Ashdown Mag300: In trouble

[Zanzoon]

Hi there,
Here a very bizzar problem. I have TWO MAG 300 amps and both with the same problem.
ps: I use a regulated psu to test the power module (+/- 64V).
The preamp is not connected.
I use a resistive load of 8 Ohms/ 600 W

1. I disconnect the load (speaker).
2. Switch on the amp.
3. Reconnect the load (speaker).
4. Amp works fine...

If I switch the amp on while the load is connected;
The positive supply rail go's in protection (= 3V) and a hefty 400mA is drawn from the psu.
Removal and reconnecting the load makes the amp work as normal.

Attached is the schematic: APC041-1.pdf
Drawing: ACD041 - PCB: APC041/1

I replaced C10 as it was bad.
I checked all other components but could not find a bad one...
Relay circuit is working perfect, when the amp is not connected to a load.
If connected the relay will not engage...

The second amp has the same symptoms...

Help, Hints, Ideas are very much appreciated.

Cheers

[jpfamps]

I remember having a problem with a MAG300 similar to this.

The amp oscillated at around 17kHz when I had my 'scope probes connected across the output (with dummy load attached).

My guess is that you are experiencing some form of oscillation in the output.

[Jazz P Bass]

Quote:"If I switch the amp on while the load is connected;
The positive supply rail go's in protection (= 3V) and a hefty 400mA is drawn from the psu.
Removal and reconnecting the load makes the amp work as normal.
What does this mean?
"The + rails go into protection."
protection (=3V) what does this mean?

Quote:"I replaced C10 as it was bad."
Was C10 shorted?

Quote:"Relay circuit is working perfect, when the amp is not connected to a load.
If connected the relay will not engage..."
This I do not get.
The relay circuit is nothing more than an "On Delay".
It should not even care that a load is connected.
Maybe you should monitor the relay circuit & see what is happening there.
Actually, I am totally confused as to how you even have the amp hooked up.
"an external + - 64V power supply"
So what is the supply for the relay circuit.
It appears to use L2 and a diode.

[Enzo]

You are firing these power modules up outside the amp? WHat happens if they are wired into the amps?

Some power amp modules expect their input grounds to be in place. With it all disconnected, is there continuity between the ground pin at the input and the bottom end of C11 or C4?


If I am reading between the lines OK< it sounds to me like your amp comes up with DC on the output.

I will further guess you mean that your bench supply goes into current limiting when you say protect. The amp has no protect. How is your bench supply set up? When power is applied, there will be surges in the power supply, moreso if the load is on. You might be tripping the supply limiter with a simple starting surge.


First thing I'd do would be reconnect that preamp, or at least run a clip wire from the ground pin of PL1 over to the rest of the power amp grounds.

The relay circuit is a simple timer off the power supply, it has no way of knowing if a load is present or not. Ther is a thermal sensor that will drop it out of the heatsink gets too warm. I susopect your relay trouble might be due to the bench supply limitations.

Before we took it apart and tried running just the power module, WHAT was wrong with the amps that we were trying to fix?

[Zanzoon]

Thanks for the replies folks.
First to start with; Englisch is not my first language so some words, sentences are a bit akward to you.
Second; The power module is connected to an external stabilized power supply (psu). I can adjust the output voltage and current.

(Amp = Power module, you have the schematic)
With no load the amp behaves normal. When I then connect a load, no problem.

If I connect the power to the amp when a load is connected, the current limiting of the psu is triggered and the positive voltage will not go above 3 V.
Current is then limited to 400mA.

Enzo: If they are in the amp; I do not have to tell you, the fuse will blow when a load is connected.
Indeed, with a load there is DC on the output. That's the problem in the first place.
With no load it will behave as explained before.

The preamp dosn't need to be conected.
I repaired a dozen or more of these amps (Mag 300) over the years.
These two are giving me serious problems.

The relay will not engage when the positive supply will remain under 13V.
The relay circuit is perfect as I isolated it from the rest of the circuit and tried it out.

Cap C10 has been replaced because an inspection with a magnifying glass showed electrolyt spill aromund the edges.
Removal conformed it and there was a steady resistance of severall KOhms.
Allthough for the moment only 1 amp has a bad C10.

Both amps have the same problem.
Even at Ashdown they are baffled...

And it has nothing to do with my testgear or cables.
I repaired in between tens of other amps in a similar way.
And I have been doing this for 40 or more years...
So we have to cancel that out -> 100%.
(And to avoid the question: Yes I modernized my equipment during the years...)

I know what I do. Here I'm running around in circles...

[Enzo]

What happens if you increase the avaialble current from the PS unit? If one rail collapses under surge, and stays that way, then there will be DC on the output for certain. I am concerned the power-on surge through the load is triggering the problem.

