Hints for Bryston 3b repair?

tronnyjenkins · Nov 6, 2015

Our 3b started acting up, and is no longer under warranty.
We aren't really interested in the full repair service available from Bryston as it is a little expensive for us and we are just using this amp for our backup/reference monitors. We got this amp basically as a freebie with the NS10s, but I know it is a quality piece. To me it's worth a shot to repair it before just using it as a "boat anchor".

My track record for repair of non-functioning equipment isn't great (couldn't find the issues), but I have built eight CAPI preamps and op-amps that worked first try, so I am OK with a soldering iron.

The amp was working fine, but a hum appeared on the R side channel. It's loud enough to render the amp unusable, but you can still hear audio behind the noise. I'd say it's a 500 hz tone or so...
Does this sound like a faulty cap?

moamps · Nov 6, 2015

tronnyjenkins said:
The amp was working fine, but a hum appeared on the R side channel. It's loud enough to render the amp unusable, but you can still hear audio behind the noise. I'd say it's a 500 hz tone or so...

It looks to me that R channel is seriously damaged. You should first check the ripple (use DMM on AC setting) on both channel power supply caps. If the ripple is higher on right channel, the fault can be on power supply or in the output stage of the channel.
Do not use it until the repair is done, you can destroy your speaker(s).

tronnyjenkins · Nov 10, 2015

moamps said:
tronnyjenkins said:

The amp was working fine, but a hum appeared on the R side channel. It's loud enough to render the amp unusable, but you can still hear audio behind the noise. I'd say it's a 500 hz tone or so...

Click to expand...

It looks to me that R channel is seriously damaged. You should first check the ripple (use DMM on AC setting) on both channel power supply caps. If the ripple is higher on right channel, the fault can be on power supply or in the output stage of the channel.
Do not use it until the repair is done, you can destroy your speaker(s).

Noted. I have had it off, just observed the audio passing when the assistant engineer brought me to that studio to hear the problem. I'll try your DMM tests.

thermionic · Nov 10, 2015

Do you have a scope? If so, short the inputs (connect pin 2 to 3), connect (assuming it's dual trace) the scope probes to L and R - and observe what you see on the scope. This will enable you to work out the frequency, as well as take a look at the shape of the 'hum'.

You could also sweep an oscillator into the bad channel and compare it against the good one on the scope. If there's a bad device in the O/P stage you may notice that the offending channel is lower than the good channel, but still relatively flat in its FR.

Bryston are good with regard to schematics: http://www.bryston.com/pages/technical.html

A bad filter cap in the power supply would affect both channels, unless it has dual-mono PSUs.

Although I imagine the O/P protection would shut it down if this were the case, you should also put your DMM across the main O/P in 'DC' mode and look for any DC offset (this is one of the first tests to do, actually).

Good luck!