Toasted 400 still toast

[roccus]

Just need to get the control board back now... how is it looking, you had a chance to check it out yet?

 

[Gepetto]

Ran the right channel on the tester all day yesterday Ray with good results. Will run the left channel the same way next.

 

[roccus]

Very good.... I am pretty sure I got all the blown stuff replaced on it, all the damage was on the right side, didn't find anything on the left side

 

[Gepetto]

Both channels seem more than fine up to 80KHz, the limit of my 8903B

 

[roccus]

Do you think you will be able to get that back to me this week? I just went over the speaker protection board and everything seems ok with it.

 

[Gepetto]

It shipped to you this morning Ray.

 

[roccus]

Cool maybe will have it up running by this week end.... We are going to get clobbered here by a snow storm beginning tomorrow evening into Wednesday

 

[Gepetto]

Yup, New England's finest coming our way.

 

[orange]

God's dandruff on the Oregon Coast and headed east as well.

 

[VSAT88]

Gonna be a big one but not down here. 60+ degrees in Phenix City Al.

 

[orange]

Break out the corduroy shorts!

 

[roccus]

Clean up is all done with the snow got 12" but drifts up to 24"

 

[roccus]

The mail came right when I finished plowing and the control board got back to me from Joe.... Thanks Joe.

I got the board all back in and powered it up at first all looked well 0V at outputs got the bias set to .355 but the rail voltage dropped from around 76V with just the backplane boards in down to about 57V I don't think that is right to me it should still be up over 70V?

 

[Gepetto]

The mail came right when I finished plowing and the control board got back to me from Joe.... Thanks Joe.

I got the board all back in and powered it up at first all looked well 0V at outputs got the bias set to .355 but the rail voltage dropped from around 76V with just the backplane boards in down to about 57V I don't think that is right to me it should still be up over 70V?
View attachment 33640

With your DBT in correct Ray?

 

[roccus]

Yea Joe same DBT I used when I did the backplanes..... to me though the DBT was dim but seemed to look just a tad bit brighter than usual

 

[Gepetto]

Yea Joe same DBT I used when I did the backplanes..... to me though the DBT was dim but seemed to look just a tad bit brighter than usual

If you turn your bias pots all the way down you should see that voltage climb back up Ray. Try it.

 

[roccus]

Only goes up to 61V with bias all the way down...

 

[Gepetto]

Only goes up to 61V with bias all the way down...

You are seeing the effect of the bias current and the DBT you have installed in line Ray. That is what I wanted to point out to you by having you adjust the bias voltage down. Even at minimum adjustment, you probably have a bias voltage measurement in the region of 0.2-0.25V present which is dragging down your B+ and B- with the DBT in place.

The effect and purpose of the DBT is to ruin the load regulation of the power transformer in your amp and turn it into a voltage source with a big source resistance.

If you want to see the effect even more, jumper each bias transistor C-E which defeats all the bias current altogether. You will see the voltage get closer to your backplane only value, although you still have the load of the control board present so it will not go all the way up.

If you do this experiment, do it very carefully as you do not want to blow up your amp with a slipped wire.

 

[roccus]

Ahhhh ok, so when I take the DBT out of the mix the voltage should go up to where it should be then right?

 

[Gepetto]

Yes Ray, the purpose of the DBT is to create a high impedance voltage source out of the transformer and give you visual indication that the caps have charged and settled back to an idle current. You will have to set you bias current again when operating without DBT. I normally dial the bias all the way down before removing the DBT and applying normal line voltage.

Again bring it up on a Variac if you have one.