Line 6 XPS-AB Schematic

[Billguitarvin]

Thanks so much gaztech! I’ll check the voltages on the caps you talked about once I get the resistors soldered in. Hopefully by Sunday.

[Billguitarvin]

Here are the voltages now that I have replaced the resistors.


C 12 +4.99 volts with black probe on negative side of cap, red probe on positive side of cap (I triple checked this - not sure what I did wrong or why it is a positive voltage).


C 11 +4.99 volts with black probe on negative side of cap, red probe on positive side of cap


C 2 0 volts (On my board this was a surface mount)


C9 +13.09 volts with black probe on negative side of cap, red probe on positive side of cap


I have an ac adaptor for my Line 6 pod xt that is the same adaptor that is used for the XPS-AB, so I tried using the pod adaptor and nothing changed.


Red power light LED is now intermittent. Thanks for taking the time to look at this!

[gaztech]

Hi Bill,

Hmm... something isn't right here... either your measurement technique is off or we have a serious issue. Let's take these readings one by one:

C 11 +4.99 volts with black probe on negative side of cap, red probe on positive side of cap

This one looks ok.

C 12 +4.99 volts with black probe on negative side of cap, red probe on positive side of cap (I triple checked this - not sure what I did wrong or why it is a positive voltage).

That one is ok too. If you look at the drawing, the "+ve" side of the capacitor is what is actually connected to the 0V line so the way you are measuring it will yield a positive voltage. What you should be doing with that one is putting your black probe on the "positive" lead of the cap to take the measurement - which will then show you -5V on the negative lead.

C9 +13.09 volts with black probe on negative side of cap, red probe on positive side of cap

This voltage looks ok.

C2 0 volts (On my board this was a surface mount)

Now, here's a problem. You should get a reading of about +7V on this point. I need to know if you are taking these measurements with the guitar connected or not. This makes a difference. Also, does the guitar work if you use it on batteries? If it doesn't then the issue could be with the guitar and not this unit!

The odd thing here is that the unit was working then it mysteriously stopped. You didn't really do anything to it when it was on. You'll need to take some readings around the FET (Q9) as this is effectively the turn on/off for the 7V supply. Since you're not an electronics guy, it's very tricky knowing what to tell you to do next. I could say "test Q9 and Q7" but I'm not sure if you'd know how to do that!

Q6 could also be a contender here as if this is short circuit, it will take the 7V line down. My money is on the TL082 (U2) or Q9 (MTP2955V). These parts supply the switch line for transistor Q7 - which looks like it turns the unit on. If the unit had been left on for a long time then the majority of the current would have been travelling through Q9 - so it may have given up. Q7 could be the cause too but I doubt it as there isn't much current actually flowing through it when the unit is on.

Bill, you need to have a look at the above but it might be time to give it to someone that knows electronics so that they can take a look and take some meaningful measurements. Using a shotgun approach and just replacing parts might yield a result but it's very easy to damage other stuff in the process.

You replaced the LM339. You put it in the correct way round - right?

There is also some switching going on around the guitar jack. It looks like there should be a plug in the socket to turn the unit on. You could try taking the measurements again with JUST an unconnected stereo jack cable attached- (nothing connected on the other end. (That's a cable with a three pole jack on both ends plugged into the guitar input socket). This may give you different results. The power LED coming on/off is intriguing me...

The tests you would have to perform to narrow this down further now get to component level and some knowledge is required to work out what each component does and then figuring out why they are not doing it!

Have a prod around with your meter but be careful. Any further info might be useful but we might have gone as far as we can without starting to remove components and checking them.

Let me know how you get on.

Cheers.

[Billguitarvin]

Thanks so much for the quick reply! Thanks too for educating me what “-ve” and “+ve” means. For some reason I thought electrolytics always had the negative going to ground, so right there is a valuable lesson for me. I can see on the schematic how the “+” is indeed going to ground or 0 volts.

All the readings I took with the guitar not plugged in and no TRS cord plugged into the guitar input on the box. Fortunately the guitar does work with batteries. I will try taking some readings with a stereo guitar cord plugged into just the box and not the guitar.

As far as testing Q9, l looked up the model number and then was able to find some info about testing P Type Mosfets by taking it out of the circuit and using a multimeter. Is this the kind of testing you were referring to, or were you thinking about testing components around Q9 or some other way of testing? If you are ok with telling me what to test, I am totally into researching what I have to do to test a part, or if you have suggestions on studying a type of circuit used somewhere in the XPS-AB box, I am fine with that too. Since I have some time on my hands, I’d love to learn more about repairing this box and understand more about what the various circuits are doing. I totally understand though if you aren’t into guiding a noob through repairing this unit.

