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?
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.
An externally hosted image should be here but it was not working when we last tested it.
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?
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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.
They are Sanyo 2SB631K and 2SD600K transistors. For this particular task, MJE243/253 should work adequately.
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?
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?
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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.
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.
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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?
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?
My old and lousy DT-830B doesn't even have the feature of checking for shorts, so I have no reliable tools for checking it. I am however ordering a new one, and was thinking of the BK Precision 2709B, but I haven't decided yet.
Anyways, back to the amp. How do I proceed in repairing this sucked? Do I just go ahead and change all the burned components, or is there a smarter way?
Anyways, back to the amp. How do I proceed in repairing this sucked? Do I just go ahead and change all the burned components, or is there a smarter way?
Id say pull and test components... however it really depends how much is toast. If most of the amp is toast, is it worth the cost?
I usually pull all the transistors, that leaves most of the other components easy to test in situ.
The biggest danger with amp faults is just replacing one faulty item without getting at the cause of the problem. Sometimes one bad component can take with it others.
Had an interesting fault on one amp, I pulled all the transistors and one transistor had a gain of 1 !
Of course it wasthe last transistor I pulled out, sods law.
I guess its not necessarily dead transistors, especially not the output stage ones, even though the soldering has been burned?
I've been thinking the same, but my idea is that I can learn from the repair (still a student), and if I get it up and running, I get a free amp. So ye, it might be worth it.
But I'm still a bit curious about how I've lost almost all the sound in the right channel. Where is the differentiation circuit wise between the two channels?
Edit:
When you say "pull transistors", do you then implide that you de-solder them, test them in a multimeter for instance, and then re-solder if it works, otherwise replacing it?
I've been thinking the same, but my idea is that I can learn from the repair (still a student), and if I get it up and running, I get a free amp. So ye, it might be worth it.
But I'm still a bit curious about how I've lost almost all the sound in the right channel. Where is the differentiation circuit wise between the two channels?
Edit:
When you say "pull transistors", do you then implide that you de-solder them, test them in a multimeter for instance, and then re-solder if it works, otherwise replacing it?
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The solder on the bottom of the PCB is burned. I have attached a handful of pictures.
http://img59.imageshack.us/img59/2237/20110605011442.jpg
http://img844.imageshack.us/img844/897/20110605011506.jpg
http://img690.imageshack.us/img690/7769/20110605011518.jpg
http://img151.imageshack.us/img151/9417/20110605011531.jpg
http://img853.imageshack.us/img853/3066/20110605011727.jpg
The fourth one shows that even the in/output connectors have burned solder, and the final one shows the leakage on the PSU caps.
http://img59.imageshack.us/img59/2237/20110605011442.jpg
http://img844.imageshack.us/img844/897/20110605011506.jpg
http://img690.imageshack.us/img690/7769/20110605011518.jpg
http://img151.imageshack.us/img151/9417/20110605011531.jpg
http://img853.imageshack.us/img853/3066/20110605011727.jpg
The fourth one shows that even the in/output connectors have burned solder, and the final one shows the leakage on the PSU caps.
The stuff you're seeing is flux residue, where the board has not been completely cleaned after soldering. Clean the flux residue away... you can scrape most of it off with a screwdriver or tooth pick. You can also get special flux cleaner that will remove it more thoroughly.
There is some evidence of the traces heating especially on Q901/902 - probably local overheating from transistors running hot and not being heat sinked. Now I think about it, these should probably either be heat sinked or TO-220 package devices rather than TO-126.
That brown stuff around the main supply capacitors is actually glue. This is used to keep those large capacitors firmly secured.
There is some evidence of the traces heating especially on Q901/902 - probably local overheating from transistors running hot and not being heat sinked. Now I think about it, these should probably either be heat sinked or TO-220 package devices rather than TO-126.
That brown stuff around the main supply capacitors is actually glue. This is used to keep those large capacitors firmly secured.
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