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Old 11th February 2009, 10:21 PM   #2871
Tea-Bag is offline Tea-Bag  United States
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Quote:
Originally posted by RobertOder
Mr. Pass:


I found them here:

http://www.hndme.com/productcart/pc/...?idCategory=13
I use handmade for parts. Very reliable.


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Old 22nd February 2009, 03:14 PM   #2872
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Need help troubleshooting - Sorry if this has been brought up before.

Just finished wiring up the F4 and was adjusting the offset & bias -

1 channel was all ok - bias & offset

For the other channel - offset was also ok.
I was also able to adjust the bias via P1, however one of the mosfets (Q5 IRFP240)was running extra hot and was found to have a voltage of 300mv across the 0.47R resistor, Q3 & Q4 are measured at 100mv across the 0.47R during that time. Unable to access R21(Q10 resistor) to measure.

The mosfets are purchased as a matched set.
What are the possible causes for this to occur - how to trouble shoot and rectify.

I do have another set of the F4 boards - should I just use the extra set? (Although it will be a pain to rewire the board and trying to match the tapped holes for the mosfets).

thanks

her shann
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Old 22nd February 2009, 03:19 PM   #2873
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It may be that the Mosfets are not matched,

or the resistor is of a higher value than the others,

or the transistors is not well attached to the heat sink thermally.

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Old 22nd February 2009, 03:39 PM   #2874
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Quote:
Originally posted by Nelson Pass
It may be that the Mosfets are not matched,

or the resistor is of a higher value than the others,

or the transistors is not well attached to the heat sink thermally.


Heat sink is warmest at the errant mosfet - so i think attachment is unlikely the issue.

Too late over here to tackle this issue - will try another day to resolve it.

thanks

her shann
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Old 22nd February 2009, 05:23 PM   #2875
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I would place a larger value resistor on the Source, calculated to
have the same current at the difference in voltage.

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Old 22nd February 2009, 11:00 PM   #2876
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Quote:
Originally posted by Nelson Pass
I would place a larger value resistor on the Source, calculated to
have the same current at the difference in voltage.

PaPa Pass,

thanks for the tip!

After doing some crude calculations - If the voltage across the source resistor is 3x (at steady state) that of the other mosfets - I will have to use a resistor 3x larger (0.47x3).

Is that it? Although I may have to rig up something to test many resistor values, to trial & error.

Thanks, I will report back (in a few days time)

her shann
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Old 23rd February 2009, 02:26 AM   #2877
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I would go with trial and error, and it doesn't have to be truly equal
current - just get the dissipation down.

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Old 6th March 2009, 06:39 AM   #2878
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I have been using the F4 for quite some time and still very satisfied with the sound. Since I am able to find some Toshiba mosfets J200/K1529 and thus I have modified the F4 as in attached to use the Toshiba mosfets. I have reduce the source resistor to 0.245 ohm (0.51//0.47) due to good match of the mosfets. Below is the modification I did on the bias circuit.

The sonic of Toshiba mosfet is more refine to me especially on the mid and high frequencies.

Spencer
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Old 6th March 2009, 10:56 AM   #2879
EUVL is offline EUVL  Europe
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> The sonic of Toshiba mosfet is more refine to me especially on the mid and high frequencies.

Could not agree more.

If you would get a chance to try 2SK1530 / 2SJ201, or even 2SK2554/2SJ554, I could only recommend that.




Patrick
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Old 6th March 2009, 05:37 PM   #2880
Gluca is offline Gluca  Italy
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Default A late builder

Should I read all the long discussion? OK I will, well I am actually, but am I allowed to ask for two questions?

1) What size for the H/S? I could recycle a pair rated at 0.33 C/W. Enough?
2) I am going for a choke input PSU (as I have a +35VAC transformer), any hint on the current each channel requires?


Thanks
Gianluca
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