DC Offset?

[dyermaker8]

Trying again,. sorry to keep buggin you guys,. I found 2v dc offset on pin 7 of an opamp (U7) in the chorus signal of my fender ultra chorus amp. The offset only occurs when the chorus is switched off. Which makes me think this is the cause of the buzz i'm getting when the chorus is switched off. With the Chorus on there is no buzz. Anybody have any good tips on locating the cause of this dc offset?

Thanks,
Daniel

[loudthud]

Just to make sure we're on the same page, I'm looking at a schematic that is titled "Schematic Diagram, Ultimate Chorus Amplifier" Drawing number 039242. U7 drives the stereo Effects loop and then the signal goes to the power amps.

http://www.fender.com/support/amp_sc..._Schematic.pdf

Always good to post a link to a schematic on this forum so everybody knows what you are talking about.

[dyermaker8]

Sorry. But, Yes,. thats the one. DC offset on pin7,. only with the chorus switched off. Where does DC offset come from generally. I've seen Enzo tell people to check for it on like a hundred post ,. but for some reason those threads never go futher than that. So i'm looking for the next step,.

[loudthud]

I've been trying to figure out how this thing is supposed to work. The first thing I would do is clean off any solder flux from around U7 and the Q6, Q7 area. Old flux and humidity can do strange things in high impedance circuits. When you replace an IC on an amp like this, I always install a socket just to prevent any future damage to the PCB if I have to replace it again.

Assuming you mean +2V of offset and U7 is good, then possibly something is pulling negative on pin 6 of U7 when the chorus is off. When the chorus is off, Q7 should be connecting the right and left together and Q6 should be shorting the chorus signal to ground. The SELB line should be clamped at about +1.2V through CR26 and the gate of Q6. The gate of Q7 should be 0V. The junction of R86 and R87 should be 0V. Look for a small negative voltage somewhere that only gets to the B side of U7.

[dyermaker8]

Thanks for the tips,. thats exactly what I was looking for. However,. i'm still coming up empty handed,. I'm not seeing that low negative voltage anywhere... board cleaned,. dip socket installed previously. I'm seeing 1v on one side of CR26 and .5v on the other. The gate of Q6 shows 0v. The gate of Q7 shows 5mv. The junction of R86 and R87 is 0V. Pin 6 on U7 is 6mv. SELB is showing 1v after R152 with chorus off. arghh,. I'm thinking about swapping the iron for a blow torch. Any thoughts?

[Gunny]

an odd bit of circuitry indeed. You say that an IC socket is installed. Did you change out the Op Amp in question?
If you have, is there a chance that the Send/Return jack "switch" contacts are dirty? I'm kinda groping in the dark, but those ideas are worth a shot.

[dyermaker8]

brand new op amp,. made no difference at all. tried some contact cleaner and multiple inserts in the in-out jacks,. no difference,. they have the clear tops and look clean inside anyways,. i'm grasping for straws here,. what else could cause +2v dc offset on pin 7 besides small negative voltage.,. my 16v rail (at pins 4 and 8,.are actually much closer to +-19v ,. would that make a difference?

[loudthud]

The +/- 19V is high but I would just verify that the 5W zeners, CR68 and CR69 are getting hot. If they are blown, they either short and you get 0V or they are open and won't get hot.

6mV on U7 pin 6 should be lower. Verify pin 5 is grounded and measure again from pin 5 to pin 6.

There should be no DC at the stereo effects loop jacks. Verify that there is no DC on R92 and R95.

You said: I'm seeing 1v on one side of CR26 and .5v on the other. The gate of Q6 shows 0v.
These should be the same point, they should have the same voltage. Check the data sheet for the J111's pin-out. Note that the Drain and Source are interchangable so they might not be hooked up exactly as shown in the schematic.

http://www.onsemi.com/pub_link/Collateral/J111-D.PDF

I would remove Q6 and Q7 and install Drain to Source jumper wires. Now if there is offset, I'd remove R87 and the Q7 jumper and pin 7 better go to 0V +/- 3mV.

[dyermaker8]

I jumpered u7 pin 5 to ground and got down to 2.5mv between pins 5 and 6 but still about 5.5 mv between pin 6 and the chassis whatever that means.
Also measured 2mv on both sides of R92 whiched dropped to 0v after regrounding pin5. 0v on R95 either before and after. pin 7 dropped from 2v to about 1.95v.

CR68 and CR69 are both hot.

remeasured q7 oops,. the gate has .6v same as CR26,. .

jumpered drain and source on q6 and q7,. noe at 2.28v offset on pin 7. amp is now on switched off the clean channel and is stuck on the drive channel. 2mv on both sides of R92 and R95.

Then,. i removed R87 and the Q7 jumper and pin 7 went to 1mv,. then the board smoked and the fuse blew,. awesome.

[dyermaker8]

Reverted changes,. powered up,. blew another fuse. oh my. how'd i mess this up?

[Enzo]

CR68,69 are 5 watt zeners, they are going to get real hot.

You already tried a new U7.

OK, you have 2v of unwanted DC on U7-7, but only when certain channel switching states are on. My immediate thought is Leaky Q6 or maybe Q7. You can't often measure things at op amp input pins due to their "virtual ground" phenomenon. You cannot measure current flowing in pin 6. Try removing Q6,7 and see if your 2vDC goes away.

Less likely but possible would be a leaky C48. The MN chips and U6 around them run off +16 and ground, NOT +/-16 SO the signal sits at +8v. if C48 were real leaky it might tryo to send some DC on to U7. But I still like a bad J111 at Q6.

Oh, I see Loudthud was already sniffing that hydrant.

[dyermaker8]

Enzo,. thanks will definatly replace those j111's ,. any thoughts on why it chose now to start blowing fuses?

[Enzo]

Not any of value.

[dyermaker8]

LOL

[loudthud]

You must have created a short somewhere. A solder splash, a circuit touching the chassis where it shouldn't, a pinched power transistor washer, something somewhere. Use an ohm meter to find the short. Use a series light bulb when you power the amp up to save the fuse and whatever else can be damaged.