ground loop issues

[MarshallJMP]

Do you have hum on all voicings?

[ang3lus]

I do, and i connected the headphone and gave a listening, the hum is there, just more silent because of the impedance mismatch of my headphones, i've uploaded the video to youtube, you can hear the mechnical hum, this is with nothing plugged in, no amp or guitar


https://www.youtube.com/watch?v=pUvc7yhCAls

end of the video i put the phone onto the chassis and you can hear the hum thru the chassis, thats the hum im getting thru anything

[ang3lus]

could it be a bad filter capcitor ? i dont see any external damage, but might be worth changing them out before changing out the transformer

[Rusty]

could it be a bad filter capcitor ? i dont see any external damage, but might be worth changing them out before changing out the transformer

Yes, it sure is pointing in that direction, A PSU filter cap or caps are going bad.

A change out, thats what I would do, it's looking that way by the sounds of things.

Also to confirm, and assuming you don't have a scope at hand, and most importantly of all, if you have experience at working on live circuits.

You could easily make up a home made DIY probe if you have some parts laying around to test out the HT and other power supply lines. This is to check audibly, if the hum is anywhere in those supply lines.

You will need an axial high voltage coupling capacitor, rated no less than about 300V and somewhere in the range between 0.022uF to about 0.1uF. This is to block the DC voltage but let any AC through which will be the noise if any of your PSU caps are on their way out the door.

A guitar lead that you can cut up and an amp to listen to.

1, Cut one end off the guitar lead. Strip it back about eight inches or so.

2, Solder one end of the capacitor to the guitar + signal wire and wrap some electrical tape around it and make it safe.

3, Straighten the other end of the capacitor lead and if need be, extend it with short piece of solid wire or a straightened out paper clip or something like that and solder it on. Then tape that up too and make it safe. Leave a little piece at the tip bare.
This is your probe tip !

3, Twist the shielding braid into a wire and wrap some electrical tape around it, but leave a little piece at the end so that you can securely connect it to ground/earth/chassis.

4, Plug the jack into your amp and keep the volume WAY down.

5, Turn everything on, go probe the + side of those electrolytes and the other + VDC supply lines and listen to the amp for 100Hz hum.
100Hz sounds like G# -ish on the sixth sting of the guitar.

Pull the tubes out one at a time. On the tube circuit board valve sockets, test pin 1 and 6, those are your annode pins, Remember, read the pin number counter clockwise when looking at them from the front of the socket.

If you have noise on those pins then I would certianly say to change out those filter caps on that board.

6, Take your time and be careful that you don't make a slip, or else blue lightning and grey smoke will appear in milli seconds if your not careful.

                          Warning
DON'T try this method if you don't have any experience at working on live circuits !


OK, that's a cheap, easy and effective way,

You should do the noise mod anyway, and change all of those electrolytes in there. They are probably about thirty years old and have drifted well out and have a high ESR.

MJMP can supply you with the full kit.

Rusty.

[ang3lus]

hey Rusty,

thanks for all the support, you  mean an audio probe, i have built some tube amps before, never worked on anything digital...

changed out the power supply caps, hum is GONE, changed out the tube boards capacitors while i was at it.

BUT

when i hook up one of the outputs to a return on my amp i still get lots of hum, no hum thru the ada SEND into my amps return
also, when i hook up the output to an amp input (small transistor amp) i get no output, but i do get output thru the SEND of ada into the amps input

any ideas?

only thing i can think of is i put  4700uf caps instead of the 2200uf and 3300uf, maybe i killed one of the regulators and there's no sound on the outputs ?
:(

edit: checked all voltages, getting -15V and 15V where needed, also +5

what i noticed just now is that the output level on the front panel is not doing anything, i checked the pot (i know it is dual ganged) and it reads 50k pins 1 from 3 and also when testing from 2 to 3 i can see the resistance changing, but it does nothing when i connect send > power amp or send > amp input

i'm going crazy with this unit, but intent on getting it fixed

[Rusty]

Hello ang3lus,

4700uf caps instead of the 2200uf and 3300uf, should be OK for now for testing purposes. It's just, the larger you go, will cause more inrush current stress on the diodes and regulators at switch on.
There is a design limit, but if you don't have the correct values in your parts box, you should get away with it for now.

Yes, I was describing earlier how to build a home made DIY audio probe, it's a little bit rough and ready I know, but they can give a good enough insight.

checked all voltages, getting -15V and 15V where needed, also +5

Thats fine. All good there then and hum is gone, excellent.


but it does nothing when i connect send > power amp or send > amp input

Hmmm 

I'm gonna have to study the schem for a while.

I'm tired with this corvid lock down thing tonight and getting cabin fever.  >:(

i'm going crazy with this unit, but intent on getting it fixed

I know exactly what you mean, it's the nature of the repair game,  :)

Hang in there, we will get some good results soon enough, be patient, talk soon.

Take care.

