Bush VHF81 hum

[Dorian27]

Hi the above radio has developed a loud hum on power up lasting about 2min then OK I think it is the can capacitor which is a 40uf by 40uf by 20uf 275volt where could I get one of these as I am not sure what separate ones I need can any one help please as this radio is a good radio and I would like to win this one

[Al (astral highway)]

Originally Posted by Dorian27

I think it is the can capacitor which is a 40uf by 40uf by 20uf 275volt

Hello Dorian,

Not quite sure what you mean by 'win' but I'm guessing you mean fix, or something...

What was the history of the radio before you had it? Was it unused for a long time? If it was, it would be quite natural for this to happen while (at least some or most of) the capacitor's original properties begin to re-emerge. If the hum disappears as you describe then it's not clear why you would want to get rid of the capacitor without doing a few more checks.

It's a shame , as well as a bit of a faff, to get rid of original electrolytics unless really necessary. They can often be 'reformed' by passing a low current through them at the required voltage. There's an article on it in the 'sticky' threads.

What's your level of experience with these things? It's always helpful to know.

Originally Posted by Dorian27

I think it is the can capacitor which is a 40uf by 40uf by 20uf 275volt

If you did have to replace it, it's quite easy to get modern equivalents which are much much smaller and can be 'stuffed' into the original can. But as I say, hold your horses for now...

[Dorian27]

My experience is so so I know a little bit. As for the radio I bought it in an antique shop it has not been used in years full of dust but cleaned up nicely a works really well apart from this loud hum on power on after it a while great sound but the hum has got longer with each power on at the beginning it was about 1min now is more like I think it is the can as it is the smoothing capacitor I think but I need help to replace it please nothing seems to be getting overheated at all that's why I use the radio

[snowman_al]

You certainly will not harm the set replacing the smoothing caps - you very well might fix the fault too.

Sadly you have next to no chance of obtaining a new or good replacement can capacitor. Only choice is to fit 3 modern types under the chassis.

You will not get 60uF or 20uF values any more either. Modern values are 68uF and 22uF and I would suggest 400 or 450 volt types. Like these for example.
68uF https://uk.farnell.com/panasonic-ele...-20/dp/1673496
22uF https://uk.farnell.com/panasonic-ele...-20/dp/1673491

Note they are Electrolytic capacitors, they have polarity markings and must be installed correctly or they go bang in a spectacular way!

It would be wise to replace C49 (0.01uF) ''that cap'' if you have not already, when in there. I know you have some of those

And finally you know the set has a Live Chassis, make sure you un-plug it before working on it. Then double check again Every Time.

Alan

[raditechman]

You can purchase and download a service sheet for this radio by going to the service data box in the top right corner of this forum.
As well as checking the smoothing capacitors you must also check C49 0.01mfd as this capacitor can go "leaky" and cause the audio output valve pentode to draw too much HT current. Check C49 ASAP as it is not good to use the set if leaky.

John

[Dorian27]

Thank you ever so much for your help much much appreciated

[Dorian27]

Hi I have looked at C49 and it looks good no leaks or bad spots also there is a resistor across the can capacitor after re forming does it need that connected .Also I would like to put new capacitors in the old can any ideas on best way to open it so as not to do to much damage please

[Freya]

File the lip of the end of the capacitor where the terminals come out, hold the file at 45 degrees to the body. It should leave a very tiny ring of aluminium. Once done the paxolin disc with the terminals can be pulled out allowing the innards to be removed. Once the new caps are inside it can be sealed again with some superglue or epoxy.

[Sideband]

Originally Posted by Dorian27

Hi I have looked at C49 and it looks good no leaks or bad spots also there is a resistor across the can capacitor after re forming does it need that connected .

You CANNOT with the best will in the world say that a capacitor LOOKS alright with no leaks....sorry but you have to check these things. You can't physically see the leaks in a capacitor (with the exception of electrolytic types that can physically leak). If it's one of the old waxy types, it will be electrically leaky and require replacement. Most testmeters are very poor at checking capacitors for leakage since to do it properly, it needs to be tested at or near it's working voltage. Even if your meter has a capacitance range, it will probably give a very high reading if the capacitor is leaky.

Best thing is to just change it.

[Simon Gittins]

Originally Posted by Dorian27

there is a resistor across the can capacitor after re forming does it need that connected

Yes, the resistor is part of the circuit.

[Half a Mullard]

I agree, I would tend to replace C49 on sight. On the model I have it was a wax paper type. It is partially hidden but the chassis is very easy to work on. It could save a lot of problems!

[Dorian27]

Thanks for the picture mine is definitely not wax type is like a yellow plastic it looks original whether they have been changed in the past as most of them are like this

[G8UWM-MildMartin]

Could you post some pictures?
Sounds like someone may have done a very neat job of replacing the usual suspects!

[Dorian27]

Here are some of the pictures you wanted

[snowman_al]

Replace it, it is original to the set and will save grief later.

[Half a Mullard]

Yes it certainly hasn't been re-capped. Looking closely at the grid coupling capacitor it does look different from the others but looks an original part nonetheless, the soldering doesn't look as if it's been disturbed. In any event it looks to be an old component. For the modest cost involved, I would replace it, far better to be safe than sorry!

[Nuvistor]

If it the yellow capacitor it’s is a TCC Supamold, original as noted in the last two posts and I agree it should be replaced.

[Glowing Bits!]

On the first picture, there is a red TCC electrolytic, that should be changed too.
The best course of action would be to change any capacitor that's red, green, yellow or wax covered.
If the hum disappears after a few minutes, the reservoir capacitor (first 40UF after the rectifier) may be iffy, a basic test would be to source a 47UF electrolytic to use in place of the original section, the resistor will have to migrate to the new capacitor too.
This may sound complex but in practice, it's relatively straightforward.
Rick.

[snowman_al]

Dorian,
correction to post no. 4.

Yes C52 and C54 are 40uF.
A 47uF cap (400 or 450 volts) will be the current value to use.

Alan
Sorry, 'head in the shed'.

[Nuvistor]

Originally Posted by Glowing Bits!

On the first picture, there is a red TCC electrolytic, that should be changed too.
The best course of action would be to change any capacitor that's red, green, yellow or wax covered.

Rick.

Careful with the red, there are lots of red ceramic capacitors in the set which will be ok.

Rick, I understand what you mean but the statement may be taken too literally.