Hi,
One of the transistors in my Rotel RB-993 has shorted.
A toshiba 2sc5200
Easy to swap in a replacement.
However had a thought, it has been a while since this amp was designed, surely there must be something better out there now.
So what's your thoughts?
This amp is used to power the centre and surrounds in music/movie set up.
The front amps are Tag 8000p.
Current amp schematic attached:
https://docs.google.com/open?id=0B0Wtap7K_Yw_a0p0blFpaENQZjg
One of the transistors in my Rotel RB-993 has shorted.
A toshiba 2sc5200
Easy to swap in a replacement.
However had a thought, it has been a while since this amp was designed, surely there must be something better out there now.
So what's your thoughts?
This amp is used to power the centre and surrounds in music/movie set up.
The front amps are Tag 8000p.
Current amp schematic attached:
https://docs.google.com/open?id=0B0Wtap7K_Yw_a0p0blFpaENQZjg
Why should there be any better transistors than the near legendary 2SC5200?
It's better to find out what made the short for You, than trying to replace with "a stronger device".
And why so? Well, neither of them will withstand a shortcirquit anyway.
And the 2SC5200 is still a very, very good transistor.
It's better to find out what made the short for You, than trying to replace with "a stronger device".
And why so? Well, neither of them will withstand a shortcirquit anyway.
And the 2SC5200 is still a very, very good transistor.
Even with no load connected both board protection fuses go pop spectacularly.
Am very basic with electronics, but a shorted transistor is my guess.
Eventually I will remember to bring my multimeter home to give it a look over.
The main protection circuits do not detect a fault, it is just the 1 amp board.
Am very basic with electronics, but a shorted transistor is my guess.
Eventually I will remember to bring my multimeter home to give it a look over.
The main protection circuits do not detect a fault, it is just the 1 amp board.
You cannot just drop in "better" transistors and get better performance. The circuit would have to be tailored to them, or problems such as oscillation will result.
There's nothing wrong with the 2SA1943/2SC5200. They are obsolete now, mainly due to rife copying and counterfeiting - Toshiba replaced them with TTA1943/TTC5200. There are some third party manufacturer clones made by Magnatec and Fairchild.
There's nothing wrong with the 2SA1943/2SC5200. They are obsolete now, mainly due to rife copying and counterfeiting - Toshiba replaced them with TTA1943/TTC5200. There are some third party manufacturer clones made by Magnatec and Fairchild.
It will be an output stage fault (and associated damage)
Are you sure your up to working on this ? Its ultimately a fairly straightforward circuit but don't be under any illusions of just swapping an odd transistor and it being fixed. Although you could get lucky its a fair bet there is other damage as well.
As a general procedure you need to,
1. Disable the vbe multiplier to reduce output stage current to zero on both channels.
2. Remove and test all the output devices and drivers as a basic first step. Whatever the outcome of the tests I would be inclined to replace the drivers anyway as they can be intermitent in failure and this can cause the problem of destroying the outputs. It's just not worth the risk of leaving them.
3. Check low value resistors that could have been damaged by transistor failure.
4. Optional step. I would then test the amp with NO outputs fitted (done by moving the feedback resistor take off point to the vbe multiplier) and confirming the DC conditions are correct.
5. Replace outputs and test and set up.
ALL the above done with a bulb tester in place.
Are you sure your up to working on this ? Its ultimately a fairly straightforward circuit but don't be under any illusions of just swapping an odd transistor and it being fixed. Although you could get lucky its a fair bet there is other damage as well.
As a general procedure you need to,
1. Disable the vbe multiplier to reduce output stage current to zero on both channels.
2. Remove and test all the output devices and drivers as a basic first step. Whatever the outcome of the tests I would be inclined to replace the drivers anyway as they can be intermitent in failure and this can cause the problem of destroying the outputs. It's just not worth the risk of leaving them.
3. Check low value resistors that could have been damaged by transistor failure.
4. Optional step. I would then test the amp with NO outputs fitted (done by moving the feedback resistor take off point to the vbe multiplier) and confirming the DC conditions are correct.
5. Replace outputs and test and set up.
ALL the above done with a bulb tester in place.
The offer of help is appreciated.
One embarrassingly basic question. I have a RB-991 amp in bits. )All it needs is one cap to be soldered in and it is perfect for resale. However that cap has been sellotaped to the top of it for a year waiting for me to fit.)
Are the boards swapable between the amps, or is there some setup that needs to be done with each board {like swapping a transistor}
One embarrassingly basic question. I have a RB-991 amp in bits. )All it needs is one cap to be soldered in and it is perfect for resale. However that cap has been sellotaped to the top of it for a year waiting for me to fit.)
Are the boards swapable between the amps, or is there some setup that needs to be done with each board {like swapping a transistor}
Well it almost went well.
I swapped out the transistors and checked over the board. All seems good.
I then remembered (OK got reminded) that I needed to recheck the bias.
I unplugged the inputs/speakers and turned the amp on.
Bias was fine on board 3 (which I was working on).
Bias on board 1 was 0....
Turns out I have now lost the output transistors on channel 1 now!!!
This seems a bit much to be a coincidence.
So what could take out the transistors on both sides?
This is WAY over my head.
p.s. I have 1 set of spare transistors left. If I lose another set this amp is dead (the odds of getting genuine spares on this are next to 0)
I swapped out the transistors and checked over the board. All seems good.
I then remembered (OK got reminded) that I needed to recheck the bias.
I unplugged the inputs/speakers and turned the amp on.
Bias was fine on board 3 (which I was working on).
Bias on board 1 was 0....
Turns out I have now lost the output transistors on channel 1 now!!!
This seems a bit much to be a coincidence.
So what could take out the transistors on both sides?
This is WAY over my head.
p.s. I have 1 set of spare transistors left. If I lose another set this amp is dead (the odds of getting genuine spares on this are next to 0)
Have the transistors gone short circuit ?
Any fault finding has to start at the begining. Maybe the bias was too high and thermal damage occured. Its impossible to say at a distance.
To fix something like this isn't a quick job. It must be gone through methodically. Everything should be confirmed as good before the bulb tester is removed and speakers connected.
Any fault finding has to start at the begining. Maybe the bias was too high and thermal damage occured. Its impossible to say at a distance.
To fix something like this isn't a quick job. It must be gone through methodically. Everything should be confirmed as good before the bulb tester is removed and speakers connected.
"you didn't by any chance swap left and right boards over and replaced the good ones with two other good transistors?" I wish it was that easy. I did have to double check.
Thermal damage.... seems likely these amps run HOT.
I checked the bias on board 3 after swap, bias was over 200 (supposed to be 7).
Bias on board 2 is 6.5, so fine. Can't measure board 1 until I swap trans.
Now also have a problem on my RB-991 too, right channel is getting no high frequency.
I have come to the conclusion that old equipment that has not moved for years does not like being touched!
Thermal damage.... seems likely these amps run HOT.
I checked the bias on board 3 after swap, bias was over 200 (supposed to be 7).
Bias on board 2 is 6.5, so fine. Can't measure board 1 until I swap trans.
Now also have a problem on my RB-991 too, right channel is getting no high frequency.
I have come to the conclusion that old equipment that has not moved for years does not like being touched!
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