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Dear fellow enthusiasts,

 

I have a pair fo Tannoy 638 D80 floor standing hi fi speakes. I am intending to replace the capacitors but I only have limited informaiton about values. Tannoy only have limited wiring diagrams for this unit.

The two grey capacitor have EVOX MMK -63 on the side and 15 uf k J8 underneath, so they are 15 uf each. I assume the 63 relates to voltage but I don't know if they are Ac or DC rated. Can I find this out by testing with a multi meter? I have a basic analogue multi meter. I'm not sure if I might be able to test the crossover circuit and find out if it is Ac or DC, maybe there is a way of working that out?

The Blue capacitor has a long serial number 7100 0114 and the value as 2.7 uF but I can't see any voltage information or Ac or DC. Is it possible to find out the voltage rating form the serial number or by testing the circuit? I have attached some pictures of the crossover arrangement. The components are glued in to the bottom not on a PCB. I hope somebody experienced on this board may be able to help me. Thank you.

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I do not recognize the blue cap.  All of these are good quality film types.  I would not replace them.

63 is the voltage rating,  Caps are rated DC, AC rating is same or higher.  Rating is testable only by destructive test.  

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Thanks GlennW,

These speakers are at least 10 years old, I am beginning to think that replacing the crossover units  would be easier than trying to find out the exact ratings for replacing the capacitors. I was advised by a speaker renovation specialist that capacitors dry out over time and that replacing them would dramatically improve the separation and response.

I replaced the bass drivers because one got damaged I have new Monacor drivers in now and everythign is ok. I'm going to look into replacing the cross overs not just the capacitors. Would you say that capacitors do need replacing after ten years or so? Thanks again for your help.

Tim

 

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Do capacitors need periodic replacement?  Depends on the type, and usage.  Film capacitors such as are used in your crossovers have extremely long life.  Unless operated beyond their rating, they should be fine practically forever. 

Electrolytic capacitors use a liquid between the foil.  This type has more limited life.  This life is also dependent on part quality, with well made parts lasting well past ten years, others perhaps not. 

I have seen failures of old electrolytic capacitors, but never film types.

If the parts were stressed by operation beyond rating their life may be shortened.  The 63 V rating of your capacitors is approximately the maximum voltage output of an amplifier rated over 100 W / channel.  This is a reasonable rating for capacitors used in a speaker network.  It is not likely they have been stressed.

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Dear Glen,

Thank you for your comments. I will not make any changes to my cross overs after all, taking into account your opinion. Incidentally I tried another amplifier which was given to me recently, an SAE (Scientific Audio Electronics A202, I think they are an american company) the amp is a good quality one from the mid eighties, it has given me a noticeable improvement in quality and now I am satisfied! There was one last thing....... there's a noticeable hum when the amp is on and at rest. I thought it might need a dedicated earth. I was thinking I might earth the case since the power cable is only two core.

I very much appreciate your comments. Please have a good day.

Best regards from London.

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Congratulations on finding a vintage amplifier that you like.  I did a quick Google search - SAE A202 looks very nice. 

A hum - is this hum that is heard through the speakers?  Or hum that comes from the chassis of the amplifier?

If hum is from chassis of the amplifier, probably not much can be done.  Sometimes transformers have a mechanical vibration that is excited by the magnetic forces of the power conversion of the transformer.  Replacing the transformer is a large expense and suitable replacement may be impossible to find.  Best solution is having the amplifier behind a door so it cannot be heard.  I have two Dynaco Stereo 150 amplifiers, one is dead quiet, the other has this issue. 

Hum heard through the speakers is one of the most difficult problems to solve.  It can come from many causes - design of the grounding scheme, routing of wires, filter capacitors getting old and tired. 

Converting to dedicated earth AC mains wiring (3 wire AC wiring with ground connection) is primarily a safety consideration.  It forces a fuse failure or home wiring circuit breaker trip if there would ever be a short circuit between the AC wiring and the case of the amplifier.  With 2 wire connection it is possible that a short circuit between AC mains and chassis can leave the chassis at dangerous AC voltage without causing any fuse or breaker trip - obviously a bad thing.

It is not likely that dedicated earth will solve a hum.  It may even cause one since it can create a ground loop.  It is a safer wiring scheme.  Be very careful if you modify the amplifier, this conversion is wiring used at dangerous voltage levels.

 

 

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  • 3 weeks later...

Tim,I too have need to replace something in the sender unit,it is not the speakers themselves,I swapped over to confirm,it sounds capacitor linked,probably a dry joint somewhere,I bought them in 94,my problem is ,I cant get the bottom off to access the the electronics,can you tell me how you managed this.

 

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