Ampeg BA115HP trouble

[jp3zone]

Hi guys, I have this BA115hp that came in blowning fuses. Ofcourse all the mosfets were toast, so I replaced them along with the drivers, Q3 bias tranny, D8 and D9 zeners, D6 and D7 diodes for good measure. I started her up on the light bulb limiter, then it which went way bright, so I aborted and removed the power mosfets & started up again to check the rail voltages.

I discovered that R68 & Q1 get super hot but Q4 & R66 stay cool, and the rail voltage drops from -44.5v to -38v over D9 zener, but doesnt drop at all over D8 (44.5v). I measured all the resistors conderned, they all check out. The preamp rails also aren't 100% either, 15v/-16.55v, but I havent replaced them yet.

I've read Enzo's posts on the BA115, and checked as far as I could, but no luck.

Why is the darn Q1 driver getting so hot that you cant touch it?

4732d1236851325-777sch_v3.pdf4731d1236851305-457sch_0.pdf

[Enzo]

Took me a minute to figure out what you were referring to. R68 and Q1 are not connected. perhaps in further discusion we can talk of overheating resistors in the power supply and then overheating transistors in the amp circuit.

Fix the power supplies. Nothing will work right in any system if its power supply is not right.

R66,68 normally get pretty hot. Should have about 50v on one end and about 16v on the other. That is 34v dropped across 330 ohms, about 3.5 watts. 3.5 watts is a lot of heat. One wonders why R66 stays cool. Is D12 not zene-ing?

Those 16v rails are not "preamp rails." The preamp uses them, but please note that your drivers also run off those.


So your +44 is all over Q1. No drop across D8. I bet there is no voltage drop across R34. In fact you report that there is +44 at one end of D9, with -38 on the other end. That says to ne that not current is flowing through the top half of the circuit. Sounds like maybe an open at D9 or D7 or R31.

Are Q1,4 exact replacement types, NOT NTE? Are they in the correct spots and istalled facing the right way?

[AtomicMassUnit]

I got one of these and replaced the same parts, the outputs and the two drivers, fired it up and it worked, but had a small bit of distortion. I turned the bias adjustment a tiny bit, the distortion went away and then the fuse blew. Any idea what went wrong? It looks like I'm going to have to replace all these parts again I guess.

[Enzo]

Don't turn the bias adjust just the hear the difference. Bias sets current through the power transistors, and regardless of how it sounds, if yo set it to high, the parts will self-destruct.

[AtomicMassUnit]

Should I start with it fully counterclockwise when I replace the parts? The note says set bias across r72-r75 at 15-20mv average.

Also, after I powered it up with no mosfets to see if the fuse would stop blowing (it didn't blow) the 200 ohm resistor r35 started to smoke.

[Enzo]

Looking at the schematic, it looks to me like fully CLOCKWISE is the coldest setting.

If R35 burns, then Q1 is very suspect.

And test all those diodes.

[AtomicMassUnit]

Will do. I have a few spares of those.

[AtomicMassUnit]

Ok, I replaced Q2 Q7 Q5 Q8, Q1 and Q4, the diodes 6,7,8,9. I have the bias pot turned fully clockwise. The only part I overlooked is Q3. Is there a way to test it?

What steps should I take to turn it on and set bias and not blow anything up?

[AtomicMassUnit]

Also, that Q3 transistor reads about 500 ohms from the center leg to either side.

[AtomicMassUnit]

Ok, the fuse blows when I turn it on. The thermistor by the fuse holder is cracked. The schematic doesn't show a thermistor on it at all, but it is a big black one, and it needs to be replaced.

Sorry to be going on and on with this thing. I'd really like to learn how to troubleshoot solid state gear. I work on tube amps all the time and never run into anything I can't fix, but I just don't get how these things work. I wish I hadn't turned that bias pot to begin with.

[AtomicMassUnit]

Researching the thermistor, is it supposed to help the fuse not blow when turning the amp on?

[Enzo]

The thermistor is an inrush limiter, it mainly protects the rectifiers, but secondarily protects things like the power switch (smaller arcs) and yes even the fuse.


Q3, 500 to either side? That sounds qwrong, pull it and test it while it is out. Q3 holds the bases of Q1 and Q4 a proper distance apart, volts-wise. If Q3 is open or not conducting, then those bases are left free to cram all the current they can into the outputs. Pooof.