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Old 1st May 2014, 11:03 AM   #1
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Unhappy Need help with a Rotel RA-970BX

I purchased my 970BX new & got quite a few years of use out of it before it died. I don't trust any local shops to do the work, so my only viable options are to send it to Rotel or to learn how to fix it myself. I could almost buy a new amp for what it would cost for shipping + repair cost to them, so I would appreciate any help I can get. I'm very good with electrical systems & have all the tools needed to get the job done. Unfortunately, I am barely a novice when it comes to troubleshooting electronics. I also already have the technical manual for it.

Anyway, the problem is that one channel has almost no output. It outputs at very low volume & has a bunch of static. Also, when I connect a DMM to that speaker tap, It shows a couple volts DC. I know enough to know this is bad, but beyond that, I don't know where to start. I don't want to have to de-solder & test every component if I don't have to. Hopefully someone can point me in the right direction.

If you need more detail, ask away.

Thanks in advance
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Old 1st May 2014, 12:20 PM   #2
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First thing that I would check is the volume pot. The wiper may not be making good contact with the track. Put the multimeter across one end and the wiper (middle pin) and check the resistance whilst varying the volume position.

I believe that an open wiper can also cause DC on the output due to the input not having a reference to ground.

edit: assuming of course it has a pot and is not digital volume!

Tony.
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Last edited by wintermute; 1st May 2014 at 12:21 PM. Reason: comment about whether it has a pot...
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Old 1st May 2014, 12:36 PM   #3
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Will do. I'm going to take it with me to the shop today. This is probably important, but it also had a blown fuse. I had replaced it at the time that it died, but it seems that it's missing one now. It's been probably 10 years since I've messed with it, so I'm a bit foggy on all the specifics of it. If I can find a suitable replacement in town, I'll check it out today. If I have to order one, it'll be a few days. Also, this amp is pure old school AB analog, so it's a kind of standard volume pot. The only difference is that it's a split pot, so left & right can be adjusted independently. It has 6 pins on it, so I'm assuming each set of 3 corresponds to one channel. I'll keep you posted.
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Old 1st May 2014, 12:53 PM   #4
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In the first instance I would check for leakage, C601,602,687 and 688
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Old 1st May 2014, 01:09 PM   #5
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Ok, 601 & 602 appear ok, unless there's a check I need to do with the meter. The two above those appear suspect. I didn't see the other 2 that you mentioned on the circuit board. 909 & 910 on the ps side also appear suspect. Thoughts? Keep in mind, I'm a total beginner at this kind of thing, so baby steps. Thanks
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Old 1st May 2014, 01:59 PM   #6
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Oh those caps look like they have leaked badly! You will probably want to at least remove and clean up the components that have had the electrolite around them as well. Test them whilst they are out!

Tony.
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Old 1st May 2014, 02:05 PM   #7
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Will do. I just picked up crc contact cleaner & I found some glass fuses. Is there any reason not to use those temporarily until I get new ceramic ones ordered? I'll pull the bad components off & give it a good cleaning. Then I'll order replacement parts.
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Old 1st May 2014, 02:14 PM   #8
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Quote:
Originally Posted by wintermute View Post
Oh those caps look like they have leaked badly! You will probably want to at least remove and clean up the components that have had the electrolite around them as well. Test them whilst they are out!

Tony.
That is contact glue.....sheesh.

Dan.
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Old 1st May 2014, 02:33 PM   #9
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That was what I first thought but I couldn't understand why they would put it there and in such a wide patch...

The resistor legs appear to be corroded, or is that just my colourblindness?

Tony.
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Old 1st May 2014, 02:40 PM   #10
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It doesn't appear to be corrosion. If the consensus is that it's just glue, then back to my other questions: crc contact cleaner to clean the board & glass fuses to temporarily substitute for ceramic? I'm at the shop now, so as long as the fuses are acceptable, I can plug it in give precise readings with the DMM
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