Finished building Aleph J: one side biases up while the other gets a bias of 0. DC offset on the speaker output is ~18.6VDC.
Voltage is there. I checked and doubled-checked for bad solder joints, including reflowing every solder joint. No shorts - as far as I can tell - and I made sure all of the transistors ZTX550 and ZTX450 are in the proper sockets. Check resistors against the working board and everything is the same.
Where to next? Can provide voltages at any junction, pictures, etc.
I'm going to go out and walk this old Labrador for a few blocks and do some thinking.
Obviously current isn't flowing through the output transistors, nor the current source, which, to me, points to an issue at the latter part of the circuit.
Voltage is there. I checked and doubled-checked for bad solder joints, including reflowing every solder joint. No shorts - as far as I can tell - and I made sure all of the transistors ZTX550 and ZTX450 are in the proper sockets. Check resistors against the working board and everything is the same.
Where to next? Can provide voltages at any junction, pictures, etc.
I'm going to go out and walk this old Labrador for a few blocks and do some thinking.
Obviously current isn't flowing through the output transistors, nor the current source, which, to me, points to an issue at the latter part of the circuit.
pictures
be sure that both mosfets are isolated from heatsink
post link to exact schematic (or insert it here , even better) , so I can download it and write some voltages , if needed
orientation of bjts , diodes etc.
also check trimpots (resistance in cold)
just in case - both inputs shorted , no load on output
be sure that both mosfets are isolated from heatsink
post link to exact schematic (or insert it here , even better) , so I can download it and write some voltages , if needed
orientation of bjts , diodes etc.
also check trimpots (resistance in cold)
just in case - both inputs shorted , no load on output
Pardon my resoldering blobs - also pulling and inserting the power / signal leads multiple times doesn't help!
Images:
Imgur: The magic of the Internet
Imgur: The magic of the Internet
Imgur: The magic of the Internet
Schematic is the original on Post #1 but with a 1K resistor instead of the pot for R8
Images:
Imgur: The magic of the Internet
Imgur: The magic of the Internet
Imgur: The magic of the Internet
Schematic is the original on Post #1 but with a 1K resistor instead of the pot for R8
Hi KStagger,
Not sure if it matters, the experts can chime in on this, both of your trim pots for bias and offset are 180 deg backwards from my Aleph J and others that I referenced when building my amp.
Not sure if it matters, the experts can chime in on this, both of your trim pots for bias and offset are 180 deg backwards from my Aleph J and others that I referenced when building my amp.
No expert here, but wouldn't reversed pots just cause direction of turn to be reversed, but not cause problem?
Russellc
Russellc
Correct. No problem other than needing to remember that adjustments will be happening in reverse. Ask me how I know. 🙂
Yeah I realized I put the pots in backwards once I was done soldering them in place (!) - but hey, I just keep fiddling with the set screw either way until it dials in.
That's the spirit. Much more important to know what to watch that to
just think a particular direction means only one thing. 🙂
BTW, please measure and post the voltages ZM asked for. Also, what are you
getting as voltages across R7 and R8?
Cheers,
Dennis
Back from work so I finally had a chance to do some measuring:
Good side, VDC measurement across resistors:
R7 = 4.95V
R8 = 8.4V
R16 (Source Resistor?) = .41V
R17 = .41V
Bad Side:
R7 = 0V
R8 = 8.4V
R16 = 0V
R18 = 0V
Good side, VDC measurement across resistors:
R7 = 4.95V
R8 = 8.4V
R16 (Source Resistor?) = .41V
R17 = .41V
Bad Side:
R7 = 0V
R8 = 8.4V
R16 = 0V
R18 = 0V
check resistance of R7 , good vs. bad channel , in cold (amp off) state
you gave convenient free pads for that on each side of trimpot
you gave convenient free pads for that on each side of trimpot
I hope there is nothing wrong with the 2SJ74s - got them from a DIY Audio mod who mostly dabbles in tube gear. So real, honest to goodness parts from his stash..
all resistor values good ?
you have installed shortie in place of R27A , good;
further - you need to have installed either R27B or R27 trimpot, not both
I prefer R27 trimpot there (sans R27B) , pre-set to half value
if you have too small (resulting) resistance there , it can be confusing for novice
you have installed shortie in place of R27A , good;
further - you need to have installed either R27B or R27 trimpot, not both
I prefer R27 trimpot there (sans R27B) , pre-set to half value
if you have too small (resulting) resistance there , it can be confusing for novice
try finding picture of bare pcb (I'm not familiar with it) , or move slightly that resistor right from it , to see it's designation
per schm , it must be as I wrote :
R27A shortie
R27B - left out
R27 -trimpot (set to half for start)
per schm , it must be as I wrote :
R27A shortie
R27B - left out
R27 -trimpot (set to half for start)
As far as I know, this board only has two R27 locations - one that is jumpered, and the 100k pot location. Can anyone else confirm that?