f5 power up issue

[acold7dusta]

can you post pictures of your setup?

 

[Bill Fuss]

Check your bias voltage at the gate traces for the output fets. You should be able to turn it down to zero with the trimmers. If you cant you have a problem in the board somewhere.

 

[Bill Fuss]

Let me try again, that wasn,t right. Check to see that you can adjust the voltage across R3 and R4 from zero to 5v with the trim pots.

 

[specialidiot]

Check your bias voltage at the gate traces for the output fets. You should be able to turn it down to zero with the trimmers. If you cant you have a problem in the board somewhere.

from what to what? with the output FETs in or not?

can you post pictures of your setup?

I put a few pics up here

An externally hosted image should be here but it was not working when we last tested it.

 

[Bill Fuss]

Across R3 and R4. Do you have the schematic? That should be the same voltage you get from the rail to the gate pin of the output fet, which is pin 1. Dont put the fets in until you have this under control.

 

[specialidiot]

Let me try again, that wasn,t right. Check to see that you can adjust the voltage across R3 and R4 from zero to 5v with the trim pots.

That works fine

Across R3 and R4. Do you have the schematic? That should be the same voltage you get from the rail to the gate pin of the output fet, which is pin 1. Dont put the fets in until you have this under control.

If Pin 1 is next to the Q3 designation and opposite the Q4 designation, then I have 5v there when I have 5v across r3/r4.

hmmmmm

 

[Bill Fuss]

Also, check your fets from 2 to 3 in both directions.

 

[specialidiot]

Looks like I've mixed up the data sheets, had the n channel and p channel mixed up. Dang I hate it when I do that.

I've swapped them but I have the same results.

Q3 is now a Toshiba 2SJ201 P channel, replacing the indicated IRF9240 P Channel

Q4 is now a Toshiba 2SK1530 N Channel, replacing an IRF240 N Channel

I guess I've blown an expensive pair of Toshiba outputs?

 

[Bill Fuss]

Were you able to turn the voltage down to zero at the gates. Do that before you try new fets.

 

[specialidiot]

I'd have to do that without the outputs in place... is that what you mean?

 

[Bill Fuss]

There is no reason not to do it now, just for added safety on start up with new fets.

I know how easy it is to mix the fets up when building, I luckily caught it when I built mine.

 

[specialidiot]

Yes, I can adjust the pin 1 pad (outputs out) vs the adjoining rail to 0V with the trim pots, both rails.

Does that mean I'm good to go for trying new FETs?

Do I leave it at 0v to start after the replacement?

 

[Bill Fuss]

Yep, just bring each one up a turn at a time til you get close to correct bias, then fine tune your bias and offset.

Have fun.

 

[specialidiot]

Thanks Bill. I appreciate your gentle guidance.

I'm at 608/614 and .007 offset right now. Heatsink is toasty but the fins are facing down instead of vertical so I think I'm gonna be fine.

Except for frying those Toshiba outputs I mean.

 

[Bill Fuss]

Any time, enjoy.