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Old 19th November 2013, 02:10 PM   #1
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Default Rotel RB-990BX Blown Channel Help

Right channel had two blown fuses. I suspect the speaker cables were shorted. I replaced them and all was ok for about a year. Now the fuses are still intact, but the right channel has lost all treble. I can still hear some bass. A starting point would be helpful.

A visual inspection showed no obvious damage. Protection circuit light is ok.

I have a multimeter and a scope, and know how to use them both. I thought I could generate some signals of different frequencies and measure along the signal path.

The service manual can be found here. It's too large to attach:
B&W Group North America Service & Support - Home

Thanks!
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Old 19th November 2013, 02:35 PM   #2
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If one channel is working, start by comparing the voltages between the two channels.
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Old 19th November 2013, 06:52 PM   #3
jaycee is offline jaycee  United Kingdom
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Check and replace C602 and C608.
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Old 19th November 2013, 10:47 PM   #4
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I'll check them. That's very specific. Thanks!

C602: Electrolytic, 50V, 4.7uF
C608: Electrolytic, 25V, 100uF
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Old 20th November 2013, 12:07 PM   #5
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Often Rotel blow rectifiers check that also
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Old 22nd November 2013, 02:25 AM   #6
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I think it's time for an electrolytic cap update as this amp is about 20 years old and the dielectric is probably dried up, so I ordered replacements for all the electrolytic capacitors. If I'm going to get inside, I may as well replace them all.

Here's the list, comments are welcome:
(2) Elna Silmic II 4.7uF 50V, C601, C602
(2) Elna Silmic II 10uF 50V, C611, C612
(2) Elna Silmic II 22uF 100V, C801, C804
(1) Elna Silmic II 100uF 16V, C802
(2) Elna Silmic II 100uF 25V, C607, C608
(1) Elna Silmic II 220uF 100V, C803
(4) Nichicon KW 1000uF 100V, C623-C626

I'm going to hold off on the 15000uF caps for now. They're accessible by just removing the cover, and quite expensive. The rest were pretty inexpensive, about $25 for all.

I'll check the rectifier at the same time. Thanks for everyone's comments.
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Old 22nd November 2013, 11:44 AM   #7
jaycee is offline jaycee  United Kingdom
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If you can get 4.7uF WIMA MKS2 capacitors, use those for C601/C602, but otherwise, a good quality electrolytic is OK
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Old 22nd November 2013, 08:23 PM   #8
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Thanks jaycee - Mouser had the WIMA MKS2 capacitors you mentioned ($2 per), so I added a couple to the order.
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Old 25th November 2013, 09:55 PM   #9
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Thanksgiving week here - hope my parts arrive by Wednesday so I can spend my days off working on the amp. I also ordered a new Edsyn soldering station (for work, really) and a couple tubes of Thermalcote.
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Old 28th November 2013, 05:47 AM   #10
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I replaced all the electrolytics (most were leaking badly). No change, but it needed to be done.

FWIW, the Nichicon KW 1000uF 100V, C623-C626 is too tall and doesn't clear the power supply.

Left channel is normal, right channel still has only some low volume scratchy bass.
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