Amplifier Quiet in Two Channels - Advice Please

Not quite a computing question but there may be a electronics boffin in here that can point me in the right direction.

I have a Rotel amp that has quiet volume in the Left A and B channels.

The Right A and B channels seem to be fine.

I am handy with a soldering iron and can most probably fix it myself but I am not skilled in diagnostics.

If anyone has suggestions on how to start a diagnostic process that would be appreciated.

Its a basic amp, no fancy chips, valves or components.But lots of resistors, capacitors and transistors.

Thanks in advance!

I will post this over at DIY as well.

geek_mss2006, Jan 30, 2:16 pm

Not quite a computing question but there may be a electronics boffin in here that can point me in the right direction.

I have a Rotel amp that has quiet volume in the Left A and B channels.

The Right A and B channels seem to be fine.

I am handy with a soldering iron and can most probably fix it myself but I am not skilled in diagnostics.

If anyone has suggestions on how to start a diagnostic process that would be appreciated.

Its a basic amp, no fancy chips, valves or components.But lots of resistors, capacitors and transistors.

I can post a photo of the IC Board if that helps.

Thanks in advance!

I will post this over at DIY as well.

geek_mss2006, Jan 30, 2:16 pm

Its a blown capacitor, happened to me when too much load is applied to one channel. Still under warranty!

geek_henry284, Jan 30, 2:19 pm

Hi, I was guessing it may be a Cap or Trans but not really sure where to start to choose which ones to start with.

Its about 20 years old.

geek_mss2006, Jan 30, 2:21 pm

This is a pretty good guide: http://www.bcae1.com/repairbasicsforbcae1/repairbasics.htm Because you have one channel working OK, if you can't see anything obviously wrong like bulging capacitors or scratchy volume controls, I would make up a soundcard oscilloscope - http://www.zelscope.com/faq.html - and go through each side of the circuit until you get a level difference. You can generate a test tone using software e.g. www.esseraudio.com/ttg.htm .

You might get away with a sensitive AC multimeter but a scope is more fun and will show you noise and distortion problems as well.

Try to find the circuit diagram online, if it's a "Classic" amp you might be surprised (though I don't think most Rotels were classics!).

geek_drsr, Jan 30, 2:31 pm

This is a pretty good guide (for car amplifiers but mostly applicable to mains amplifiers as well): http://www.bcae1.com/repairbasicsforbcae1/repairbasics.htm Because you have one channel working OK, if you can't see anything obviously wrong like bulging capacitors or scratchy volume controls, I would make up a soundcard oscilloscope - http://www.zelscope.com/faq.html - and go through each side of the circuit until you get a level difference. You can generate a test tone using software e.g. www.esseraudio.com/ttg.htm .

You might get away with a sensitive AC multimeter but a scope is more fun and will show you noise and distortion problems as well.

Try to find the circuit diagram online, if it's a "Classic" amp you might be surprised (though I don't think most Rotels were classics!).

geek_drsr, Jan 30, 2:31 pm

Oh well then. Mine was a brand new Sherwood RX-4503. Take it shop to get fixed or get a new one. What is the channels wattage and system RMS. how many channels and what is the wattage of your speakers on that channel

geek_henry284, Jan 30, 2:32 pm

ROFL, WTF are you on about! RMS is a measurement of wattage, so why ask the same thing twice in a sentence! And wtf does speaker wattage have to do with the system! The only thing that matters is impedance of the speakers. Stop pretending to know what you're talking about. ROFLMAO.

geek_lostdude, Jan 30, 4:44 pm

If the wattage of the speaker on one channel exceeds more than 50% of the RMS you have a real problem.

geek_henry284, Jan 30, 4:46 pm

WTF!
RMS is a measurement of Wattage. (Root-Mean-Squared)
So your statement in #7 is saying "if the power of the speaker is greater than the channel wattage you have a problem."
It is totally normal for speakers rated power to exceed the rated power of the amplifier. The other way around you have a problem.

geek_pheonix, Jan 30, 4:56 pm

Back to the original question .
How quiet is quieter! Can you hear the quiet channel clearly or have to have the ear against the speaker!
Have you tried swapping the left and right speakers to eliminate the speakers as the cause!

geek_pheonix, Jan 30, 4:59 pm

I think this guy just likes talking from his a*$e. Thankfully drsr has made a handy script for idiots like these.

geek_lostdude, Jan 30, 5:02 pm


Clearly you have no clue about any of this and a simply pulling things from your arse.
If the wattage rating of a speaker is less than the rated output of the amp then you run the risk of damaging the speaker - Not the amp.
Providing the impedance matches, you could quite safely connect a 5000W rated speaker to a 50W Amp. But a 5000W amp on a 50W speaker could blow the speaker inside out.
RMS is simply root mean square of the peak wattage to give a more realistic figure. I'm not going to explain that, you can google it.

As for the OP question, obviously ignore henry. If you can, I recommend the sound card oscilloscope suggestion. If there is nothing obviously wrong (like damaged caps) then signal tracing is the easiest way to do it. You could also try signal tracing with a small speaker if the oscilloscopes to hard to set up. Just start from the source and work your way through until you find the spot that does not match the other channel. Those old amps are pretty easy to work on.

geek_oclaf, Jan 30, 5:08 pm

A big thank you everyone for all your help.Having read your posts it is clear I am completely out of my depth so I think the time has come to admit defeat and take it in for repair.But thanks again for all your advice!

geek_mss2006, Jan 30, 8:41 pm