Mains filter cap - Bush DAC90A

[Donald_S]

I've been looking for an X2-class mains filter cap for the Bush DAC90A. The service data for the radio indicates that it is a 0.1uF 500V cap. However, I have not been able to find any 500V rated X2-class caps. All the ones I have found have been rated 250 or 275 VAC. How critical is it to stick to the original specification for this cap? can I get by with the lower voltage rated caps?

Don

P.S. I found a couple of X2-class caps in a defunct computer power supply with the following markings: 0.22K 275V and 0.33K 275 volts. I am presuming that the 0.22K cap is a 0.22uF and the other is a 0.33uF cap; am I correct? Can these be used instead of the 0.1uF cap in the radio?

[ppppenguin]

Originally Posted by Donald_S

I've been looking for an X2-class mains filter cap.....

By definition an X2 rated cap is up to the job. You don't need to look at the voltage rating. Before X2 caps we tended to use caps rated for at least 600VDC and often 1000VDC.

[Donald_S]

That was an awfully quick response! you replied before I posted my edited message. What do you think about the 0.22 or 0.33K caps that I have mentioned?

Don

[paulsherwin]

You can use the 0.22uF cap, or use both in series. The value isn't critical. Most (all?) scrap switch mode PSUs will contain suitable X2 caps.

In most cases the radio will work perfectly well with this cap removed altogether.

Paul

[kalee20]

Although without it, you may get more interference, buzz, clicks, from your DAC90A when you switch on items like fluorescent lights.

[paulsherwin]

Originally Posted by kalee20

Although without it, you may get more interference, buzz, clicks, from your DAC90A when you switch on items like fluorescent lights.

The nature of mains borne interference has changed somewhat since the 50s though. There is much less inductive switching - the only source in most houses today will be fridges, and maybe central heating thermostats - and most nuisance interference is hash from switch mode PSUs and digital processors which a simple cap across the mains won't do much about.

In the 50s there were still lots of trams and trolley buses about, all with electromechanical speed controls

I would replace this cap myself as a matter of principle, but I've run sets for extended periods with it removed and haven't noticed either excessive impulsive interference or modulation hum.

Paul

[Donald_S]

Originally Posted by paulsherwin

You can use the 0.22uF cap, or use both in series. The value isn't critical. Most (all?) scrap switch mode PSUs will contain suitable X2 caps.
Paul

So, the 0.22K or 0.33K markings DO indicate that they are 0.22uF and 0.33uF respectively?

Don

[paulsherwin]

Originally Posted by Donald_S

So, the 0.22K or 0.33K markings DO indicate that they are 0.22uF and 0.33uF respectively?

Yes. This is one of the (many) standard ways of marking capacitors. Any X2 cap with a squarish appearance about 2cm by 1cm by 5mm is likely to be between 0.047uF and 0.47uF.

Paul

[Nickthedentist]

It's neater and easier to mount the new cap on the back of the mains switch thus:

It's also cooler there.

Nick

[Station X]

I have to confess that before I knew better I replaced the filter cap in my DAC90A with a 1000V Polypropylene type like the yellow one in the picture. It fitted the clip nicely. I have no intention of changing it.

I use the correct X2 items on new restorations.

[Donald_S]

Originally Posted by Nickthedentist

It's neater and easier to mount the new cap on the back of the mains switch thus:

It's also cooler there.

Nick

Nice photo there, Nick. I think that's where I'll put my X2 cap. An added benefit of this is that you will not have a modern component on the topside of the chassis, but will be hidden from view. Thanks for sharing the picture.

Don

[Colin]

I wouldn't worry Graham, it may not meet current safety requirements but I'll bet it's a much better component than the original cap, even when it was new!

[paulsherwin]

Originally Posted by Station X

I have to confess that before I knew better I replaced the filter cap in my DAC90A with a 1000V Polypropylene type like the yellow one in the picture. It fitted the clip nicely. I have no intention of changing it.

I would imagine that modern 1kV plastic film caps would comply with the X2 standards if they were actually tested against them. They certainly self heal like X2s. There's little difference in price if you have to buy them though, and X2s are very easy to scrounge from dead equipment.

Paul

[thermionic]

Originally Posted by paulsherwin

The nature of mains borne interference has changed somewhat since the 50s though. There is much less inductive switching - the only source in most houses today will be fridges, and maybe central heating thermostats - and most nuisance interference is hash from switch mode PSUs and digital processors which a simple cap across the mains won't do much about.

In the 50s there were still lots of trams and trolley buses about, all with electromechanical speed controls

I would replace this cap myself as a matter of principle, but I've run sets for extended periods with it removed and haven't noticed either excessive impulsive interference or modulation hum.

Paul

I would not wholly agree with this. We tend to live with an increasing amount of inductive electrical apparatus in our homes. Low energy light fittings with magnetic ballasts, microwave ovens, induction hobs, vaccuum cleaners, etc.etc. However, I have found that the removal of mains filter caps have little effect on the sets with todays power supplies.
Mains filter caps fitted to modern equipment (such as computer power supplies) are fitted to help prevent 'hash' from the SMPSU getting back onto the mains.

Cheers. Simon.