Rotel RB980BX power amp hum and "pop"

[metako]

I just plugged in for the first time a Rotel RB980BX power amp that I bought off ebay and there is a hum in both speakers (this is with preamp connected to power amp but the preamp not turned on). Its louder in the left speaker and when I pull out the interconnects from the preamp (which has its power off) the hum gets louder. The Rotel only has a 2 pin AC plug. Does this sound like failed power supply caps? Is there an easy way to test the caps without complex equipment? (I only have a mulimeter). Why do the interconnects affect the level of the hum?

Also, there is a loud "pop" when I switch the power amp off! The amp has a delayed power on so I didnt think it would pop on switch off. Would cleaning fix this or has a part failed?

Seller said the amp was in excellent working order so has misrepresented it. Like to know what I'm up for before I contact him. I swear this is the last time I buy something off ebay without hearing it! Thanks.

 

[Mooly]

Hi, It's going to be more involved than that, for sure. You need a meter to do measurements and possibly a circuit-- depends on how experienced you are.

 

[metako]

Havent been able to find a circuit for it except the clone given in the thread below and I dont know how close to the Rotel it is.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=40432&highlight=

Also I dont know alot about amps so I guess I'm in trouble!

 

[Mooly]

Have you a meter

 

[metako]

A multimeter yes.

 

[Tube_Dude]

Hi

Have you inspect the obvious?... the fuses!

 

[metako]

Can the fuses cause hum???

 

[Mooly]

Check with meter on DC volts and with no speakers connected whether there is any voltage present at the speaker terminals.

 

[Tube_Dude]

Can the fuses cause hum???

IF the amp is working with only a voltage rail, yes!

 

[metako]

measuring across pos and neg of each speaker terminal there is 0.026V on right and 0.048V on left.

Tube_dude can I measure the fuses or just replace?

 

[Mooly]

That sounds good ! You say it's just a power amp. Do the speakers hum if you now connect them to the amp with no inputs connected at all. Leave the inputs floating.

 

[metako]

yes they still hum-even louder than with no inputs. The sound seems to be wavering slightly or oscillating.

 

[Tube_Dude]

Tube_dude can I measure the fuses or just replace?

Inspect with a multimeter for continuity...

Inside, see if any connector is disconnect during shipment.

Even with the hum does the amp work properly ?

 

[Mooly]

Sounds intriguing-- I have to dissappear now ! Will look in again later today hopefully, sounds a bit strange really.

 

[metako]

Will check fuses and inside. Yes amp works OK (well as well as can be expected with a Tandy/Radio shack preamp ).

Thanks for you help Mooly and Jorge.

 

[Tube_Dude]

Is the switch near the RCA inputs in the Stereo position and not in the bridge position?

 

[metako]

yes its in stereo position.

 

[metako]

I tested the four internal fuses for continuity (in situ) and they're all OK. No obvious loose wires either.

 

[Tube_Dude]

I tested the four internal fuses for continuity (in situ) and they're all OK. No obvious loose wires either.

With the amp disonnected, inspect for continuity (0 Ohms), between the outer side of the input RCA and the output speaker minus binding post...

 

[metako]

well when I touch the pos and neg probes of the meter together I get 0.4 ohms. Then when I connect the outer phono socket and the neg speaker post I get 0.5 ohms, so I guess the real value here is 0.1 ohms. Also its strange but I sometimes get continuity between the outside of the right phono socket and the POSITIVE right speaker post, but its not always - its intermittant! That seems weird. There shouldnt be any continuity there should there? (its now happening with the left too)