Replace 25 Year Old Metalized Polycarbonate Film Cap?

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PreCD

PreCD

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Joined 2011
I have a older preamp in which I replaced all the electrolytics in. I have two 4.7uf 100v MKCs that are used as bypass for my power capacitors, 6800uf 50v.

Is it necessary to replace those caps? The replacement for those are a MKP and I have been looking at those from Wima.
 
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davidsrsb

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Joined 2005
Polycarbonates can degrade with age - sometimes the metal film corrodes away and mains types can lose value as spikes burn holes in the film. I have seen them go leaky in high impedance circuits where moisture has got in. All of these faults can be tested with a capacitance meter and checking dc leakage
 
PreCD

PreCD

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I have decided to replace the old MKC caps in the preamp with the recommended MKP. The old MKCs are WIMA brand. WIMA recommends a MKP4 as the replacment. The problem is that they do not have the cap with 30mm lead spacing. Any suggestions for a replacement 4.7uf 100v bypass cap for preamp power supply?
 
Elvee

Elvee

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PreCD said:
I have decided to replace the old MKC caps in the preamp with the recommended MKP. The old MKCs are WIMA brand. WIMA recommends a MKP4 as the replacment. The problem is that they do not have the cap with 30mm lead spacing. Any suggestions for a replacement 4.7uf 100v bypass cap for preamp power supply?
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That makes me think of people "restoring" ancient furniture by replacing the precious walnut veneer by formica, just in case it might be damaged but not visibly.
 
PreCD

PreCD

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Elvee said:
That makes me think of people "restoring" ancient furniture by replacing the precious walnut veneer by formica, just in case it might be damaged but not visibly.
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I favor Walnut! Thanks for the input. Please give a suggestion as to what would be a good bypass capacitor in a preamp power stage. 6800uf 50v power caps currently bypassed with WIMA MKC 4.7uf 100v cap.

From yours and Andrews suggestion that the caps are timeless if proven not to be out of spec I may just leave them alone. I am trying to improve the sound quality of my system and this is one of the easier items to replace to see if it improves. I have a nice audible "hiss" when the preamp is switched on.Get a foot away and you can't hear it but I am concerned that it is a symptom that could be displaying itself at other times of the music. Could be many things but without proper test equipment replacing caps is a good option for me.
 
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AndrewT

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Joined 2004
Listen to and measure the hiss when the input of the amplifier is shorted.
Now substitute a 1k0 resistor for that short and listen and measure hiss again.

Now go though your various sources connecting one at a time and one channel at a time and listen and measure the hiss.

Are any of your source/s noisier than the 1k0 resistor?
Are any as quiet as your 1k0 resistor?

Choose the quietest source and now listen and measure the hiss with both channels connected.

Any difference?
 
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PreCD

PreCD

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AndrewT said:
listen to and measure the his when the input of the amplifier is shorted.
Now subtitute an 1k0 for that short and listen and measure hiss again.

Now go though your various sources connecting one at a time and one channel at a time and listen and measure the hiss.

Are any of your source noisier than the 1k0 resistor?
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Andrew:

Thank you for the trouble shooting procedures. Headed out to breakfast but will complete this when I get back in.....
 
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AndrewT

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Joined 2004
It should be just as quiet (measured).
When you exceed ~10k then you will measure and may hear more noise.
By the time you get to 100k you should clearly hear more noise.

But, I am suggesting 1k0 as a starter just to check that the power amp stays quiet when connected to a typical Rs.
 
PreCD

PreCD

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davidsrsb said:
So long as the old Wima capacitors are not damaged and out of spec, they will not be causing the hiss. The best way to spoil "box" capacitors is to fit the wrong size for the pcb and bend the leads too close the case, cracking the seals
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Thanks for the input. Yes I agree about trying to bend the leads on the box caps.

Still trying to find the time to experiment with the input leads as Andrew suggested.
 
PreCD

PreCD

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Andrew:

It has been awhile since we spoke about this issue and you were very kind to give me advice about using shorting plugs. I found the time to craft a pair, plugged into the inputs of my power amp, and all was quiet using a 1K resistor.

I did as you suggested and started down the line of my Adcom GFP-565 preamp inputs with the shorter plugs. None of the inputs I shorted made a difference in the hiss. As far as I could tell the level of hiss stayed consistent with the ear, not measured.

Again the hiss is not audible from a distance of a foot, maybe two if the house is dead quiet. That will give you an idea of the amount I am hearing.


AndrewT said:
It should be just as quiet (measured).
When you exceed ~10k then you will measure and may hear more noise.
By the time you get to 100k you should clearly hear more noise.

But, I am suggesting 1k0 as a starter just to check that the power amp stays quiet when connected to a typical Rs.
Click to expand...
 
PreCD

PreCD

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Joined 2011
I will make this last comment and leave this alone since I posted it in the parts section. I believe the preamp is just a bit noisy. I have it connected to an Adcom amplifier and these should pair well together.

Thanks Andrew for the shorter plug lesson. Slowly but surely my knowledge is building thanks to people like you.



AndrewT said:
That leads me to make two tentative conclusions.
1.) the system gain may be too high for the music Source you have.
2.) the pre-amp is a bit too noisy for the following equipment.
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