A couple questions about blown output fuses, and protection (Rotel RT-990BX) - diyAudio
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Old 2nd August 2009, 12:57 AM   #1
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Default A couple questions about blown output fuses, and protection (Rotel RT-990BX)

Hi,

This is my first post. I'm not really a DIY type unless things break down.

I recently fixed a Dual 1264 turntable and I've got a Bosch 6805 dishwasher in the middle
of my kitchen that I'm still debugging. Otherwise, I'm a EE with really no practical skills to speak of. I've enjoyed reading thru postings of others here, and it's helped me a lot just to get to this point.

I have a Rotel RT-990BX Amp whose right channel failed recently.

I was playing music, not very loudly, while moving the right speaker (B&W 802 series 80) around a little bit while it was attached and playing, and then it just...went out. I'm the original owner for 13 years and have just used it in a normal manner.

I got the schematic of it emailed from Rotel (they are very responsive). I opened up the Amp and found both of the 2 fuses blown on the right side. That alone has been confusing. They are both listed as T6.3a (slow blow) on the schematic. However, on the parts list, one is listed as (U)F6A and the other is (S)T6.3A. To confuse things further, there are stickers on the actual board behind both fuses that say 125V 6A (not 6.3 A).

I pulled the 2 non-blown ones from the left side and both say "BEL UL SA" on them, although the A could be a "B". Nevertheless, they both appear to be identical, so I went ahead and assumed that the schematic was right and the parts list is wrong and replaced them with T6.3A.

I eyeballed the solder side of the PCB's the best I could and didn't see anything obvious, so I decided to power up with the top off, and just watch it for awhile.

So far so good. I attached the speakers, and played things at normal volume. Right channel working again. So here are my 3 questions

1. Did i make a reasonable choice on the fuse situation, or should I have been more conservative?

2. The protection light *seems* to stay on a little longer before going off, right now. Is that anything to be alarmed about? What is a reasonable length of time?

3. I don't even own a multimeter, so I havent checked any of the output levels or anything pointed out on the schematic (to check the resistance across output transistor, and the voltage across the output diode). Do you highly recommend I do that before putting the cover back on and calling it a day?

Any other suggestions? I realize that blown fuses are usually indicative of a more significant problem, but I also know that old fuses just blow sometimes and simply replacing them is (infrequently) all that's needed.

Thanks ,
Henry
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Old 2nd August 2009, 07:14 AM   #2
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Join Date: May 2008
It is quite likely that you produced a short across the speaker terminals, when you moved the speaker around. Or the speaker wires were not tightly connected and on making and breaking contact produced arcing that made the fuses blow. You should have a look there. Make sure no unisolated wire is looking out of the speaker terminals (on the speakers and the amplifier side) and that the connections are stable. Tighten screwable terminals. Add lugs or sleeves to spliced or stranded cables.

The difference between 6 A and 6,3 A should not be of any concern. The difference between a fast-blow (F6A) and a slow-blow fuse (T6.3A) is significant. Although it sounds as if you did everything right, you should take another look at the old fuses. BEL UL SA indicate their compliance with those regulations. On the other end of the fuse the characteristics and current rating should be printed. FF=super-fast F=fast M=medium T=slow TT= super-slow.

The protection light should be described in the owner's manual. As long as the amplifier works, don't worry too much.

Oh, and get yourself a multi-meter.
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If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 2nd August 2009, 10:19 AM   #3
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Thanks Pacificblue.

I did fiddle around with the connectors, since moving it around was causing the connectors, which are very stiff (music wave connectors) to keep popping off, so it is very likely that I was making and breaking contact several times. Next time I do that I will turn the stereo off before doing those things. I also neglected to mention that I was also switching out a pair of speakers (B&W 803's for 802's), while the music was playing, unconnecting, reconnecting, etc...what was I thinking?

And you are correct indeed about the fuse. I didn't see writing on the other end of the fuse cylinder; however, now I look and I do see writing on the the part of the metal that goes around the cylinder. It says 5MF 6A 125V. Unfortunately, I googled that and 5MF Bel is fast blow.

So, it looks like I will have to replace the fuse with the Fast. Also, there were two fuses on each output and I already misplaced the other one. Worse, I removed both from the working left side and put them in again later, but didn't pay attention as to the order since I thought they were both the same. Hopefully both fuses on each output are the same.

Protect light is on for about 4 seconds. Used to be about a second. But it operates correctly.

Thank you very much for the info. It was very helpful. Best Regards.
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Old 2nd August 2009, 11:50 AM   #4
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Join Date: Nov 2002
Just for the heck of it pick up a digital mulitmeter (one from Harbor Freight will work fine and the cost is about $3.00) Check the DC offset on each channel upon start up. The protection light issue may be one channel with DC offset a little high upon turn on. If this is the case there may be a pot to adjust this and if not its probably not a real reason to worry.
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