Amplifier power supply capacitor removal advice needed!

[Chaggy78]

Hello all,

I have a hifi power amplifier that has 4x 6800uf 50V capacitors in the power supply and one of the caps is bulging so i am intending to replace all 4 of them. My question is do i need to discharge them before removal or can i just go ahead and de-solder them without any worry? Obviously with the amplifier disconnected from the mains!
Any help or advice would be greatly appreciated!
Many thanks

 

[tedstruk]

"discharging caps can be a real chore!"
"got any good enemies?"
generally regular power caps discharge overnight by themselves when the board they are on is disconnected from the power source. You should only replace the one blown cap, because anything you do, might ruin your amp. use a low power, soldering gun, and don't try any electrical insta-melt systems on electronics.
replace it with the exact size, voltage, and farad.
YEAH, you might want to discharge them, if you can figure out a safe way.... ever been shocked by one? I always discharge mine ever since...., I put a ground on one terminal, then touch the ground to the other. Hasn't failed me yet....

 

[(*steve*)]

Yes, it is probably a good idea to replace all 4. Chances are that if one is affected the others are almost at the same stage.

Read the voltage across them (carefully) with a voltmeter. If the voltage has fallen (say) to below 12V then there's little risk from them.

If leaving the device turned off overnight hasn't discharged them, it's better to discharge them through a resistor than to short them out. A resistor of around 1k will discharge them in under a minute.

When you replace them, make sure that:

1) you get replacements of the same or higher voltage rating
2) that they physically fit
3) That the capacitance is similar (+/- 20% isn't going to make any real difference)
4) that you wire them back the right way around
5) that if they're 105C rated capacitors that you replace them with 105C rated capacitors.

 

[Chaggy78]

thanks for the replies, very good advice thanks.
How long would it take for them to discharge if the left the amp turned off? are we talking days or weeks? how would i attach the resistor to the capacitor?
should i replace any other components while i'm at it or just the capacitors?
Many thanks

 

[(*steve*)]

With the amp turned off and unplugged, it may only take a few minutes, or it may take a lot longer. That's why it's always important to measure.

At relatively low voltages like this, leads with insulated alligator clips on each end clipped to the resistor. Then clip one free end to one side of a capacitor and then getting the other free end and clipping it to the other end to one side of the capacitor.

At this stage I wouldn't replace anything else.

Was the amplifier still working?

 

[Chaggy78]

Yes the amplifier is still working even with the one bulging capacitor but i would like to replace it soon before it starts leaking. I will leave it turned off for a couple of days or more before i measure them with a multimeter.
Thanks for all the good advice, much appreciated.
Would it be ok to change the 4x 6800uf 50v caps for 4x 10,000uf 50v?
thanks

 

[Chaggy78]

I have found some cheap capacitors that are the same physical size as the ones i need to replace but the capacitance is higher also is the voltage, they are 10,000uf 63V.
Would these be ok to use?
many thanks

 

[KrisBlueNZ]

They should be OK. They will put a slightly higher load on the rectifier diodes at the moment you turn on the power, but it's unlikely to cause any problem.

 

[Chaggy78]

Ok thanks, so it would probably be safer to use the same UF and voltage caps as the originals? Would there be any improvement in sound by using larger UF caps?
cheers

 

[KrisBlueNZ]

Voltage rating isn't an issue as long as the replacements are equal or higher.
Using higher capacitance replacements will cause that side effect but really I would not worry about it.
Using higher capacitance will provide more sustained energy for peaks but I doubt the difference would be noticeable.

 

[Chaggy78]

So do you recommend that I stick with the same 6800uf 50V or use 10,000uf 50v?
or is that too much of a capacitance difference?
I really don't want to damage the amp at all.
Many thanks

 

[KrisBlueNZ]

I recommend you use the 10,000 uF 63V capacitors you found, if they will fit.

 

[Chaggy78]

Ok thanks for the reply, I can only get Jamicon 10,000uf 50v in my price range will these be ok?

 

[KrisBlueNZ]

Yes...

 

[Chaggy78]

Hi all just a quick update, I have replaced the capacitors with the 10.000uf 50v and the amp powers up fine but there seems to be a strange static/hissing noise that comes in randow waves through the speakers, i have no idea what could be causing this?
Sometimes there is no noise and then after a while it seems to come back then goes away?

 

[KrisBlueNZ]

Nothing comes to mind from your brief description.

Can you tell us the brand and model number of the amplifier?

Do you have a link to its circuit diagram?

Does this noise happen even with the inputs disconnected?

Does the position of the volume control make any difference?

Does the amplifier have tone controls and/or a balance control? If so, does changing them make any difference?

Does the sound come through both speakers identically?

Can you record the sound using a microphone and post an MP3 or WAV file here or on a file sharing site?

 

[Chaggy78]

Thanks for your reply!
I have a feeling it's the two output volume poteniometers on the front of the amp so that you can adjust the output of each channel 1&2.
I tried cleaning the pots but it did not help, at some positions while turning the pots it makes a pop noise and some hissing and in other positions no output at all.
The are 20k Noble pots.
The make of the amp is HH electronics VX-200, they are really old amps from the late 80's made here in UK.
I ay try to replace these pots but would there be a way to bypass them completely so the amp would just be at full volume all of the time?
I don't have a circuit schematic available sorry.

 

[Chaggy78]

Link to pdf about the amp here: www.hhamplification.co.uk/dbfiles/techpdfs/HHBroadcast.pdf

 

[KrisBlueNZ]

Thanks for the PDF. It's really helpful to have some idea of what we're talking about; there are so many different audio amplifiers around.

If the noises happen as you turn the volume pots, and come from the speaker that's associated with the pot you're turning, it's probably bad spots on the carbon tracks in the pots. These happen when the amp is left with the pots in one position for a long time. You can prevent them by turning the pots to minimum when you finish using the amp.

Yes you can bypass them. Turn them to maximum and link the wiper (middle pin) to the end pin that corresponds to maximum volume. If you view the pot from the front (looking at the knob or into the shaft), with the terminals downwards, it will be the terminal on the right, which connects to the end of the carbon track that the wiper will be at when the pot is fully clockwise (maximum volume).

I guess it's possible that the pots have failed in a way that will still cause problems when you do this, but most likely it will stop all the hissing and bumping.

 

[duke37]

Could the problem be another faulty capacitor?