Time for capacitors to discharge?

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four_corners
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Time for capacitors to discharge?

Post by four_corners » Fri Oct 30, 2020 1:19 pm

Hey everyone,

I've got a Juno-106 from Japan that has a 100v transformer, and instead of using a step-down transformer, I'm thinking of looking for an old 110/120v North American transformer to replace it.

I've done lots and lots of low voltage synth building (tons of eurorack modules and stand alone synths), but not a lot of mains voltage work. I know no one wants to be liable, but is there a rough amount of time it takes for any stored voltage in something like an (unplugged of course) 80's Roland synth to discharge to less dangerous levels? Basically, are we talking 5 minutes, or 5 hours?

Thanks!

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KSS
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Re: Time for capacitors to discharge?

Post by KSS » Fri Oct 30, 2020 1:39 pm

Minutes, or much less. Often that's a requirement of UL or equivalent safety testing and qualifications.

Having said that, you never trust it. If you're inside a synth, it's almost always because something's not working right.

Keep one hand in your pocket is the oldtimer's rule, and it still holds even though the voltages of synths are much less than the tubes of old.

Best advice is to learn to identify the areas of potential concern <--See what I did there ;)
And then also how to safely discharge ANY capacitor that can hold a dangerous charge. Don't just short with a plain wire or screwdriver! Use a properly sized resistor.

That way you're not relying on some timeframe for assumed safety.
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Most of the Rolands were set up for different PSU 'entrance' PCBs to account for the various mains worldwide. Be sure to look at the Service Notes for your Juno-106. There might be an easy and already planned way to do what you want.

There are also quite a few new PSU upgrades for many classic synths which can do a *much* better job than the originals.

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mrand
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Re: Time for capacitors to discharge?

Post by mrand » Fri Oct 30, 2020 3:09 pm

My understanding is that if they synth is working, then when you power it off, the analogs should continue to draw stored power from the power filtering caps till they are drained, which will take seconds or less.

You can also pull out your multimeter and probe the power rails to see if a voltage remains after you've unplugged it.
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four_corners
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Re: Time for capacitors to discharge?

Post by four_corners » Fri Oct 30, 2020 3:23 pm

mrand wrote:
Fri Oct 30, 2020 3:09 pm
My understanding is that if they synth is working, then when you power it off, the analogs should continue to draw stored power from the power filtering caps till they are drained, which will take seconds or less.

You can also pull out your multimeter and probe the power rails to see if a voltage remains after you've unplugged it.
Good point, I'll try that out, and if it seems like they are holding onto the voltage for a significant amount of time, I can maybe make a little voltage bleed resistor or something.

If I probe where the power supply connects to the main pcb, should that give me an idea of remaining voltage, or will I need to literally probe the capacitor?

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MikeDB
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Re: Time for capacitors to discharge?

Post by MikeDB » Fri Oct 30, 2020 7:17 pm

KSS wrote:
Fri Oct 30, 2020 1:39 pm
Minutes, or much less. Often that's a requirement of UL or equivalent safety testing and qualifications.
Yes it is now. But in 1984 all the regulations we're used to now were still evoling, which of course led later on to the proliferation of wall warts to avoid having to get equipment certified. So as you rightly pointed out, don't trust anything.

To the OP, are you sure there isn't a different voltage tap on the transformer ?
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four_corners
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Re: Time for capacitors to discharge?

Post by four_corners » Fri Oct 30, 2020 7:23 pm

MikeDB wrote:
Fri Oct 30, 2020 7:17 pm
KSS wrote:
Fri Oct 30, 2020 1:39 pm
Minutes, or much less. Often that's a requirement of UL or equivalent safety testing and qualifications.
Yes it is now. But in 1984 all the regulations we're used to now were still evoling, which of course led later on to the proliferation of wall warts to avoid having to get equipment certified. So as you rightly pointed out, don't trust anything.

To the OP, are you sure there isn't a different voltage tap on the transformer ?
It is being shipped to me at the moment so I'm not sure. My understanding though is that the 80s Roland synths meant for the Japanese market usually don't have the multi-tap transformer. That being said, I will definitely check inside before just blindly ordering a transformer.

If I do have to order one, do you think it would be wise to purchase one that came out of another 106, or try and find the specs and find a modern replacement?

