NAD! Mesa MPulse 600

[tavi]

Well. New to me. I've been wanting one for years. But could never justify the purchase. Found one locally on marketplace for a steal. It has some road wear. But sounds exactly the way I hoped it would.

The only real issue I've found with it. It seems to be stuck on the Solo channel, (or Solo mode, or Solo volume, if you prefer). It didn't come with the foot switch. The manual states the Solo will be bypassed if the foot switch is disconnected. According to the schematics. The foot switch is just a switch LED and 100 ohm resister. Using a 100 ohm resister to jump the other three options on the foot switch and they each engaged. But didn't switch to master volume when I jumped that one. Hopefully someone has some insight.

Oh. And if it makes a difference it's an early model.

Thanks guys!

 

[bassboysam]

@agedhorse should be able to shed some light.

 

[tavi]

@agedhorse should be able to shed some light.

I was hoping he would chime in.

 

[agedhorse]

I'm not that familiar with that switching circuit, but I thought the master is normally engaged and the footswitch changes from the default to solo?

There's a lot going on within the switching, does the solo LED on the amp turn on when engaged?

Troubleshooting this requires understanding FET switching, it uses both P and N channel FETs and opposite polarity switching.

 

[tavi]

I'm not that familiar with that switching circuit, but I thought the master is normally engaged and the footswitch changes from the default to solo?

There's a lot going on within the switching, does the solo LED on the amp turn on when engaged?

Troubleshooting this requires understanding FET switching, it uses both P and N channel FETs and opposite polarity switching.

That is my understanding. Master is default. But on mine, Solo LED is always illuminated and Solo knob is controlling volume.

 

[Wasnex]

My read of the manual is the Master volume is normally on. With a footswitch you can toggle to the Solo volume. This is consistent with your expectations, but not how the amp is working.

Here's my attempt at circuit analysis:

If I am reading this correctly, the footswitch is on the left and circuitry on the right is inside the amp. The two circuits are connected with a 5 Pin DIN cable.

To toggle the Master to Solo you close SW-1 (in left image). This sends a ground through the 5-pin DIN connector to R85 (bottom of right image). The LED in IC11 lights and toggles the transistor on. IC11 is called an optocoupler; you can see a datasheet here: Link Removed. The output of IC 11 toggles between 15V and -15V.

The signal is applied to the FETs in the Master circuit (see comment underlined in red below). The FETs act as switches, so only one of the FETs passes signal a time...and it depends on whether the circuit sees 15V or -15V from IC11

I don't remember FET theory, but I believe if the footswitch is not attached, the circuit sees 15V and Q5 passes signal. 15V is supplied at the top of R73 and R72 on the upper left corner, so it does not matter if the footswitch control circuit also supplies 15V. However when the footswitch is toggled to provide -15V, LED 3 lights, Q5 toggles off, and Q4 toggles on.

Notice the notes on the FETS. Note for Q5 says ".4V on, -13V off." Note for Q4 says "13V off, .6V on."

If I am reading the circuit correctly, the Master and Solo volumes actually work sort of backwards. Rather than passing the signal to the next stage, they provide variable negative feedback from the output of IC7A to its inverting input. Increasing negative feedback decrease gain.

Found this blurry image and labelled where I think the optocoupler ICs are:

The image was taken from: Mesa M-pulse 600 - Music Electronics Forum

I don't think it would be too difficult to troubleshoot and repair this. But if you plan to keep the amp and not use a footswitch, I am not sure it's worth the bother unless you are compelled by OCD. Both the Master and Solo controls are 100K log, so it really does not matter which is active.

Good Luck!

 

[tavi]

@Wasnex that appears to be very helpful. You might be right about the ultimate end usefulness. If I ever get a foot switch, it will probably be a few years. And even with one, I don't know that I would be switching all that often. So the desire for the amp to function as designed is more of a personal preference. So if I, personally, can't get it working. its not worth it to me to have it fixed.

Unfortunately. I don't have time to open it up today. But, to get an idea where to start. Looking at the diagrams you provided. My understanding of your explanation and Master volume being defeated, (only one volume working). Is it safe to assume the FETs are receiving the +15V to properly function. And could it be as simple as IC11 is either receiving a 5V signal or the 4N33 itself is bad/stuck closed.

 

[agedhorse]

The FETs receive either +15 or -15 depending on the FET.

Before replacing parts, it’s important to know what isn’t actually working correctly.

Or if not using the foot switch, you can simply use the solo control in place of the master control.

 

[tavi]

@agedhorse I definitely appreciate the words of caution. The last thing I want to do is make my working equipment not work. Because I wanted a feature, that I my never use, to work. I don’t plan on bringing out the soldering iron anytime soon.

