Musicman 2275-130 - tremolo issues

[Mars Amp Repair]

Hello folks,
See attached schemo:
The tremolo oscillator is running (tr-2), but there was no output from the N channel FET driver TR-1. Being mindful of the lead config differences, we replaced it with another N-channel FET, but cannot get the FET to concuct...ie, no oscillator out of the Drain.

The original FET is a 2N4091 & we tried using a J111, 112, 113, as well as a 2N5458. I don't have all that much 'in my head' knowledge of good subs for FET's like I do Bipolar xistors. Frankly from the specs, it seems to me that most of these Fets should work in here.

Anyone have any ideas for a better sub or what else we should be looking for here?

thanx, glen

[loudthud]

J111 is a near perfect sub for the 2N4092. On this family of FETs the Drain and Source are interchangable so don't work about that. That circuit is unusual. The FET acts like a varable resistance to ground. The trim pot gets the gate voltage in range where the oscillator can wiggle it effectively. You might need to tweek on the 150K and 220K in the gate circuit to get in range of the trim pot. When the gate voltage is near zero, the FET is on and there is no trem. A negative voltage turns the FET off letting the guitar signal through where is cancels itself at IC-7. Inbetween the oscillator can change the gain of the guitar signal. That's the way I see it.

[loudthud]

The 2N4092 has a gate cutoff range of -5V to -10V but the J111 is rated -3V to -10 so you may have to increase the 220K.

[Mars Amp Repair]

Great info...I'm pretty sure we tweeked the trim pot, but perhaps it's very touchy & only has effect in a very small range. we might have missed that...thanx, glen

[Mars Amp Repair]

Well we're embarassed!
Seems I was expecting to see an oscillator output from this FET & we also didn't have a signal going through the amp while troubleshooting...times like this ya wonder what the heck you were thinking.

Anyway apparently after replacing the FET the tremolo was working fine...we just didn't know it. What cha gonna do ;-[

We did have to tweak the trimmer for optimum operation with the J111, but that's to be expected with the different ratings on the Fets. Actually kind of nice the adjustment is there.

Thanx for all the help. I've always been a bit weak on FETS...this little exercise has helped to cure that!

glen

[Enzo]

Now you know, the JFET is a variable resistor.

[Mars Amp Repair]

Yeah,
Had originally thought the output of the Fet was modifying the one input of the OP-amp, until we realized the output of the FET was actually varying the level of the channel 2 signal to the opamp.

Knew about the resistive mode of the FET, just wasn't thinking about that here. I guess in older times that device would have been the bug or an LDR.

Too much to work on & working too fast to get it all done!

Thanx, glen

[otherdreams]

It sounds like we're going through the same repair on this same amplifier...

The original transistor had two shorted pins.. I tried replacing it with a J112 and ended up (again) with a dead transistor- the Gate shorted to one of the other two pins.

I have some more J112s on hand. Should they work? Or do I need to order a 111 (or PN4091) from Mouser?

Any idea what might cause this? Is there likely some other problem?

It does appear that the oscillator circuit is working, at least up to the other side of the .33 cap.

Oh, one other clue! - Turning the tremolo "intensity" control has an effect on the volume. At 10 the signal is coming through fully.. As I turn it down to 4-ish the level drops smoothly until it is quiet.. turning it further (down to 1 and 0) brings the level back up a bit.


Thanks !

-Robert

[loudthud]

Oh, one other clue! - Turning the tremolo "intensity" control has an effect on the volume. At 10 the signal is coming through fully.. As I turn it down to 4-ish the level drops smoothly until it is quiet.. turning it further (down to 1 and 0) brings the level back up a bit.

I'm not sure about this. I'll have to think about it, study the schematic and get back to you. Here's some words about JFETs in general.

It's normal for two pins of a JFET to "look" like they are shorted when testing with an ohm meter. That's because the JFET is "on" until you bias it "off". When a JFET is out of circuit, the Gate is floating and the JFET is in a low resistance state between Drain and Source.

Identifying the pins of an unknown JFET with an ohm meter:

Two of the pins will have a lowish resistance between them. Reversing the ohmmeter leads will have little or no effect on the reading. These will be the Drain and Source. In most cases, they are interchangable. (It doesn't matter to the FET or the circuit which is which.) The third lead will be the Gate. It will act like a diode between the other two leads. Usually when a JFET is blown, the Gate will be a low resistance to the Drain and/or Source or not read over-range when reverse biased.

P-channel or N-channel:

Most ohm meters apply a small amount of DC (or pulsed DC) to the unknown resistance. This DC comes out of the meter with the polarity the same as when measuring DC volts. Red is positive, black is negative. You can test this if you have two meters. Set one on DC volts, the other on Ohms and connect the leads red to red and black to black. The meter set to Volts should indicate a positive voltage. So when you measure a diode with the Ohm or Diode-Check function, Red to Cathode (striped end of a diode) is the high resistance polarity and Black to Cathode is low resistance polarity.

The symbol for an N-channel JFET has the Gate arrow pointing towards the Drain/Source. The Ohm meter should read a high resistance when the Black lead is connected to the Gate, a low resistance when the Red lead is connected to the Gate.

The symbol for an P-channel JFET has the Gate arrow pointing away from the Drain/Source. The Ohm meter should read a high resistance when the Red lead is connected to the Gate, a low resistance when the Black lead is connected to the Gate.

On many JFETs the Gate is the lead that you would normally think was the collector of a 2N3904 or 2N2222.

[otherdreams]

Thanks for the reply!
Let me know what you come up with on the 'intensity rotation' clue.,

In my tests, I'm pretty sure the short was between the Gate and and the Drain (or Source).

Should an analog meter show changing voltage at the Gate (middle pin) of Q1 (the FET)?

Is it voltage or current controlled?

Do you have any suggestions as far as further testing?

Do you think there'd be a problem with using a J112 and adjusting the trimpot? Or do I need to get the PN4091/J111?


Thanks!
-Robert
[ quietchannel.blogspot.com ]

[loudthud]

There is bound to be some interaction of the intensity control and the volume control when no footswitch is connected or footswitch is open. If the tremolo is operating, you probably wounldn't notice. When the intensity is on zero, the circuit acts just like the footswitch is shorted.

I dug a 2275-130 out of my rainy-day amp pile. It needs a new OT so I couldn't run a signal through it but I took some measurements that my help. Turn the Intensity control to 10. Remove the oscillator transistor. With the power off the center terminal of the Intensity pot measures 24.5 ohms to ground. (The JFETs on resistance). With the power on the same point measures 4.42K. As the pot is adjusted clockwise, the resistance won't increase but as it is adjusted counter-clockwise the resistance will decrease so adjust right to the point where the resistance stops increasing. This doesn't make total sense to me but it's a starting point.

According to my 1982 Siliconix Small Signal FET book the J111 and 2N4091 have an On resistance of 30 ohms (max). The J112 is 50 ohms max and the threshold is different. The J112 will probably work but may require some tweeking to get in range of the pot.

[beaglebone]

hi loudthud,
i am going to build a diy (2475-65) musicman (built 4 tube amps now) and got the schematics and the LM307H op-circuits. i read, the 2N4391 is an ideal replacement for the 2N4091 JFET. what is your opinion, do i need to tweek the 220K resistor at all?

[loudthud]

The 2N4091 and 2N4391 are almost identical as far a specs go. But the specs on JFETs are so loose you might have to tweek the 220K even if you got the correct part. Build the amp with a socket for that part (like MusicMan did), you may have to swap parts to get the thing working.

[kayopi4a]

I have the same problem with my amp.Did you solve the problem with yours?
Thanks
Nick