Ashdown MAG200 Dead - Blown Main Fuse

[stuckinstandby]

Hi

I've just acquired an Ashdown MAG200 combo (C410T-200) which is completely dead. I've not spent a lot of time on it yet, but so far I've discovered that the main fuse has blown, two of the four output transistors have scorch marks and one of the large caps has a very slight dome.

I've never repaired an amp before (aside from dodgy sockets) but I've dabbled with various electronics over the years and I'm keen to learn more. I just thought I'd post to see if anyone has any insight/advice for this amp or similar before I get started. I've requested some info from Ashdown and found a similar circuit diagram (power board is ACP008) and just starting to study it.

I've ordered new fuses and I've been looking at ordering replacement output transistors - BUZ900P and BUZ905P seem a bit expensive so I'm considering ECX10N20, ECX10P20.

I'll add some pictures when I can.

Thanks

[stuckinstandby]

A few photos....

Power board:


Burnt output transistor, right:


Slightly domed cap, right:


Schematic, page 4:
MAG300.pdf

Have replacement fuses now. Off to build a light bulb limiter...

[Enzo]

The domed cap is likely OK, Slit the plastic sleeve on the can and free the black plastic disc that is actually domes. If the aluminum can under it is domes, then the can is bad. But if the can itself is flat, then the dome is just warped plastic.

So you will need to replace the power transistors, and check all resistors associated with the blown transistors. Even if they look OK, they could have been burnt open.

[stuckinstandby]

Thanks Enzo. I've not had any time to look at this but hopefully this week. I have an ESR meter somewhere so should be able to test the caps with that. Cheers

[stuckinstandby]

So 9 months on and I've finally got round to having a go at this. Blame housing renovations

Before changing anything, when powered on with the light bulb limiter, the bulb (100W) glowed brightly.

I tested the resistors and couldn't find any issues. I bought a couple of replacement transistors from profusion: ECX10N20 for the BUZ900P and ECX10P20 for the BUZ905P. Installed and tested a few minutes ago with a light bulb limiter and no speaker.

Now the bulb is bright initially and then fades to off. The VU meter on the amp is illuminated now.

So it's looking good but I'm wondering if there's anything else worth checking before I go full power? I don't have variac or scope, just a multimeter and I never did track down my ESR meter.

Thanks!

[stuckinstandby]

Decided to measure for any voltage on the speaker output, with no speaker connected. On power up with the light bulb limiter, I get 50mV DC. When I switch the amp off, that jumps to 250mV and takes a while to come down.

In contrast, I tried the same on a Laney bass combo (8ohm output) although I had the internal speaker connected. It gave 20mV on power up with the limiter and went to 0mV when powered off (audible pop through speaker).

I'm being a bit tentative about reading other voltages with the Ashdown powered up (DMM only has probes, so I should probably wire up some croc clips). I've not really messed with higher voltage stuff before.

I see suggestions of checking the rail voltage. Looking at the schematic I guess that would where V+ and V- are marked and this would be a DC voltage measured wrt ground rather than earth?



How would I work out what the correct voltage here is? and what should I expect when plugged into the limiter?

Thanks

[Enzo]

A few millivolts at output is not a concern. As to a small offset at power off, it may take a while to go down with no load, but when a load is present it will discharge almost immediately.

I have no idea what you main power rails should be. Look at the AC they are made from, and expect ROUGHLY 1.4 times that voltage, but DC. Usually they are present or absent, seldom wrong. If they are wrong, usually they read real low because the filter cap is not working.

The limiter is used to protect the circuit better than a fuse can, but any voltage readings are largely meaningless. Proper readings are taken not with the bulb.

[stuckinstandby]

Thanks Enzo.

I measured V+ and V-. Around +60V and -60V DC respectively. Not measured the AC yet.

Plugged in the speaker with the limiter still in place which resulted in a loud hum from the speaker. Would the limiter cause that or does it sound like another problem?

I read filter caps are the main suspect for a constant loud hum, so I plan to remove the board again and resolder just in case. One of them does look a little domed but it seems like that's just the plastic cap as you said above. I'll slit the sleeve and check that too.

[stuckinstandby]

I re-flowed the solder on the larger components including the filter capacitors but it's made no difference.

I'm seeing 46V AC from the transformer and +61.4V DC for V+, -61.4V DC for V-, which is in that x1.4 range you mentioned.

I've posted some info on another forum so hopefully between here and there we'll be able to get this going.

[g1]

Plugged in the speaker with the limiter still in place which resulted in a loud hum from the speaker. Would the limiter cause that or does it sound like another problem?

Was the amp turned off when you connected the speaker? If so, then yes the limiter can cause the problem.
Power up on limiter lamp without speaker. Should still be same low level (20mV) DC on output, and dim bulb.
If so, connect speaker while amp is running.
Still get hum?

[stuckinstandby]

The amp was turned on when I plugged in the speaker.

[Enzo]

A loud hum in solid state amps COULD be filter caps, but really, more often it is a sign of DC on the speaker output.

[stuckinstandby]

I measured the voltage on the speaker output again just now - 54mV DC.

[g1]

How about AC volts on the speaker while humming?

[stuckinstandby]

No AC on speaker output, but a breakthrough!

I'd been a bit timid about leaving it connected to the speaker after hearing the loud hum. While re-taking the measurements earlier today, I noticed that this time, the hum was much quieter to begin with. After 10 seconds or so, there was some crackling and then it reverted to loud hum.

I just tried again and it seems like the hum was due to crackly pots. This doesn't quite make sense though as I'm sure I played around with them while connected before and noticed no change. Anyway, I attacked them with contact cleaner and for now the hum is gone

Still a few issues to sort:
- input control is still a bit crackly - it's difficult to access from the front so I'll have to take the board out
- seems to be a grounding problem with the master control - hums when I touch the metal of the pot
- quiet hum audible as you turn up the master control

Getting there.