Sunn 1200s solid state

[lowell]

This amp works in the fx return so the PA is good. I also get signal from U10 pin6. I am not however getting signal from the junction of R71/72 that go to jfet Q5's drain. I tested the jfets and they seem good. When I turn off the amp I get continuity from drain-source. However when the amp is on I do not get continuity AND there is -14v on the gates, even with the compressor OFF. I then followed the Comp1/Comp2 headings. I tested Q42/Q43 and they're good. There is ~+14v on the base of Q42. I'm not sure how U18 functions. I read the datasheet and it's a CMOS component. I'm pretty sure it turns Q42/43 on/off, therefore turning jfets Q5/6 on/off.

http://music-electronics-forum.com/t27591/

[Jazz P Bass]

Confused am I by this post.
Stated is "I also get signal from U10 pin6"
Pin 6 is an input pin!
It is difficult to read opamp input pins.(virtual ground?)
I always look at the output pin.
Next: "I am not however getting signal from the junction of R71/72 that go to jfet Q5's drain".
Same thing. that is the pre input to pin 6.
You should be testing the signal over at the test point where the signal comes from the EQ.

[lowell]

JPB,
Sorry about the confusing post. What I mean is that I'm getting signal OUT of the amp's output when I apply signal to said points.

[Jazz P Bass]

OBoy.
So what is the issue with the amp?
The power stage works. That was proven by going in the loop return.
Here is what the jfets are doing.
"The FETs Q5 and Q6 select between pre- and post-compressor via the front panel switch (S8) or footswitch. After
passing through op-amp U10B and master-gain control R116, the signal is sent to output jack J3, effects-loop send.
The signal input at J4, effects-loop return, goes to differential receiver U11B."
That is from the SM.
Are you having a preamp problem.
If so, track it down to a stage.
The area that you indicated should be checked at TP 33 & 34.
After TP 34 is where the signal splits.
Then the jfets pick which one gets passed to U10.

[lowell]

JPB,
Don't you think that both jfets Q5/6 having -14v on their gates is a problem? Are they not BOTH off? Therefore not passing anything on to U10B. As I stated, maybe not so clearly, that signal passes from U10B pin6 to the speaker out. However, signal injected at the drain of EITHER Q5 or Q6 passes nothing to the output. Thanks!

[Jazz P Bass]

You need to go through Test Point 18, 19 & 20.
Exactly where indicated.

[lowell]

That's exactly what I'm getting at. With the compression OFF, both test points have -15v.

[Jazz P Bass]

Now we need to find out why both of the jfets have -15V.
I would measure TP 18.
Comp Sw NOT depressed = +15V
Now depress the switch & it should change to -15V.
All good?
Test Point 19.
Sw NOT depressed = -15V.
Depress the switch & it should change to +15V.
All Good?
Test Point 20.
Sw Not Depressed = +15V.
Depress the switch & it should change to -15V.

[Enzo]

OK, so both paths are turned off. WHat turns both of them off together normally? The tuner switch.

Got -15 at the cathodes of CR22, CR24? That is the TUNER_L control signal. That comes from upper left of the power amp page. Is the tuner switch pushed in? Is it shorted? Is R138 open? Right there is CR27 to Q7, is Q7 off too? SO line out is dead in either pre or post?

You need to see +15 on the TUNER_L control line.

[Jazz P Bass]

Thankyou for the concise clarification, Enzo.
That one slipped right by me.
The Tuner turns both jfets off.

[Enzo]

I surely don;t KNOW that is his problem, but it suggests itself from his report of condition.

[Jazz P Bass]

Most certainly does suggest it.
"Rule Out The Obvious"

[Enzo]

I ask about the button, but I recall one Marshall head I had in here once. NO sound out, wouldn;t bias up right either. The two side of bias seemed to adjust each other. I spent far too much time on it, until my troubleshooting revealed two sides shorted together, and I tracked it down to... The mute swich on the panel was pressed in.

[lowell]

Sheesh... yep that was it! Dog garn tuner button was depressed. (now I'm depressed) Originally the amp came in w/ no sound but 1 of the preamp tubes was leaning in its socket. The owner had dropped the amp on the floor. After reseating that tube, it still didn't work so I began to troubleshoot the circuit. Sorry, I'm an idiot sometimes. I love this forum and greatly appreciate your help guys.

[Jazz P Bass]

Lowell, thankyou for fessing up.
I for one, was really curious what the issue was.
Having blown it on the reading of the schematic (I missed the importance of the Tuner input to the jfets), I felt obligated.
Anyway thanks.
(I must add that your admission is a good sign of your character, which compliments this awesome site)
Hey, if someone else can learn from others mistakes, that makes this a community.