Ashdown ABM 500 EVO Issue

[Billy Penn]

Hi all. I have an Ashdown ABM EVO that blows the mains fuse. I had the amp open and noticed that TR4 was burnt. I tried replacing it but it burned again. Maybe I need to replace TR4, TR5 & TR6? Has anyone ever had this experience before? Any input would be greatly appreciated. Thanks!!

[nickb]

These should help:
ashdown-abm500-bass-magnifier-schematic.pdf
ashdown-abm500-preamp-evo11-2003.pdf

Best to replace TR1 thru TR6

[Billy Penn]

Thanks for the reply and the schematics. Why do you think I would need to replace all 6? Do you think anything else would need to be replaced?

[nickb]

That should do it. There is a risk they are all damaged and it's just easier to replace all of them at once. When you power up, don't connect a speaker at first. Check that the voltage across the speaker terminals is less than 0.1V before you do.

[Billy Penn]

That should do it. There is a risk they are all damaged and it's just easier to replace all of them at once. When you power up, don't connect a speaker at first. Check that the voltage across the speaker terminals is less than 0.1V before you do.

Is there a typical reason for these to burn up? Also is there anything else I should replace? I'd hate to replace all six and have them burn up. They are about $13 each.

[nickb]

There are some suggestions on alternatives here that might help http://music-electronics-forum.com/t9948/

If you really want to attempt to replace the minimum you could check TR1-6 for drain/source shorts (pins 2 and 3) with a meter on ohms range Replace only those that are read low. Handle any replacements with care as they are static sensitive. Start with only TR1 and TR4 in circuit. Bring the amp up slowly, no speaker, with a variac if you can while monitoring the power supply current, power rails V+ and V- and the voltage at the speaker terminals for <0.1V, a lamp limiter is second best.

If all looks OK, plug in a speaker, power up again and test at low volume. If that passes you can repeat with TR5/TR2 and then TR6/TR3. If not OK we'll need to troubleshoot that.

Edit: I forgot to say,the most common reason for blowing up is a short on the speaker leads. There is no current limiting in this design so those output transistors will blow almost instantaneously.

Another Edit: When checking the output voltage and bringing up with a variac the output relay might not be engaged so you should monitor at the junction of D3/Z1 for output voltage. It might do odd things until the V+/- rails are high enough for correct operation.

[Billy Penn]

I see now that it is easier to replace all at once. Thank you for your help. I'm more of a tube guy and don't do much with SS amps. I will check the alternatives and order the parts. Then post when I get it back up and running. While I'm ordering is there anything else I should replace? Any other parts failure reason that any of the TR's would burn? Thanks again!

[nickb]

...is there anything else I should replace?

Well, here's a trick I use sometimes. You can remove the output transistors and link the feedback path, check for <0.1V on the output and check the power rails, no speaker. This will tell you if the rest of the amp is OK for a DC point of view before you pop in the replacements. See attachment on how to do this.

is there anything else I should replace?

If you do the test above, you'll know better.

Any other parts failure reason that any of the TR's would burn?

Undoubtedly! Any component failure on that main output that results in a low resistance path will do it. Power supply problems can do it do. Best you check for these before popping in the new parts. And I'm sure there's another ten reasons too.

The most important thing is to bring the amp up gradually as you don't want to destroy your new FETs.

[Billy Penn]

Well, here's a trick I use sometimes. You can remove the output transistors and link the feedback path, check for <0.1V on the output and check the power rails, no speaker. This will tell you if the rest of the amp is OK for a DC point of view before you pop in the replacements. See attachment on how to do this.[ATTACH=CONFIG]21894[/ATTACH]

If you do the test above, you'll know better.

Undoubtedly! Any component failure on that main output that results in a low resistance path will do it. Power supply problems can do it do. Best you check for these before popping in the new parts. And I'm sure there's another ten reasons too.

The most important thing is to bring the amp up gradually as you don't want to destroy your new FETs.

Ok, so remove the FETs, jumper at the yellow highlighted red line, read .1v AC or DC at the output jack? Then check the V+ and V- power rails? Do I have this correct? Probably get back into this later today or early tomorrow. Thank you so much again!

[nickb]

Ok, so remove the FETs, jumper at the yellow highlighted red line, read .1v AC or DC at the output jack? Then check the V+ and V- power rails? Do I have this correct? Probably get back into this later today or early tomorrow. Thank you so much again!

DC, not AC, and no signal going in.You got it

[Billy Penn]

Ok, thanks again. I will try to post results today. Drumroll.....

[nickb]

Are you aware of these equivalents ECX10N20R and ECX10P20R ? Quite a bit cheaper.

[Billy Penn]

Yes, much cheaper. Wonder what the shipping is from the UK and how long it would take....

[Billy Penn]

Ok, so remove the FETs, jumper at the yellow highlighted red line, read .1v AC or DC at the output jack? Then check the V+ and V- power rails? Do I have this correct? Probably get back into this later today or early tomorrow. Thank you so much again!

I did exactly as above and the amp fired up and I got .98mv DC at the speaker jack. Also got 182v DC on the rails. Does this mean that all else in the amp is ok and all I need to do is replace the FETs? I also used my light bulb current limiter and got no light... Thanks!!!

[nickb]

Looking good, Billy. Go for the FETs. I still advise bringing up cautiously - no speaker. Very carefully check there are no solder blobs or anything wired other than it should be before restoring power. Don't forget to remove the link you put in for this test.