I replaced U2 as it looked like it had some heat damage. When I replaced U2 and U6 I replaced them so the etched line on the replacements were on the same side as the notch in the silk screen on the pcb. This was the way they were placed before I replaced them.

I just took out Q9 the P Mosfet and you were right. If I did the test properly, with the mosfet in the on position, the multimeter showed “OL” between drain and source. Also only 56 ohms of resistance between gate and source.

Please feel free if you have any other suggestions and I will be happy to carry them out. All the best and take care!

[gaztech]

Bill,

Did you check Q6? You could try removing it to see if the 7V comes back...

You should also have around 2.5V across two of the pins of Q8. This voltage is used as a reference elsewhere. If its not present that coukd be a problem.

[Billguitarvin]

Thanks gaztech!!!! Got my Q9 replacement yesterday so hope to put that in the next couple of days. I’ll check the voltage on Q8 and remove Q6 to see if I get the 7V back. I really appreciate the help!

[Billguitarvin]

Greeting gaztech! I installed the new Q9 then took some measurements. I did these all with the negative probe on ground. Here is what I got

Q1 - 4, -4.9V
Q1 - 8, 1.6V (Should be 5V)

Q8 2.49V

R16 9.5 (Should be 7V)

I made some other measurements around the board where the schematic showed some voltages:
U1 - 4, -4.9V
U1 - 8, -.6V (Should be 5V)

D9, -9.9V

L2 10.8VAC (Should be 9VAC)

D6 -4.9V

After the above measurements I took a stereo guitar chord, inserted into TRS IN and got these measurements with the negative probe on the sleeve and the positive probe on either the ring or tip. Did I do this backwards, as I got a negative voltage reading? I thought on a TRS cord the ground or common is usually the sleeve so that’s why I put the negative lead on the sleeve.
Sleeve to Ring -.02V
Sleeve to Tip -8.3V
Ring to Tip - 8.3V

So the good news is after replacing Q9, I am now showing a voltage on the TRS cord, but unfortunately did not power up my guitar. I also noticed it stating on the schematic “min 7.02V max 7.06V” by where the +7V arrow is between C2 and R16. Is this staying what the voltage of the +7V should be, or is referring to something else? Thanks again for all your help!



I will try taking out Q6 tomorrow and see what that does. Thanks again for all your help!

[gaztech]

Hi Bill,

I'm glad to see you're sticking with it! Ok, so we now have some volts. What you should NOT have done at this point was to plug in your guitar! If the supplies are not 100% correct, don't do this. You can damage the guitar. Hopefully you haven't already...

I think there are some measurement errors in there - probably due to probes being the wrong way round when you took them...

The thing that bothers me now is your statement : U1 - Pin 8, 0.6V. This is wrong. It really should be +5V. If it isn't then everything will be wrong. You need to check the voltage regulator U4 (or the anode of diode D7). Pin 1 of the regulator should be +5V. This supply feeds several of the chips in this unit and it is required.

If it's not 5V then you "could" have a bad regulator (LM78L05). If not the regulator then U1 or U3 could be bad and are dragging the supply down. It's likely that there will be some decoupling capacitors on these lines too which could also be at fault (these may not be shown on the schematic).

However, you replaced U3 already I think. I would start there. You might have shorted a couple of pins together when you soldered it in. Check the soldereing with a magnifying glass.

If Q9 really was the major culprit of thie unit going off (looks like it might have been) then I doubt this would have affected the other parts so we need to assume that somethng else happened here. Replacing the only part that is tied to the 5V rail seems likely... Check this lot and let us know what you find. Good luck.

[Billguitarvin]

Thanks gaztech! I am glad you have stuck it out with me. I don’t totally trust my readings as I have to fool around with the probes to get any readings. I don’t know if some thinner probes would help. I’m using the stock probes that came with the meter which are a bit big. I’ll see if some thinner probes help and remeasure as well as check out the parts you suggested. Many thanks and I’ll let you know what I find out.

[Billguitarvin]

Thanks for the warning about waiting to plug in my guitar. I was lucky nothing got messed up. So I took some voltage readings from U4, U1, U3 and anything else in the schematic that showed 5 volts coming out of it. I got some thinner probes which seem to be helping getting the readings.


U4 pin 1 1.6V (should be 5V)


U1 pin 8 1.6V (should be 5V)


U3 Pin 3 3V (should be 5V)


R26 3V (should be 5V)


R27 -10.3V (should be 5V)


C11 1.6V (should be 5V)

I checked most of the caps on the top of the board and here is what I found. I didn’t see any caps that weren’t on the schematic. Not sure if I need to take these out of the circuit to get an accurate reading, or if I was doing something wrong, but here goes.

