Beomaster 2400 radio hum

[jansjobe]

Hi all,

I am restoring/repairing a Bang&Olufsen Beomaster 2400.

After a minute or so, I get a hum from tuned station, no hum between stations. Hum is propably 50Hz.
I have recapped all electrolytes.

I suspect something wrong in the stereo decoder section. Could it be the IC itself?

Has anyone any ideas?

Best Regards,
Jan Sjöberg

 

[JonSnell Electronic]

Check for ripple on the supply to the radio section.
Are you sure it is not interference?
If 50HZ it is not a capacitor as the smoothing capacitors are running at 100HZ due to a bridge rectifier.

 

[jansjobe]

 

[jansjobe]

Since the hum does not appear until after about a minute, it seems to me that it is either a cap or a heat problem. But I could be wrong...

 

[jansjobe]

I thought that I should replace C202,C203 and C302,C303 with 1uf MIKA polyester caps. Is that a good idea?

 

[Mooly]

I wouldn't replace anything until you have fixed the fault. C302 and 303 need only a single film cap. If you replace both with 1uF you will end up with just 0.5uF total and lack of low frequency response.

Freezer spray might help you find the problem. Where does that 15 volt rail come from? There could be an issue with a regulator or something like a diode going open circuit in a bridge rectifier.

 

[jansjobe]

Sorry, was unclear. Of course I will replace 2 electrolytes with 1 polyester (no polarity) I am not sure why they designed it with two polarized instead of one bipolar in the first place.

 

[Mooly]

Cost is usually the over riding factor in such choices.

 

[tomchr]

+1 for excessive supply ripple. I bet one of the main supply caps has gone soft.

Tom

 

[jansjobe]

I have replaced ALL electrolytic caps in the amp.

 

[Mooly]

Did you replace the caps for this fault? or has this issue occurred since? If there is hidden history we need to know 🙂

I'm going to say the next logical step has to be to look at the supplies on a scope and also look at this 'hum' and see what it really is. I've had weird things like commutation noise from rectifier diodes cause a harsh sounding buzz (that was on an Hitachi radio).

 

[jansjobe]

I actually don't know if there was a hum before i recapped... I don't think so. It was a while ago.
I will check with an oscilloscope and get back.

 

[Audio1Man]

More people get into trouble when they “REPLACE ALL CAPS”. DON’T REPLACE CAP(S) unless the single cap is bad. If you want to recap, take pictures/notes and or replace one cap and check if still works, then next one.

Duke

 

[Boydk]

This secondary supply is, where you should find the error 😉

 

[jansjobe]

It turned out to be the bridge rectifier in the 15V supply.
Thanks everyone for your time and effort!

 

[jansjobe]

As for replacing all caps;

• It's a pain to disassemble this thing. While I had it apart, I could as well replace all.
• I imagine that one prolongs the life of it at least 10-20 years
• To sell a fully recapped device gets me a better price.
• Opinions does not help me the slightest...

Again, thank you all for helping me!

 

[Boydk]

So.. it was the bridge in the left of my circle 🤣 ...... you´re welcome.
A thing to know about cap´s of these vintage B&O thingies. The ROE (Roedestein) covered with yellow film last forever (usually).
The deep red (also ROE) are totally dried out like 30 years ago. You can see the tops are cracked. If you see one, replace it.
You may run into some receivers that instead of the red Roedersteins have same size bright orange instead of red (also ROE).
In my 50 years of repair service, I never had any of these orange fail. Not one single time

 

[as_audio]

I agree, the red Roe fail frequently and earlier in items running hot.
This is not true for the orange and grey coloured Roederstein caps
which are even older, 60s to mid 70s. I never saw one of these fail.