If at all possible. I would like to diagnose the problem. Even if for my own peace of mind. If the process is simple enough.

@agedhorse and @Wasnex if I could private message either of you to get a one on one about possible steps to take.

 

[agedhorse]

Everything you need is already on that schematic.

 

[Wasnex]

@agedhorse I definitely appreciate the words of caution. The last thing I want to do is make my working equipment not work. Because I wanted a feature, that I my never use, to work. I don’t plan on bringing out the soldering iron anytime soon.

If at all possible. I would like to diagnose the problem. Even if for my own peace of mind. If the process is simple enough.

@agedhorse and @Wasnex if I could private message either of you to get a one on one about possible steps to take.

Most of the time, we crowd source trouble shooting publicly. I will say up front that I am not an active tech, but decades ago I was a radio repair person in the military for about 5 years. I don't normally get involved in solid state trouble shooting, but this seems basic enough. However, my experience and knowledge is not deep enough to describe the frequency of various failure modes for all of the components.

If the LED is lit, I believe it indicates the Master circuit is seeing -15V, and this should only happen with a footswitch installed. So the expected fault is constant -15V on the output of IC11.

I suggest reviewing the schematic and the 4N33 datasheet, and then tracing out the circuit in the amp. Do a visual inspection to see if you can identify any obvious damage. Finally, confirm you are getting the constant -15V. I suggest printing out the schematic and taking notes as you go along.

If the -15V is confirmed, the next step is to try and reason out out why.

Theoretically IC11 could be shorted. IC11 could also be receiving a false key. For example, maybe a metal shaving is causing a short somewhere. Or maybe someone installed a jumper on the associated pin on the DIN connector, because the FET in the Master circuit is dead.

 

[agedhorse]

Or somebody modified it because they read about how all the cool kids were doing it on the internet

 

[tavi]

I got a chance to poke around tonight. I keep forgetting to tell the story of the acquisition of the amp: While out of town for work. I found the amp on marketplace of my residence. In the add the seller mentioned a possible bad tube. I had my wife make contact and pick it up. The seller pointed to the Solo knob and told my wife it had been doing something and had been to a GC tech. But wasn't having the issue when it was taken in. The seller hasn't responded to me since I received the amp. So I'm unsure of what the actual issue was. When I used it for the first time I just assumed the issue was the amp being stuck on Solo.

On to what I was able to figure out to this point:

I'm quite positive IC11 was right where @Wasnex suspected. Conveniently labeled FS MAST. Which I'm assuming is for Footswitch Master.

I checked for continuity between Master and ground on the foot switch input. Meter indicated it was open. Checked for continuity between master and leg 1 of IC11. And found minimal resistance. On IC11 there was no continuity between any of the legs. (For the record, not my finger prints)

On IC11, there was a 5V signal across leg 1 and 2 and a -15V across 4, 5 and 6.

While in there I looked at Q4 and Q5. But didn't take any readings. I did notice another finger print in the dust next to Q5. This highlighted some delimitation on the circuit board. I Don't think this is a contributing factor. But haven't ruled it out as a potential.

@agedhorse I truly hope it wasn't intentionally modified to be cool. Otherwise I will absolutely have to fix it or sell it. I was never one of the cool kids.

 

[Wasnex]

I got a chance to poke around tonight. I keep forgetting to tell the story of the acquisition of the amp: While out of town for work. I found the amp on marketplace of my residence. In the add the seller mentioned a possible bad tube. I had my wife make contact and pick it up. The seller pointed to the Solo knob and told my wife it had been doing something and had been to a GC tech. But wasn't having the issue when it was taken in. The seller hasn't responded to me since I received the amp. So I'm unsure of what the actual issue was. When I used it for the first time I just assumed the issue was the amp being stuck on Solo.

On to what I was able to figure out to this point:
View attachment 4403958

I'm quite positive IC11 was right where @Wasnex suspected. Conveniently labeled FS MAST. Which I'm assuming is for Footswitch Master.

View attachment 4403945
I checked for continuity between Master and ground on the foot switch input. Meter indicated it was open. Checked for continuity between master and leg 1 of IC11. And found minimal resistance. On IC11 there was no continuity between any of the legs. (For the record, not my finger prints)

View attachment 4403959
On IC11, there was a 5V signal across leg 1 and 2 and a -15V across 4, 5 and 6.

View attachment 4403938
While in there I looked at Q4 and Q5. But didn't take any readings. I did notice another finger print in the dust next to Q5. This highlighted some delimitation on the circuit board. I Don't think this is a contributing factor. But haven't ruled it out as a potential.

@agedhorse I truly hope it wasn't intentionally modified to be cool. Otherwise I will absolutely have to fix it or sell it. I was never one of the cool kids.

