Ashdown Mag 300 evo II problems

[Jimmy74]

One of my customers has brought me in an Ashdown Mag 300 Evo II. He's had a weird and quite unique encounter with it, I must say, I've never come across anything similar. Basically for some reason he attached the output lead to the tuner\line out jack and after a few seconds realizing his error, proceeded to pulling out the cable while the amp was on causing a massive spark. I have opened the amp and noticed that on the rear of the preamp board, just behind the small 2 wire plug that connects the preamp board to the output board, there's a small section of the path that has obviously burnt out and separated from the board.

Just out of curiosity, I would like to know what you think an accident like this could cause? Just damage to the preamp or could the output transistors have blown too? The amp produces a loud hum when powered up.

thanks
J.

[Mick Bailey]

The output transistors may have failed, but would usually cause the fuse to blow, as their normal failure mode is to fail shorted. Any fault of this nature can be unpredictable. You could also have a failed power supply rail and/or a whole bunch of problems.

Don't proceed with connecting a load to the amp.

Is there DC on the speaker output?

[Jazz P Bass]

Been there.
Done that.

Had the exact same occurrence.
In mine, the ground buss melted.
A few IC's where fried.
If I remember correctly, the main board took a hit also.

I do remember it was a PITA to finally get it up & running.

[drewl]

Yikes.
Going from memory of the one I repaired last week,
Probably fried the tuner out amp, and maybe the 15v supplies which the regulators are on the preamp.

I could see that damaging the the voltage feed connector to the preamp.

You might get lucky and the power amp may be okay.

[Jazz P Bass]

The one that I had, the issue did not go as far as the power amp.

[Jimmy74]

I repaired the severed wires and bypassed the burnt path with a length of wire and powered it up to test the output for DC voltage...it's starts up at around 40mV and quickly goes down to around 1mV and eventually -ve. I also tested PL-1 but no voltage on those pins and I tested the AC 15v line and that's working and is giving roughly 20vDC after the rectifier.

any thoughts?

[Jazz P Bass]

So the output section is placing -VE on the output jack.
Not having a voltage on PL-1 is a good sign I guess.
But any dc voltage should not get past C 13.

Before diving into that, can you specify what tracks where scorched?

At this point, I would start at the beginning.
Check all four of the power supply rails (+ & - VE and + & - 15V).

Which power amp do you have?

Mag 300.zip

[Jimmy74]

Only one track was scorched just after the PL3 connector on the preamp board where the black wire connects and the track passes to R98, SK6 and so on. I'm guessing this is the ground path.
I'm finding it difficult to follow the schematics all the ones I have differ in many ways to the actual board and component numbers. The main board states Ashdown APCO41 Issue 4 Clive Button 23\07\09 MAG300 EB180. The Preamp Board states Clive Button 28\09\2006 Ashdown Design - Electric Blue\Mag APC060 Issue A so if you can shed some light as to where exactly I should test the power supply rails and what readings I should expect. I did test the PL1 AC 15v connector on the preamp board and that is giving me correct readings AC and also DC after the rectifier diodes. I'll test C13 for DC. I don't see any smoke or bubbling parts anywhere, and I can hear the relay click in a few seconds after I power the amp up.

thanks

[Jazz P Bass]

At this point, do not connect a load )ie: speaker or resistive load) to the output.

Can't help on the power rails as I don't have the amp.

The big caps are + & = Vdc.

The ic's are powered by voltage regulators.
Pin 4 (-) & pin 8 (+).

[Jimmy74]

No dc on C13.

Tested some IC's...on some I'm getting only the +ve 14.6 volts, on an other I tested it was giving me half voltages 7.2V +ve and -ve and on others I tested it was giving me correct +ve and -ve 14.6V

[Jazz P Bass]

"I have opened the amp and noticed that on the rear of the preamp board, just behind the small 2 wire plug that connects the preamp board to the output board, there's a small section of the path that has obviously burnt out and separated from the board. "

The two wires that go to the output board are the speaker connections.

The one I worked on the ground buss there was burnt.

Here are the two supplies.
I was incorrect as to the + - 15Vdc supplies.
They are using 15 volt zener diodes.

ashdown MAG300 15 Vdc supply.pdf

ashdown MAG300 V+_ V- Supplies.pdf

[Jimmy74]

I did test all the TL072's and all tested ok except for IC12, IC11, IC9 and IC7 on which I was getting correct positive voltage but no negative voltage on pin 4. There are another 2 or 3 IC's which are not TL072's

[Enzo]

C13 is across the AC terminals of the main bridge rectifier, yes? I would not expect DC there.

I don't see any ICs on different power, so all ought to have +/-15v. 14.6 is close enough. What IC measured low?

[Jimmy74]

"I have opened the amp and noticed that on the rear of the preamp board, just behind the small 2 wire plug that connects the preamp board to the output board, there's a small section of the path that has obviously burnt out and separated from the board. "

The two wires that go to the output board are the speaker connections.

The one I worked on the ground buss there was burnt.

Here are the two supplies.
I was incorrect as to the + - 15Vdc supplies.
They are using 15 volt zener diodes.

[ATTACH]41500[/ATTACH]

[ATTACH]41501[/ATTACH]

No not the speaker output board but the small plug in wires that run from the main power supply board to the preamp board.

Those schematics are different from the amp I have on the bench....C10 is an electrolytic cap on the main board and C23 is a tantalum cap on the preamp board

[Jimmy74]

Enzo: four of the TL072's had no negative voltage readings on pin 4.