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Old 10th April 2014, 01:39 AM   #1
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Default Rotel RA-840BX3 repair?

I got this amp and when i took the cover off the temperature was very high and 2 resistors almost gone :

Click the image to open in full size.

I adjusted the bias current and dc offset according to the manual and temperature dropped significantly on the transistors.

The area next to the faulty resistor is still very hot (70C) same thing on the resistor itself.

This is not happening on the other pair of resistors (max 32C, right side in photo) but the values are very strange, 1kΩ for the broken one and around 5.7MΩ for the other three.

The manual says that they are 2W 1K...

So i need your help on what to replace, obviously the broken one for start.

I know that these amps running hot but at the moment i guess the heat is because of the faulty resistors.

The amp is working though and sounds great

Is it worth trying to repair?
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Old 11th April 2014, 01:44 PM   #2
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Anyone?
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Old 11th April 2014, 06:54 PM   #3
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There may be a few people able to help but you need to be more forthcoming with what parts you mean. To get help, post a schematic or a link to the one you have, assuming it's available free on the net. Then specify the resistors with their part numbers.
I found one here: B&W Group North America Service & Support - Service Manuals

Taking a guess, because there aren't that many 1k resistors, you must be referring to R642, 644 - the R channel driver transistor emitter resistors. These do waste a lot of power in this type of output stage so we expect them to get hot but if you read 5.7M, it will just be the circuit resistance, as the resistor will be history.

The hard question is, why? The reason is most likely the associated driver has failed and possibly 1 power transistor too. You can replace the resistor but the shorts will just cause it to run hot and fail again. When unpowered, test the transistors for shorts (both power transistors and the driver) then check the other 3 of opposite polarity as well. You can verify readings from the good channel if in doubt as to what to expect but don't apply power until you locate any such problem short(s) and replace the necessary parts.

Let's see what measurements show first.
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Last edited by Ian Finch; 11th April 2014 at 07:22 PM.
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Old 11th April 2014, 06:56 PM   #4
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this is the position of the resistors on the schematic The actual use of them I am not familiar with so i cannot comment on why these are getting hot ( topology i have never noticed before ) R641,2,3,4

Some other member may illuminate about this

If you need a schematic i have one

expect to replace them all use better type FP style resistors if the amplifier balances after all these should get warm but not hot ..

Thinking that this amplifier is designed to run hot is wrong warm may be but hot i dont thing so

Kind regards
Sakis
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 11th April 2014, 07:02 PM   #5
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Before any repair attempt set up voltage in the correct position Rated voltage in your country should be 240 and not 220

If originally was set up for 220 these few extra volts is enough to disturb bias up to a point and you may wana take a look at it again

Kind regards
Sakis
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 11th April 2014, 07:09 PM   #6
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Ian seems that we posted in the same minute ..>Do you have any idea on the actual use of this topology ?

good to see you !!
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Old 11th April 2014, 08:16 PM   #7
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The second yellow circle under the resistors indicate the area that gets really hot.
Yes Ian, i'm refering to resistors 644 & 642.
With the amp off R644 reads 0.88MΩ and R642 1KΩ.
R641 and R643 6.7MΩ but no hot area on the other channel

Can you guide me exactly which transistors to check?

I have the service manual and keep in mind that the amp is fully operational
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Old 11th April 2014, 08:54 PM   #8
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Check Q 623, 625, 629, 633, 627, and 631.

I've seen driver transistors get so hot they sort of desoldered themselves from the board over time, resulting in further catastrophe. On a big old Sony amp I was able to pull a couple of them without even desoldering them.

Check the board traces too. It's common for them to be toast.
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Old 11th April 2014, 08:59 PM   #9
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Use 2 watt resistors too. Replace all of them.
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Old 11th April 2014, 09:11 PM   #10
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Transistors are ok, but
there are many discoloured soldering points probably caused by high temp as Fast Eddie says.
Also some traces look with different colour.

Whats the solution here?
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