fet help please 2SK135

[SA-12]

Ok so I am going to try the mosfet power supply mod to a scrap Dynaco st120. I know the amp is more or less garbage but it is cheap and fun to work with. I have tried many times with this power supply upgrade but keep falling short on regulated voltage. I have checked and re-checked a dozen times the circuit and have found small errors and fixed. My voltage is to low, about 58v on cap 12 when I should have about 70v or so. The control zenner is fine but I changed it just to be sure a couple times. I have also changed the FET a couple times with the same result. This is what I have that is not working, any advice would be nice. I am having problems finding a sub for a 2SK135 and that is why I used the IRF241 and it has a nice TO3 case that I like.

FET used IRF241 which should work right?
Now I admit I cheeped out on the rectifiers and used NTE components, could this be a cause?. They are NTE588 a 200v/3a ultra fast and should work perfect.

I think my entire problem lies with the FET being used which on paper should fit nice but in reality is not giving me the regulated voltage I need.

 

[SA-12]

I know I could use 2sk1057 but as stated I wanted to maintain the original TO3 case style. If any member here have some experience building this design and can offer some advise that would be greatly welcome. Thanks all.

 

[woody]

Sure would help if you posted the circuite.

 

[SA-12]

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

 

[SA-12]

there ya go, very simple but get it to work right

 

[SA-12]

fyi cap 12 3300uf@100v
cap 9 and 11 500uf@100v and 1000uf@100v
diode 4,5,7,8, 200prv@3amp.
cap 16 .01@1kv.

I have this supply built exactly like the one in the pic, I have built it many times the exact same way and have never reached anything over 60v on cap 12.

 

[Tube_Dude]

Hi

If memory serves, the IRF241 have not the same pin out of the 2SK135...

 

[SA-12]

yeah, figured that out with destroyed fet number one , I think I have pinout right. I downloaded the spec for this fet and pinout was clearly shown. I had it wrong at first. When i had pins reversed I was getting 98vdc on cap 12.

 

[Tube_Dude]

yeah, figured that out with destroyed fet number one , I think I have pinout right. I downloaded the spec for this fet and pinout was clearly shown. I had it wrong at first. When i had pins reversed I was getting 98vdc on cap 12.

In the 2SK135 the case is the source, In the IRF the case is the drain...

 

[SA-12]

ok so turns out the first way I had it was right, and I got 98v on cap 12, zenner is good. 1n4760. what am I doing wrong

 

[Tube_Dude]

ok so turns out the first way I had it was right, and I got 98v on cap 12, zenner is good. 1n4760. what am I doing wrong

The Mosfet is shorted (burned)...

 

[SA-12]

I thought so too, but it is not. I had five of these fets can they all be shorted. It is not a overvoltage issue thats for sure. I got 2 left, will try again I guess.

 

[SA-12]

unless these fets are super sensitive to heat and static. I do ground myself before touching them, but I also solder direct to the pins. They have a huge pin, twice the size of normal pins on say a 2n3772.

 

[Tube_Dude]

Inspect again the pins layout.

The pins position is the same of this one:

http://pdf1.alldatasheet.com/datasheet-pdf/view/67474/INTERSIL/IRF244.html

 

[SA-12]

yep that the pinout I used all along, still get 98v on cap 12. I have to check some other component values but I am sure it is all right. I might have to make my own pcb as I just can't trust the original pcb even though I checked it now about 5-6 times to make sure I got it right. To many traces not being used it is a little confusing.

 

[SA-12]

I thought I would remove the original caps from circuit and aligator clip some newer versions in still no difference. 98v cap 12. what could cause the high voltage?, a zenner right well checked and re-checked and replaced for the heck of it

 

[SA-12]

it is funny because some of the kits I have built and amps i have worked on without problems, then this exceedingly simple circuit I can't get right. I have been trying this off and on for the past couple years, it is my shame.

 

[Tube_Dude]

Have you 70 Volts at the Zenner?

 

[SA-12]

going to hook up again tomorrow, keep following the thread please and i will report back. thanks for your help

 

[Tube_Dude]

Try with a 100 uF capacitor in the C12 position.

The value of 3.300 uF is to great and the mosfet can exceed the maximum gate source voltage at turn on...