[ang3lus]

thanks, you also take care, hectic situation with this COVID

[rnolan]

Hey ang3lus, so just to confirm: you get signal (hum free) from the MP1 fx send (into either amp return or transistor amp input) BUT you get no signal and just humm from the MP1 output (both A and B outs ??).
The MP1 fx send is just after the gain (& eq) stage and controlled by the Fx send pot.
So next in line is the chorus (where it splits into L/R) and then patch master vol > front vol out A/B
If you do have signal at the Fx send (yes???), then it seems the issue could be in the chorus or after it.  A possibility to check is the MP1 Fx return jack, make sure it's clean as it breaks the signal chain when the loop is ON in a patch and there is a jack inserted in it.  So make sure the loop is Off in the patch you are using (remember loop On/Off does not affect the Fx send, it always has signal)

[ang3lus]

well i connected the headphone out and am getting good output and also the chorus works well, so the chorus is good i think.

i also plugged the RECIEVE jack into an amp, very low output but am still getting sound and the patches are working well.

so what's left ? volume pot and NE5532 opamp ? any way to test in circuit ?

[Rusty]

Good,

Plug in two spare unwired jack plugs into the output jack sockets, chop an old guitar lead up or something like that, you need plugs in there to do this.

1, Work your way backwards upstream from the outputs.

Check that wire loom, it's solder connections and that header connector KB going from the outputs back onto the main circuit board right beside U10,  NE5532 opamp.

2, Plug in some signal to the input, use you mobile cell phone headphone out, computer headphone out or any thing to provide some signal. (I'm assuming you haven't got a function generator.)

Keep the input signal down very low, adjust the volume's, and just pre-clip the LED's, using a clean channel setting on the MP-1.

3, Refer to the link below.

Test the voltages, V+ & V- DC first on the NE5532 opamp.
If you have the top Inst/Line switch set it to Line out. Then test for the AC signal voltages,
You should be seeing readings in the high ACmV range if you have a decent enough meter.

http://www.ti.com/lit/ds/symlink/ne5532.pdf

If all seems ok there, then we are moving on up the stream towards R98, R99, C48, C48, then heading for the top switch to test.

After there you should have identified where the problem ends going upstream. You say the headphone is working OK. The problem should be somewhere from U11 pins 1 and 7 to the output jacks. Checkout the diagram below.

In my opinion from here, it does look likely that NE5532 opamp U10 might be the perpetrator. That high ripple voltage from those poor capacitors wouldn't have been friendly.

If the problem happens to be U10, and you want to get it out the fast way, then you might be able to snip each pin leg with a decent mini set of side cutters. Then de-solder each pin out one at a time.
If not, you can get brutal and mash the silicone up with a pair of pliers, without damaging any of the surrounding components or the circuit board, and again, take the pins out one at a time.

Make sure before you re-install the new op-amp that you install an IC socket for it first. 

Make life easy,   ;D

Give the test procedure above a go first and let us know how you get on, OK.

Rusty.


   

[MarshallJMP]

Well I agree with Rusty on the U10 opamp, or it could also be a bad topswitch, try wiggle it and see if sound comes through. Another thing but very unlikely is the red connector near the front pot is not plugged in like it should.

[ang3lus]

well i checked quite a few things and also socketed and replaced NE5532 (with a TL072 cause i don't have the NE on hand) and still no sound :(

am getting voltage to it though, also replaced the electros near it, mad no different, i'm at a complete loss.

the only suspects are the pot and top switch, but i did test them with a multimeter and they seem to work fine so that's really weird

[Rusty]

A TL072 will be ok. 

Could be a dry joint.

Have you tested for continuity and re-soldered all of the pin connections for that wire loom, it's solder connections and that header connector KB going from the output back board onto the main circuit board right beside U10, NE5532 opamp?
Re- solder them, and meter them. Also re-solder the inst/line switch, and spray clean it,
Check all the tracks too, meter them from point to point per component as per schem.


Did you test for signal voltages as I have described in detail above?  from U11 right through to the output jacks, (with two jack plugs plugged in to the A/B outputs)

The problem has got to be right around there. Has to be.

Measure it out with an input signal on the clean channel and set the switch to line out, if your meter cannot go down to ACmV range then build the audio probe that I posted.

( (edit) You definitely have new the op-amp in the correct way around, sorry, just asking, )

Keep us posted.

[MarshallJMP]

Did you ohm everything out? Meter set to ohms and than start from U11 pin1 to the switch then to C56 etc...

To test the pot turn it all the up and measure from C56 to C49 and from C57 to C48.

[ang3lus]

THANK YOU GUYS !!! i've found the problem, but no solution ???

Rusty, thank you so much, that tip with the AC voltage was gold, i traced the signal right up to the output jacks and then nothing

so i suspected the jack, but then i said, it can't be that two jacks are dead.

i plugged in a cord and OHMed tip to GND and i got 120k, so i figured, ok bad trace somewhere, then i saw they all go to the 2 connection.

the wiring harness i have has the red wire cut, and i searched on the forum and saw this is a factory upgrade, probably to prevent loops ?

so i jumpered it to GND and BAAAAM, sound, clear and beautiful and POT works beautifully.

but if i don't need to have gnd refrence from the cut wire, where should i be getting the GND refrence ? is there another component bad somewhere ? or a trace that lost its gnd ?