Diagnosing a broken MIDI input

[guyatone]

I recently purchased a used DT50. When trying to find out if needed a firmware update, I was unable to get Line 6 Monkey to recognise the amp. I tried a different MIDI interface, with no success. I am able to receive MIDI signals from the DT50, for example, the channel selector sends CC19, but I cannot get the DT50 to recognise the same CC when I send it via MIDI controller to it.

 

Now, I know my way around the insides of a tube amp (discharging caps, etc), and would like to diagnose further why the MIDI input isn't working rather than having to send it in for (costly?) repair. Looking at the I/O PCB, it looks like the optocoupler is a likely candidate - are those known to fail? Other than this and the connectors between the I/O and the digital board, are there any other components that I should be checking?

[jakeman19]

On 3/19/2021 at 4:16 PM, guyatone said:

I recently purchased a used DT50. When trying to find out if needed a firmware update, I was unable to get Line 6 Monkey to recognise the amp. I tried a different MIDI interface, with no success. I am able to receive MIDI signals from the DT50, for example, the channel selector sends CC19, but I cannot get the DT50 to recognise the same CC when I send it via MIDI controller to it.

 

Now, I know my way around the insides of a tube amp (discharging caps, etc), and would like to diagnose further why the MIDI input isn't working rather than having to send it in for (costly?) repair. Looking at the I/O PCB, it looks like the optocoupler is a likely candidate - are those known to fail? Other than this and the connectors between the I/O and the digital board, are there any other components that I should be checking?

I’m sure psarkissian would advise against poking around. Having said that, I’m not at home with my amp now but when I get home I’ll double check. I don’t know for sure if monkey would see the amp, it’s been a long while since I connected to mine. 
 

Jd

[guyatone]

11 hours ago, jakeman19 said:

I’m sure psarkissian would advise against poking around.

I'm pretty sure he would. However, the closest Line 6 service point is not only more than two hours away, but I'd also have to cross two international borders to get there.

 

I took out the I/O board earlier today, hooked up my bench power supply and meter and was able to verify that the optocoupler is working - MIDI signals coming in through the DIN connector read correctly at the PCB connector.

 

I've seen people refer to "safe mode" in this forum, including someone writing that they were able to use DTEdit when the amp was started up in safe mode. Safe mode, when I try it, leads to an amp that gets stuck with both Class A/AB lights on and does not respond to any switches, MIDI signals or guitar input. Is that the intended behavior or does my amp have a software problem?

[psarkissian]

Opto-isolators in MIDI circuits are usually very reliable. MIDI in the DT50 is

for connecting to Line 6 Monkey.

 

Be aware that DT Edit is  3rd party app and is unsupported by Line 6.

 

And yes, don't poke around in there.

 

[jakeman19]

1 hour ago, guyatone said:

I'm pretty sure he would. However, the closest Line 6 service point is not only more than two hours away, but I'd also have to cross two international borders to get there.

 

I took out the I/O board earlier today, hooked up my bench power supply and meter and was able to verify that the optocoupler is working - MIDI signals coming in through the DIN connector read correctly at the PCB connector.

 

I've seen people refer to "safe mode" in this forum, including someone writing that they were able to use DTEdit when the amp was started up in safe mode. Safe mode, when I try it, leads to an amp that gets stuck with both Class A/AB lights on and does not respond to any switches, MIDI signals or guitar input. Is that the intended behavior or does my amp have a software problem?

So I was able to connect to my DT25 via the DTedit software and YES it does show up in Line 6 Monkey. I did have to redo my MIDI setup on my Mac for the M-Audio 1x1 to be seen by the DTedit software. Have you tried doing a reset on the DT25? Other than that, the board level stuff is beyond my expertise. 

[guyatone]

I have done a reset, several times. It does reset properly, both channels are back to voicing I. I just can’t get the DT50 to react to any MIDI signal. Not from Line 6 Monkey, not from a hardware controller, not from DTEdit, not from Pure Data. The optocoupler is working, there isn’t really anything else between it and the MCU, so I’m leaning towards firmware issues. Besides the “pinch” reset and the channel B safe mode, are there any other diagnostic/reset possibilities that I have from the outside, other than breaking out the soldering iron and adding a JTAG connector?

[jakeman19]

3 minutes ago, guyatone said:

I have done a reset, several times. It does reset properly, both channels are back to voicing I. I just can’t get the DT50 to react to any MIDI signal. Not from Line 6 Monkey, not from a hardware controller, not from DTEdit, not from Pure Data. The optocoupler is working, there isn’t really anything else between it and the MCU, so I’m leaning towards firmware issues. Besides the “pinch” reset and the channel B safe mode, are there any other diagnostic/reset possibilities that I have from the outside, other than breaking out the soldering iron and adding a JTAG connector?

Sorry, I've got nothing at this point. 

