Help Biasing BRYSTON 4Be

[angelothewolf]

Hi everyone!

i cant figure out how to bias this amplifier, one side is heating more than the other, i got a 10k blue trimpot (uper left corner on the schem) and a 2K2 big carbon trimpot (1k on the schem), and two test points PT1 & PT2, any thoughts?

thanks!

[g1]

1-800-632-8217
I haven't talked to them for a long time but last time I did they were very helpful.

[Jazz P Bass]

I cannot make out any test points.
Everything is smudged.

Anyway, as far as I can see the 2K trim pot is the bias adjust.
The 10K trimmer appears to be a DC offset adjust.

I don't think either will help if one side is hotter than the other.

Bias checking would be accomplished by measuring across the 0R15/ 5W ballast resistors.
I would measure those two points before doing anything.
You want to see millivolts.

No signal, no load.

[g1]

The 4BE name became 4B NRB. I've read that the bias is set the same as the SST, but suggest you check with the factory. The 1K trimmer should be the bias adjust.

[Enzo]

Jazz, I read it as one side as in one channel is running hotter, not one side of a particular channel. The bias trim ought to help that.

[Jazz P Bass]

Agreed.

If it is off.

[angelothewolf]

Well i start by measuring the 0R15/ 5W resistors, in the left channel of the amp (the good one) i get around 15mV on each resistor, now in the right channel i measured 30mV on the + side and around 65mV on the negative side resistors, so i get my screwdriver and turn the bias pot one millimeter, and then the mains fuse blow up, and now i got 2 shorted output transistors

So, i think the next step is to replace all output transistors (BR6521/BR6522) with a decent replacement maybe MJ15024/MJ15025?? what you think??

thnks!

[angelothewolf]

hello hello!

[The Dude]

Those should work fine as subs. Start the bias all the way off and bring it up slowly after replacing the shorted outputs.

Edit: You may also want to build a light bulb limiter until you get it working to save yourself making a pile of shorted transistors.

[Dandy Don]

Bryston Bias Pots

Those should work fine as subs. Start the bias all the way off and bring it up slowly after replacing the shorted outputs.

Edit: You may also want to build a light bulb limiter until you get it working to save yourself making a pile of shorted transistors.

Bryston says they haven't had problems with their 2.2k ohm bias pots that are used in many of their amps, but in my limited experience, I have had two amps blow output transistors due to bias issues. Corrosion occurs at the wiper arm pivot (see the photo). In most of my amps, I have replaced the bias pots with multi -turn sealed Bourns pots. The only thing is you have to be certain to set the pot to give minimum bias current before powering up. The original pots can be modified as shown in the photos to act as a carrier for the Bourns 3299 model pots.

[angelothewolf]

Bryston says they haven't had problems with their 2.2k ohm bias pots that are used in many of their amps, but in my limited experience, I have had two amps blow output transistors due to bias issues. Corrosion occurs at the wiper arm pivot (see the photo). In most of my amps, I have replaced the bias pots with multi -turn sealed Bourns pots. The only thing is you have to be certain to set the pot to give minimum bias current before powering up. The original pots can be modified as shown in the photos to act as a carrier for the Bourns 3299 model pots.[ATTACH=CONFIG]33447[/ATTACH][ATTACH=CONFIG]33448[/ATTACH]

Thanks Dandy Don, yesterday arrived the full set of mj15024/25 transistors from Mouser. i replace all transistors on both sides of the amp and all electrolitic caps. unfortunately i have several transistors shorted again. i was trying to set bias, one channel at a time, with light bulb on the mains. measuring mV from V+ to test point on the emitter resistors, i started on the begining of the trimpot reading just a few mV, then pfff shorted, maybe i have that trimpot problem, what you think?

[Dandy Don]

Thanks Dandy Don, yesterday arrived the full set of mj15024/25 transistors from Mouser. i replace all transistors on both sides of the amp and all electrolitic caps. unfortunately i have several transistors shorted again. i was trying to set bias, one channel at a time, with light bulb on the mains. measuring mV from V+ to test point on the emitter resistors, i started on the begining of the trimpot reading just a few mV, then pfff shorted, maybe i have that trimpot problem, what you think?

Angelo, "just a few mV" is all you need. If the bias pot has an intermittent contact, this could be what is causing it to eat output transistors (I've been there!) Find out the bias spec for your amp. You should be able to find instructions on the Bryston website. For older amps, biasing instruction are in http://www.bryston.com/PDF/Other/3B_...B_TECH_REF.pdf. Typically, the bias current is set to half its final value and then, after the amp is fully warmed up, the bias is slowly increased in small steps until you reach the final value.

Check your pots with the power off using an ohm meter connected between the wiper and one of the other terminals. It should change smoothly as the pot is turned. If it is jumpy, turn the wiper back and forth several times. If you have contact cleaner with a lubricant, this would be a good idea. If there is a bad contact, it will likely be in the wiper pivot, not where the wiper contacts the resistive element. Since the Bryston bias pot is exposed to the atmosphere, this is why I like change them for sealed pots.

If you can't figure it out, send it to Bryston. Their service is excellent. If your amp is over 20 years old, it won't be under warranty, but in my experience they do great work at a very reasonable price. And they fully test amps after they repair them.

[Jazz P Bass]

It would be a better approach to ditch the limiter when trying to set the bias.

[angelothewolf]

It would be a better approach to ditch the limiter when trying to set the bias.

of course but the last time i did that the transistors shorted out just by turning the trimpot one millimiter. the trimpot seems ok, but i think its a good idea replace it with a sealed one.
i was reading the same ¨tech reference¨ sheet that J P Bass suggested, wich indicates seting the bias around 4mv to 6mv, but my measure never drops below 10mv, so i think ive got another problem here...

thanks!

[The Dude]

You'll want to check over the entire circuit THOROUGHLY before you fire it up again. Check for open resistors, shorted/open driver transistors, etc. Spend some time checking the rest of the amp and not just the outputs. It'll save you some grief. Also, when you do fire it up again and before you touch a bias pot, measure the output for DC (no speaker). If you detect any DC at the output, there's no sense fiddling with bias. The amp is still broken.