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Old 6th August 2008, 08:48 PM   #1
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Default Jfet Questions

Can a Jfet (2sk170) be damaged by having power applied to it with the gate not connected?

The reason I ask is I am having a problem with the Uno preamp from Decibel Dungeon...............

http://myweb.tiscali.co.uk/nuukspot/...clonepre2.html

It's the second I have done on stripboard, the first worked first go and has played at least 12 hours a day since, but I am having problems with this one.

I neglected to check the voltages as it was put together as I was waiting for a transformer, but once it was connected I only got 2 volts after the regulator and the LM317t was very hot (not surprisingly). If RS1 and RS11 are disconnected the regulator remains cold to the touch and the voltage reads 24VDC after the regulator. This is still down on the 41V that should be there, so I assume the regulator to be damaged by the "short"

The circuit was incomplete when I powered it up as I just wanted to check the voltages, the gates of Q1 and Q11 just being soldered to the stripboard. Could this have allowed the transistors to dump all the available power straight out to RS11 and RS1 killing them and the regulator?

I've checked for shorts and mistakes and can find none so it's time to ask the wise, that and resist the temptation to be hasty as it's come back to bite me this time.

John
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Old 7th August 2008, 12:15 AM   #2
CBS240 is offline CBS240  United States
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Hi

A J-fet is a normally on device and requires -Vgs to turn off. If the gates are left not connected, they will be fully on. It may be possible they were damaged. 2SK170 is rated for 40V so using 41V is risky, IMO. RD1/11 at 75R seems a bit low to me for a common sourse amplifier.


BTW, I found this calculator for the LM317.
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Old 7th August 2008, 11:02 AM   #3
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CBS240

Thanks for the info, that would explain what happened, I've learned something at least! I'm still very new to this stuff.

As I say the first one has played for hundreds of hours so I got over confident with this one. I took the precaution of ordering extra Jfets and an extra regulator when I bought the parts so I can swap them out and try again.

Once again thanks for the help and education.

John
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Old 7th August 2008, 01:28 PM   #4
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There must be another problem going on with my layout of this.

I swapped the LM317 and still have 24VDC on the output.
I removed the resistors and measured them, both are the correct value (470R and 15K). I think it may be time to plunge into etching my first pcb and do away with the stripboard.

Once again thanks for the help

John
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Old 7th August 2008, 01:34 PM   #5
AndrewT is offline AndrewT  Scotland
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15k/470r should give around 41Vdc from a 317.
You have wired it up wrong or the reg is damaged/overloaded.

Build up the PSU alone and test it. load it with resistors and see how well it regulates.

When you are happy it is doing as intended, only then add on the amplifier.
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Old 7th August 2008, 01:43 PM   #6
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AndrewT

That's the plan, probably after trying to etch a board. I checked with the calculator and the figures are way off to give 24V. It was old stripboard that took an awful lot of cleaning before use, so maybe there's a higher resistance in the copper strip itself than there should be.

If nothing else I have learned a lesson for little cost. Don't be hasty and measure at every stage.

Cheers

John
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Old 7th August 2008, 08:03 PM   #7
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AndrewT

I replaced the resistors I had removed and reflowed the joints around the regulator area. I now have 41.8V from 242VAC mains so I am back on track and have a bit better understanding of the LM317. I can only assume it was a dry joint giving much more resistance than needed and messing up my values.

The wisdom of the lightbulb test is proved once again in that it appears the original regulator survived being repeatedly shorted almost directly to ground.

Slow progress but progress none the less.

John
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