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Old 7th September 2013, 11:47 AM   #51
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hallo andrew,

thanks for your answer. but i donīt understand "Rth c-s". please, can you explain?
thanks, mario
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Old 7th September 2013, 03:42 PM   #52
AndrewT is offline AndrewT  Scotland
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Rth c-s = Thermal Resistance from Case to Sink.
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regards Andrew T.
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Old 7th September 2013, 04:04 PM   #53
wrath is offline wrath  Poland
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Thermal resistance between mosfet and heatsink. What can mean no or wrong paste,wrong type of insulator, loose screws, convex or concave surface of heatsink near mounting point or broken thermometer ;-D
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Old 7th September 2013, 06:23 PM   #54
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Quote:
Originally Posted by Weymario View Post
Hi! i am confused. hope someone can help.
i canīt bias my F5 to 0.6 v (R11/R12). when i have .36 v the voltage at R3/R4 is 4.6 v. what is wrong?
the temperature of the heatsink (which is 300 x 150 mm per channel) is after (2 hours) 46 celsius, the transistors about 72 celsius.
thanks, mario
regarding the bias. are you sure that R11/R12 are 0.47ohm?
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Old 8th September 2013, 05:13 PM   #55
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yes, R11/12 are 0.47 ohm (= R47 ohm, right?). other idea?

i think, the insulator between transistor and heatsink is a (pink) silicon-pad, no paste. if the surface is ok, i have to check in the next days.

thanks, mario
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Old 10th September 2013, 07:08 PM   #56
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Hi!
meanwhile, i tried some things:
a) decrease/increase R3/4 from 1.1 k to 9k: 1.1k was worst (bias: 0.07v), 9k also bad (bias: 0.12v); best 2.2k with bias o.24v and 4.3v over R3/4
b) decrease/increase R19/20: nothing happened
c) decrease/increase R13/14: nothing happened
d) power supply is +/- 25.7 v

maybe the jfets (sj74/sk170) or transistors (IRFP240/IRFP9240) are the problem?

any ideas are welcome

thanks mario
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Old 10th September 2013, 07:24 PM   #57
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just measured some temps on my F5. sinks baseplates run at 45c right at the middle outputs (2pair version) transformers run at 34c.
ambient is 25c and bias is 1.7A (0.85A pr Device)
looks like i need to pull up the bias some more
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Old 12th September 2013, 05:50 PM   #58
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so you are a luky one ;-)
have you tried a higher bias? any changes in sound?
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Old 12th September 2013, 06:11 PM   #59
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no. i have not tried any higher bias yet.
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Old 12th September 2013, 11:39 PM   #60
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Quote:
Originally Posted by AudioSan View Post
just measured some temps on my F5. sinks baseplates run at 45c right at the middle outputs (2pair version) transformers run at 34c.
ambient is 25c and bias is 1.7A (0.85A pr Device)
looks like i need to pull up the bias some more
Take it up...you won't be sorry
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