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Old 5th February 2009, 11:20 PM   #2711
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Thanks guys.

1. Mosfets are properly isolated - I'm using those bergquist thermal pads and and isolating washer on the screw even though these mosfets apparently don't need them.

2. Components are all definitely in the right way around/correct values have checked this twice.

3. I'm reluctant to swap parts over as I now only have one functioning channel that i can use as reference.

4. I will fit new pots just in case, but they seemed to function fine and not "twitchy" when setting the bias.



Thinking about this as logically as I can, somehow there must be a sudden large current draw thats causing this. The only devices in there (other than a short to GND) that can draw that sort of current are the mosfets. So somehow it seems that there must be this sudden current draw, enough to burn out the source resistors. Thing is I thought I would have seen a gradual rising voltage across R11, but bias was steady through the falling volume level until R11 goes open.

Can someone check my reasoning?


Will look more at the board tomorrow night...

Fran
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Old 5th February 2009, 11:29 PM   #2712
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Quote:
Originally posted by woodturner-fran

So could anyone advise what to do?


Do you have any voltage measurements from the bad channel?


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Old 5th February 2009, 11:39 PM   #2713
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Sure, I can measure whatever you want.

I have nice +/-24V supply AOK to the board. After the resistor goes I measure maybe 2 or 3V across R11/12. Before this happens gs voltage is above 3V, can't remember the exact value, but its 3.433 or something like that.

Tell me what you want measured and I'll do it!

Important note: this amp is running without current limiting (I know, i know!) or thermistors. I could refit the parts for current limiting, but I undertand the amp should be stable without them. I don't want to fit them to plaster over whatever this problem is.


Fran
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Old 6th February 2009, 12:04 AM   #2714
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What are the gs-voltages (or voltages at the drains of Jfets) after the R11 and 12 have blown off?
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Old 6th February 2009, 02:51 AM   #2715
ichiban is offline ichiban  United States
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I quit for tonight. It's 10F outside and 55F in my basement.
I've got the half watters placed on two of CVillers boards.
Tommorow, I'll select/matchup the 0.47 & 100 Ohm source &
feedback 3 watters and place them. Check the placement a
second time vs. the schematic then shoot the lead to 'em.

You'll notice in the picture that I've placed the grid oops! gate
stoppers as close as I could as per EUVLs suggestion many
posts back. Don't want dem os-kelations startin' up.
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Old 6th February 2009, 04:56 AM   #2716
cviller is offline cviller  Denmark
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Quote:
Originally posted by bobodioulasso
Cviller
i would like to order two F5 PCB but , as it is my first post in this forum though i read it for a while, i can't email you. May be you could email me instead? It seems that i need your email adress to send you a payment with paypal. Do i need to fill the wiki too?
You don't have email activated so I cannot write to you, but you can find my email address on the wiki in my signature.


Quote:
Originally posted by ichiban

You'll notice in the picture that I've placed the grid oops! gate
stoppers as close as I could as per EUVLs suggestion many
posts back. Don't want dem os-kelations startin' up.
Looks like this is going to be a really nice build!
I bet you are more likely to get oscillations if you place the gatestoppers dangling... and if you do get oscillations in this amp, it would most likely be due to incorrect wiring.
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Old 6th February 2009, 05:12 AM   #2717
Zen Mod is offline Zen Mod  Serbia
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@ Fran - have you possibility to look at output ( with setup as is - cables , source) with CRO ?

seems that you have oscillations , and not having current limiter parts doesn't help you with this ......
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Old 6th February 2009, 07:23 AM   #2718
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Yes, I have an oscilloscope but I'm not sure exactly how to use it. If you can tell me exactly what to measure I can do that ok.

Source into the amp is from a 24V aikido pre (with output caps) and The gate stoppers are mounted right on the gate of the mosfets.


Will set up and take the measurements tonight.

Fran
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Old 6th February 2009, 10:24 AM   #2719
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One other thing.... all of this setup works fine with my other amps.... so I'm kind of ruling out source or speakers as the problem.


EDIT: Would it be ok to attach a dummy load to the outputs rather than hooking up my quads? I really wouldn't like to damage them.


Fran
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Old 6th February 2009, 12:28 PM   #2720
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Quote:
Originally posted by woodturner-fran
One other thing.... all of this setup works fine with my other amps.... so I'm kind of ruling out source or speakers as the problem.


EDIT: Would it be ok to attach a dummy load to the outputs rather than hooking up my quads? I really wouldn't like to damage them.


Fran

I would just let the amp run with no speakers connected until it stabilizes.
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