 

Jd

[guyatone]

4 hours ago, psarkissian said:

And yes, don't poke around in there.

If I can avoid it, I'll be happy to keep the thing closed. What other options to I have to get MIDI input (and with it Line 6 Monkey) to work?

[psarkissian]

Don't go adding a JTAG header,... that would be bad. Don't do that.

 

 

 

[guyatone]

What should I be doing? A bit of reading in this forum tells me that I’m not the first one who can’t get Line 6 Monkey to recognize a DT50/DT25, and I have tried everything that was suggested in those threads, such as different cables, different computer, different MIDI interface, safe mode and reset. I would love nothing more than a solution that doesn’t require me to take it apart, but I don’t know of one.

[psarkissian]

-What computer OS are you using?

-Is there a USB hub connected to the computer? If so, disconnect it while connecting to Monkey or Workbench HD.

-Use a MIDI Uno interface.

 

[guyatone]

- Windows 10, all updates installed

- no USB hub

- I don't have the UNO model, but my MIDI interface is made by M-Audio

 

Again, this is not just limited to Line 6 Monkey or the computer USB, but even with simple hardware interfaces (no computers, operating system or USB involved at that point) sending nothing more than CC19 on MIDI channel 1 I get no reaction from the amplifier. According to all the documentation I read so far, that MIDI command alone should cause the amplifier to switch channels, which it doesn't.

[guyatone]

I know the script. Try resetting the amp. Try rebooting the computer. Update your drivers, update your operating system. Try a different MIDI cable. Try a different MIDI interface. Try a different computer. That's what the script for "troubleshooting firmware update" says. I have done all that.

 

I'm not even at the point where the firmware update is my concern. The current status is that there is no reaction, whatsoever, to any MIDI input of the amp.

 

MIDI is a dead simple protocol. UART, 31250 bits per second, 5V. The MIDI command to change the channel on a DT50 is a whole three bytes, nothing more. Sending that simple signal to the amp does nothing - I can send it from a hardware MIDI controller, I can even send it from an Arduino - nothing. If I can't get that simple command to work, there's no point in even going through operating system and driver updates, because they aren't even playing a role yet.

 

And, my question from above again: What is the expected behavior of the amp in safe mode? I see others reporting that when they start the amp in safe mode, it will respond to MIDI signals and switch channels and topology. When I boot mine in safe mode, it does not respond ot anything. Safe mode on mine means that "B", "Class A" and "Class A/B" light up and stay lit, the lights on all the other switches stay off and the amplifier responds to nothing. Is that how it is supposed to be or does my amp have trouble with safe mode?

[psarkissian]

It may be a hardware issue. When was the last time the tubes were changed out? Old or failing tubes

can pull down on a circuit and create a number of issues. Also the incorrect tubes and bias can result

voltage pull-down issues.

 

Tubes choices to use for the DT Series,...

DT-50 Series:

12AX7EH Electro-Harmonix approved for the DT50 pre-amps.

EL34EH Electro-Harmonix (matched pairs) for the power amps.

 

DT-25 Series:

12AX7B China for pre-amps.

EL84EH Electro-Harmonix (matched pairs) for the power amps.

 

Power amp tube choices are crucial, and must be properly biased by a tech or Line 6 authorized service center who knows the product.

Don't do this yourself, the shock hazard is too great,... and it hurts when you get electric shock. Leave it to a tech trained who is for this.

These valve amps are particular about tubes and tube brands. Do Not use substitutes, No Mesa, Groove Tube, Tung-Sol or JJ or (etc) substitutes.

No hot rodding of an already Bogner hot rod amp.

 

Your unit was built way after the tech bulletin range, so that won't be an issue.

 

[guyatone]

Thanks for your help. I know we technical users aren't the easiest :)

 

I got this amp used just a couple of days ago, I don't know how old those tubes are. They are EH tubes, so either a correct replacement or they're 10 years old... I tried replacing the 12AX7s with known working EH tubes that I have, with no difference. I don't have any EL34s for testing. The amp, when I got it, did have a blown heater filament fuse, so there's that...

 

Voltage drop due to high current draw was also on my mind. I tried it with all tubes removed, but that didn't make any difference either. Getting new tubes put in by an authorized dealer, well, that's the aforementioned half-day trip to Switzerland in which case I might just as well have them look at the MIDI port while they're at it. For any other amp, I'd have service points nearby to get new tubes in (Bogner authorized even), but as you say, the DT50 has a special procedure that goes beyond a basic bias adjustment and those specs aren't public.

 

One thing I might take a look at, which I know is not recommended, is to check the test points on the digital PCB for correct voltages. Line 6 is doing a great job at labeling their boards, I have to say.

 

Still, the one question that I'd like to get answered: What is the expected behavior of an amp in safe mode? Is it supposed to be "frozen" and quiet or should it still be amplifying sound and react to the switches?

[psarkissian]

I've never had to try safe mode.