http://4.bp.blogspot.com/-jE4pIrQWStc/U_yamS0kAUI/AAAAAAAAAZI/R4RlYfz1WlY/s1600/alephJ.png
I hope this link actually gets us to the schematic...
-Graham
I hope this link actually gets us to the schematic...
-Graham
Hi Graham,
If that's the schematics you're using then the voltage across R8 is good and it
should mean 8.3mA going to Q1A and Q1B. If you only have 1.7V across R7 then
the current flow through Q1A isn't right (or at least badly mismatched.)
Do you have an actual 2sj109 dual jfet or are you using a pair of 2sk74? Is there
a way for your to test the jfet?
Dennis
If that's the schematics you're using then the voltage across R8 is good and it
should mean 8.3mA going to Q1A and Q1B. If you only have 1.7V across R7 then
the current flow through Q1A isn't right (or at least badly mismatched.)
Do you have an actual 2sj109 dual jfet or are you using a pair of 2sk74? Is there
a way for your to test the jfet?
Dennis
Evidently 2sj109's are hard to come by, so I've got 74's. I know the 74's were matched when they went into the board, but I haven't tested them since the reverse polarity incident. I have about a half dozen extras, and I'm sure I can figure out how to match a couple. Would testing them be as simple as pulling them from the circuit and making sure I have large resistance with the gate low and low resistance with the gate high, or is there a more sophisticated method?
-Graham
-Graham
Hi Graham,
I suggest trying to measure the Idss, since you will have to do that
if you need to match new ones.
Please have a look at the (P channel) diagram here:
https://www.cavalliaudio.com/diy/exstata/main.php?page=matching
You can use a 9V battery. Also, if you're unsure about your ammeter,
you can replace the meter with a 100 ohm resistor and measure
the voltage across it. (The idss current is then roughly the measured
voltage/100)
If you are using 2sj74BL devices, your measured idss should be in
the 6 to 10mA range.
Cheers,
Dennis
Well, new JFETs are installed, and I'm still getting low voltage across R7. It seems to fluctuate between 1.8 and 0.8V, on a time scale of maybe 60 seconds.
A question about schematics: I'm only using the schematics I am because of the options I got using google image search, those were the most printable. The boards I'm using are Peter Daniel's. They were prototype boards but I believe only some spacings changed between mine and the production units. Could I be working off of the wrong schematics?
-Graham
A question about schematics: I'm only using the schematics I am because of the options I got using google image search, those were the most printable. The boards I'm using are Peter Daniel's. They were prototype boards but I believe only some spacings changed between mine and the production units. Could I be working off of the wrong schematics?
-Graham
Here are some photos, including one for fun of the chassis and forced air cooling system. I take no credit for any of the chassis. That was entirely the work of long time diyaudio member Francolargo. Let me know if I need to get closer/better lit shots of anything relevant. I can switch from the phone to the DSLR if necessary.
https://goo.gl/photos/jrqhzuxkmaMmoVR59
-Graham
https://goo.gl/photos/jrqhzuxkmaMmoVR59
-Graham
Dennis,
One of my multimeters (I don't recall the make/model, and I'm not at home presently to look) has slots on the face for measuring idss. I don't recall the units, but the 74s I tested all fell between 800 and 1400 [arbitrary units]. The two jfets I chose both read 1225 +- 5. I was careful to only handle them with tweezers, and to wait for a count of 2 before taking the reading. So I'm confident they're a good match, but I'll have to report back this evening on the actual measurement.
-Graham
One of my multimeters (I don't recall the make/model, and I'm not at home presently to look) has slots on the face for measuring idss. I don't recall the units, but the 74s I tested all fell between 800 and 1400 [arbitrary units]. The two jfets I chose both read 1225 +- 5. I was careful to only handle them with tweezers, and to wait for a count of 2 before taking the reading. So I'm confident they're a good match, but I'll have to report back this evening on the actual measurement.
-Graham
While I was visiting my dad and we were working on audio stuff, he mentioned how he'd struggled with phony parts, and how hard it was to find genuine units. So I'm sure that the parts I have are the real McCoy. (In fact, if I recall correctly, he ordered them from another DIYAudio member.)
-Graham
-Graham
Grahambo, here is what you put in: Linear Systems LSJ74 Ultra Low Noise Single P-Channel JFET (8 Pieces) – diyAudio Store (BETA)
Well, I've done some more messing about, and here's the summary.
I've replaced
Q1A&B
Q2
Q3
Q4
R7 (I also tried swapping R7 with a 470 ohm +trimmer, to no avail.)
Zener diode
I've got ~14mV on the source pins pins of Q1A&B relative to ground. (I think it should be slightly negative?)
I've done some probing of voltages and resistances and compared to the good channel I've got, and I think I've found a problem.
R9, 10, 11, and 12 are all 221 ohm units, but on the bad unit, R9 was reading ~180 ohms. I pulled the resistor, and it measured spot on 221 out of the circuit, so I measured across the pads, and found 1057 ohms. This was with the power lines disconnected. Looking at the schematic, I don't see another path for those electrons, unless Q5 is misbehaving? Maybe (probably) I'm missing something...
Should I suspect Q5?
Should I toss a 280 ohm unit in there to get 221 effective?
Should I suspect a shorted trace, and order a new board?
Something else?
Not really sure how to go about nailing this down, so advice will be much appreciated.
Thanks in advance,
-Graham
I've replaced
Q1A&B
Q2
Q3
Q4
R7 (I also tried swapping R7 with a 470 ohm +trimmer, to no avail.)
Zener diode
I've got ~14mV on the source pins pins of Q1A&B relative to ground. (I think it should be slightly negative?)
I've done some probing of voltages and resistances and compared to the good channel I've got, and I think I've found a problem.
R9, 10, 11, and 12 are all 221 ohm units, but on the bad unit, R9 was reading ~180 ohms. I pulled the resistor, and it measured spot on 221 out of the circuit, so I measured across the pads, and found 1057 ohms. This was with the power lines disconnected. Looking at the schematic, I don't see another path for those electrons, unless Q5 is misbehaving? Maybe (probably) I'm missing something...
Should I suspect Q5?
Should I toss a 280 ohm unit in there to get 221 effective?
Should I suspect a shorted trace, and order a new board?
Something else?
Not really sure how to go about nailing this down, so advice will be much appreciated.
Thanks in advance,
-Graham
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Another thought too Grahambo - check to be sure the power transistors are properly insulated from the heat sink. There should be mica insulators with thermal paste, but the mica itself is practically transparent so who knows if something went wrong during assembly. ZM will know if that could produce what you are seeing...