Ashdown MAG300 EVO2 problem

[m1989jmp]

Hi all, my first post here.

A couple of friends accidentally plugged speaker-out from one amp into speaker-out of the other amp while trying different cab/head combinations.

Now both heads produce nothing but loud hum. I'm keeping my focus on the amp in the thread, because the other one is a small SS practice amp.

So far I've tested all of the output transistors with DMM's diode tester function while still in the circuit and they're fine. For other transistors I guess I'll have to remove them from the circuit and test, because the nearby components are interfering with the resistance measurements.

All fuses are fine.

Primarily I'm wondering what exactly happened at the moment of the fault? Did the amps short each others output transistors? Can the power supply be affected?

Keep in mind I have a lot of experience with building guitar tube amps, just not that much with troubleshooting. Later today I will measure DC on the output, V+, V- and will try to confirm it's indeed a power amp problem (hopefully the LINE OUT jack produces sound without hum).

Thanks for the answers, the schematic is below:

[J M Fahey]

accidentally plugged speaker-out from one amp into speaker-out of the other amp while trying different cab/head combinations ...... Now both heads produce nothing but loud hum.
So far I've tested all of the output transistors with DMM's diode tester function while still in the circuit and they're fine. For other transistors I guess I'll have to remove them from the circuit and test, because the nearby components are interfering with the resistance measurements.

Not too sure about power transistors being undamaged.
To be certain they should be measured disconnected from other stuff, and a single diode or resistance test is not enough, in fact thereīs 6 measurements per transistor: BE - BC - EC both ways
Only forward BE and BC should show around 0.7V drop, all 4 others must be fully open.
Driver transistors should be tested the same way, and you need to check ceramic emitter resistors (0.33 ohms)
Your meter wonīt be accurate with such low values, might show more than 1 ohm each, but as Enzo always says, we are not checking precision here, just that they are not open.

That said, this amplifier surprisingly has NO output short protection, itīs an accident waiting to happen ... and it finally did.
Original Ashdown used expensive MosFets, which "do not need protection" (famous last words) but at least were robust enough to stand brief ones ... this seems to be a later cheaper "update" using bipolar transistirs.
Fine with me but they should have added protection.

Start by plugging the amp in a lamp bulb limiter, all controls set to 0, no speakers or load of any kind and check whether it turns on, bulb blinks or stays brightly lit (red or dark orange is normal), what rail voltage you get and whether you have DC at the speaker output.

A gut picture showing board and power transistors may help.

All fuses are fine.

Ceramic resistors might have blown open and worked as de facto fuses.

Primarily I'm wondering what exactly happened at the moment of the fault? Did the amps short each others output transistors? Can the power supply be affected?

A short to ground is bad, like a train hitting a mountain.
An amp to amp short is even worse, like a train hitting another going same speed the opposite way, meaning one amp will *fight* the other for control, both dying in the process.

Keep in mind I have a lot of experience with building guitar tube amps, just not that much with troubleshooting. Later today I will measure DC on the output, V+, V- and will try to confirm it's indeed a power amp problem (hopefully the LINE OUT jack produces sound without hum).

Yes, the power supply might also have been affected, only measuring will tell.

[Mick Bailey]

Hi and welcome.

I've encountered this 'plug and play' scenario a few times. What happened at the moment of failure is possibly each amp saw a short on the output. You could be lucky and just one of the internal fuses is blown and skewing the output towards the opposite supply rail. You're right to check the voltages, but make sure that the two fuses are OK first. Don't run the amp with a load connected until the fault is fixed. If the voltages are present you need to check to see if there's any DC on the output and report back.

EDIT, ah - just re-read and you say the fuses are fine. Did you measure them?

[g1]

Also do a visual check for burnt or blown out traces (especially ground tracks).

[m1989jmp]

I will definitely post more info and pictures today. I do know that there are no burn marks anywhere (components or traces) and no blown fuses, so something must have given up the ghost fairly quickly and non-violently.

[m1989jmp]

So, I fired it up with bulb limiter; it flashed and then no light, I guess that's fine.

