I’ve started Bias’ing up the front end & am not getting up to 1.5V across R5/R6 on both channels, one channel reaches 1.24V and the other 1.3V (after warmup) with one of the P3 or P4 pots fully clockwise on the channels.
How would you suggest I solve this, do I increase R5/R6 and with how much or swap-out the P3/P4 for 1KOhm pots?
Also I still measure a high supply voltage 31V between G-T6&T8 and T3&T5 measure 21.4V. Are these acceptable?
How would you suggest I solve this, do I increase R5/R6 and with how much or swap-out the P3/P4 for 1KOhm pots?
Also I still measure a high supply voltage 31V between G-T6&T8 and T3&T5 measure 21.4V. Are these acceptable?
Thanks Dave and Walter,
One of the P3/P4 pairs is at 473/463 Ohm's, the P3 being as far as it will go. I'm using the Fairchild FQP3P20/FQP3N30's. Since I can't get the 1x turn pots dialed in accurately either, the tiniest adjustment would still offset the bias by 1V, I've gone ahead and ordered some 4 turn pots and some higher value Dales to replace R5/R6 (which I hadn't lying around).
One of the P3/P4 pairs is at 473/463 Ohm's, the P3 being as far as it will go. I'm using the Fairchild FQP3P20/FQP3N30's. Since I can't get the 1x turn pots dialed in accurately either, the tiniest adjustment would still offset the bias by 1V, I've gone ahead and ordered some 4 turn pots and some higher value Dales to replace R5/R6 (which I hadn't lying around).
I changed the suspected P3 trimmer to a new 500R single turn one and also replaced R34/35 with 1K ohm. Still I am not able to get any voltage across T6-7 with the P3 trimmer turning. Now the last resort is that I need to replace the FQP20/N20 mosfets which I have ordered from element14 and might take another 10 days to arrive.
After the P3 pot change the below are the test voltages:
G-T6 = +32V G-T8 = -32V
G-T4 = -23.7V G-T2 = +23.8V
T6-T7 = 0V T8-T9 = 0.232V
T6-T11 = 11.95V T8-T13 = 11.62V
G-T18 = -31.8V (seems to be a issue)
G-T14 = +15.3V G-T15 = -15.3V
G-T19 = +13.4V G-T20 = -12.3V
G-T17 = -21.2V (seems to be issue) G-T16 = 5.6V
The above values with P34/35 with 1K ohm and the P1/2 at the center position, no other changes. Also I checked for continuity of the FQP20/N20 mosfets and I get good amount of resistance on the DMM for both and does not seem to be any issue. Based on the above values like the G-T18 and G-T17 being abnormal values compared to the optimal values as per the guide any suggestion was could be the problem as I have cross checked for any loose soldering/connection as well as the wrong resistance values and everything seems to be correctly placed and also the orientation.
Thanks
G-T6 = +32V G-T8 = -32V
G-T4 = -23.7V G-T2 = +23.8V
T6-T7 = 0V T8-T9 = 0.232V
T6-T11 = 11.95V T8-T13 = 11.62V
G-T18 = -31.8V (seems to be a issue)
G-T14 = +15.3V G-T15 = -15.3V
G-T19 = +13.4V G-T20 = -12.3V
G-T17 = -21.2V (seems to be issue) G-T16 = 5.6V
The above values with P34/35 with 1K ohm and the P1/2 at the center position, no other changes. Also I checked for continuity of the FQP20/N20 mosfets and I get good amount of resistance on the DMM for both and does not seem to be any issue. Based on the above values like the G-T18 and G-T17 being abnormal values compared to the optimal values as per the guide any suggestion was could be the problem as I have cross checked for any loose soldering/connection as well as the wrong resistance values and everything seems to be correctly placed and also the orientation.
Thanks
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An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I was able to adjust P1 to get to my Vgs+1 = 11.41v but the P2 turns are out to get down to 10.3v Vgs and it stops at 20.2v with the turns being lowest (counter clockwise as the voltage starts at around ~30v).
Yes used thermal insulation pads for Q5/6.
I just checked every resistor on the board and compared one by one with the board which is working fine. And the values are exactly the same without any discrepancy. I cross checked the J74/K170 orientation and they are fine including ZTX450/550. Q5/6 are also mounted correctly. The only values being different on both the working board and this board is the R34/35 where the good board I have used 562R and the bad board 1K after checking with 221R/470R/562R and went all the way to 1K. I have ordered the FQP20/N20 pair to replace if they are damaged with excess voltage during the setup initially which I doubt as I checked the continuity and they seem to have almost the same resistances around pair of legs when compared to the good board Q5/6.
And I hope the jfets Q1/2 are not damaged during this voltage checking process as they are matched ones that I had since last year for this project
Now this is getting little complicated as everything is same and with couple of test voltages being abnormal rest seems to be on par with the published values.
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