Initial setting of F5 bias--help, please! - Page 3 - diyAudio
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Old 23rd November 2012, 11:37 AM   #21
Nikitas is offline Nikitas  Greece
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Old 22nd June 2014, 05:14 AM   #22
kkerwin is offline kkerwin  United States
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hi
i am in the process of firing up (probably not a good choice of words) my new f5. i have successfully biased one channel and am having difficulty on the second channel.
with both p1 and p2 turned fully counterclockwise, they each measure 0 ohms resistance. when i power up the channel, r7 reads around 190mv while r8 is at 0mv. turning p1 does not affect the voltage across r7, and p2 cannot be adjusted to result in more than about 30mv across r8. i have double/'triple checked the resistor values and they all seem ok. is it possible one of the transistors is bad?? any suggestions for troubleshooting this problem?

thanks
k
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Old 22nd June 2014, 09:36 AM   #23
Zen Mod is offline Zen Mod  Serbia
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recheck all solder joints

recheck trimpot
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Old 22nd June 2014, 09:59 AM   #24
AndrewT is online now AndrewT  Scotland
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Kk,
as you adjust the trim/bias pot from zero ohms you can measure the voltage across it.
That voltage across the VR is the output device bias voltage.
Measure and see if the P1 change in voltage roughly matches the P2 change in voltage.
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Old 22nd June 2014, 11:56 AM   #25
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Might seem daft but check that you have the correct MOS-FETs in the correct positions.
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Old 23rd June 2014, 04:15 AM   #26
kkerwin is offline kkerwin  United States
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found that r7 and r8 were damaged. i may have done this on hook-up. when i first connected that channel, i had the power on and connected the negative and positive a few seconds apart. the rest of the circuit seems to check ok. i have a couple of .47r resistors on order. the other channel is working very well.
thanks
k
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Old 23rd June 2014, 11:17 AM   #27
AndrewT is online now AndrewT  Scotland
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ALWAYS use the bulb tester to power up a mains powered project after EVERY modification.

There is no excuse for damaging new equipment due to laziness.
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Old 23rd June 2014, 12:43 PM   #28
kkerwin is offline kkerwin  United States
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hi andrew,
less about lazy, more about inexperience. how do you use the bulb tester?
thx
k
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Old 23rd June 2014, 01:35 PM   #29
AndrewT is online now AndrewT  Scotland
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plug your equipment into the bulb tester.
plug the bulb tester into the mains socket.
switch ON.
Does the bulb flash briefly, or a prolonged dim down, or glow bright?
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Old 23rd June 2014, 02:03 PM   #30
kkerwin is offline kkerwin  United States
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thanks andrew,
i will check this out.
k
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