Hello,
I have a Bryston 4B with some problem.
I need help in order to repair this power amp.
I got this amplifier (the version is around 1983) with a dead channel.
My first test has to test transistors and change dead transistor ( I found 4 of them in short circuit). I bought transistors (2N3773 and 2N6609) in asian store.
I also change RIFA brand spark suppression capacitors by MKP X2 , and the fuse (dead of course).
With these new transistors, the amp is alive 🙂
But the bias is too low (1.4 mv instead of 8-10mv), so I decided to modify the trimmer potentiometer position.
Immediately, the fuse blow up.....and I have 6 dead transistors ..... 🙁
Three possibilities :
- there is another problem on the channel that kill transistor
- It is my mistake because, I haven't respected Bryston requirements for bias adjust (i don't short channel input to ground and I haven't set load at output)
- I have received fake transistor (they seems good but ... )
=> If you have an idea , tell me please
So I have decided to make a complete check-up before a new try.
I have decided to change :
- 470µF/16V by 470µF/24V (5 by channel)
- 220µF/6.3V by 220µF/10V (2 by channel)
- 2k/1W resistor (4 by channel), their value are OK but resistors don't look good (like too hot)
- 1R as 7R as value, strange. I will change it too
- The 1k trimpoter by a multiturn bourns
Are you agree with my choice?
There are tantalum capacitor on the board (1.5µF/ 35V).
I have to change them ? They look good and they are not in short-circuit.
If yes, any 1.5µF/50V electrolytic capacitor is good ? (I have bought Panasonic but I hesitate to change tantalum by electrolytic , not exactly the same functions).
There is some test or other change to perform before power on this amp ?
Schematic are here :
http://shal.free.fr/Bryston/3B_4B_Pre-NRB_TECH_REF.pdf
I am a beginner in electronic, so any help is welcome .
English is not my native language, sorry for mistakes.
Thank you.
Olivier
PS : If you want look some photo, I can post here. Photos are also available on a french audio forum : Il faut sauver le soldat Bryston.. • LS3/5a le forum
I have a Bryston 4B with some problem.
I need help in order to repair this power amp.
I got this amplifier (the version is around 1983) with a dead channel.
My first test has to test transistors and change dead transistor ( I found 4 of them in short circuit). I bought transistors (2N3773 and 2N6609) in asian store.
I also change RIFA brand spark suppression capacitors by MKP X2 , and the fuse (dead of course).
With these new transistors, the amp is alive 🙂
But the bias is too low (1.4 mv instead of 8-10mv), so I decided to modify the trimmer potentiometer position.
Immediately, the fuse blow up.....and I have 6 dead transistors ..... 🙁
Three possibilities :
- there is another problem on the channel that kill transistor
- It is my mistake because, I haven't respected Bryston requirements for bias adjust (i don't short channel input to ground and I haven't set load at output)
- I have received fake transistor (they seems good but ... )
=> If you have an idea , tell me please
So I have decided to make a complete check-up before a new try.
I have decided to change :
- 470µF/16V by 470µF/24V (5 by channel)
- 220µF/6.3V by 220µF/10V (2 by channel)
- 2k/1W resistor (4 by channel), their value are OK but resistors don't look good (like too hot)
- 1R as 7R as value, strange. I will change it too
- The 1k trimpoter by a multiturn bourns
Are you agree with my choice?
There are tantalum capacitor on the board (1.5µF/ 35V).
I have to change them ? They look good and they are not in short-circuit.
If yes, any 1.5µF/50V electrolytic capacitor is good ? (I have bought Panasonic but I hesitate to change tantalum by electrolytic , not exactly the same functions).
There is some test or other change to perform before power on this amp ?
Schematic are here :
http://shal.free.fr/Bryston/3B_4B_Pre-NRB_TECH_REF.pdf
I am a beginner in electronic, so any help is welcome .
English is not my native language, sorry for mistakes.
Thank you.
Olivier
PS : If you want look some photo, I can post here. Photos are also available on a french audio forum : Il faut sauver le soldat Bryston.. • LS3/5a le forum
Be very careful about buying tranies from Asia. There are a lot fakes out there.
I used to repair Brystons at my shop and on a few occasion Bryston sent me the output tranies for free.
Now this is a very old unit and I'm not sure that Bryston would still support it but if you can get the tranies from Bryton this is best. They computer match all there output transistors.
If the bias won't set up then the beta and or forward BE drop of the transistor is wrong. Possibly fake.
You shouldn't have to modify the bias circuit to set bias. A load is not required for this initially.
I used to repair Brystons at my shop and on a few occasion Bryston sent me the output tranies for free.
Now this is a very old unit and I'm not sure that Bryston would still support it but if you can get the tranies from Bryton this is best. They computer match all there output transistors.
If the bias won't set up then the beta and or forward BE drop of the transistor is wrong. Possibly fake.
You shouldn't have to modify the bias circuit to set bias. A load is not required for this initially.
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When you don't have a load attached, the input short is not strictly required either, though you may see output offset fluctuating too much for a precise adjustment (I don't remember whether these amps even have that though).
@davada :
Thank for your response.
I have already bought new transistor (asian too, they are very cheap 😱 ), so I will perform next test with this tranies (after change capacitor , resistor and trimpot).
If problems continues, I understand that , with high probability, the problem is fake tranies.
So, in this case, I have to bought "official" transistors (not cheap at all 2N3773 )
Are you OK with the other change (capacitor , resistor and trimpot ) ?
What do you think about change the tantalum capacitor or not....?
@sgrossklass
In Bryston technical data, there is Check-out procedure (see my first link)
They said to set a dummy load (8ohm / 500W) and "short channel input to ground"
The offset should be measured "with input still shorted to ground"
Olivier
Thank for your response.
I have already bought new transistor (asian too, they are very cheap 😱 ), so I will perform next test with this tranies (after change capacitor , resistor and trimpot).
If problems continues, I understand that , with high probability, the problem is fake tranies.
So, in this case, I have to bought "official" transistors (not cheap at all 2N3773 )
Are you OK with the other change (capacitor , resistor and trimpot ) ?
What do you think about change the tantalum capacitor or not....?
@sgrossklass
In Bryston technical data, there is Check-out procedure (see my first link)
They said to set a dummy load (8ohm / 500W) and "short channel input to ground"
The offset should be measured "with input still shorted to ground"
Olivier
If the tant are not shorted them leave them alone.
Bryston was overly fussy about setting the bias. The offset should be nulled first so you don't interpret it as bias current. otherwise just subtract it from your measurement on one side or add it to the other side accordingly. Depend on the offset polarity.
Someone wanted bias instruction so this is what Bryston came up with.
It just doesn't matter that much.
As power increases a bias set for 1 watt will break up at 50 watts. So decide how you want to do it. For 1 watt or 50 watts.
Also stick a light bulb in series with the AC cord untill you know you have thing right. This will help save you output transistor.
If the bias goes too high the light build will ight up and limit the current. Cheap mans variac.
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