• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

How to discharge caps?

Roypercy

Roypercy

Super Member
I'm trying to get to the bottom of why my Akai AA-1200 has no output; I started removing the output transistors to test them and got a spark when I started unscrewing the first one (this was with the receiver unplugged, but it had been plugged in for a short while before then). How can I safely discharge what I assume is residual energy left in the caps?

Be aware that I am a total noob at doing this kind of repair, so please keep it simple...

Thanks!

Roy
 
Register to hide this ad
quick and dirty, short them with a screwdriver.
Proper way is a 1 meg ohm resistor across terminals.
tal
 
Roypercy said:
I'm trying to get to the bottom of why my Akai AA-1200 has no output; I started removing the output transistors to test them and got a spark when I started unscrewing the first one (this was with the receiver unplugged, but it had been plugged in for a short while before then). How can I safely discharge what I assume is residual energy left in the caps?

Be aware that I am a total noob at doing this kind of repair, so please keep it simple...

Thanks!

Roy
Click to expand...

Get a 100-150 ohm resistor and short it across the caps for 30-60 seconds. then with a DMM measure the DC volts across the caps--Keep shorting and measuring until the caps are fully discharged.
 
qboneus said:
quick and dirty, short them with a screwdriver.
Proper way is a 1 meg ohm resistor across terminals.
tal
Click to expand...

Well, I got sparks three times on three different output transistors. This is a 120 wpc receiver, so the caps seem like they're holding a heck of a lot of current. If I discharge the caps with a screwdriver, won't I run the risk of destroying them?
 
quick and dirty, short them with a screwdriver.
Click to expand...
Tends to leave black marks on screwdriver and capacitor terminals and sparks quite a bit.. ---not recommended..
 
I've been meaning to rig up a 25W lamp with some alligator-clip leads to do just what you need to do. Simply haven't gotten around to it.

In the meantime, I got a 22 ohm 20W resistor here that gets it done.
 
notice I said that there was a proper way as well, I believe the formula for resistor values to discharge a cap is roughly: 50 ohms/V : so for a 100v cap 1 meg is overkill anyway; more like a 1/2 meg or 500K resistor across the terminals.
 
EchoWars said:
I've been meaning to rig up a 25W lamp with some alligator-clip leads to do just what you need to do. Simply haven't gotten around to it.

In the meantime, I got a 22 ohm 20W resistor here that gets it done.
Click to expand...

I agree, lower resistance value, higher power dissipation is the way to go.

With some of the caps they put in these outrageous amps these days, shorting them with a screwdriver tends to shorten the length of said screwdriver:D.

Anyhow, my resistor is soldered to a sturdy bit of wire, with an alligator clip on the end. I clip the alligator to ground, then go around to the un-grounded terminal of every cap I think might "bite" me.
 
There is also enough current to easily blow a 5 amp fuse..Don't ask me how I know..:D One of those oops moments..
 
I use the lamp method as echowars stated. I have a $2 accent lamp that uses a type b bulb, cut of the plug and put clips or an old set of test leads. Good to go.
 
qboneus said:
I was under the impression that a higher value of resistance (for voltage holdoff ) is more desirable and from a search of this good article about cap discharge/testing would appear to agree with my earlier formula posted except that i am at the high range.

http://www.repairfaq.org/REPAIR/F_captest.html#CAPTEST_008

5 to 50 ohm/V
Click to expand...
Fine if you want to wait about 4 or 5 hours. I'd just as soon be done with it in about, oh, 3 seconds. (Put a 1M resistor across a fully charged 22,000µf 80V cap and monitor the voltage. You'll grow old waiting for it to discharge). The link is talking about discharging many KV's on a TV...lots of volts on relatively small caps.

Don't confuse discharging a HV TV or tube cap with a typical cap in SS gear (where you rarely see anything over 100V). Two different animals. What you use depends on what you're working on. OP was working on a SS Akai, so that is what I addressed.
 
EchoWars said:
I've been meaning to rig up a 25W lamp with some alligator-clip leads to do just what you need to do. Simply haven't gotten around to it.

In the meantime, I got a 22 ohm 20W resistor here that gets it done.
Click to expand...


Fantastic idea! And an easy project! Thank you for sharing!
 
Thanks everybody, for the advice! I used the resistor method tonight, and it seems to have worked.

According to my multimeter one of the power caps was reading around 64 v before I discharged it, after having been unplugged overnight. The others had an initial charge of 2 or 3 v or less before discharge. Does this suggest anything about where my problem might lie? I pulled all the output transistors on the unit (an Akai AA-1200) and they all check out okay with the meter.

thanks!

Roy
 
EchoWars said:
I've been meaning to rig up a 25W lamp with some alligator-clip leads to do just what you need to do. Simply haven't gotten around to it.

In the meantime, I got a 22 ohm 20W resistor here that gets it done.
Click to expand...

I use alligator clips connected to the plug of a 100W lamp I had handy. Worked for me just fine.
 
bricktop said:
I prefer the tongue method, but people always say I ain't right. :no:
Click to expand...

Electricity tastes like burning

tastes_like_burning.gif
 
akforum said:
I use alligator clips connected to the plug of a 100W lamp I had handy. Worked for me just fine.
Click to expand...
I prefer to suck the life out of the caps a little slower. If I could find a 15W lamp, I'd use it instead. Even this small value of bulb would discharge a fully charged 22,000µf 80V cap in 3 or 4 seconds.
 
haha, I think this idea is good. Personally, I tried using a small value resistor (I think I used a 100 ohm 0.5W) and end up burning my finger. Large value just wait too long. in general, if resistor is the only way, I would use a 3.3K ohm resistor and wait about 3-10 mins. Obviously, we can always start with something like 3.3k and then 10 seconds later switch to something like 1k and then another 10 second switch to 500 ohm and so on to avoid burning your finger and still do it quickly. :tears:


EchoWars said:
I prefer to suck the life out of the caps a little slower. If I could find a 15W lamp, I'd use it instead. Even this small value of bulb would discharge a fully charged 22,000µf 80V cap in 3 or 4 seconds.
Click to expand...
 
You must log in or register to reply here.
Back
Top Bottom