I am not totally familiar with how the 4N33 works. I found this tutorial that says the Base (pin 6) can be left unterminated for maximum sensitivity. If you want to modify the sensitivity, then the Base can be terminated. Link Removed


I pulled the pin out from the datasheet and created a composite with the switching control schematic.

I think measuring 5V on pins 1 and 2 makes sense. The voltage at pin 2 should drop if you connect and engage a footswitch. So it appears you are not getting a false key.

The schematic suggest pin 6 is unterminated. I believe pin 4 is connected to the -15V supply and pin 15 is connected to the bottom of the 1M resistor (R82) R82 is connected to 15V. Pin 5 is the output sent to the Master circuit to control the FETs.

I am a bit surprised that you read exactly -15V on all three pins (4,5,6), and suspect this would not be normal even if IC11 was keyed to activate. But I could be wrong.

I suggest you make sure 15V is present at the top of R82. Also you may want to make sure R82 is not open circuit.

Regardless, my guess is IC11 is bad.

 

[agedhorse]

There should not be 5V across pins 1 & 2. It should be about 2.5V with the opto on and about 0V with the opto off. Measuring the off state can be a little difficult with some meters though.

If you replace the opto, be sure you install a socket, and don't damage the PCB as it's no longer available.

 

[Wasnex]

There should not be 5V across pins 1 & 2. It should be about 2.5V with the opto on and about 0V with the opto off. Measuring the off state can be a little difficult with some meters though.

If you replace the opto, be sure you install a socket, and don't damage the PCB as it's no longer available.

Guess I missread...I thought he meant both pins 1 and 2 both measure 5V; I.E. 0V across .

If there's a voltage across pins 1 and 2 it sounds like there is a false key coming from somewhere...which would result in the constant -15V output.

Also, I believe it would be closer to 5V if the circuit is seeing a ground on pin 2, rather than the voltage drop you would expect across the 100 ohm resistor and LED in the footswitch. Still have the voltage drop across R86 which is 470ohms.

 

[agedhorse]

Guess I missread...I thought he meant both pins 1 and 2 both measure 5V; I.E. 0V across .

If there's a voltage across pins 1 and 2 it sounds like there is a false key coming from somewhere...which would result in the constant -15V output.

Also, I believe it would be closer to 5V if the circuit is seeing a ground on pin 2, rather than the voltage drop you would expect across the 100 ohm resistor and LED in the footswitch. Still have the voltage drop across R86 which is 470ohms.

Correct, I’m talking about the voltage across pins 1 & 2.

 

[tavi]

Sorry I took so long. I unexpectedly got called back to work and service has been spotty. @Wasnex, I miss spoke. The voltage is not across pins 1 & 2. But measured to ground on each. And yes. There was a voltage drop of approximately .1V.

I haven't had a chance to poke around again since.

I'm beginning to wonder if @agedhorse was correct about it being intentional. But because of an issue with master power.

At this point, I won't be able to get back at it for another two to eight weeks. I definitely appreciate both your help.

 

[two fingers]

Do yourself a favor. Sent that puppy to Mesa. Their customer service is superb. Their fees are reasonable. Turnaround is quick. Communication is fantastic.

I sent an older Walkabout to them for servicing a couple of years ago. Everything went well. And I have complete confidence in the amp knowing the guys who made it went through it.

Once you get it back, you simply won't have to worry/wonder about it. One slip of the solder iron and you could damage something that can't be replaced. Not worth the risk in my view.

 

[Wasnex]

Sorry I took so long. I unexpectedly got called back to work and service has been spotty. @Wasnex, I miss spoke. The voltage is not across pins 1 & 2. But measured to ground on each. And yes. There was a voltage drop of approximately .1V.

I haven't had a chance to poke around again since.

I'm beginning to wonder if @agedhorse was correct about it being intentional. But because of an issue with master power.

At this point, I won't be able to get back at it for another two to eight weeks. I definitely appreciate both your help.

If the IC is receiving a key with no footswitch applied, it could definitely be a mod. But the only reason I can think of would be if something is wrong with the Master volume signal path.

If both pins 1 and 2 read 5V with respect to ground, there is no false key and the output of IC 11 should be 15V. If you measuring pin to pin, I don't think there will be a voltage drop. If you measure from the pins to ground, I believe pin 2 will read lower because the meter will provide a high impedance path to ground that will cause current to flow through the LED in the chip.

So if there is no key across pins 1 and 2, I suspect IC11. I would still check for 15V on the top of R82, but I don't necessarily expect a problem with the 15V or R82.

No rush...work on the amp and post when you can. Or send the amp to a tech, so you don't have to worry about it.