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Old 28th April 2012, 08:17 PM   #1
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Default Stuck on F5 Build power-up...

I'm stuck powering up the F5 (2.0 from cviller)....

I get no heat, and zero volts across R11/R12. Turning the pots has no effect on anything.

If I had to guess, Q1 and Q2 are doing nothing. I see about 30-40 mV across R1/R2.

I have replaced them (Q1/Q2), but from the same stock (yes, I bought crap from China via Ebay).

Any thoughts? Thanks. ~Dave
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Old 28th April 2012, 09:19 PM   #2
Zen Mod is offline Zen Mod  Serbia
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pics , parts origin info etc.
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Old 28th April 2012, 09:21 PM   #3
6L6 is offline 6L6  United States
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Ebay jfets are most likely fake.

Are you sure turning the pots has no effect? They sometimes take a bunch of turns to do anything.

Are your V+ GND and V- measuring properly at the amplifier boards?

And very important, please post some photos.

Thanks!
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Old 28th April 2012, 09:47 PM   #4
SirByrd is offline SirByrd  United States
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Make sure your power supply is working properly, and that you have all jumpers installed and soldered in on the boards (I don't know if there are any on the cviller boards)

Then, go over each of your boards and make sure everything is in the right place.
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Old 29th April 2012, 10:38 AM   #5
AndrewT is offline AndrewT  Scotland
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Have you measured the Idss of the jFETs?
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Old 29th April 2012, 01:40 PM   #6
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Well, it looks like a 2.2K resistor got installed instead of a 22K for R21/R22.

I will build a new board (using new transistors) and give that a try...
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Old 29th April 2012, 02:18 PM   #7
6L6 is offline 6L6  United States
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Measure the Idss of your current Jfets (as Andrew suggested) if you have a chance to take them out - if they are genuine, they are pretty durable.

Make the changes to your existing board -- save yourself a bit of work. You will probably be able to get this one working.

Edit - One last question, what schematic are you using? The version with P3? If so, is it on one side of it's range or approximately centered?

Check your PMs.

Last edited by 6L6; 29th April 2012 at 02:47 PM.
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Old 29th April 2012, 02:56 PM   #8
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Quote:
Originally Posted by 6L6 View Post
Measure the Idss of your current Jfets (as Andrew suggested) if you have a chance to take them out - if they are genuine, they are pretty durable.
I have a couple new ones from the Chinese source... I can measure those.

I see that 2sk170s are still available in quantity for Mouser (and I assume they are the original Toshiba). Is there a source for the 2sj74?

Quote:
Edit - One last question, what schematic are you using? The version with P3? If so, is it on one side of it's range or approximately centered?

Check your PMs.
This is the latest version of the PCB that I can find, but it contains components for other versions: http://www.diyaudio.com/forums/blogs...OyZxkX3kU-.png

Is there a better one? The boards are v2.0 2010-10-23 (standard F5).

Thanks!
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Old 29th April 2012, 03:21 PM   #9
6L6 is offline 6L6  United States
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Off the top of my head I know that you can get 2SJ74s (and K170s) from member " h_a ", and also from the AMB.org site in his store. There are probably other sources as well, I just can't think of any right now.

Your schematic link is really small... and I wonder if the schematic has different part numbers than the original schematic... Do you have a bigger image of it?
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Old 29th April 2012, 03:23 PM   #10
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What voltagge if any at gates of mosfets?
maybe resistor pot combination is running out of steam as it does happens same time and resistor value need to be changed

Is voltagge changing as you turn the pots?

Mosfets need 3.5 to over 4 V to start conducting so ....
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