Faulty radio power supply.

[Maker]

I've got two of these power supplies out of two radios (Roberts MP sound 43) and neither work properly. One has no output power and the other causes the radio to cycle through it's functions (This seems to be a common fault).

I have just spotted a fuse under a piece of heatshrink which could be the cause of one of them having no output, I'll test it later.

The other one one doesn't appear to have anything wrong with it, but it clearly does have a fault. I don't know much about this kind of think but I'm guessing a component (filter capacitor?) has died and there's a lot of noise on the output? I don't have an oscilloscope to test this (yet).

Anyone know what the problem could be?
And more importantly, how to fix it?

Roberts don't sell them separately and they want £80 just to have a look at it so that's a no.

 

[Seadog]

At a guess, if the fuse has gone then the most likely problem is a short-circuited bridge rectifier. A few more clear pictures might help diagnosis.

 

[MattF]

Replace the small electrolytics, especially the ones near that heatsink. Get that hot melt off as many things as possible too. That stuff has a bad tendency to carbonise over the years if it's running hot. Unless they've left all the components proud of the board due to stupidly long lead lengths, (hence they lathered on the hot melt to hold them steady), it's not needed generally. Check for dry joints too, especially on the primary side and on the TX, plus on any components on that heatsink and high wattage resistors etc. A better photo would be handy, btw.

As Seadog says with the dead one, check the bridge rectifier and thereabouts for shorts first, before replacing the fuse.

 

[WorkshopChris]

Do you have a better picture of the top side?
If you find a blown fuse if you don't have one build your self a lamp limiter, before you power it back up.

http://www.vintage-radio.com/projects/lamp-limiter.html

Some useful advice on that site but most from pre switch mode years.

 

[Maker]

Dad took the dead one to work to see if anyone there knew anything, and he left it there, so I won't have it till next week.
The hot melt stuff is harder than normal hot melt and is really hard to get off.

 

[MattF]

The hot melt stuff is harder than normal hot melt and is really hard to get off.

Some of that stuff you literally have to scrape or pry off. Why anyone still uses the stuff these days is beyond me. That stuff can cause no end of damage if it crozzles. If it all looks clean though, there's no problem leaving it for the time being. If you notice any darkened patches though, those definitely want shifting. If it gets to dark brown/black in colour, it's usually becoming conductive else exacerbating localised heating.

Generally, that board looks decent enough, judging from the photo's. I'd just go with the electrolytics to start with.

 

[Maker]

If you notice any darkened patches though, those definitely want shifting.

There's two resistors and a small cap behind the big green cap with this crap all over them, and it's gone brown exactly as you describe. I'll scrap it off and test.

 

[zx9]

Some of the control processors that I look after have an off the shelf switch mode power supply board, easily replaced from Farnell for 1/4 the cost of sending back to the european service centre and only 1/2 an hour to swap out. Has your faulty board any part numbers or maker's branding, could be an easy fix if not so much fun/learning experiance.

 

[Maker]

There's two resistors and a small cap behind the big green cap with this crap all over them, and it's gone brown exactly as you describe. I'll scrap it off and test.

I desoldered them (to get better access) cleaned them, refitted them and...


Now there is no power output, I broke it.

Some of the control processors that I look after have an off the shelf switch mode power supply board, easily replaced from Farnell for 1/4 the cost of sending back to the european service centre and only 1/2 an hour to swap out. Has your faulty board any part numbers or maker's branding, could be an easy fix if not so much fun/learning experiance.

Good idea, I'll have a look.

Would a simple linear power supply do the trick, I could quite easily make one.

 

[zx9]

Would a simple linear power supply do the trick, I could quite easily make one.

I would have thought a 'plug top' power supply or wall wort as our American friends call, them would be an easier option.

Edit: That is how our Roberts Stream 83i was shipped.

 

[richardeblack]

I would have thought a 'plug top' power supply or wall wort as our American friends call, them would be an easier option.

Edit: That is how our Roberts Stream 83i was shipped.

Hello there,
I have the same problem with my Roberts MP sound 43. On checking I found the fuse blown with the glass broken. The two resisters in the top picture are very discoloured. I can't see any other obviously damaged components. I am assuming that the power supply is 5V and 2.5amps from the name. Would I just be able to use any 5 volt ps rated for 2.5 amps or does it need to be special? I also assume that the V out pin is the +5 and the other one is 0V. Do you think that is correct please?
Thank you very much,
Richard.

 

[Maker]

Hello there,
I have the same problem with my Roberts MP sound 43. On checking I found the fuse blown with the glass broken. The two resisters in the top picture are very discoloured. I can't see any other obviously damaged components. I am assuming that the power supply is 5V and 2.5amps from the name. Would I just be able to use any 5 volt ps rated for 2.5 amps or does it need to be special? I also assume that the V out pin is the +5 and the other one is 0V. Do you think that is correct please?
Thank you very much,
Richard.

As it's for music, I'd imagine it wants to be a nice smooth output, a cheap switchmode probably wouldn't do. I would also guess that Vout is 12 volts.

 

[Maker]

Oh, just realised this was my thread. Both radios got sold on eBay as spares or repair before I could get a new PS sorted.