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 ;-)
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.
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.
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.
Buzzb, if this is addressed to me, then I am far away from having any control on the situation. Just one change of jFETS has altered the entire voltage rating across the PCB. I have removed the PCB and checked the P1 and P2. They are set to zero ohms. I do not see any dry solder. I resoldered all of them. Checked component values - no errors. I need to check tracks of the PCB, if there is any flakiness associated with the board. I will report back.
Last option is to discard the PCB and build another board.
Regards
Thanks. Will compare every line. The PCB is from a India forum group buy. It has been designed by fellow member Om Mishra. While the PCBs look well made, I am probably the first guy to use these boards. There could be manufacturing issues or may be not. With time at premium, this debugging hogs quite a bit of that.
Some people do crossword puzzles, others debug circuits.
Consider it time well spent.
Papa, pardon me for using the wrong word 'debugging'. Who am I to debug a perfect design. I am just trying to find out assembly errors committed by mortals like me. While I prefer playing around with circuits than crossword, the anxiety to listen to a F5T in a hurry is driving me nuts.
Thanks.