Amplifier fuses

[JBR]

Suddenly, and for no apparent reason, internal fuses blow when I turn on my amplifier (Rotel RA840B).

These are the two 5A fuses immediately between the (two) large mains transformers and the Wheatstone bridge.
The other two internal fuses (4A) just before the loudspeaker outputs are unaffected.

Any ideas of the likely cause?

 

[OwainDIYer]

more likely to be a bridge rectifier than a Wheatstone bridge

Smoothing capacitors failed short-circuit?

 

[JBR]

more likely to be a bridge rectifier than a Wheatstone bridge

Smoothing capacitors failed short-circuit?

Thanks for your reply.
Yes, I'm sure you're right. I'm no expert, as you might have guessed.

Looking at this schematic, are C903 and C904 the capacitors to which you refer? If so, I might try to obtain replacements and fit them myself.

Untitled

• JBR
• 5 Jul 2013
• 1

I should add that when I tested it with new fuses, I disconnected speakers and all inputs before switching on with the case open. Both fuses (F901 and F902) glowed and blew immediately.

 

[Sam Gangee]

You must never, ever "test with new fuses". Locate and replace the faulty part then replace the fuse. By "testing" you are likely to kill even more components.

(If your brakes failed and you damaged your bumper, would you replace it then drive at a wall to see if the problem was fixed?)

Visual inspection will sometimes reveal a bulging capacitor. However, I've never known one to melt a fuse. What usually happens is that the failed capacitor causes a semiconductor to fail and that causes the protection fuse to melt. It's essential to locate the faulty part(s) by inspection and measurement.

 

[JBR]

You must never, ever "test with new fuses". Locate and replace the faulty part then replace the fuse. By "testing" you are likely to kill even more components.

(If your brakes failed and you damaged your bumper, would you replace it then drive at a wall to see if the problem was fixed?)

Visual inspection will sometimes reveal a bulging capacitor. However, I've never known one to melt a fuse. What usually happens is that the failed capacitor causes a semiconductor to fail and that causes the protection fuse to melt. It's essential to locate the faulty part(s) by inspection and measurement.

I've had a look and can see no bulging capacitors or any evidence of burning, etc. Further 'testing' is beyond me, I'm afraid.

 

[Sam Gangee]

Take it to a professional.

 

[danechip]

Test the rectifier for short

 

[JBR]

Sorry not to have replied earlier; we've just got back from France.

My wife has given me permission to buy a new amp, and I've decided to go for a Marantz PM6004. The old Rotel has given good service for many years. I don't suppose it's worth much now.

 

[JBR]

Test the rectifier for short

I think you may be right. I've just had another look and there appear to be some black areas on the PCB where the 'legs' of the bridge rectifier are mounted. I'm not sure why the marks should be there and not on the rectifier itself, though.

As I mentioned, I'm going to get a new amp but if I can obtain a replacement rectifier I might try to replace it when I have time, although I presume it would involve removing the main PCB to get at the connections. If this solves the problem I could perhaps sell it on.

Anyway, thanks for the advice.

 

[Sam Gangee]

Remove the rectifier and measure it. Don't simply replace items at random. If you guess wrongly you'll blow another fuse and possibly cause more damage. That's NOT the way to fault-find. Leave it to a professional.

 

[danechip]

and if you do find the rectifier faulty, chuck it and make your own.

http://www.ebay.co.uk/itm/SR5100-5-...al_Components_Supplies_ET&hash=item257dfb6f18

£4.50, for a pack of 10, and you only need 4.

 

[Sam Gangee]

Yes, instead of ordering the correct replacement for 99p, spend £4.50 to order ten Schottky diodes from Hong Kong, wait three weeks for delivery, then make your own bridge rectifier.

 

[danechip]

Didn't notice the 'Hong Kong'. Damn you beer...

 

[JBR]

Well I've bitten the bullet and bought a new amp.

It occurred to me that, as I've had this amp for perhaps thirty years, even if the present problem were to be fixed there's no guarantee that something else won't go wrong later.

Anyone want to buy a thirty year old Rotel RA840B going cheap? Well, it's not actually making any noise at the moment!

 

[Monkeh]

I'd take it off you if I were in the area. They're usually easily repairable.