Next, I've measured the voltage at the output; near 0V, guess that's fine too. After I turn the amp off, it rises to 10VDC and then drops again to 0VDC.

First issue I've noticed is that V+ voltage is intermittent, from 0 up to 65VDC, after a few seconds it stuck to 0VDC. V- is fine, -65VDC.

[m1989jmp]

Hi and welcome.

I've encountered this 'plug and play' scenario a few times. What happened at the moment of failure is possibly each amp saw a short on the output. You could be lucky and just one of the internal fuses is blown and skewing the output towards the opposite supply rail. You're right to check the voltages, but make sure that the two fuses are OK first. Don't run the amp with a load connected until the fault is fixed. If the voltages are present you need to check to see if there's any DC on the output and report back.

EDIT, ah - just re-read and you say the fuses are fine. Did you measure them?

Yes, I've measured and they're fine.

[J M Fahey]

So, I fired it up with bulb limiter; it flashed and then no light, I guess that's fine.

Next, I've measured the voltage at the output; near 0V, guess that's fine too. After I turn the amp off, it rises to 10VDC and then drops again to 0VDC.

First issue I've noticed is that V+ voltage is intermittent, from 0 up to 65VDC, after a few seconds it stuck to 0VDC. V- is fine, -65VDC.

Ok, there you found the first abnormal symptom.
Follow track backwards up to and including the main rectifier bridge, to find where you lose it.

[m1989jmp]

Ok, there you found the first abnormal symptom.
Follow track backwards up to and including the main rectifier bridge, to find where you lose it.

Does 0VDC at the output mean the output transistors are fine?

AC voltage on the bridge rectifier is normal too, and bridge rectifier tests ok. Does that mean that C14 and/or C16 are shot?

[The Dude]

As Juan pointed out, you have an obvious problem. Your positive rail is disappearing. Start there and find out why.

[m1989jmp]

As Juan pointed out, you have an obvious problem. Your positive rail is disappearing. Start there and find out why.

Sure, that's why I asked what's balancing out the DC voltage at the output to zero to possibly eliminate TR16 and TR17 as the reason for defective + rail.

I guess the most obvious thing to do is to disconnect everything that is supplied from the + rail? If it's still 0VDC the source is somewhere in the power supply? If the voltage returns to normal, I start connecting components back till it drops again, so I can zero in on the problematic area?

I'll repeat, I'm a complete noob when it comes to troubleshooting methods, so any advice will help.

[The Dude]

If there was a shorted component on the rail, I suspect you'd be blowing fuses. That said, I don't think you need to disconnect anything just yet. Where exactly did you check the +V? On the outputs, at the rectifier, etc. My bet is that you are looking for something open- not shorted. Something like a burnt trace, open fuse, etc. If you haven't, check directly at the rectifier. You can easily check for a rail short without disconnecting anything. Just check rail to ground for continuity.

[J M Fahey]

I guess the most obvious thing to do is to disconnect everything that is supplied from the + rail?

No, the most obvious thing is to grab a meter, set it to 200VDC scale, connect black probe to ground and follow the path of the +65V rail end to end with the red probe, measuring every single junction, to find which have +65V and which have 0V.

That previous to anything else.

[m1989jmp]

If there was a shorted component on the rail, I suspect you'd be blowing fuses. That said, I don't think you need to disconnect anything just yet. Where exactly did you check the +V? On the outputs, at the rectifier, etc. My bet is that you are looking for something open- not shorted. Something like a burnt trace, open fuse, etc. If you haven't, check directly at the rectifier. You can easily check for a rail short without disconnecting anything. Just check rail to ground for continuity.

I've checked on the output's collector, but I'll check at rectifier too later.

I'd presume the intermittent voltage at V+ before falling to 0 is symptomatic of a failing capacitor; I'll borrow an ESR meter and check it out regardless.

[g1]

More likely a failing connection somewhere.
Don't get stuck on the idea there must be a failed component, failed connections are even more common.
Juan's post #13 is the most direct course of action.