Carver C1 going through rework

[Gepetto]

Yeah 2 thoughts:

1. The positive bias to the JFET gate is way positive (as in +10V+) during the time you are seeing the 33 and 23 mV and then switches way negative when you see it go back to the level before power on. It could be leakage from the junction FET gate into the drain terminal. This JFET is being biased hard to an on condition during your 8 second period by IC8 (it only takes 0 V at the gate to turn on a JFET, this design is applying ~+10V).
2. The ground that the source of each JFET is attached to is really not ground but a ground that is offset by 33 or 23mV from the real ground. This would be a board layout related issue if it is the case. (This is probably unlikely but could exist)

Now that we have a bead on this, what I would recommend is that you increase the value of R261 and R361 from 2.2K to ~100K. Try it on the left channel first which has the worst offset to see if it makes a difference.

If it works, you can call it the Gepetto fix. Otherwise not

A MOSFET N-Channel enhancement device may also do a much better job than the current N-channel JFET depletion device that is in there now, but that can be reserved for a future discussion.

 

[Gepetto]

Note: N channel enhancement mode MOSFETs probably only existed in the lab in the days this preamp was created. JFETs were it in those days.

 

[WOPL Sniffer]

If you planned on changing the old 4136's and you "THINK" the FET's are good, I'd change them out first before modifying the circuits. But hey, that's just the way I'd try it first. The current TI issue 4136's won't alter the sound if that's what you were worried about.

 

[WOPL Sniffer]

IF the FET's or FET is leaking, change them out.

 

[Mohawk]

Hey Joe, Perry

I just got back to the bench and made a discovery this morning ... If I have the amp on and a CD player playing...
Power off the C1 for 10 seconds or so ... Then power back on the C1 and I hear low level music coming through prior to
IC 8 switching the FETS.... If I quickly adjust the volume during the mute circuit on time I do hear it get louder.... After the fets switch, music is back and everything seems ok....

So what Joe said seems totally plausible to me ....

"Yeah 2 thoughts:"

1. "The positive bias to the JFET gate is way positive (as in +10V+) during the time you are seeing the 33 and 23 mV and then switches way negative when you see it go back to the level before power on. It could be leakage from the junction FET gate into the drain terminal. This JFET is being biased hard to an on condition during your 8 second period by IC8 (it only takes 0 V at the gate to turn on a JFET, this design is applying ~+10V)"

*"Now that we have a bead on this, what I would recommend is that you increase the value of R261 and R361 from 2.2K to ~100K. Try it on the left channel first which has the worst offset to see if it makes a difference."

I'm gonna' swap out R262 / R361 to 100k and post back !

Thanks !

M

 

[Gepetto]

JFETs typically don't have very low channel "ON" resistance.
The reason you still hear music playing while muting is for the above reason. It attenuates but does not really mute it altogether. I think the PL manual tells you this.

 

[Mohawk]

Got it...

Will the enhancement mode 2N7000 MOSFET upgrade you suggested work to mute the signal more completely ?

M

 

[Gepetto]

Got it...

Will the enhancement mode 2N7000 MOSFET upgrade you suggested work to mute the signal more completely ?

M

yes but it is complicated...

 

[Mohawk]

yes but it is complicated...

Aside from swizling pins are the not a direct replacement?

 

[Gepetto]

You need to back to back 2 MOSFET devices in an application like this which is why it is complicated.

Pictures show what you have to do. Assembled to hopefully show what you have to do. An 1/8W resistor would have been better but could not readily find one.

 

[J!m]

Looks like six monkeys humping a football...

 

[laatsch55]

Well, it's a MOSFET so........

 

[stetter]

Lee, did you catch any fish on the river

 

[Mohawk]

This could work .........

 

[laatsch55]

Lee, did you catch any fish on the river

Yes I did...

 

[Mohawk]

Hey Joe,

Seriously, thank you for the post !
I appreciate your time !

M

 

[Gepetto]

You have proven to yourself there is no bump noise in the headphones which is driven by this same circuit, the simplest solution is to remove the two 2N5458 JFETs

 

[VSAT88]

Well, I told you the same thing yesterday. You asked for my "Expertise" and I gave it to you. You can do with it as you like. The first 6 times, I told you nicely, not the second 6.... I'm a jerk, and you are a hard head............. No winner either way

LOL !!!!!!!!!!!!!!!!!!!!!!!

 

[VSAT88]

If NONE of that works, Power it on in the correct sequence.

Sniff you are going to KILL ME LAUGHING SO FUKIN HARD !!! I am about to fall OUT my chair !!

 

[Mohawk]

Op amps and FETS , grab bag of other stuff is on order from Mouser .....
Do you bother replacing X/Y caps ? I there a better option now a days ?

M