Presonus MP-20 mic pre

[kdawg]

Hello - I have an older MP-20 mic pre that stopped working, I couldn't get any help from Presonus other than a schematic. It's pretty much a Jensen dual servo design w/Jensen input transformer The Q1/Q2 (which are actually U441 dual JFETs which I can't find anymore) appear to be bad, I did replace with two 2N4392 but did not match them and it worked for a while and then stopped. So my questions are:

1. What voltages should be set at TP1 and TP2? I'm seeing like 10VDC on TP2 which I'm pretty sure is bad. The RV2 trim pot sets some kind of bias right?
2. Do I have to have matched JFETs? What's the best replacement for dual JFET, some other dual JFET or two 2N4392 or ?

While I had one working channel, I compared a lot of voltages and couldn't find anything different except TP1 / TP2 - which were around 0V on the working channel

Thanks!

[Enzo]

How did you determine the two JFETs Q1,2 were bad?

As I look at it, RV2 trims the voltage at TP1 to zero DC. Notice Q1, Q2 circuits are essentially the same, strung between +18 and -18vDC. Inevitably the parts will vary a little, so the trimmer is there to adjust one side so it balances the other. Adjust RV2 so you have zero DC on TP1.

[MarkusBass]

Q2 (together with R21 and RV2) form a current source and RV2 sets its current. I think that you should have voltage close to 0V on TP1. But you did not provide this voltage. You may have U1 failed. To check it measure the voltage on the output of U1 (pin #6).

Mark

[kdawg]

Thanks Enzo & Mark - I had to step away from this one for a bit, I did find that Q1 was reading same resistance both directions from source to gate and drain to gate, so that's why I replaced it with two 2N4392's.

Channel 2 is working fine. Both channels are passing audio to the outputs on the scope, but Channel 1 has a DC offset on the main out from -0.2 to -5 V depending on gain knob. With the new transistors, I can't get TP1 to anything lower that -0.21v - could this be just because of JFET variability? Would I have to adjust the 121 ohm resistor to get more range, or think something bigger wrong? I tried different U1/U2, no changes. Here's some voltages w/no input:

Channel 1
TP1 = -0.21 VDC
TP2 = 11.71 VDC (why is this positive, when CH2 is negative?)

Q1
G = 0.012 VDC
D = 18.3 VDC
S = 1.369 VDC

Q2
G = -17.1 VDC
D = -0.268 VDC
S = -15 VDC

U1
6 = -0.24 VDC

Channel 2 (working)
TP1 = 0 VDC
TP2 = -6.63 VDC

Q1
G = 0.012 VDC
D = 18.3 VDC
S = 1.11 VDC

U1
6 = ~0 VDC

[g1]

CH2 Q2 voltages?

[kdawg]

Sorry Q2 got cut off. I am able to set TP1 = 0 with trimpot on channel 2. I've changed U1/U2 so they should be fine.

Channel 1 U1 Pin 6 = -0.25 to -2.6 depending on gain knob
Channel 2 U1 Pin 6 = 0 on any gain knob setting

Channel 2
Q2
G = -18
D = 0
S = -17.16

[g1]

I think your 2 fets in ch.1 are just too far unmatched.
If it's not too much hassle, swap Q1 and Q2 around in Ch.1. Probably the offset will flip polarity at the test points.

[g1]

And if you'd rather try to do some matching, here is a simple circuit courtesy of JM Fahey:
http://music-electronics-forum.com/t24947/#post375805

[kdawg]

I had no luck getting any of the JFETs I had close to zero, guess I'm stuck buying some more 2N4392 unless anyone can suggest a better equivalent (hate to buy 10+ just to get 2 matched)? I guess the U441 they used originally eliminated the need for matching, but I can't find any.