Mains filter unit - Defiant MSH 954

[SKEXIS1]

Dear All,

I'm after some advice please!

The (rather sorry-looking) item shown in the attached photos is a "mains filter unit" from a Defiant MSH 954 (AC) set. I couldn't find a circuit diagram for this particular model, so have been referring back to one for a MSH 953 instead (it's a slightly different layout, but the filter unit appears to be the same).

As you can see from the photos, some of the insulation has broken down and it needs a bit of a tidy up.

Here are my questions:

1. Should I do this tidying, or should I simply remove the two chokes and replace the capacitors (which I assume are inside the metal can) with new ones?

2. I guess that as these are across the mains input I'd be looking at the "X" type, mains-rated capacitors for this purpose?

3. In the circuit diagram, C26 and C27 are 0.01 mfd each. Is this why the metal can has 0.02 mfd written on it, i.e both of them are wired in parallel, therefore total capacitance = 0.01 + 0.01 = 0.02?

4. What does the "inner" bit mean on the circuit diagram?

5. My understanding of these fiter units is that they effectively reduce "modulation hum" i.e cancel out the 50hz present in the mains supply. Is this right?

I'm sure I'll think of other things, but if you have any suggestions in the meantime, that would be great!

Thanks

Andy

[Station X]

Inner means the inner end of the winding.

I can't explain the value of C26/27. They aren't in parallel.

[Steve_P]

The chokes are probably OK.
The caps should be X2 types.
0.01 in both cases, which does make 0.02, yes.

They filter all sorts of shash off the mains. There could be as much then as now. They can reduce hum, but there may be a humdinger cap on the set too for this. As well as smoothing capacitors, etc.

Make sure C30 is an X2 cap. Taking an aerial to mains is not something I am fond of I must admit, but if you must keep this option, let it fail open circuit. If it fails closed circuit, then it could cause problems.

Cheers,

Steve P.

[Al (astral highway)]

Originally Posted by SKEXIS1

1. ... should I simply remove the two chokes and replace the capacitors (which I assume are inside the metal can) with new ones?

Leave the chokes. I'll come to them in a sec. Replace the flex with three core and run Y rated caps between each phase and earth: one from live to earth and the other from neutral to earth.

To answer your 4th question, 'inner' is a way to identify the way the chokes are facing round their shared former.

Why does this matter? Because they are placed in anti-phase.

[Herald1360]

Looking at the circuit, the purpose of the filter is two fold- the low pass LC filter in each line will reduce the amount of noise on the mains reaching the transformer plus it looks like a high impedance for RF on the mains side so it will stop any "wanted" signal from the mains aerial from being shunted away by stray capacity to earth in the same transformer.

C26 and C27 are simply the shunt condenserss to ground for each leg of the pair of L section low pass filters. The circuit is confusingly drawn- it would be easier to follow if the bottom of each condenser simply had an earth symbol on it (and possibly the chokes were shown "in-line" with the mains input.

Depending on whether the two chokes are coupled magnetically or not the filter will either be common mode only or common and differential mode in its effect.

Chris

[SKEXIS1]

Chaps - thanks for your help.

From your responses, I think I'm concluding:

1. I'm going to ditch the mains aerial completely. I don't like the idea of using these and I'm thinking I won't really need it anyway as long as there is a decent external aerial.

2. I'm going to keep the two chokes, but get rid of the two canned capacitors, replacing them with two separate X2, 0.01 mfd caps. Each will join to one of the main input phases at one end, with the other end going to earth.

3. Replace the two-core with three-core main cable, using the chassis as the earth.

I'm afraid I don't understand the concept of the chokes operating in anti-phase, common or differential mode! My (very basic) understanding of the function of the coil is that in conjuntion with a capacitor it acts as a high impedance to certain frequencies, allowing current to flow for those that are wanted.

What does the anti-phase / common / differential concept mean? This might be a little too technical for me, but I'd like to understand it!

Thanks!

Andy

[ppppenguin]

Originally Posted by SKEXIS1

2. I'm going to keep the two chokes, but get rid of the two canned capacitors, replacing them with two separate X2, 0.01 mfd caps. Each will join to one of the main input phases at one end, with the other end going to earth.

These must be class Y or Y2 capacitors. This is because failure could cause an immediate shock hazard. Typically class X are used between live and neutral while class Y are used between live/neutral and earth.

[SKEXIS1]

OK - I'll use the X2 class instead - thanks!

[Al (astral highway)]

Originally Posted by SKEXIS1

OK - I'll use the X2 class instead - thanks!

No, it's Y, not X!

[Herald1360]

Hi Andy,

A common mode signal is one which is the same on both lines- hence "common" (most RF interference is like this since the capacitive coupling between the lines in cables etc tends to make them look like just one circuit at high frequencies). If you look with a meter or 'scope from each line to ground you see the same signal and from line to line you see no signal.

A differential mode signal is one where either each line has a different signal on it so if you look with a meter or 'scope across the lines you will see a signal which is the difference between the signals on each line (hence "differential"). The mains itself is like this- though nominally all the mains signal is on one line and none on the other!

If two chokes are wound in the same direction on a common core, because of the coupling in the core between the two windings, they only look like an inductor to a common mode signal, to a differential mode signal they effectively cancel each other out. This can be useful in high (mains) current filters where the core would otherwise be saturated by the mains signal (and hence not useful to increase the inductance beyond what an air cored coil would have) since most of the problem signals are common mode anyway. The filter will still have some differential mode effect, thanks to the capacitors and any stray line impedances on the interference side.

I hope this is helpful- I'm sure it could be put better, but I thought I'd have a go, having raised the issue in the first place.

Looking at the Defiant filter, unless there is a common slug coupling the two coils, they are probably efectively independent air cored inductors, and will give similar common and differential mode filtering performance.

Chris

[SKEXIS1]

Thanks for all the help chaps, time to get some capacitors (Y2!) ordered and get soldering!

Cheers

Andy

[Framer Dave]

Could I suggest that, while you're dong this work, it might be a good idea to place the on/off switch in the incoming live lead instead of its present position? As things stand, mains is present across the filter unit whenever the set is plugged in.

Dave