Advice on burned transistor replacement?

[Alaza]

Hello,

A friend of my handed my his Rotel RA-840BX4 which had almost lost all sound in the right channel. After taking the casing of it was clear that two of the transistors were burned, and possibly two caps as well.

The schematic can be found in the service manual here (login required):

Rotel RA840BX4 service manual

For simplicity sake I've added a screenshot of the circuit.



The components concerned are Q901, Q902, C911 and C1912. I'm not sure about the purpose of the circuit, but I reckon it's some sort of pre-amplification for the right channel?

My idea is to replace the caps with some Nichicon I have spare, but the transistors I'm not so sure of. Which one would you suggest?

[sofaspud]

It appears to be part of the power supply. Difficult to read the characters, but Q902 looks like 2SD500M.

[Alaza]

I've uploaded a higher res screenshot. It's a 2DD600K and 2SB631K.

[jaycee]

In this circuit they work as power supply regulators. Note that if they are burned, it most likely means there is a fault in the circuit they are powering. In this case they provide power to the opamps, so I'd replace those.

They are Sanyo 2SB631K and 2SD600K transistors. For this particular task, MJE243/253 should work adequately.

[Alaza]

The op amp is connected to the volume pot, so could it be the old user's been pumping to much juice through the amp? The op amp i btw. a NE5532AN. Is there an obvious replacement for that?

Regarding the transistors, is there something that could resemble a universal replacement archive or the like?

Edit:

The MJE253 doesn't have a VCEO of 120V, but 100V - could this become a problem? And as a note, I'm a bit surprised that they don't have any heatsink on them?

[richie00boy]

They may have falied as they have no heatsink.

The voltage rating is fine as the device will only ever see 47 volts. You may find MJE340 and MJE340 easier to source. 2SA1209 and 2SC2911 should be available from Farnell in your country and would be a better choice though as they have higher gain.

I would speculate that as C903 and 904 are intact then the transistors failing has not allowed the full 47V to the pre-amp circuitry, as those caps are only rated 25V. However as Jaycee said there may be a fault down the line that caused the transistors to blow.

[Alaza]

Fair point, I should have noted that myself.

Anyways, after looking more thoroughly through the amp, it is clear that it has been massively misused one way or the other. There are burn marks on almost every solderpoint on the bottom of the PCB for all the transistors - most severely on the output transistors with heatsinks. But also the transistors C909 and C910 in the power supply have been leaking liquid (which seems almost corroded).

What on Earth could ever have resulted in such major failure throughout the amplifier?

[jaycee]

Possibly a mains surge or lightning strike

[Alaza]

So I take that replacing the more or less burned components would possibly restore the amplifier to its proper state?

[nigelwright7557]

Looks like a power supply.
Its possible the output transistors are gone and taken the supply with them.
Look for shorts on the output transistors.