Discharging a capacitor - should I??

[cederholm]

Hi all,

I'm new here and new to messing around with a tube amp and I'd love some advice. I own a Create Vintage Club 30 guitar amp that cuts in and out once warned up. I changed the power tubes over a year ago thinking that was the issue, but it wasn't. In my frustration I stuffed the thing on a shelf (unplugged).

No one to give up, I've recently decided to look into it again and I've found multiple posting on the web about this amp and this problem. Many say to check the solder joints as they're know to come loose and ack up.

I feel very comfortable resoldering any issues, but capacitors scare the heck out of me. Should I attempt to discharge them and have a look around or just take the amp to a pro?

Thanks for any and all advice.
Carl

[nigelwright7557]

Capacitors need to be discharged through a resistor or you will start welding things !

Be careful because the voltages on them can kill you.

[astouffer]

Most amps have bleeder resistors built in so after a few minutes the voltage should be almost nothing. If your amp has been sitting on the shelf its most certainly discharged by now.

Lastly you can always measure the voltage with a meter to be sure.

[Michael Bean]

Cederholm,
First, make sure unit is unplugged and disconnected from everything else. If you have a volt meter, or can borrow one if necessary, I recommend checking for voltage on caps, BUT BE CAREFUL! Set voltage range to highest range (if it's not an auto rage meter), connect one lead (usually negative) to chassis and/or ground, put one hand (left or right) in your pocket, and use other hand to probe cap terminals. By following this proceedure, you won't electrocute yourself. If caps are dis-charged, you wil have no or low voltage readings, then you can poke around and solder connections.
Good luck,

Mike

[Globulator]

Originally Posted by astouffer

Most amps have bleeder resistors built in so after a few minutes the voltage should be almost nothing. If your amp has been sitting on the shelf its most certainly discharged by now.

Lastly you can always measure the voltage with a meter to be sure.

FIRSTLY measure with a meter to be sure. I have an old Pioneer SX-838 that holds about 80V for _weeks_ - and that's transistor!!!

[CanAm Man]

Originally Posted by Michael Bean

put one hand (left or right) in your pocket, and use other hand to probe cap terminals.

Probably doesn't need clarification, but here goes:

DO NOT use your other hand to probe the cap terminals. Use this free hand (i.e., the one not in your pocket) to carefully hold the voltmeter probe, which you will touch to the cap terminals looking for residual charge.

Why place a hand in a pocket? To hopefully avoid an "arm to arm" discharge path should you touch a charged lead--the arm-to-arm path would cross the chest and heart, which is very unhealthy to do!

[Miles Prower]

Originally Posted by cederholm

I feel very comfortable resoldering any issues, but capacitors scare the heck out of me. Should I attempt to discharge them and have a look around or just take the amp to a pro?

Before doing anything, it's essential to use your DMM to see if there's any residual voltage across the filter capacitors. Most hollow state equipment can retain enough of a charge to give nasty shocks. For solid state, residual charge can poof sensitive transistors and ICs, such as any MOSFET, or anything based on CMOS.

It's a good idea to prepare a soft discharger from a 500R / 10W resistor fitted with alligator clips. Clip that across any hot capacitors to bleed down the voltage before shorting across the terminals. It's also a good idea to keep that short in place while working since some types like electrolytics can "recharge" themselves if left open circuited (though this is more of a problem with solid state and its static charge sensitivity). Just remember to remove that shorting link before powering up.

[grenadier]

Originally Posted by Miles Prower

Before doing anything, it's essential to use your DMM to see if there's any residual voltage across the filter capacitors. Most hollow state equipment can retain enough of a charge to give nasty shocks. For solid state, residual charge can poof sensitive transistors and ICs, such as any MOSFET, or anything based on CMOS.

Heh, that made it over here from 4HV.

[cederholm]

WOW - very helpful guys - Thank you!!

And yes, the amp has been on the shelf and unplugged for at leaste a year, maybe even two. ...but I will still follow all your advice.

~ Carl

[Printer2]

Want to drain your capacitors without those messy resistors and test clips? Crank your amp playing something loud and hit the off switch. You will hear the sound fad off pretty quick. That is your capacitors discharging. The heaters will be glowing long enough to use up the power in the capacitors. Much easier with amps that have a standby switch but in the end the same thing happens. Empty capacitors. I am assuming you will head the others and check the big ones though.