C1 4.4 microfareds (should be 4.7) 1M ohms


C2 OL (should be 2.2), .3ohms


C3 19 nanofareds (should be 10 nanofareds), 43 M ohms


C4 2.3 microfareds (should be .1microfared) 1M ohms


C5 50 nanofareds (should be 10 nanofareds) 3.6 M ohms


C15 2 nanofareds (should be 10 microfareds) 1 M ohms

I replaced U6 and U2, but haven’t replaced U3. I’ve looked closely with a magnifying glass and don’t see any solder bridges on U2, U3 or U6. Thanks again for all the help!

[gaztech]

Hi Bill,

I think that you're going to have to start removing components to find this...

There should be +5v on the regulator. Therefore, I would start by removing U1 and U3. Do that to see if either of these chips are dragging the supply down. Take them out, then measure the output of the regulator. If it comes back to +5v then it's likely a chip is at fault.

Be careful not to damage any tracks when removing these parts! It's very easy to mess things up with too much heat...

BTW, you can't measure caps accurately with them in circuit - you have to remove them. Resistance measurements for caps are meaningless.

[Billguitarvin]

Hi gaztech and thanks for the quick reply! I’ll try removing U1 and U3 and see what happens. Thanks too for the info about capacitors. One question I have is if a capacitor shows very little resistance (like .3 ohms on C2) could that be a possible sign that the capacitor may have a short? Or if the ohms read OL In circuit it could mean there possibly is an open circuit in the capacitor? If not in circuit, would this apply to out of circuit, or am I not correct?

[DrGonz78]

Hi gaztech and thanks for the quick reply! I’ll try removing U1 and U3 and see what happens. Thanks too for the info about capacitors. One question I have is if a capacitor shows very little resistance (like .3 ohms on C2) could that be a possible sign that the capacitor may have a short? Or if the ohms read OL In circuit it could mean there possibly is an open circuit in the capacitor? If not in circuit, would this apply to out of circuit, or am I not correct?

The readings that you got on C2 caught my eye too. However, I am more confused by the readings than anything. So one way I read it is that C2 measures 0.3ohms and the other way I read it is that it measures OL. There seems to be a contradiction in that it can only be one or the other if it is in circuit. So to clarify, are we talking about two different measurements? One in circuit and the other out of circuit.

Edit: Also, I can read this as that when checking with capacitance setting it reads OL and it reads 0.3ohms with resistance setting. Perhaps the problem reading it in circuit is due to the Q6 Jfet in parallel to C2.

..."C1 4.4 microfareds (should be 4.7) 1M ohms


C2 OL (should be 2.2), .3ohms


C3 19 nanofareds (should be 10 nanofareds), 43 M ohms" ,,,

[Billguitarvin]

The readings that you got on C2 caught my eye too. However, I am more confused by the readings than anything. So one way I read it is that C2 measures 0.3ohms and the other way I read it is that it measures OL. There seems to be a contradiction in that it can only be one or the other if it is in circuit. So to clarify, are we talking about two different measurements? One in circuit and the other out of circuit.

Edit: Also, I can read this as that when checking with capacitance setting it reads OL and it reads 0.3ohms with resistance setting. Perhaps the problem reading it in circuit is due to the Q6 Jfet in parallel to C2.

..."C1 4.4 microfareds (should be 4.7) 1M ohms


C2 OL (should be 2.2), .3ohms


C3 19 nanofareds (should be 10 nanofareds), 43 M ohms" ,,,

Thanks again for the quick reply! Yes, both of these readings were taken in the circuit, and yes, per your edit, with the capacitance mode on the multimeter I got OL, while with the resistance mode on the multimeter I got .3 ohms. Sorry for the confusion on that. One other weird thing is I can not find Q6 anywhere on the top or bottom of the circuit board. It’s probably staring me right in the face, but I’ve gone over both sides of the circuit board with a magnifying glass as well as enlarging pictures I took before I started mucking around with the electronics but can’t find it. I see it on the schematic, but not on the circuit board.

If I may, one other question is why are some parts on the board in what I would call a “grayed out” font? For example C1 and C2. Another thing is “Do Not Stuff” written for C1. I googled it and it said it was for something to use for testing at the factory but was to be taken off the board after completing final inspection. If that is true, C1 is still on my circuit board. Not that that has anything to do with getting this unit working, but just curious about that. Thanks again!

[gaztech]

Hi Bill.
I’m baffled by the readings too. You can of course show a shorted capacitor using a resistance test. A reading of 0.3 ohms usually shows that it is short circuit but it does depend on the components in parallel with it. Can you post some photos of the board so that we can see what you have. It may help.