[gates_2]

Hey Guys,

So, I just built a pair of jisbos buffers to drop into a soha. One of them seems to work fine, biases correctly and dc offset set at 0. Unfortuantely, the other channel is pumping out 6.23 V DC, yet it biases fine.


thoughts?

EDIT: the board looks fine to me, as in no blatant bridges or anything like that...

 

[amb]

Poorly-matched input JFETs?

 

[luvdunhill]

gates_2:

I'd recommend looking at the circuit diagrams on amb's website that have the various voltages listed at each node. In my opinion, this type of documentation is invaluable (amb++). I just built 8 JISBOS and haven't installed them yet, but I hope too by the end of the weekend.

 

[gates_2]

wait- where is that documentation? I get redirected to steinchen's site, but I can't seem to find the voltage listing?

also, if the jfets were poorly matched, adjusting the trimpot would still affect the DC offset output right?(I remember having matching problems on a B22- offset would change, just not go to zero) Here, that voltage remains constant no matter that I do with the trimpot

 

[amb]

JISBOS operating points (±12V rails)
JISBOS operating points (±15V rails)

 

[amb]

Quote:

Originally Posted by gates_2 /img/forum/go_quote.gif also, if the jfets were poorly matched, adjusting the trimpot would still affect the DC offset output right?(I remember having matching problems on a B22- offset would change, just not go to zero) Here, that voltage remains constant no matter that I do with the trimpot

Yes, you should see a change as you turn the trimpot. If not, then something is not right. Check the soldering on the trimpot and measure it with a DMM in ohms mode to see if the trimpot is good. If it checks ok, then check the rest of the board for cold joints. Check each of the BJTs using the diode check mode of your DMM between the B-E and B-C junctions. For NPN devices you should read about 0.6V with the red probe on the B pin and the black probe on the C or E pins. Swap the probes for PNP. If these check out ok then perhaps one of the JFETs is blown.

 

[regal]

I have the same problem. Odds are you have a blown transistor. Also make sure you +-12V are correct on the SOHA board.

 

[gates_2]

well, i ended up swapping out both 550, 560, and then each JFET in order,(except for q7- hard to get to) and I'm getting the exact same reading, -6.23V between the output and ground. The pot tested good- any other possibilities? should i switch Q7?

 

[amb]

Have you checked your circut against the operating points PDF I linked to above? What voltages do you get at the various nodes? It would be nice if you could take the PDF, convert into a PNG or GIF file, mark your actual measured voltages and post it here. All voltages listed are relative to ground.

 

[gates_2]

I love it how I always mess the simplest thing up

I had the ground connected to the negative at C11, thinking it was the same ground as the test point(continuity test showed it was)- apparently its not

 

[gates_2]

btw- it sounds pretty ridiculous- ridiculously good

Thanks for all the help

 

[amb]

I am glad you solved the problem. Sometimes it is indeed the simplest thing that gets us.