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Old 17th November 2012, 08:42 AM   #521
anilva is offline anilva  India
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Quote:
Originally Posted by buzzforb View Post
Bad jfet is easy. Test V drop across source resistors. Take voltage readings at various places, starting with FE. Report back. All this after taking a break, as ZM suggested. It is amazing how many mistakes I have found after taking some time away from a project.
Continuing on my post where the left channel is showing very low gain compared to right channel. There is proper reproduction from the right channel and the left channel, while the sound is clean is barely audible at full preamp output.

Both the channels are biased at 350 mv at the outputs and seem to be stable and warming up nicely.

I measured the voltages compared both the channels. They seem to be identical everywhere. Especially:

Left Channel (faulty) ... R3 - 63 mV, R4 - 57 mV, R11 - 1.9V, R12 - 1.45V
Right Channel (OK) .. R3 - 65 mV, R4 - 57 mV, R11 - 1.855V, R12 - 1.377V

Both channels measure similarly, but there is a huge difference in gain. I have swapped the speaker & preamp inputs to eliminate issues external to the F5T.

I tweaked around a little bit with P3 but no change in the gain level. Measured the R1, R2 and they are OK with 1K and 47.5K.

What else to look for?

Thanks
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Old 17th November 2012, 10:11 AM   #522
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sorry if this is already asked - where you bought your Jfets ?
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Old 17th November 2012, 10:43 AM   #523
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ZM. they are from spencer i think.
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Old 17th November 2012, 10:46 AM   #524
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start swapping things between channels

recheck bias and offset after every change

of course , first thing is that you're sure that you measured everything , including possible mishap in resistor values
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Old 17th November 2012, 10:53 AM   #525
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Something is stealing signal. Had this once with preamp. Had it wired wrong. Have you checked input and output wiring for shorts? As Zm said, check all values, and all voltages. Makes sure diodes sinks are not contacting with trace or anything else.
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Old 17th November 2012, 11:18 AM   #526
anilva is offline anilva  India
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Originally Posted by anilva View Post
Continuing on my post where the left channel is showing very low gain compared to right channel. There is proper reproduction from the right channel and the left channel, while the sound is clean is barely audible at full preamp output.

Both the channels are biased at 350 mv at the outputs and seem to be stable and warming up nicely.

I measured the voltages compared both the channels. They seem to be identical everywhere. Especially:

Left Channel (faulty) ... R3 - 63 mV, R4 - 57 mV, R11 - 1.9V, R12 - 1.45V
Right Channel (OK) .. R3 - 65 mV, R4 - 57 mV, R11 - 1.855V, R12 - 1.377V

Both channels measure similarly, but there is a huge difference in gain. I have swapped the speaker & preamp inputs to eliminate issues external to the F5T.

I tweaked around a little bit with P3 but no change in the gain level. Measured the R1, R2 and they are OK with 1K and 47.5K.

What else to look for?

Thanks
I don't know whether I should open a separate thread instead of cluttering this one.

I bought the jFets from Spencer. I believe they are genuine.

I took things from bad to worse. I removed the PCB from the chassis and desoldered the jFets and put in a new pair of 8mA IDSS set. Turned the bias presets to zero resistance (just to be safe) and fired the channel which was giving me the problem. On the output there is voltage appearing at -17V, which should have been zero. Voltage across R4 measures 33V and R3 measures 1.29V. There is no voltage across any of the Output source resistors.

I think it is easy for you guys to decipher what is going wrong with such bad voltages all over ;-)
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Old 17th November 2012, 11:54 AM   #527
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just fiddle with pots ( read several tutorials around ) and you'll get it where intended
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Old 17th November 2012, 01:18 PM   #528
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Hi Anil, I have one suggestion. Can you please remove that P3 and measure again?

Also continuity measured from part leg to part leg, here I want to suspect PCB tracks as well holes and dry solders around input circuitry. This could be done for all part including P1 & P2 IMO.

Last edited by omishra; 17th November 2012 at 01:22 PM.
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Old 17th November 2012, 01:39 PM   #529
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Soundslike you have your bias arrangement heavily weighted to one side. If it were me, i would pullthe pots, make sure they are at similar point, trimmed low, then pu them back in the circuit. Turn them both equally. If you track both top and bottom RS while doing this, you will see that turning just one, will increase bias of both polarities of devices, trimming the second will bring the ouput back into balnce, and decrease offset.
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Old 17th November 2012, 01:53 PM   #530
anilva is offline anilva  India
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Quote:
Originally Posted by omishra View Post
Hi Anil, I have one suggestion. Can you please remove that P3 and measure again?

Also continuity measured from part leg to part leg, here I want to suspect PCB tracks as well holes and dry solders around input circuitry. This could be done for all part including P1 & P2 IMO.
Thanks. In fact it did cross my mind that there could be some problem with PTH holes or tracks on the PCB. I will take it out and recheck everything, including the P1, P2 and P3.

Quote:
Originally Posted by buzzforb View Post
Soundslike you have your bias arrangement heavily weighted to one side. If it were me, i would pullthe pots, make sure they are at similar point, trimmed low, then pu them back in the circuit. Turn them both equally. If you track both top and bottom RS while doing this, you will see that turning just one, will increase bias of both polarities of devices, trimming the second will bring the ouput back into balnce, and decrease offset.
This is my 5th F5 type build and I am quite familiar with setting up of bias and about the P1 and P2 being lowest before starting the setup. In any case, I will check all over again.

Thanks.
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