Musicman 2275-130 - tremolo issues revisited

[telecaster]

Re: Prior forum post http://music-electronics-forum.com/t12356/
Schematic: 2475-130_and_2275-130.pdf

One of our MusicMen, a 2275-130, had the tremolo stop working. I read the prior posts on here, and I referenced the one above that had great information. I verified that the oscillator is working and feeding the FET gate properly. Both + and - 16V supplies are fine. So I ordered a new FET, a J111. Replacing it and twisting the trim pot throughout its range still yielded no tremolo. I compared the specs of the original and the J111, and saw that the cutoff voltage is -5 to -10 volts for the 2N4091, and is -3 to -10 for the J111. So if I understand correctly, the gate should swing in the 0 to -3 volt range using the J111. I increased the 330K resistor (schematic shows it as 220K by the way) to 470K and was able to get the gate input to swing in the 0 to -3V range. Still no tremolo. I temporarily replaced the resistor with a substitution box and tried ranges from 0 to -10 volts, still no tremolo in any range. I verified that the resistors, intensity pot, and caps after the FET that feed the op amp were all good. So I pulled the FET and wiggled the intensity knob back and forth and induced a working tremolo. I believe that validates everything after the FET, so I am left confused on what to try next. I appreciate any insight or help.

[52 Bill]

Are you certain that the gate is connected to the circuit correctly? The drain and source will not matter much here, but you must be certain that the gate is getting the control voltages.

[Enzo]

Just in case: you know all JFETs are not wired the same? The gate can be on any leg, unlike common 2Nxxxx or 2SCxxxx types which will be consistent. if you replaced a bad 2N4091 with a good J111, it is possible the legs are now in the wrong holes.

In these JFETs, the source and drain are interchangeable, but the gate is the controlling element. A JFET is ON until turned off at its gate. That means the part siting in your hand will have low resistance between source and drain legs. I don't know, 100-200 ohms maybe. When the gate is energized, the thing shuts off and that resistance goes way high - like OFF. So whenever I replace a JFET, I ALWAYS check the pinout. Use an ohm meter to find the two legs with low resistance between. The remaining leg is the gate. Make sure your new J111 has its gate pin in the hole receiving the LFO signal.

The J111 datasheet says the far right leg is the gate. 2N4091 seems to be the same, but always check.

[telecaster]

I'm pretty sure I have it right

Just in case: you know all JFETs are not wired the same? The gate can be on any leg, unlike common 2Nxxxx or 2SCxxxx types which will be consistent. if you replaced a bad 2N4091 with a good J111, it is possible the legs are now in the wrong holes.

In these JFETs, the source and drain are interchangeable, but the gate is the controlling element. A JFET is ON until turned off at its gate. That means the part siting in your hand will have low resistance between source and drain legs. I don't know, 100-200 ohms maybe. When the gate is energized, the thing shuts off and that resistance goes way high - like OFF. So whenever I replace a JFET, I ALWAYS check the pinout. Use an ohm meter to find the two legs with low resistance between. The remaining leg is the gate. Make sure your new J111 has its gate pin in the hole receiving the LFO signal.

The J111 datasheet says the far right leg is the gate. 2N4091 seems to be the same, but always check.

52Bill and Enzo - I'm pretty sure I have the gate correctly plugged in the socket, but I will double check tomorrow and report back. I will use the resistance measurement suggestion to be certain. Thanks!

[telecaster]

52Bill and Enzo - I double and triple checked and I have the FET plugged correctly in the socket. I even lifted the board to view the circuit traces and verify that the socket pin locations were correct. I did the resistance check on the original FET, and in addition to having the same pinout as the J111, it seems to check ok. 30 ohms across the S-D, and no continuity from gate to either S or D. So maybe my problem is further downstream going to the op amp. Since all the resistors and caps check ok, maybe that input of the op amp is bad? Although I was able to get that input to respond by pulling the FET and rolling the intensity pot up and down.

[52 Bill]

So maybe my problem is further downstream going to the op amp. Since all the resistors and caps check ok, maybe that input of the op amp is bad? Although I was able to get that input to respond by pulling the FET and rolling the intensity pot up and down.

Is there continuity from the fet socket to the rest of the circuit?

[telecaster]

Yes, there is continuity through the socket

Is there continuity from the fet socket to the rest of the circuit?

I ohm'ed from each leg of the FET plugged in the socket to the various connections on the board, all checked ok.

[52 Bill]

Okay, let's do a quick D&D circuit analysis here. The tremolo effect is based on changing the gain of IC7 by varying the resistance to ground of the non-inverting input.

This is controlled by 1-the footswitch, 2-the value of the intensity control and 3-the resistance value of the fet. When C25 is directly grounded, the gain is at the maximum, so no tremolo. So if the footswitch is grounding the connection, the gain will be at max. The same will be true if the intensity control is set to zero ohms.

You said that without the fet in circuit, you could manually change the volume by rotating the intensity control. This would prove that the footswitch was not grounded and that the intensity pot was okay as well.

That leaves the fet and the oscillator circuits. The LFO runs all of the time and the fet changes resistance all of the time as well. The changing resistance of the fet will not change the gain of the stage as long as the drain and source are grounded by either the footswitch or the intensity pot. The fet resistance will only vary the gain when the intensity pot is no longer set to zero ohms.

So either the LFO is not running, or the fet is not working. Can you read a varying voltage coming from the LFO? Do you have another fet to try in circuit?

[telecaster]

52Bill - I tried another J111 FET, it does not work either.
The LFO is oscillating, I measure a swing of approximately +14V to -14V. FWIW, I am using a multimeter measuring DC with the MM's speed pot set to its slowest. At the FET gate, the swing is only about 1/2 volt (-3.3 to -2.8). There is a 5.6M resistor in between (and a 0.33 cap). Maybe the FET is not being driven hard enough? Not sure if it's valid, but I floated the FET drain, leaving the gate and source connected in the socket, and then stuck my meter across source-drain and measured 36-39 ohms as the gate varied as described above. FYI, The J111 measures 32 ohms across source-drain when it's out of the socket (circuit).
I will look into the rest of the components between the oscillator and the FET. I'm curious if you agree there should be more drive at the FET gate.
Thanks

[52 Bill]

...across source-drain and measured 36-39 ohms as the gate varied as described above. FYI, The J111 measures 32 ohms across source-drain when it's out of the socket (circuit).

So the fet is never going high resistance. How have you set the bias trimmer?

At the FET gate, the swing is only about 1/2 volt (-3.3 to -2.8).

I don't know if the variation should be greater or not. The bias trimmer is supposed to make the fet go high resistance and then the oscillator output is supposed to make it change resistance. If you don't get the fet to go high resistance the trem effect will never work.

Check the 0.33 cap and the 5M6 resistor, but with that high a value resistor, I wouldn't expect to see a huge amount of oscillator signal getting to the gate.

[telecaster]

Problem found and solved!
Actually two problems. First, the resistor off the bias trimmer, R42, needed to be changed to work with the J111. The schematic calls for 220K, but this amp had a 330K there originally. I experimented and ended up using a 150K. Second, I did have to alter R49, the 5.6M coming off the oscillator, to get the signal swing up to a little more than 1V at the FET gate. I ended up with 2.5M for this one. Thanks to 52Bill for your suggestions - it helped me focus on the right parts of the circuit.