Ashdown MAG 600 Blows Fuses

[earache]

Hello Folks

I am looking at an Ashdown Mag 600 bass head which blows it's main 8 amp fuse when turned on. This is the fuse which is built into the IEC connection where the power cable plugs in.

The amp doesn't blow the fuse when turned on if connected to a light bulb limiter. The bulb glows brightly however.

In that state, there is no DC across the speaker jacks.

All other fuses internal to the amp test ok.

How would I continue to find the source of the problem?

In another post on this forum in the schematics requests section, member "tealrex" requests a schematic for this same amp as he was having difficulty finding one thru Ashdown.

In a later post he states "Just a quick heads up if any one else has the same issue with finding the schematic. the schematic labeled "abm output board"--drawing # acd033 can be used. Its about 99% accurate. All i've noticed so far that is different is that one of the signal input connector pins is unused."

Here are the schematics

Bass Magnifier Power Amp_Bipolar (2002) {Scheme}.zip
Bass Magnifier ABM300_500 Complete Power Amp_MOSFET (1999-2000){Scheme}.zip

Thank You In Advance!

Earache

[g1]

There are two 3pin connectors shown coming from the power transformer. One feeds the HV supplies (by the T10A fuses), and the other feeds the LV supplies (by the T1A fuses).
Disconnect the one for the HV supplies and see if the light dims.

[Jazz P Bass]

With such a gross short, it shouldn't be that hard to find the problem.

With a discharged power supply, a simple ohmeter check of the basics will help.

The Bridge rectifier should not show a short.

The main filter caps should not show a short.

None of the output MosFets should show a short.

Dig in.

[earache]

G1 - With the amp connected to the light bulb limiter and the HV connectors removed from the PC board, the lite bulb limiter doesn't seem to glow much at all, but the lights on the front of the amp and in the meter turn on.

[Enzo]

Yes, those things run off the low voltage supplies.

Do what was suggested above. Look for shorted main rectifiers, shorted filter caps, and shorted output transistors.

[earache]

Hi Folks - the bridge rectifier and one of the filter caps tested bad (shorted) out of circuit.
I didn't take the mosfets out of circuit, but they showed no shorts across any combination of their legs. Is that an adequate way of testing the mosfets?

[Jazz P Bass]

Quote:"I didn't take the mosfets out of circuit, but they showed no shorts across any combination of their legs. Is that an adequate way of testing the mosfets? "

Good enough for now.

[earache]

I replaced the bridge rectifier and the filter caps and the amp works great now.
Thanks to all who helped!

I am still curious about the failure mechanism behind this problem. What happened , did the capacitor fail and take out the rectifier, or vice versa? And why did either part fail to begin with?