No input front or rear MP-1

[gtmm]

BTW, I should mention that the board stuffing diagram and schematic were gotten off this site, which makes this site a fantastic place. :thumb-up:

All i did was to outline U4 in red. Thanks to all that help keep these great preamps alive! :thumb-up: :thumb-up:

[Mxzx]

Awesome, thanks for the help all!!

I'll get on it and see what I find.

Thanks!!

[gtmm]

Mxzx,

Hah, hah.  ;) I'm such an idiot!

If you look at the schematic, there's a lot of front end before U4 that I missed, due to the fuzziness of many copies. I missed the double line of the input jack which sends the signal to the 1Meg ohm input impedance, then to a FET, then to a bipolar transistor -- then to U4.

Sorry, but at least you know where U4 is now.  :thumb-up:

[Mxzx]

No problem, thanks again for the help!

I'll dig into it tomorrow afternoon and report back.

Thanks!!

[Mxzx]

Checked U4 and all the voltages check.

This opamp was replaced by the previous owner as it now has a mounting socket.  I don't have that opamp but I do have a TI one, it has 24AJTRW then under it has NE5532P, will that work to replace it with?

I did notice some things under the board. The 2 middle pins on the left side of U4, where it mounts in the first pic, have continuity, is that correct.  Looks like a drop of solder in between to me?

Also C31, next to the opamp socket, appears to have a resistor soldered to it in second pic, is that right?

Thanks

[MarshallJMP]

First the U4 opamp is actually a dual opamp,so 2 opamps in one housing.The one with the red rectangular is not the one you need,that is used to feed the level led "input".You need the other one,which is a bit more to right (with pins 1,2 and3).

The NE5532 will work,just make sure you insert it the right way!!

Yes these are pins 6 and 7,they are supposed to have continuity (in the schematic you see pin 6 and 7 connected to each other).

The resistor is correct,this is R73 (100k).

[Mxzx]

Ok, which way is the right way for opamp?

So is U4 the socket there or not?  I'm confused. The voltages you said to check on the pins are correct, so I though that was U4?

[MarshallJMP]

I'm at work but email me and i'll send you a pic how to insert it.

The socket is U4, that is correct, but i was refering to the schematics gtmm provided.

[Mxzx]

Changed opamp and still no improvement.

On a side note, when the headphones are plugged in and turned all the way up, I can hear a faint guitar signal, its not much but it is there.

[Mxzx]

Not to bring up a dead thread, but I now have an oscilloscope if anyone has any more ideas on this problem.

Thanks!

[rnolan]

Hey Mxzx, hopefully someone can help, I'm sure MJMP or others will again chime in, unfortunately under the hood isn't my area, but the thead is at least "alive" again  :thumb-up: . and sorry we fell off the perch as it were...

[Mxzx]

Thanks, it was my fault as I didn't have an oscilloscope.  MJMP has been helping me along!

Thanks!

[rnolan]

No worries, MJMP will no doubt (as always, go MJMP  :thumb-up: )  help you out. Hope you get it sorted soon !!

[Mxzx]

I have both an oscilloscope and a signal generator now.

Thanks

[MarshallJMP]

Ok insert a signal into the mp-1 of 2kHz 700mVpp and start measuring from the input,then see if you have a signal on U4 pin1.
Then see if you have a signal on the input and output of the tubeboard.

Let me know.