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Old 2nd March 2009, 03:22 AM   #1
SA-12 is offline SA-12  Canada
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Lightbulb fet help please 2SK135

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.
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Old 2nd March 2009, 03:54 AM   #2
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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.
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Old 2nd March 2009, 05:45 AM   #3
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Sure would help if you posted the circuite.
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Old 2nd March 2009, 02:23 PM   #4
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Click the image to open in full size.
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Old 2nd March 2009, 02:26 PM   #5
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there ya go, very simple but get it to work right
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Old 2nd March 2009, 02:33 PM   #6
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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.
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Old 2nd March 2009, 11:02 PM   #7
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Hi

If memory serves, the IRF241 have not the same pin out of the 2SK135...
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Old 2nd March 2009, 11:49 PM   #8
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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.
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Old 2nd March 2009, 11:58 PM   #9
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Quote:
Originally posted by 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.

In the 2SK135 the case is the source, In the IRF the case is the drain...
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Old 3rd March 2009, 12:07 AM   #10
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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
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