Rotel amplifier buzzing, please help

[lt cdr data]

Hi I have a rotel power amplifier, its buzzing,

symptoms:

no smells
no charred resistors
no dodgy looking caps.
no dodgy looking wires or connections visible.

no dc offset on the output

with speaker leads connected and no inputs, amp is silent.

only when phono leads are connected to amps inputs, a low level buzzing/humming occurs, no more dc offset, ac offset on outputs seems to go up about 2 mvolts

no untoward ac or dc on inputs.

amp plays music fine.

I am fairly sure its not earth loops.

I bought 2 of these offa someone, and both amps exhibit similar symptoms, it seems as though something must have happened simultaneously to trigger this, but he couldn't point to a specific event. They were ok b4 afaik, unless he didn't notice it until lately.


any ideas, as to diagnosis and cure, I suspect its not much at all?

[mitch90]

Look for dry caps or leaky transistors in phono section.


Regards,


MITCH.

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no ideas???

[singa]

Hi,
Buzzing can be caused by loose plates of EI power transformer.Confirm by listening to the transformer.If so, a little super glue or clear cement/craft glue will to the trick.

OR you have ground loop problem with turntable.

[lt cdr data]

Its got no phonostage, and very few electrolytics, which all appear fine.

I suspected earthing, its also unusual for a jap amp to develop a glitchy fault, usually, they are excellent at fully developed products.

I have 2 and both are showing the same symptoms, very odd.

schematic here:



pretty simple?

that's about it, the little network on the lhs is a bridging one.

its just 2 monobloc amps in one box, with a common toroidal power tx.

the only elcaps are the feedback one, and the bias one, both of which I will shortly replace (any benefit to be had in upping cap values?)

repeat, it only starts to buzz (its not a mechanical buzz at such, it thro the speakers)

when a phono input is plugged in, and it happens on each channel independantly.

hope someone can help.

[homertooties]

Hi there i have had a few rotels in the past my favorite was the rb981 it would drive anything and not even break a sweat,however i had an ra960bx which had a humming problem i replaced the main psu capacitors normally in pairs fed by the bridge rectifier or 4 discrete diodes ,this worked on mine also check all the phono sockets and speaker terminals,best to resolder the lot as they dry joint hope this helps.

[djsbriscoe]

Hi,
Does this happen on all the inputs?
Have you checked the phono sockets for continuity or intermittent contact.
It could be a simple electrical connection fault, eliminate this possibillity before suspecting anything else.

David.

[Leolabs]

"Print Screen" method. Check for possibly dry solder joints.

[lt cdr data]

hi thanks for replies, its just a power amp, so only 1 pair of phono sockets in, and the speaker sockets out

I have 2 of the same amps, both are giving the same problem.

yes the print screen methoe

There are no bad signs optically anywhere, no duff components, no dodgy looking joints, they all look in very good order,

but I will resolder the lot, the board appears fine underneath

the sockets 'mate' ok, but that doens't preclude dry joints.

They are great amps, I suppose I will have to get the resevoir caps, too, I am going to need 8 10k uF, so its gonna cost a little.

[gni]

Look for mechanical first; loosen, tighten, little clear finger nail polish.
Second, my amps "sing" at higher powers; oscillations!
Bad Caps. . .just replace all that you can get your hands on.
Check solder joints. . .
Short all inputs. . .noise? no noise? My preamp on every other
odd day will throw extra "noise" into the signal at high gain. . .short
any inputs not being used. . . quiets down the system.

Take it step by step. . . short inputs. . .mechanical. . . solder. . . .caps!

If that doesn't clean up the signal. . .might be bad transistors. . . see if
you can still get them. . . but I'll bet it is a cap. . .somewhere in the
signal path.

Chris