Safe Way to Discharge 18,000uf Caps on B-2

Klipschornman

Yamaha V-Fets
Hello to all. I'm thinking about re-capping the power supply
board on a B-2. It has developed a hum. Need advice on easy
way to make sure the 18,000uf caps are discharged. Thanks in advance
and kudos to all V-fetters.:D
K-man
 
take a screwdriver and short-circuit both taps! (joking, mostly)

well, we do that as a demo in physics lab, not sure if it's safe for the amp or the caps. if you have a spare resistor laying around, use that instead. it'll only take a brief bridging of the posts to dissipate the charge. just make sure you're not touching the resistor with bare skin when you do it.
 
Last edited:
Well, that's the quick and dirty way - many say you should use a resistor to do this - say around 5K ohms or less - loads of amps have them included anyway, they draw just a tiny current - but enough to discharge the caps gently when the amp is turned off.

Short em out ? It's a bit hard on the caps and the screwdriver to do that IMO

And what if you forget to turn the amp off before discharging with that screwdriver :yikes:

Just my 3 ha' pence worth
 
Last edited:
take a screwdriver and short-circuit both taps!

well, we do that as a demo in physics lab, not sure if it's safe for the amp or the caps. if you have a spare resistor laying around, use that instead. it'll only take a brief bridging of the posts to dissipate the charge. just make sure you're not touching the resistor with bare skin when you do it.

Not recomended.Tends to leave weld marks on cap terminals and screwdrivers.
 
i edited my previous post - i should've been more clear that i was mostly joking about the screwdriver. even if it isn't that dangerous, i'm not enough of an expert to know.
 
Most PS electrolytics are 50-80V, so a 110V light bulb will discharge them safely in about 4 sec. providing a visual indication that the job is done.
 
Most PS electrolytics are 50-80V, so a 110V light bulb will discharge them safely in about 4 sec. providing a visual indication that the job is done.
I set up the light bulb procedure and hooked neg to ground and positive
test lead to every point across power supply board as well as each solder
post on said caps. Bulb did not light at all. The amp was set to DC instead
of normal on the back and has not been plugged in for weeks. Could the
charge already be drained off? Thanks to all suggestions. What a great
and helpful site.
K-man:scratch2:
 
i edited my previous post - i should've been more clear that i was mostly joking about the screwdriver. even if it isn't that dangerous, i'm not enough of an expert to know.
That's another thing I like about this site. The humor around here
is hard to beat. Thanks for making me chuckle, cdfac.
 
I vote for the screwdriver method. <big grin>

A light bulb is a good choice, because its resistance is very low unless it is lit up.

Once you think you are done, simply measure with a volt meter. If you read a couple volts or less, all is well.
 
The safe way

The technique I recommend is to use a high wattage resistor. I use a 1000watt 10 ohm ceramic resistor with insulated clip leads attached. This isn't critical - a bit more or less will be fine but will affect the time it takes to fully discharge the capacitor. The use of a current limiting resistor will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).
 
The B-2 discharges its caps just fine without any external help. Wait 1/2 an hour after power-down and you're ready to go.
 
The B-2 discharges its caps just fine without any external help. Wait 1/2 an hour after power-down and you're ready to go.
Thanks EW. No wonder it didn't show any voltage. Thanks
to everyone who responded as well. :thmbsp:
Gonna get this baby going again.
K-man
 
I think we have discussed this a while back, but there are a couple of 2SC458 transistors on the regulator board (on the +/-85V supplies) that need to go away.
 
I set up the light bulb procedure and hooked neg to ground and positive
test lead to every point across power supply board as well as each solder
post on said caps. Bulb did not light at all. The amp was set to DC instead
of normal on the back and has not been plugged in for weeks. Could the
charge already be drained off? Thanks to all suggestions. What a great
and helpful site.
K-man:scratch2:

FYI, Yamaha amps' "DC" switch is just a switch for some coupling caps that filter low (dc level) freqs from entering the input terminals.
 
The technique I recommend is to use a high wattage resistor. I use a 1000watt 10 ohm ceramic resistor with insulated clip leads attached. This isn't critical - a bit more or less will be fine but will affect the time it takes to fully discharge the capacitor.

I know this has been answered (aka OP dont' need to in this case) but I do the same - see pic below of the rig I use. Since I do tube work where, yeah, you better be sure and no, you don't want to spark the screwdriver with 400-600V...

Just a fat power resistor soldered to clip leads with a big heavy shrink over the top of the whole thing. Can't recall exact ohmmage but mine is bigger so the discharge is slow... I usually leave it clipped on for a while (hey I'm patient... sometimes...) and if it's a big deal (like tube HV levels) I measure to confirm before touching stuff.

With ancient tube gear caps have been known to develop 'memory' and re-gain potential after being drained so I often leave it attached while working especially on a long-term project. One reason for keeping it a big value is in case you stoopitly power up with it still attached, hopefully you won't blow anything up but just drain off a bit of current aka it's more like an OEM cap-draining resistor.

capdrainer.jpg
 
I think we have discussed this a while back, but there are a couple of 2SC458 transistors on the regulator board (on the +/-85V supplies) that need to go away.
Thanks EW and to all that took time to share their advice.
Replaced all the "Leaksistors" as EW called them and did the recap
and replace the relays. A person could learn a lot from this place.
Got her going and she sounds great again. Moving on to my next
B-2.
K-man:D
 
Back
Top Bottom