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newbie question about psu smoothing capacitors discharging
newbie question about psu smoothing capacitors discharging
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Old 3rd January 2018, 03:40 AM   #1
joaoveludo is offline joaoveludo  Portugal
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Default newbie question about psu smoothing capacitors discharging

Hi.

Is normal for 2x10000uf capacitors to be completely discharged right after i turned off and opened the cover an yamaha AX-400 SS amp? I measured right at the caps pins on downside of the pcb.

The caps are new, the amp was recapped. I see no bleeding resistors anywhere on the schematic or the pcb.

I thought they would hold charge for longer but probably they are discharging to the circuit. I don't think both caps were sold damaged to me, but i could be wrong. Just want to get that out of my head.

thank you for the patience.

Last edited by joaoveludo; 3rd January 2018 at 03:43 AM.
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Old 3rd January 2018, 05:42 AM   #2
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by joaoveludo View Post
I thought they would hold charge for longer but probably they are discharging to the circuit.

Your amp have a bias idle current at the most stages.
This current discharges caps in a couple or tens of seconds.
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Old 3rd January 2018, 06:09 AM   #3
PRR is offline PRR  United States
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How fast can you open the cover?

I can, some gear, play soft, turn-off, un-screw, and lift cover and the amp is still playing on stored charge (no speaker relay). I've practiced. I need to be careful.

If you are very curious, and careful, use clip-leads to connect meter to caps, then keep your hands out. Power-up, watch voltage rise; power-down, watch voltage fall. Depending on the amp type and details, it may take from 10 seconds to 10 minutes for voltage to fall to half. If it falls fast and the cover takes a while, you may be way-way low by the time you get in there.

I assume the amp works OK. A short in the amp could discharge caps real fast. It would probably also make the amp run hot.

A case you may never see: BIG electronics, such as radio/RADAR transmitters, sometimes have a relay which closes on power-down and applies heavy bleeders, so a frantic technician can work inside quickly. But such systems also usually have a Short Stick wired to the door, so the tech can hook the main power and be SURE it is grounded. (Belt and suspenders.)
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Old 3rd January 2018, 06:34 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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My amp will play at low volume for about 30 seconds after power is disconnected.

Depends on volume and bias current.
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Old 3rd January 2018, 09:25 AM   #5
AndrewT is offline AndrewT  Scotland
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Many amplifiers discharge their PSU are different rates as the PSU voltage falls. Often you will find that one polarity is at virtual zero volts and the other polarity still has many volts remaining.

This inequality of discharge is generally not a safety issue to the Builder, the voltages are too low by the time you can get your fingers/probes in there.

Reconnecting a partially discharged PSU to an amplifier can cause sparks and near instantaneous voltage rise and near infinite current transients that can damage components.
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Old 3rd January 2018, 10:08 AM   #6
joaoveludo is offline joaoveludo  Portugal
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Amp works and sounds good. Normal bias and temperatures, very cool actually.

I guess it is normal then. Thanks!
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