[jtwrace]

I know nothing about Bryston but I thought one of the main selling points was it had 20 year warranty.  Why wouldn't it just get sent back to them for repair if this is within the 20 year timeframe? 

[werd]

Don't use house 120V to trouble-shoot an amp. Too much current on tap. That is actually bad practice.

[Johnny2Bad]

I know nothing about Bryston but I thought one of the main selling points was it had 20 year warranty.  Why wouldn't it just get sent back to them for repair if this is within the 20 year timeframe?

If it's an original 3B that means built possibly as early as the 1970's. No 20-year warranty relief unless your Time Machine is up and running (mine is broken, in the shed waiting for a rainy day repair, right next to my non-working Mind Reader).

Having said that, Bryston are not the kind of people to leave their customers to fend for themselves. I strongly recommend you ship the unit to them and have them contact you with their recommendations.

I say this for many reasons, (and with a nod to user werd) but one reason is you don't seem to have a Variac, which would go a long way in helping you troubleshoot this particular issue, as well as adding some (not total) electrical safety margin. Really no-one should be opening up a vintage amplifier (this 3B could be as much as 40 years old) without one, even if it appears to be a working unit.

Finally, the age of the unit is itself an issue, as it would be with any audio component of the era it was manufactured, even if no problems obviously manifest themselves (such as in the working channel), and I once again strongly recommend having Bryston give it a go-over to insure it's in good operating condition for continued use in the future. At a minimum, it costs nothing to send them an eMail.

[KV4LQ]

Are you just plugging it in and using the house 120 volt to power the amp while you do this?

Yes.  I've got a 3.5 amp fuse inline with it though, and it's on a sensitive 15A breaker.

[KV4LQ]

If it's an original 3B that means built possibly as early as the 1970's. No 20-year warranty relief unless your Time Machine is up and running (mine is broken, in the shed waiting for a rainy day repair, right next to my non-working Mind Reader).

Having said that, Bryston are not the kind of people to leave their customers to fend for themselves. I strongly recommend you ship the unit to them and have them contact you with their recommendations.

I say this for many reasons, (and with a nod to user werd) but one reason is you don't seem to have a Variac, which would go a long way in helping you troubleshoot this particular issue, as well as adding some (not total) electrical safety margin. Really no-one should be opening up a vintage amplifier (this 3B could be as much as 40 years old) without one, even if it appears to be a working unit.

Finally, the age of the unit is itself an issue, as it would be with any audio component of the era it was manufactured, even if no problems obviously manifest themselves (such as in the working channel), and I once again strongly recommend having Bryston give it a go-over to insure it's in good operating condition for continued use in the future. At a minimum, it costs nothing to send them an eMail.

email.  Good idea.  Where can I find the contact info?

And I'm not worried about the components's age so much, because my grandfather replaced most of the power supply section.  Would it be a bad idea to fire it up without the resistor in place?

[KV4LQ]

Don't use house 120V to trouble-shoot an amp. Too much current on tap. That is actually bad practice.

What would you recommend?  I don't have access to something else (unless I grab my 300W 600W peak inverter)

[srb]

What would you recommend?  I don't have access to something else (unless I grab my 300W 600W peak inverter)

Variable Autotransformer (VARIAC).  Lots of units on the market, some very expensive - this Tenma 72-110 10A unit is popular among audio hobyists.  Best price seems to be from Newark (just under $150).

 

Steve

[KV4LQ]

Variable Autotransformer (VARIAC).  Lots of units on the market, some very expensive - this Tenma 72-110 10A unit is popular among audio hobyists.  Best price seems to be from Newark (just under $150).

 

Steve

I know what these are, I just don't have one and can't afford one.

[werd]

What would you recommend?  I don't have access to something else (unless I grab my 300W 600W peak inverter)

You do nothing, do not use the wall for power. Its dangerous on so many different levels. You already know its busted. Also using a meter which means both hands will be over the chassis. You do not want current through both hands.. Its just bad... Does that amp have a rocker fuse?

You need a fused Variac power supply designed for this, then power the board.  Until then forget it.

[KV4LQ]

You do nothing, do not use the wall for power. Its dangerous on so many different levels. You already know its busted. Also using a meter which means both hands will be over the chassis. You do not want current through both hands.. Its just bad... Does that amp have a rocker fuse?

You need a fused Variac power supply designed for this, then power the board.  Until then forget it.

So nobody's actually going to tell me anything as to why that resistor is blowing?

[audio.bill]

Anyone can tell you that the resistor is blowing due to excessive current flow, the real question is what is the cause of such high current. That can be due to a number of possibilities, and as others have suggested you would be best advised to have it serviced by Bryston or other qualified service personnel. With all due respect, unfortunately having a multi-meter and soldering iron does not make one qualified.

