4b blown capacitor?

Discuss the dual-mono 4BSST² and all previous versions of the Bryston 4B amplifier

4b blown capacitor?

by blackknight2072 » Tue Jul 21, 2015 5:30 pm

Love Bryston amps; love bryston amps!

I've got a early 80's 4b consumer amp that has seen a pretty rough life but still sounds so sweet; at least until vicious static took over in both channels. the static is constant in volume, regardless of the volume of the underlying music. it seems to be stronger on one channel than the other, and started quite suddenly and is consistent. I popped the top cover off and one of my big main capacitors (100v) is bulged on top with a 1/2" slit/crack centered on the bulge. This does not look right. i've done some research, and this seems consistent with a 30 year old capicitor baking off. my system has had several mods to keep it kicking, including a hodgepoj of main caps, i think this is the only remaining original cap (one of the 100v units that blew), with an adcom 75volt and another brand 100v on the other channel. all the replacement caps seem to match up with the power ratings on the schematics. The power switch was replaced with a generic toggle switch, there is a replacement power cord attached, and one of the larger resistors had been replaced with a lattice work of small resistors. i had it checked over by a semi talented local tech a few years back when the group of resistors came un-soldered, and he said everything else checked out fine.

My assumption, needs a new main cap.

my questions: could Bryston send me a replacement capacitor to replace my blown unit? should i replace all the mismatch units with new units for more uniform sound and better imaging? Costs? Should i scrounge online eletronic parts stores for a matching cap? I leave this unit switched on and attached to a power outlet that has a light switch to turn on the power. Am i overwhelming the system with a power surge that is not buffered or controlled by the turn on switch built into the amp? i had assumed these older units didn't have any kind of soft start circuit, but this blown cap has me questioning my assumptions. This is part of a secondary 2 channel system that sees high volume play (105db) for only 20-40 minutes a day (this is my shower stereo setup), so pure hifi excellence is not a top priority.

I've pulled the unit from service, and have temporarily replaced it with an AB Systems 410. The AB is a fine sounding amp, much sweeter than the adcom 555 that had previously been in the system; but nothing images like a Bryston.
blackknight2072
 
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Joined: Tue Jul 21, 2015 4:34 pm

Re: 4b blown capacitor?

by Mike Pickett » Thu Jul 23, 2015 3:12 pm

Hi;

The symptoms you describe don't seem consistent with a cap failure on one channel, but if one of the caps has vented it definitely needs to be replaced. If the static noise persists after replacing the cap, the likely suspect is the differential input pair transistors.

When we repair an amp of this vintage that needs a filter cap, we replace all 4 chassis mounted caps with a large circuit board containing 8 smaller solder-in caps. A larger number of smaller caps will always work better, and we've found that most chassis mounted caps available today are old stock, which is not desirable...

The cost of the filter capacitor board is $200.00 plus shipping. There are two versions with different mounting patterns, so I would need to see a picture of the bottom of the amp to ensure we send the correct board.

If you're interested in purchasing this assembly, please send me a private message with your shipping address, phone #, and the picture mentioned above.

Thanks,

Mike
Mike Pickett
 
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Joined: Mon Aug 25, 2014 5:20 pm


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