dealing with are all under 12V so pfft.

I was going to guess at using some resistors, though what values to use ???


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 Post subject: Re: Electronics question
PostPosted: Mon Jun 22, 2015 12:33 pm 
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BDAqua wrote:
Does it have DC voltage that high?

Yes it seems so...see pic below.
Attachment:
Scan 5.jpg
Scan 5.jpg [ 262.35 KiB | Viewed 1371 times ]


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 Post subject: Re: Electronics question
PostPosted: Mon Jun 22, 2015 1:16 pm 
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What voltage do you get for them then?


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 Post subject: Re: Electronics question
PostPosted: Sat Jun 27, 2015 8:05 am 
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Waited a week. Checked voltage and found it within a safe range to work on. Lowered bias setting. All seems to be working fine now.

Just to clarify a bit. I bought this Hafler amp (and partner preamp) back in the seventies and built them as a kit. I can follow instructions and do pretty well with an iron but thats about the extent of it. I later bought a kit to modify (and hopefully improve) those components. They are both Hafler's modified by me using a kit from Musical Concepts (now Musical Designs I think). Unfortunately, that puts me in the position of not really having a clear path for repairing things when they go wrong these 40 years later. It's really pretty impressive that this amp still works as well as it does. I did replace many of the capacitors about 3 years ago. Including those 2 10,700 Mf guys.


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 Post subject: Re: Electronics question
PostPosted: Wed Jul 01, 2015 2:28 pm 
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Most Hafler amps use 100V DC filter caps. Actual voltage will be less than
that (usually around 50 or less).

A 20,000 ohm 5 watt resistor will safely dissipate up to 100V.

Here is a handy online ohms law calculator:
http://www.bowdenshobbycircuits.info/ohmslaw.htm

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 Post subject: Re: Electronics question
PostPosted: Wed Jul 01, 2015 2:59 pm 
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Thanks. :)


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 Post subject: Re: Electronics question
PostPosted: Wed Jul 01, 2015 3:04 pm 
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Hi BD: :)

I found some free Hafler service manuals here:
http://www.eserviceinfo.com

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 Post subject: Re: Electronics question
PostPosted: Thu Jul 02, 2015 2:28 am 
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kjk555 wrote:
Hi BD: :)

I found some free Hafler service manuals here:
http://www.eserviceinfo.com

Great! Thank you kjk555.


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