Ashdown Evo Mag 300 pulling current

[Mr_bibbles]

http://bee.mif.pg.gda.pl/ciasteczkow...G300_schem.pdf

Hey y’all,

Have a strange issue. Working on an Ashdown Mag 300, shows bright on the limiter WITH load, but not without. About 70 mv DC offset, but measuring 54V AC at the output at idle... this seems off.

Replaced the OT trannies with 2sa1943/2sc5200, still having the same issues.

Does any of this seem off? In my experience, failure when connected to output has been DC offset, indicating half a burnt ot section.

Thanks.

[Mick Bailey]

Measuring AC at idle means oscillation. DC offset is OK. Are R18/C16 and their solder joints OK? I would disconnect PL1 and ground the input to the power amp to see if the problem still persists.

[glebert]

Someone had given me a non-working Evo 300 a while back with rail to rail AC oscillation. My poor little bench speaker didn't even know what hit it I wish I could tell you how to fix it, but I spent a few hours on it and couldn't get it figured out and scrapped it. I remember not being able to see what was happening with the output waveform because it was too big for my fast digital scope and too fast for my slow analog scope. Should have made a voltage divider for it. Crazy waveform with very fast rise/falls and sharp peaks.

[Mr_bibbles]

Mick, you’re definitely onto something. AC disappears when I disconnected the preamp board, grounding the input has no effect though. Resoldered c16/c15 and everything around r18 to no effect. Solder points look pretty good overall.

[Enzo]

So scope every power rail, both high and low voltage. Any show signs of oscillation? Or are they all smooth DC?

Look on your schematic at the output. Should find a small cap and resistor in series across the output. My drawing says 4.7 ohm 4W resistor and 220nf cap. Is your resistor open?

[glebert]

Mick, you’re definitely onto something. AC disappears when I disconnected the preamp board, grounding the input has no effect though. Resoldered c16/c15 and everything around r18 to no effect. Solder points look pretty good overall.

So if AC goes away when you unplug the preamp, do you see any AC at the Line Out? Maybe also look at make sure the Output pot hasn't gone open, I have had amps oscillate with an open master volume. Maybe look at the powerup mute on the preamp output too?

[Mr_bibbles]

Thanks Enzo. I don’t have a scope, but I do have an audio probe. The resistor is intact and measure a pretty well.

Perhaps there’s an other way to check the rails??

[Mr_bibbles]

Thanks Glebert! Will check those and get back.

[Mr_bibbles]

So, Output pot measures well, no AC on the output DI or the turner line output. The mute seems like it functions, No AC on the output until the first relay click during startup.

[g1]

shows bright on the limiter WITH load, but not without.

Are you still on the bulb?
If so, recommend you try connecting the load only while unit is turned on (when using bulb limiter). Power up without load, then connect load without turning unit off.
The symptom of AC at the output is not typical for this type testing issue, but it's worth a try anyway.

[Mr_bibbles]

Hey G1, good thinking. I’ve already done that and that’s what made me test for AC at the output in the first place... I saw an arc across the speaker cable when plugging it in! As soon as you add a load the bulb goes bright, just as with a failed OT.

[g1]

I don't think we are looking at the right schematic, so it's unclear when you call out component numbers.
Your unit does not have FET outputs does it?
Maybe the preamps are the same, but it needs to be verified. No bi-polar output devices in schematics from post #1.

[Mr_bibbles]

I think it is the correct schematic, I could be wrong though.

Page title: ABM output board
Project: Bass Magnifier

There’s text at the bottom to leave out tr3/tr6 c21/c211 on the 300 watt version, this is definitely the “Mag 300”. The 400/500 watt version pcb: acd001, but my board is APC041. It seems very similar but some things are off, I’ll look around.

[Mr_bibbles]

Here, found the correct PCB version:https://music-electronics-forum.com/...3&d=1371880709

And another similar thread!:https://music-electronics-forum.com/...ad.php?t=28046

[Mr_bibbles]

Found it! Now that we have a correct PCB schem, I went back to the other suggestions by Enzo, checked the ACTUAL resistor/cap he meant, (r3/c3). They measured fine, but there was no continuity to ground, so I traced it all the way back to the preamp board where I found a lifted trace with a little discoloration.

After reconnecting the ground, no AC on the output! Plugged in a load and it sounds fine.

To further my understanding, am I right I guessing that those components are part of the NFB? A disconnected ground here would inject the oscillation straight to the speaker while leaving the rest of the amp alone?