How can I understand if it is time to replace capacitors?

[beppe61]

Dear Sirs,

I have bought a 30 years old integrated amp.
How can I assess if the capacitors (originals) must be replaced?
Aestetichally they appear in good shape.
There is no form of leakage or other.
They are SPRAGUE 36DX and rated 7200 uF /50 VDC.
There is also a serial number 7552.C.

Thank you so much.

Kind regards,

beppe61

[wintermute]

One of these can help ---> http://www.dse.com.au/cgi-bin/dse.st...uct/View/K7214

basically if the ESR is too high then the cap probably needs replading... this is something I need to add to my test gear.

Tony.

[RetroAudio]

I think the question is: do you WANT to replace them?

[beppe61]

Originally posted by wintermute
One of these can help ---> http://www.dse.com.au/cgi-bin/dse.st...uct/View/K7214
basically if the ESR is too high then the cap probably needs replading... this is something I need to add to my test gear.
Tony.

Dear Mr. Tony,

thank you very much for your kind and extremely valuable reply.
If I understand well your point is that ESR tends to get higher with the time (plaese excuse my naive english).
So the charging and discharging speed slows down.
Have I understood well ?
Thank you again very much.

Kind regards,

beppe61

[beppe61]

Originally posted by RetroAudio
I think the question is: do you WANT to replace them?

Dear Sir,

to be honest, yes.
Please let me explain a little and then kindly ask you a question.
I have noticed that all the power amps with great dynamic properties have very big capacitors.
On average I can find 20.000 uF /channel of capacitance in the power supply.
In my 70+70 W stereo amp there are only two of this very good Sprague 7200 uF caps.
It seems too little to me indeed.
So yes, I would like to increase the capacitance at least to, let's say, 15.000 uF x 2 , doubling the total capacitance in the pS.

Then let me to ask you a question.

I have still a big doubt about power supply.
With short term impulsive signals which are more important: the caps or the transformer?
A friend of mine told me that only caps deal with short term impulsive signals so the dynamic capability of the amp are related to the electricale properties of the caps (ESR, capacitance).
I really do not know if this is true or not.
But if it is true, then fitting very good big caps should improve things in an evident manner.

Thank you so much for your kind reply.

Kind regards,

beppe61

[RetroAudio]

well beppe61, this is partially true and thus part of the story. Good caps are always welcome in a power supply no matter what, but one needs to remember that the transformer isn't exactly invisible here. It does refresh the caps at a rate up to 120 times/sec and is therefore in parallel with the caps supplying current to the circuit's demands. Although signal peaks or dynamics can be quicker than that rate, they can occur at the same instant that refreshes take place. Also something else to think about is that bigger is not always better, a notion not too popular with some around here to be sure. Perhaps doing some light reading on the subject to form your own ideas would be a positive step.

[I_Forgot]

Originally posted by beppe61
I have bought a 30 years old integrated amp.
How can I assess if the capacitors (originals) must be replaced?
Aestetichally they appear in good shape.
There is no form of leakage or other.
They are SPRAGUE 36DX and rated 7200 uF /50 VDC.

There are several things to consider:

How old is the equipment?
30 years is old enough to warrant wholesale replacement of all
electrolytic caps. Electrolytics are the least reliable electronic
components.

How much will it cost?
This size cap is dirt cheap. In fact, put in a bit more capacitance
while you're at it. Modern electrolytic caps are about 1/2 the size
of 30 year old caps, so even if you get 2X the capacitance, it will probably fit in the available space. All this assumes you will do the work yourself. If you have to pay someone to do it, it probably isn't worth it unless the equipment is something really special.

How easy are they to get?
You can get them from any electronics distrbutor or surplus outlet.

All things considered, I'd say replace them.

I_F

[wintermute]

Hi beppe61,

Yes the esr gets higher as the cap gets older.... I'm not sure whether your description of why this is a problem is correct or not I just know it isn't desirable

here is another link, which tells the different ways you can use an esr meter to tell if a cap is bad. This one is a bit closer to home too!

http://www.qsl.net/iz7ath/web/02_brew/15_lab/06_esr/

Tony.

[anatech]

One thing you can try. Hang an oscilloscope on the capacitors and look at the waveform. If the ESR is getting high, you will see "pips" on the leading edge of the waveform. This gets more pronounced as the ESR and inductance goes up. So will any music waveforms. Try it.

-Chris

[beppe61]

Originally posted by RetroAudio
well beppe61, this is partially true and thus part of the story. Good caps are always welcome in a power supply no matter what, but one needs to remember that the transformer isn't exactly invisible here. It does refresh the caps at a rate up to 120 times/sec and is therefore in parallel with the caps supplying current to the circuit's demands. Although signal peaks or dynamics can be quicker than that rate, they can occur at the same instant that refreshes take place. Also something else to think about is that bigger is not always better, a notion not too popular with some around here to be sure. Perhaps doing some light reading on the subject to form your own ideas would be a positive step.

Dear Sir,

thank you sincerely for your extremely kind and above all valuable reply.
My idea of increasing capacitance in the PS is driven by some considerations.
Some very well regarded power amps sport really huge capacitance in their PS.
The Krell KSA 50, if I am not wrong, has 4 x 40.000 uF/each caps.
Odyssey Audio offers a caps upgrade in their amps.
This must mean something.
I understand that caps is not the full story.
There is also the trasformer that acts.
My problem is:
if I want to upgrade the PS do I have to start with caps or with transformer?

Thank you very much again for your kind and valuable suggestion.

Kind regards,

beppe61
ITALY