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billken 31st August 2004 11:46 AM

Noisy rotel ra500
 
Looking for some advice here. I use an old Rotel RA-500 amp which I've had for years, I like the sound and I'd hate to part with it. But it's developed a fault. Most of the time it plays fine, but then noise starts coming through the speakers, usually increasing in volume till I have to shut the amp off it's so loud. It's not affected by the volume control, and will happen whichever input or speaker channel is selected. It'll do it even if no inputs are connected. It's like a huge "rushing" noise, totally random, no pattern to when it occurs. I know this is pretty vague, but anybody got any ideas how to cure this?

cunningham 1st September 2004 11:44 PM

Re: Noisy rotel ra500
 
Quote:

Originally posted by billken
Looking for some advice here. I use an old Rotel RA-500 amp which I've had for years, I like the sound and I'd hate to part with it. But it's developed a fault. Most of the time it plays fine, but then noise starts coming through the speakers, usually increasing in volume till I have to shut the amp off it's so loud. It's not affected by the volume control, and will happen whichever input or speaker channel is selected. It'll do it even if no inputs are connected. It's like a huge "rushing" noise, totally random, no pattern to when it occurs. I know this is pretty vague, but anybody got any ideas how to cure this?
It sounds like occilations. It is possible that some RF or some type of noise is being introduced and bouncing around like a tank circuit, and increasing in amplitude with respect to time. Or there could just be a cold solder joint someware, probably in the amplifier circuit.

billken 3rd September 2004 09:23 AM

Thanks for the contribution. I have to confess that due to the random nature of the problem, and the fact that it works perfectly otherwise, I have kind of suspected the dry solder joint scenario. Maybe time to bite the bullet and resolder the whole thing!

ashok 3rd September 2004 09:44 AM

If you say that the noise is coming from BOTH channels then the cause is common to both channels. This might clear semiconductor or parts from the amplifying section of the amp.
I assume that the sound is equally loud on both channels.
The power supply section is common to both channels.

Cold solders and defective devices usually will show up only on one channel. It's very unlikely that both channels have the same problem unless those are common to both channels.

Based on this , do some more checking and tell us what you find.
Another way to find defective transistors or IC's is to heat them with a soldering iron when the amp is behaving. Just 3 to 4 seconds should do. Make sure you do not touch any metal encased transistors. You could blow them. For RF the easiest method is to check it on an oscilloscope. But if you did have such loud noise and it was RF it might have blown your tweeters by now and additionally the power transistors would be really hot - touch it and see. If it is as warm as usual it seems unlikely that it is RF. Do check out parts in the power supply. What kind of noise is it. A hiss or hum or crackling sound or like escaping steam?
Let us know what you find.
Cheers.

anatech 4th September 2004 04:08 AM

Hi billken,

It's common for some older caps to open up with age. This includes some early film types used for supply bypassing. This will lead to oscillation, which the other channel may pick up.

The best thing to do is use a 'scope to see what's going on. One thing you can look at is the zoble network to see if the resistor has been running hot. Of course, the cap in the zobel may be opening up and causing instability. The resistor may not overheat in that case.

If the amp is not oscillating, look for some common component. Something like a common supply for tail current or voltage amp regulator. (I'm not familiar with this amp)

-Chris

billken 4th September 2004 07:56 PM

Thanks for all your replies, guys. I've now realised that my initial post was slightly misleading, or at least contained insufficient info.
This amp was only recently pressed back into service, the problem actually arose quite some time ago. I'm not an experienced electronics tech, but I did buy and fit a replacement for the main amplifier IC - you know, the 40 pin STK unit that drives the whole thing. It didn't fix the problem (and I've lost the original, which was obviously OK).
In addition, when I said the sound was present on both speaker channels, I meant that this amp has outputs for two sets of speakers, and it didn't matter which set you used. In STEREO terms, the sound is only coming through one channel. In fact I'm presently using it with both speakers wired into the good channel, (the amp has a mono switch), but it's hardly the way to go!
Sorry about this!

anatech 4th September 2004 08:06 PM

Hi billken,
I have had new STK devices oscillate when replaced with the exact same part (from Japan). You may need to increase the size of the compensation caps if they check good. Oscillation normally begins as the amp warms up in this case.
Do you have a pre-out jack on this? This would be an easy way to check the preamp section if you do. If you have a "Y" connector you can try to run both channels off one preamp channel at a time. This is provided you have main amp input jacks. Otherwise you may have to short the amplifier signal inputs together (before the DC blocking caps!).
The main task now is to decide if the fault is in the preamp or the main amp.
-Chris

billken 6th September 2004 08:05 PM

I thought I'd cracked it yesterday. Examining the power supply board, I found a capacitor (one of the flat ceramic disc types) that had was basically unsoldered. You could push the leg through the board no bother. (But deep down, from your posts, I knew that a power supply fault shouldn't affect only one channel). So I fixed it, powered it up, and of course after half an hour the fault reappeared. No surprise, except I'm amazed it was working at all with this joint.
So today I stripped it down and resoldered every connection on the main board - just desperation really, I don't have your skills.
But lo and behold, it's been playing for five hours now without a hiccup, and I can't remember the last time it did that. So here's hoping.
Only one snag now - I got so p****d off I bought a NAD 3020 off Ebay, so it looks like a listening test now!

anatech 6th September 2004 08:22 PM

Hey billken,
Congrats! Sometimes resoldering will work. From your description I still suspect the small bypass caps. And now you get to compare a Rotel with a NAD.
Stick one unit in the bedroom, or use it as a sacrificial party machine (using someone else's speakers)
-Chris


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