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Old 7th February 2013, 11:15 AM   #1
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Default F5 bias problems

Hi all,

I'm having problems biasing one channel:
P2 does not affect bias !
When I measure R across R3 & 4 resistance drifts
I've changed Q4 and the jfets, I've changed the 5K trim pot , I've checked all connections and they are all well.

Is there anything else to look for- pleaseÖ
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Old 7th February 2013, 01:58 PM   #2
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R3 and R4 from wich schematic? across the pot's?
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Old 7th February 2013, 03:16 PM   #3
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R3 & R4 in the original schematics. they should start at 0 and go up when turning the pot
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Old 7th February 2013, 03:25 PM   #4
Zen Mod is offline Zen Mod  Serbia
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pics could help
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Old 7th February 2013, 04:13 PM   #5
6L6 is offline 6L6  United States
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Yes, Please take good photos and post them.

Question - what effect does P1 have? I ask only because you mention that P2 doesn't do anything.

Are you sure you have no cold solder joints on the PCB? That seems to be the Occam's Razor answer...
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Old 7th February 2013, 05:00 PM   #6
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also. check the source resistor on that half.
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Old 8th February 2013, 07:47 AM   #7
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Thank you all for trying to help

I'll do some checking this weekend
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Old 8th February 2013, 10:36 AM   #8
cjkpkg is offline cjkpkg  United States
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Do you have any DC voltage on the output of that channel?

Do feedback resistors get really hot?

Check voltages on the pins of your MOSFETS and JFETS.

Are you using the speaker protect circuit?
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Old 8th February 2013, 01:14 PM   #9
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If you really get stuck trying to bias the F5 -- unsolder the source leg of the mosfets, take a DVM and measure the voltage across the drain resistor of each input JFET. From the manual you'll see that this voltage should be ~4.2V -- set it to ~3.5V, Oftentimes it will be too low or too high. Then you solder the MOSFET back in, and gradually adjust the trimpots for bias current and offset. It requires a bit of patience.

Remember that P1 is in parallel with R3 AND R15+TH1. (from the original schematic). If you can't get 4.2V with a lower IDss/gm JFET like a 2N5457, you may wish to consider changing R3 to a higher value, or placing another JFET in parallel with the original.
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Old 9th February 2013, 07:31 AM   #10
cviller is offline cviller  Denmark
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The most frequent issue I have heard of with F5 is caused by people trying to adjust only one of the pots first. Both pots need to be adjusted half turns at a time with constant monitoring of the mosfet source resistor voltages.
Resistance drifts due to the active components and capacitance, so focus on the mosfet source resistor voltages.
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