BA283AV problem

Hi all,

I have a BA283 that's drawing too much current (300mA) and oscillating at 75kHz, I think. R7 is getting very hot and scorching the board. I swapped out the 2N3055 and it didn't help. As far as I can tell, the other components on the board seem to be good. All I can think of to do is to start swapping out other components until I get the one that's bad. I think I have enough good polypropylene caps around to replace most of the polystyrene caps, and I have some BC107 transistors I can use to replace the BC184Cs, or some of them, anyway. It doesn't seem very efficient, and I wonder if I'll change the sound by changing the caps. Anyone have any ideas?

Here is a pdf of the schematic:

http://sethglassman.com/BA283AV.pdf

BTW, mine is a filled 283, not the half-filled one in the docs.

Thanks


:sad: :?

do you happen to have another one that you can compare voltages with, to see where things are going awry?

ju

Is this a DIY unit or an old module that you are racking?
Is the input jack shorted when you are testing?
Is there a load on the output transformer?
Does it do it at all gain settings?
Play around with termination resistors on the output trans.
Get a .1 cap at 50 v and hook one end to a pieceof wire.
Ground the wire to the chassis and go around with the cap and just start putting it on various locations.
I wouldn't swap out components until you found the ones that are related to the problem.
good luck!

:guinness:

I found the problem, thanks to Joe Malone. I just needed to tweak the trim pot so the bad card drew the correct current. Live and learn. I'm afraid that wasn't obvious to me. They run pretty good now - listening tests tomorrow.

Thanks Jon and Chris. I love this place.

:guinness: :sam: :guinness: :sam: :guinness: :sam:

:thumb: :thumb: :thumb: :thumb: :thumb:

-So what's the total current draw now, and will the home-brew PSU run it now?

Keith

ooh ooh....gettin close Seth.

The total current draw is about 230 mA, and the home brew psu is working happily and quietly!

Thanks, Keith.

:thumb: :thumb: :thumb: :thumb:

> The total current draw is about 230 mA,

It is hard to see how that amp wants more than 100mA-150mA, or 5-6 volts across the 47 ohm resistor. At 230mA you waste almost half the supply voltage in the resistor.

I like to run things rich, but there is enough degeneration in this stage that 100mA or 300mA in the 2N3055 will work the same.

> I just needed to tweak the trim pot ...that wasn't obvious to me.



OK, they talk about TR1 without explaining that the bias flows through to TR3.

On the Seventh circle Audio Neve 1272, the draw current should only be
less than 60mA.

If I remember it right a half filled BA283 with correct bias adjustment is around 90-95mA, 80mA through the prim windings!
How much current can the LO1166 handle?