My Take on X-BOSOZ

[BrianDonegan]

The FETs are very static-sensitive, and they tend to fail shorted. Check continuity between the pins of Q2. Most likely Q2 is dead. You compare these measurements against another FET, but any low resistance readings indicate failure.

[BobEllis]

It's possible that you have damaged Q2 but IMHO it's more likely Q4.

My first thought is the gate-source voltage on Q2 is a bit high. Ref2 is working properly, but there isn't much voltage across Q4, driving the source of Q2 to1.8V. You're pretty much turning it full on with 4.2V Vgs.

What is the voltage on the gate of Q4? It should be 0. Compare voltages to the other side and figure out what could cause the differences. Here I think either Q4 is shorted or you've got some DC on its input driving it to full on.

Hope this helps.

[JoeyDD]

Thank you for your help guys!

I checked Q2 and Q4, and they seemed fine, but I replaced them anyway. The problem was still there, so I poked around a bit more, until I checked the second half of the circuit, where I found -4V on the Q5 gate. So I changed Q5, and then Q3 also, which fixed the problem.

When I checked the second board, the same issue appeared, so I changed all of those FETs. Everything was working and measuring well, so I left it at that. I connected the last few wires of the controller, and checked the voltages. The same issue came back once more, but on only one of the two boards.

Any ideas on what can cause these repeated failures of the FETs?

Paul

[thomaseliot]

Hi,

I finished the first channel and it works, but R2 and R7 1.5K 3W resistors get really hot, too hot to touch.

I discovered this post later:

Originally posted by Russ White

So to wrap things up, the BOM has been corrected and future kits will include the 10K R5 also as Brian said we will happily send 10K resistors to anyone who bought a kit and needs one. Any good 10K metal film resistor should do fine. You could actually use any value from 1.5K to 10K or so. The value is not really critical, just make sure you stay below .15W or so dissipation to keep it running cool.

So I replaced R5 with 10K (0.25W) and R2 and R7 are hot the same, less than before, but enough hot to be afraid these will burn.

The rails measured on V+ and V- on the preamp are +78.5V and -19.5V.

Any ideas?

[BrianDonegan]

They will always get rather warm. Mine idle at about 85C. No problems after more than a year though.

[thomaseliot]