Fender Ultimate Chorus issue

[Randall]

I have a '92 Ultimate Chorus amp in with no output. Quick check reveals sound out when going into FX return jacks (it has a mono and a stereo, both work). I trace signal on the clean channel thru U1 pins 1 and 7 up to the treble pot side of R20, but nothing on the other side of R20, and thus nothing to U4. On the boost channel, I see signal on U2A on pin 1 up to the junction of C16 and R31, but on the Q2 side of R31 nothing.

I suspect something to do with channel select switching, but I am unclear on this. Q1, Q2 and Q3 are involved I believe, but I don't get the role of the double diodes CR4/5, CR3/15 and CR13/14. The channels appear to be switching, as the LEDs are functioning, albeit with a thud. The chorus appears to be functioning, as the LED comes on and modulates between red and green with the rate control.

The 16v rail is good, but the -16v rail is at -13.2 volts. And the amp passes signal for a brief second at powering off.

Any ideas?

http://www.electronicstudio.net/sche..._Schematic.pdf

[52 Bill]

Can you link the correct schematic? None of your part numbers make any sense.

[Randall]

Sorry. Fixed.

OK, I think I found why the clean channel is dead. With SEL A voltage not applied to the junction of CR3 and CR15, the clean channel goes to pin 2 of U4. With SEL A voltage applied it switches on Q1, shunting the clean channel to ground, and Q3 which sends the gain channel to pin 2 of U4. I think I have a shorted Q1, because I jumpered from the treble wiper to pin 2 U4 and got signal.

But still not knowing why gain signal stops at R31 at Q2. I tried jumpering from C16/R31 to pin 6 U2 but nothing happened. Then I found -8vdc on pin 7 U2. Then I jumpered to junction of C17/R33 after U2, and got signal with all drive channel controls functioning, except now drive channel is passing signal regardless of channel switch position or color of LED. I suppose I could have a shorted Q3 as well?

Off to Mouser for some J111s and TL072s.

[The Dude]

Oh, you can't do that to us. Please tell us what you found.

[Randall]

Sorry if I wasn't clear. I haven't taken it out of the chassis yet, but my guess is shorted Q1, possibly shorted Q3, and bad U2B. I'm told it took a power surge. I live in the lightening capital of the country.

[Randall]

I got the drive channel to work by replacing U2 and Q3, but I request some help with the clean channel. Something doesn't make sense. I have clean channel signal all the way up to R20 after the treble pot, then it stops. I replaced Q1, no change. I measure 26 ohms to ground at the other side of R20 with SEL A switched off , (this is the clean channel mode), and 22 ohms switched on. So the signal appears to be getting shunted to ground all the time. I removed Q1 and the clean channel works. I put Q1 back in and lifted CR3, and clean channel does not work. How can this be? A J111 is just a switch, right? Off with no gate voltage?

Also, I forgot to mention that in clean channel mode, some of the drive channel is getting thru. When switched to drive channel it is much louder.

[Jazz P Bass]

Quote:" A J111 is just a switch, right? Off with no gate voltage?"

My take: a JFet is considered 'normally On' when there is no gate voltage.

The J111 has an On resistance of 30 ohms.

To turn it off you have to apply about -3 Vdc to the gate.

[Randall]

I see, I had it backwards. Thank you.

If that is the case and channel switching FETs Q1 and Q3 are conducting with no voltage from the channel switch , then the clean channel is shunted to ground, and the drive channel passes a slight signal onto pin 2 of U4. The problem is, this is when the green clean channel LED is on. When the drive channel is selected the LEDs change from the green one to the red, and +14.5 volts is applied to gates via CR 3 and 15, and this is when the drive channel comes full on.

Doesn't make sense to me.

Edit: I applied -9v with a battery to the gates of Q1 and Q3, and the clean channel came on loud and clear, and the green LED stayed on. So now the question is, why is there -0.4v in clean mode, and +14.5 v in drive mode? On the left side by the footswitch circuit, if I read it correctly is U10 which supplies the turn on voltages for SEL_A (channel) and SEL_B (chorus). I measure +14.5v and -15.5v at the two output test points. Where is my negative turn off voltage?

[Randall]

Bump. Sure could use some help here, I'm at a loss.

[nickb]

Bump. Sure could use some help here, I'm at a loss.

Have you checked to see what pin 1 of U10 is doing? Is should be about -14V in clean.

EDIT: Oh duh..you did! I just missed it. In that case something on the SELA line is pulling the voltage up Lift one end of CR17, CR20, CR3 then CR5 to find the offending path.

[Randall]

nickb, I read the schematic as the SELA line to be +15v at pin 1 of U10.

[nickb]

nickb, I read the schematic as the SELA line to be +15v at pin 1 of U2.

U2??

SELA should be about +15V in drive and -15V in clean. I suspect one of the FET source/gate junction is shorted. Instead of lifting the diodes you might see it using a ohmmeter across the G/S on each of the associated FETS.

[Randall]

Sorry I meant U10. This is helpful because I wasn't getting this part of it. I will look at what you suggest and report back. Thank you.

[Randall]

Brilliant advice nickb! I tracked down a partial short on gate/source on Q2, replaced it and now I have the proper voltages on SELA as you said, and channel switching is working. THANK YOU.

But I still don't understand what INH is at Q3 and input jack 1, or what effect this may have on why I don't get Clean channel with nothing plugged into either input jack. Also, there is a loud thud or pop when the amp is switched off. I am wondering if this is normal for this amp?

[nickb]

Brilliant advice nickb! I tracked down a partial short on gate/source on Q2, replaced it and now I have the proper voltages on SELA as you said, and channel switching is working. THANK YOU.

But I still don't understand what INH is at Q3 and input jack 1, or what effect this may have on why I don't get Clean channel with nothing plugged into either input jack. Also, there is a loud thud or pop when the amp is switched off. I am wondering if this is normal for this amp?

The purpose of the INH line is to mute the amp with no input. That function shares the same FETS and so overrides their normal purpose.
IIRC correctly the thump is a 'feature' of these amps. You can try plugging a jack into the return socket and then switching on to see if it is before the power amp. Not that you can do much about it without mods, I expect.