GEC BC5445 Advice Needed

[GrahamN]

I have a GEC BC5445 which has had the capacitors changed and which works quite well - but I have an annoying problem that I would like to resolve.

Whenever a switch elsewhere in the house operates, there is a loud thud from the radio. There is also a little hum that doesn't seem to clear. While neither problem is acute, I wuld quite like to sort them out.

I've got the manufacturers service sheet (from up top), and notice there is no capacitor across the incoming mains. Would it be sensible to add one? Also the dual electrolytic (32uF / 16uF) is original - should I look to change this?

Or am I expecting too much from this set? There is a built-in aerial, and it would be quite difficult to add an external aerial where the radio generally sits, but I've temporarily connected a wire from the aerial socket to the screen of a TV aerial lead (not connected to a TV!) with no noticeable difference.

Thanks for any advice.

[Ed_Dinning]

Hi Graham, its certainly worth adding a 0.1 or 0.22 uF (Class X type) across the mains input, that should suppress anything coming along the mains wiring. You can also try moving the set to see if its being picked up by the frame aerial.
Slight hum is usually evident on a set such as this; you can always try additional capacity in parallel with the smoothing cap (not the resevoir, connected to the rectifier).
You can also try replacing the cap (if there is one) on the output valve cathode resistor.

Let us know how you get on.

Ed

[Boom]

Isn't the BC5445 an AC only set? If so I can't see any point at all in putting capacitors across the transformer. The windings will have plenty of their own capacitance.

As usual I could be wrong. (Or is that I could as usual be wrong?)

[GrahamN]

Yes - it is an AC only set.

Does that mean adding a capacitor would have no effect?

[Boom]

Originally Posted by GrahamN

Yes - it is an AC only set.

Does that mean adding a capacitor would have no effect?

Well I wouldn't have thought so and neither apparently did the makers as it was common practise not to place a cap' across the transformer. I wouldn't say that no maker did it but I've never seen one.

[GrahamN]

Okay - so is it likely there is a fault with the set?

Surely all AC radios of this vintage didn't make a loud noise whenever a light switch or whatever turned on? Or did they?

This radio even makes a thud when I turn on the tap in the kitchen - presumably something to do with the boiler switching (but dead impressive nevertheless!)

[Boom]

Originally Posted by GrahamN

Okay - so is it likely there is a fault with the set?

Surely all AC radios of this vintage didn't make a loud noise whenever a light switch or whatever turned on? Or did they?

They certainly do. That's AM for you. You should hear mine once the central heating gets going and no amount of capacitors change it!

[Station X]

The "thud" could be entering the set via the mains lead or via the aerial. Therefore it will do no harm to try a suitable capacitor across the mains input.

The only way to overcome interference entering via the aerial socket is to increase the signal input by using a decent external aerial. By external I mean outdoors, ie not just a piece of wire up to the picture rail.

I can confirm that this is a common problem. Often the thud or pulse will cause the receiver's AGC to operate reducing the gain a lot. This will result in low volume until the AGC recovers. How long this takes depends on the time constant of the AGC.

[Geoff 555]

Hello mine go off with a real crack when the fridge cuts in and out. Also as a matter of interest perhaps is that I have a clock that uses 'Nixie' tubes that are multiplex driven and this wipes out a radio in the next room on the other side of the wall ( by the clock) if you see what I mean.
Cheers.
Geoff.

[PJL]

AC sets will also suffer from RF noise on the mains. The capacitance between the primary and secondary windings will essentially represent a short to common mode interference. My 1935 AC mains HMV set has mains suppression. In AC sets you can suppress this by wiring earth to the chassis and putting Y rated capacitors from L-E and N-E. 22nF is the maximum allowed. Better still try a decent commercial mains filter with a choke.

Interesting about the AGC. You could try slowing the AGC attack time.

[GrahamN]

Thanks for the advice. I tried this morning fitting capacitors both across the mains and from L and N to E as suggested. There was no difference at all. I also tried disconnecting the Earth, but again no difference.

I think I'm just going to have to live with this, as adding a better aerial to where the radio sits is really not an option - and perhaps look at sorting out a different radio for listening to Gold while having my breakfast!

[geofy]

It might be better to put suppressors on the switches causing the interference, mains rated suppressors can be bought with connecting leads to put across the offending switch. Obviously observe safety to make sure the electricity supply is fully disconnected before attempting to fit suppressors which usually consist of a 0.1uf cap in series with a 120 Ohm resistor.

Regards

Geof

[GrahamN]

To be honest - it probably isn't worth the expense and time. The problem occurs whenever any light switch in the house is activated, as well as when the boiler, oven, shower etc. switches on or off. As no other radio or TV I have is affected, I can't justify that sort of expense when I can just use a different radio.

[Station X]

It might be worth trying the receiver in someone else's house. This would show whether the problem was with the set or your house wiring.

[Geoffrey]

Checking it elsewhere is a good idea. I used to have a similar interference problem on an audio amplifier. Eventually I found a neutral to earth short half way round my ring main. This had no visible effect until the panel was updated with an RCD, but when it was traced and rectified the interference ceased.

[Ed_Dinning]

Hi Gents, theory is that there is plenty of inductance (and possibly an earthed screen) in the transformer to suppress the interference; however, it will do no harm and may prove beneficial.

Ed