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
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
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.
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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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.
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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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
That glue looks typical of the type that can become conductive as it ages. Definitely clean it off... most should "pull" of with pliers/cutters... and then clean it all up.
Ceramic fuses are OK but I would recommend powering up with a bulb tester and not having speakers connected until you have fixed the DC offset issue.
Ceramic fuses are OK but I would recommend powering up with a bulb tester and not having speakers connected until you have fixed the DC offset issue.
If you don't get ceramic fuses just go ahead and use glass encased ones. Check if they slow blow or regular types.
Might be a good idea to remove the cap where that brown 'glue' is. Then clean up the area thoroughly and check the resistors to see if their lead are corroded. If necessary replace them and certainly the electrolytic cap. You need to clean the area thoroughly. You don't want any more corrosion with time.
Might be a good idea to remove the cap where that brown 'glue' is. Then clean up the area thoroughly and check the resistors to see if their lead are corroded. If necessary replace them and certainly the electrolytic cap. You need to clean the area thoroughly. You don't want any more corrosion with time.
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