Ashdown MAG300 Problem

[Bigdrums]

I have a Ashdown MAG300 amp that when hooked to a load draws too much current (perhaps 2-3 amps) upon ramping on a variac. It was reported to be blowing fuses. I've been using a light bulb limiter for testing. Without the LBL the current rises rapidly using the variac, and has a slight constant hum through the speaker. I have checked all the parts on the board, transistors, diodes, resistors, and caps and can't find any suspect parts. There is no DC offset. There is no AC after the main filter Caps.
Would like to know why the excessive current draw with a load and next to nothing without.
The schematic I have is not the correct one and can't seem to locate the correct version.

Also a curious detail: at the line plug into the back incorporates a fuse holder. Printed on the chassis says FUSE RATING 120V T5AL 250V, 230V T3 15AL 250V. It was my understanding that 220V operation usually required a lower rated fuse due to lower current draw with higher voltage?

[Helmholtz]

There is no DC offset.
Would like to know why the excessive current draw with a load and next to nothing without.

Are you sure there's no DCV at the output without a load?

Also a curious detail: at the line plug into the back incorporates a fuse holder. Printed on the chassis says FUSE RATING 120V T5AL 250V, 230V T3 15AL 250V. It was my understanding that 220V operation usually required a lower rated fuse due to lower current draw with higher voltage?

120V: 5 amps fuse
230V: 3.15 amps fuse.

[g1]

The schematic I have is not the correct one and can't seem to locate the correct version.

Can you be more specific about which schematic you need? What kind of output devices?
If outputs are Fet's, try these schematics: https://music-electronics-forum.com/...etch?id=840042

[Bigdrums]

Are you sure there's no DCV at the output without a load?

Yes, there is no DCV without a load, there is significant DCV with load connected, 15VDC at 30% on variac. Could be as high as B+ variac full up but obviously I can't due to current draw



120V: 5 amps fuse
230V: 3.15 amps fuse.

Thank you for clarifying

[Bigdrums]

Can you be more specific about which schematic you need? What kind of output devices?
If outputs are Fet's, try these schematics: https://music-electronics-forum.com/...etch?id=840042

Great, apparently the one I have matches the EB150 MAG250 Output schematic with 2SA1294 outputs

[g1]

2SA1294 outputs

This schem. has a few additions that may (or may not) be in your unit.

[Mick Bailey]

The amp may be oscillating, but only when the load is connected. This can result in excessive current draw, causing excess PSU ripple (hence hum). Scope the output to check for ultrasonic oscillation. Make sure C11/R22 are good

[Bigdrums]

The amp may be oscillating, but only when the load is connected. This can result in excessive current draw, causing excess PSU ripple (hence hum). Scope the output to check for ultrasonic oscillation. Make sure C11/R22 are good

There does appear to be a significant signal on the output, initially a sine wave of 4ms. Even at 20% at the variac. When I reverse the scope leads it looks more like high freq oscillation
C11 and R22 are both good. I get the same results with the pre amp unplugged from the main amp board. Same when the +/- leads reversed coming from transformer

[Helmholtz]

When I reverse the scope leads it looks more like high freq oscillation

You should use a real scope probe. Probe/scope ground must be connected to amp circuit ground. So you can't reverse scope leads.

Same when the +/- leads reversed coming from transformer

A transformer delivers ACV, so no +/- leads.