C10 seems like an unusual failure to me. Is that cap rated below the 128v total of the power rails? So in my view, either that caps saw excess voltage or something caused its failure. As I see it, if we short across C10, the amp will become biased very cold. SOme crossover distortion will occur, but the amp should work in stable manner. That might be a way to see if the amp behaves differently.

Welcome to the forum Zanzoon. I have no doubt of your ability, but any experienced technician can find it hard to see a problem from time to time. It happens to me also. SO if I question your set up, it is not because I have no trust, it is because I cannot make assumptions. For some unknown reason your approach is not working in this case, and we need to find out why that is. SO trying different techniques is one way to help discover the answer.

[Jazz P Bass]

[QUOTE=Zanzoon;245092
(The Power Modules) If they are in the amp; I do not have to tell you, the fuse will blow when a load is connected.
Indeed, with a load there is DC on the output. That's the problem in the first place.
With no load it will behave as explained before.
[/QUOTE]
Yeah, solid state output sections can be a treat
First, I would put the module back in the amplifer, using the amps power supply.
Next, limit the current to the amp. (lamp limiter)
Now you can probe the module & look for what is outputing Vdc.
Specifying the polarity of the unwanted output Vdc may help narrow down the fault.

[Zanzoon]

Thanks Jazz P Bass,
But that's noty the way to do it.
I wrote clearly that I have a regulated psu that will do the job...
There is a positive DC at the output.
I'm just back at my ps.
I checked every component on the pcb.
Every track.
All OK.
It must be a component that during startup is leaking and then sets the amp in a certain state.
(Positive voltage at the output)
Because the psu is current limiting (as your bulb... but in a controlled manner) the output voltage is only +3V.
If I raise the current to approx 800mA for the positive side, the output voltage will go up to +6V.
The negative side is behaving normal (I = 50mA).

I tried a sollution and it seems to work; but that's for tomorrow...

Cheers

[Zanzoon]

As said, Here the solution.
In fact; it isn't THE solution.
The problem or cause of the problem couldn't be found.
I checked every single component and pcb track.
All tested OK.
I changed the electrolytic caps as it could be one that is leaking at the initial start up.
No change at all.

So I had to be drastic and here it is.

On the right you see the parts in red. They have been added.
The cap C5 has been increased to 22uF to make sure that the amp is stabilised when the speaker is connected.
It also avoids a plop when the amp is switched off and within a second or so switched on again.
Resistor R16 is placed after the termo switch. At that point another resistor (1K2/5W) is inserted to bring doen the positive supply to 24V.
This 24 volts feeds the speaker relay that in turn is actuated when RL1 is set.

Both amps are under test now and I keep them on the bench for another 24 Hours.
Output power is a hefty 350W at 4 Ohms.
Until now they behave very wel.
Let's hope that they will last. Fingers Crossed.

Not an elegant solution. The electronic problem is not solved. I lost a battle...

[Jazz P Bass]

Thanks for the reply to your fix.
The way I see it, you did not "Lose A Battle".
This is not your design.
The increased time delay may be just what the doctor ordered.

[lolbou]

Hi,

Could someone please repost the schematic please (looks like it's been lost as some other files from the database)? I have a dead unit there (MAG300H, the head only), and I get no response from Ashdown...

Thanks for your help,

Laurent

[Zanzoon]

Ashdown is usually verry helpfull.

Here the schematic:
MAG300.pdf

To be sure that it's the correct version; there is a APcxxx or ACDxxx code printed on the pcb.
If it's not the one as posted , let us know.

Cheers

[lolbou]

Ashdown is usually verry helpfull.

Here the schematic:
[ATTACH]23084[/ATTACH]

To be sure that it's the correct version; there is a APcxxx or ACDxxx code printed on the pcb.
If it's not the one as posted , let us know.

Cheers

I know they are (they did provide me a FA-60 schem some time ago), but considering they're silent know, I keep wondering.

Thanks for this schem. It doesn't match my PCB though, I'm afraid. I did not mention these because the file in your OP is named ACP041-1.pdf, which matches my PCB number (although the issue number seems different).

For further information, PCB numbers are:

- APC041 issue 3 (03/12/04) (power amp section)
- APC044 (sub board)
- APC021 issue 1 Electric Blue/Mag (main board)

Thanks a lot again, the basic blocks should help the debugging anyway, but if you've got something closer, I'll happily take it...

[lolbou]

Hi,

Some schem appeared here at FSB. Looks close, though the SUB schem is definitely different.

No reply from Ashdown yet.

[KCblues]

Hi folks,

I have one on the bench with the same problem.

Zanzoon do you have a copy of your first schematic with the modifications marked in Red ?

Thanks!