Aleph 3 ->! help

[Mad_K]

Hi!

I have built an Aleph 3, but whein I fire it up I get: SMOKE!

I am only using one output-mosfet (and one in the current source) per channel, and I have resized accordingly.

What happens it that both R118 and R109 (aleph 3 scematic)smokes (even glows in the dark), and I turn it off ...

I am troubleshooting it now, but can't seem to locate the problem.

I am using a bc550c instead of the mpsa18, and I have checked them.

The rails are +/- 26VDC unloaded.

Some help would be greatly appreciated

[jh6you]

Hi, Mads

It seems that R118 and 109 are short-circuited.

[Holger Barske]

A short on resistors? I don't think so. And, if they would be shortened, they would not burn because there would be no voltage drop across them.
---
Holger

[Mad_K]

I thought so too, but I have been looking for solder-bridges (I am using pcb from PCB-DESIGN (Kristijan), and coldn't find any. But maybe you're right anyways.

Any other suggestions?

[stefanobilliani]

What value did you used for R122?

[Mad_K]

R122 is 0,25 ohm

[kristijan-k]

Hello,

What happens it that both R118 and R109 (aleph 3 scematic)smokes (even glows in the dark), and I turn it off ...

If R118 glow, than you probably damaged the Q 108.
What voltage reading was across R122 ?

[Mad_K]

Hi Kristijan!

R109 burns first, then R118.


I wasn't able to check any voltages, because it happened so fast.

PS -it is the same on both channels. My theory now is that maybe I have wired something incorrectly. Refering to your Aleph 5 board I am only using the outhermost MOSFETS and the resitors for them (the one on the left, and the one on the right side of the board). As far as I can see this should work fine, as the banks of 3 MOSFETS are in paralell, but maybe I have missed something. I'm at work , so I can't check it right now.

Since I am not paralelling MOSFETS, I haven't tested the MOSFETS (IRFP250). Maybe they are faulty

[Mad_K]

to make a long story short: IT WORKS!!!

Kristijan, you where right, Q108 was damaged (along with Q106 and R122).

The reson for this is that Q104 & Q105 was incorrectly oriented at the first try.

All voltages check out fine, except across Q105 = 3,2 V
And the voltages across R120 and R122 is only 0,3V.

So it is severly underbiased, and I can hear some ugly distortion when I try to push it a little (increase the volume). At goes away at low volume. At least I hope it is distortion due to the very low bias (R120/R122 measure too much with my multimeter also 0,3/0,4 ohms).

I will try another type of source resistors( paralelled 0,47 ohms 5W ww), and also leave R113 out of the circuit to try to increase the bias. Worst case I get 2,8A bias on a single MOSFET: BWAAHAAA!!

Thanks for your input guys!