Aleph 2 Frustration

I have just completed construction of 1 channel for the Aleph 2 project, but have a problem. There is no Bias .Its weird theres none at all!

I am using Wayne's boards and every thing appears correct including the voltages.

R11 5.32 Volts,
Q 3 bias 3.44 volts to Q1/Q2 Source
R14 4.19 volts
Q5 4.22 volts, Q5 base 0.66 volts.

The output is 0.00 volts

These are very similar measurements for my Aleph 5.

The power fets are IRFP244s from Allied which I recently matched for Vgs of 4.48- 4.46 across a batch of 24.

As you can appreciate, I feel a bit stumped, My only other thought is that the 244's are some how cactus, meaning somehow the gates are damaged, although I took all the normal precautions.

How do you test for Gates?

I would appeciate any ideas, I will leaving for a few days and have another look at it.

best regards


I had the same with my Aleph. After 2 hours of looking for the problem I had found I had connected the 'sense' wire to the two banks of 6 mosfets symmetrical. So the sense was on one of the banks connected to the wrong pin of the mosfet. This gave all the right voltages but no bias whatsoever...

Perhaps you did the same?

Whenever I need to double check the output connections, I will always look at this wiring diagram.

Also, if you are using output boards like Wayne's, check the four connections on the two output boards and their associated connections on the main board. I would suggest to mark the four holes on the output board and main board with a marker pen - to indicate D, G, S & sense and then trace the wires to make sure they landed at all the right places. This can get confusing if all the 4 wires has the same color! Once you're done with one output board, do the same for the other output board.
macka said:

Thanks for the tip.

If I understand you correctly you had the sense connected to all 6 fets on both sides.

I have just checked this and I'm fairly sure my sense wire is only connected to 1 fet (source connection) but I will have another look at it.



I had it connected to one fet on each bank. However as I have 6x Fet then my PCB and then again 6x fet, I soldered one wire of the first sense from the center PCB to the left first fet, and one wire to the right first fet. I soldered both wires to the first pin of the first fet next to the pcb (left and right). This was wrong, but my symmectrical way of thinking in my head thought this was corrected!

Its Going

Edwin & Fcel,

Thankyou for your assistance,

I located the error after carefully re checking everything tonight.

The wiring to the boards was correct but your are right, it could easily be confused as both sides are not symetric.

The error was in fact the the value of R17 , the Dale in the board was marked 4750 which of course is 470R, as such Q5 was not biased correctly.

I swapped it for the 4.75K and all is well, Q5 now as a healthy 4.7 Volts, all other paramters are within specification.

Thankyou again.

PS I will post some pics in a few days guys.