[KV4LQ]

Anyone can tell you that the resistor is blowing due to excessive current flow, the real question is what is the cause of such high current. That can be due to a number of possibilities, and as others have suggested you would be best advised to have it serviced by Bryston or other qualified service personnel. With all due respect, unfortunately having a multi-meter and soldering iron does not make one qualified.

Mere possession of the implements does not make one qualified, but I have been working on equipment for years.  I know what I'm doing.  I acutally *use* my soldering iron.  I've restored several old amateur transceivers and receivers.  I've restored many items.  All I'm asking is if there's a common mode of failure for this amp.

[werd]

At first i thought it read the transistor started on fire. That would be DC. If an amp is going to blow i want to see flames out the back...  .

No, seriously put it away until you get the right electronic stuff. You are good to do it but don't turn the thing on anymore using house power. You may not have to send the whole amp either if you want Bryston to fix it. Maybe only send that channel. I would talk to them first. 

[KV4LQ]

At first i thought it read the transistor started on fire. That would be DC. If an amp is going to blow i want to see flames out the back...  .

HAHA! Nope, that final would make a lot larger flame .  This is just a 1/4W resistor!

[*Scotty*]

First have you checked for the presence of DC offset  at the speaker terminals of the channel that is blowing the resistor.
Not knowing the circuit design and having no circuit diagram to refer to I couldn't tell you the location of the resistor in the circuit. One of the transistors in the dif pair could have failed. You may have lost a transistor in the pre-driver, driver stage or output stage of the amp. You could also have lost a transistor in an active current source or this could be a resistor type current source and it is blowing due to a failure of a component down stream of it.
 Multiple failure modes are possible, I would advise getting a circuit schematic to identify the resistors location in the amp and work backwards from the output stage until you find failed components. A scope, DMM and distortion analyzer would be handy to have. The amp could be oscillating at ultrasonic frequencies which can cause many things in the circuit to go up in smoke. The scope will show this. Be sure you are using shorting plugs on the inputs of the amp before powering it up to test for circuit faults.
Scotty

[KV4LQ]

First have you checked for the presence of DC offset  at the speaker terminals of the channel that is blowing the resistor.
Not knowing the circuit design and having no circuit diagram to refer to I couldn't tell you the location of the resistor in the circuit. One of the transistors in the dif pair could have failed. You may have lost a transistor in the pre-driver, driver stage or output stage of the amp. You could also have lost a transistor in an active current source or this could be a resistor type current source and it is blowing due to a failure of a component down stream of it.
 Multiple failure modes are possible, I would advise getting a circuit schematic to identify the resistors location in the amp and work backwards from the output stage until you find failed components. A scope, DMM and distortion analyzer would be handy to have. The amp could be oscillating at ultrasonic frequencies which can cause many things in the circuit to go up in smoke. The scope will show this. Be sure you are using shorting plugs on the inputs of the amp before powering it up to test for circuit faults.
Scotty

Thank you for the first really useful answer.  I'll make sure to check all of this when I get the time.  I've got a scope, but no distortion analyzer.  What would the analyzer be used for, and what could I substitute?

[KV4LQ]

UPDATE:  I checked, and it appears that one leg of the resistor is connected to +55VDC.  The other leg is connected to a capacitor, which leads me to believe that this is the resistor:





This is page 3 of http://bryston.com/PDF/Schematics/3B-8BST_SCHEMATICS.pdf

[*Scotty*]

While the harmonic distortion analyzer is nice to have you don't have to have it to successfully repair the amp.
The distortion analyzer is used to check the working channel to get a baseline of the amplifier's THD+N figure. when the other channel is fixed you can check it against the one that was still functional. Sometimes when you replace a transistor that is supposed to be a correct substitute part for the original transistor the THD figure goes to hell and you are looking for yet another one in the list that is supposed to work in this application. The amp may be old enough that finding the original parts used by Bryston may not be doable.
 I'm not saying that you are going to be in this position be it's possible.
Scotty

[KV4LQ]

While the harmonic distortion analyzer is nice to have you don't have to have it to successfully repair the amp.
The distortion analyzer is used to check the working channel to get a baseline of the amplifier's THD+N figure. when the other channel is fixed you can check it against the one that was still functional. Sometimes when you replace a transistor that is supposed to be a correct substitute part for the original transistor the THD figure goes to hell and you are looking for yet another one in the list that is supposed to work in this application. The amp may be old enough that finding the original parts used by Bryston may not be doable.
 I'm not saying that you are going to be in this position be it's possible.
Scotty

Ah I see.  Well, my grandfather had used that replacement transistor in other brystons, so I'm assuming that it worked fine.  I'll throw the scope on it eventually, assuming it will stay on for a bit without going up in flames.  Last time, the resistor went up in flames instantly.

[*Scotty*]

I think you should go looking for failed transistors in the amp before powering it up again. Burning up the resistor repeatedly is not likely to reveal which transistor is blown and could result in further damage if possible.
Scotty