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MikeDB
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Re: Time for capacitors to discharge?

Post by MikeDB » Fri Oct 30, 2020 8:11 pm

four_corners wrote:
Fri Oct 30, 2020 7:23 pm
If I do have to order one, do you think it would be wise to purchase one that came out of another 106, or try and find the specs and find a modern replacement?
Either has possible negative consequences. Another option is to totally remove the whole PSU and supply the required DC power lines (no idea what they are I'm afraid) with a pre-built and tested power supply module, either internal or external. Generally a modern switchmode PSU will be smaller, quieter and cooler so it probably can be fitted inside.
Personally I hate cats ! Charlie the jackdaw on my shoulder also hates cats as his mother was killed by one so I had to raise him until he set off to procreate his lineage.

four_corners
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Re: Time for capacitors to discharge?

Post by four_corners » Fri Oct 30, 2020 10:47 pm

MikeDB wrote:
Fri Oct 30, 2020 8:11 pm
four_corners wrote:
Fri Oct 30, 2020 7:23 pm
If I do have to order one, do you think it would be wise to purchase one that came out of another 106, or try and find the specs and find a modern replacement?
Either has possible negative consequences. Another option is to totally remove the whole PSU and supply the required DC power lines (no idea what they are I'm afraid) with a pre-built and tested power supply module, either internal or external. Generally a modern switchmode PSU will be smaller, quieter and cooler so it probably can be fitted inside.
Looks like -15v, +15v, 5v, and maybe 9v?
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KSS
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Re: Time for capacitors to discharge?

Post by KSS » Sat Oct 31, 2020 8:24 am

Looks like you're gonna have 117 tap since you've got the 100v version. According to that PCB *drawing*.
Is the PSU in the photo yours? Or a photo found online somewhere?

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MikeDB
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Re: Time for capacitors to discharge?

Post by MikeDB » Sat Oct 31, 2020 11:15 am

KSS wrote:
Sat Oct 31, 2020 8:24 am
Looks like you're gonna have 117 tap since you've got the 100v version. According to that PCB *drawing*.
Is the PSU in the photo yours? Or a photo found online somewhere?
I hope it isn't his since the wires to the mains transformer have been cut.
Personally I hate cats ! Charlie the jackdaw on my shoulder also hates cats as his mother was killed by one so I had to raise him until he set off to procreate his lineage.

four_corners
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Re: Time for capacitors to discharge?

Post by four_corners » Sat Oct 31, 2020 1:22 pm

Not mine luckily, just an image I found online.

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franziskano
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Re: Time for capacitors to discharge?

Post by franziskano » Sat Oct 31, 2020 1:24 pm

Here is a video on how to discharge the filter capacitors of a Guitar Tube Amplifier, the same technique should apply to any high voltage device:

.

You can always check if there is still voltage left with a multimeter.

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fragletrollet
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Re: Time for capacitors to discharge?

Post by fragletrollet » Sat Oct 31, 2020 1:45 pm

Big caps in tube designs can hold current easily 15-20 minutes and alot longer, as far as I understand depending on if there is a current drain in the system, some resistance that slowly (or quickly) drains the circuit after power is shut off. Wouldn't really know what to look for (big resistor to earth somewhere?), and Im guessing most transistor circuits would keep any energy left after shutdown in the reservoir caps in the powersupply, and that the circuit being powered itself is pretty safe to poke around in. But don't take my word for it, I'm an amateur!

Unplug, have a coffe, open unit and fasten your common on your meter to ground and poke the red probe where you can reach close to the caps in the psu with the other hand on your back. Probably nothing to read.... but now you know! Helps to have a few meters aswell :eek:

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KSS
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Re: Time for capacitors to discharge?

Post by KSS » Sat Oct 31, 2020 3:17 pm

It's worth saying that not every capacitor is between some voltage and 0V or GND. Voltage doubler and triplers used in some tube circuits are good examples.
Poking around with one lead clipped to GND may or may not be a good idea!
If you don't know what you're doing, get someone who does.

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wallyp
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Re: Time for capacitors to discharge?

Post by wallyp » Sat Oct 31, 2020 6:10 pm

Get a current limiter, isolation transformer, a variac, a 5mm rubber all in one and a chopstick.

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