Aleph-X builder's thread

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#845
Hi,

I need help on my Aleph-x.

I am working on Kari's Aleph-x boards.
25V rail. Vgs = 3.2V.
There is no current pass through the source resistor.
My questions are:

1 Is 3.2V too low for mosfet to operate?
2 What is the typical value of vgs on A-x?
3 What may go wrong on my boards?

Thanks

Wing
 
#848
Wing,

If that is the voltage you are measuring in circuit, chances are it is too low.

I don't kow Kari's board, but there should be a way to adjust the current in the input differential. The IRF9610 that is NOT part of the input pair is the CCS, there is probably a pot connected to its source. Adjust that to bring the voltage on the gates of the output gain devices up to somewhere around 4v.

Once you get it up there you'll see current flowing in the output devices and you'll tweak further to adjust the absolute offset.

If there is not enough adjustment, you may have to change some of the resistor values in the CCS (R24 and R26 in Grey's schematic)
 
#849
Hi William

yep, Vgs on the current source.
I am using IRFP240
Both sides are the same.
I have tride to adjust the bias pot.
It only moves the Vgs a little bit <1V
Kari used kk's schematic, I just used the same resistors' value
http://www.kk-pcb.com/alephx.html

Hi Nelson
I don't remember the exact value.
DC between OUT+ and OUT- is less than 1V
DC between OUT+ to ground is ~9V. (I check this one several days ago. Not 100% sure about it.)

Hi Bob
I will check that part tonight.

Thanks

Wing
 
#850
The pot you need to adjust is the 200R pot.

It is an iterative process - first get the absolute offset close with the 200R pot, then adjust the relative offset with the 50K pots, repeat until it all stays steady. of course once you start heating up, you have to wait for an hour or so between adjustments.
 
#851
Hi Guys,

Well as Bob can tell you, I have been having a similar problem. No matter how I adjust V2 I still have 12.5V at Q6 source. The resistance of V2 changes and so does the reading across R26, but the reading from +rail to the source of Q6 remains at 475R.

Any ideas? I have checked for bad solder joints and can't find any.

Thanks, Terry
 
#852
I found that you really need to match the differential pair for VGS.

Matching is VERY important, recheck your other FETS too. I was
having the same problem, and getting the diff pair matched helped a lot.

Oh, and make sure you check the values of matching with the
FETS in the circuit, on the heatsink, completely warmed up.
I worked up a little "jig" so that i could keep swapping FETS
until things worked. Also, try to keep conditions on the bench as
close as you can to how they'll be in the final build.

If everything is working fine the circuit will settle down and behave.
 
#855
Hey guys,

With Bob Ellis' help, I figured out my problem. I had 332K in R26 instead or 332R. Replaced it and now I have it running at .5V across the output source resistors with 50mv offset. I need some hints as to what to do to bring down the offset a little. The offset from out to ground is about 3.25V.

Thanks, Terry
 
#856
Wing,

The current source irf9610 silkscreen looks backwards to me. Turn it around so that the source is on the left, gate on the right.

I don't know the BC's off the top of my head, but they should be CBE top to bottom if I read the layout correctly.

Terry,

Your relative offset is fine, but you should work on absolute offset so you don't lose so much of your available swing. More current in the input differential should bring it into line. See the PM I sent.
 
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