Ashdown MAG 300 PA

[crane]

Hi guys!
I'm having problems with my band mates bass amp head.
It stopped working one day.
I found that one of the driver transistors was dead (it was not short circuited but hFe could not be measured) so I changed that. The fuses were blown and some 0R resistors as well (they are used as jumpers on the PA pcb).
This far it was straight forward but now the frustrating part comes.
If I turn it on (with a light bulb in series with 220VAC cable) without any speaker load and attach speakers later - it works fine. If I turn it on with speaker connected - it trys to jump to positive rail voltage (the light bulb limiter does not allow it to do that). Judging from the burned 0R resistors it has been run with only one rail connected for a while. All the trannies are fine now (desoldered and measured hFe for all of them).
Has anyone expirenced such a strange PA behaviour?

[J M Fahey]

You still have more dead transistors.
Ask them for the schematic or try to find it online and post it here, or we are stabbing at the dark.

[crane]

Check the attachment.
I'm pretty sure that I've checked all the trannies and all the jumper resistors (they are not shown in the schematics)

[J M Fahey]

*Many* (most?) amps can not be turned on with load connected, they tilt.
Yours *seems* to be already repaired , but let´s recheck for safety.
1) turn it on with bulb, without speaker.
Check that you have close to 0V DC at the output (less than 100mV).
Then that you have less than 30mV across the ballast emitter resistors R4/13/20/23
If all fine:
2) without turning off connect the speaker.
You should hear *something* in it: slight hum, hiss, that show it´s alive.
If so, plug yourBass and play a little, at very low power.
If you play loud, it might tilt again.
3) If all normal, plug the amp straight into the wall, no bulb.
Play sometime and check with your digital temperature sensor (your finger) that, say, most transistors are just warm but one sizzles or something like that.
Post results.
And ... good luck  :dbtu:

EDIT: what I find weird is I don't see anu short circuit protection. Strange.
At least on the official schematic.
Doesn't it have an added daughterboard or something like that?

[crane]

*Many* (most?) amps can not be turned on with load connected, they tilt.
Yours *seems* to be already repaired , but let´s recheck for safety.
1) turn it on with bulb, without speaker.
Check that you have close to 0V DC at the output (less than 100mV).
Then that you have less than 30mV across the ballast emitter resistors R4/13/20/23
If all fine:
2) without turning off connect the speaker.
You should hear *something* in it: slight hum, hiss, that show it´s alive.
If so, plug yourBass and play a little, at very low power.
If you play loud, it might tilt again.
3) If all normal, plug the amp straight into the wall, no bulb.
Play sometime and check with your digital temperature sensor (your finger) that, say, most transistors are just warm but one sizzles or something like that.
Post results.
And ... good luck  :dbtu:

Thanx for your help!
Output DC voltage - 20mV
voltage on ballast resistors <1mV
It plays alright if I attach the speaker when it's already on (as I said on my first post).
When I tried it straight from the wall - it was ok the first time. Than I turned it off for a while and it blew my apartment's fuse (sorry - I don't know the correct word in english. probably you don't call them "Fuses"). I'm starting to wonder - may be it's because my fuse is 10A B type. May be C type would not blow?

[J M Fahey]

Ouch !!
Back to square one.
Retest with the bulb again and no speaker.
Obviously still we have other problems. 

[crane]

Is it normal that voltage on ballast resistors is so small - it means that trannies are biased really really cold.

[J M Fahey]

Yes.
Bipolars don´t need more than a few mA to get into the linear region.
Audio (lateral) MosFets are worse and (vertical) Switching MosFets the absolute worst.

[joecool85]

*Many* (most?) amps can not be turned on with load connected, they tilt.

What?  I must be misunderstanding because it sounds like you mean that amps can't have the speaker connected until the amp has been powered up.  Unless it's disconnecting electronically somewhere that I don't know about, any amp I've had works fine leaving the speaker connected all the time.

[crane]

*Many* (most?) amps can not be turned on with load connected, they tilt.

What?  I must be misunderstanding because it sounds like you mean that amps can't have the speaker connected until the amp has been powered up.  Unless it's disconnecting electronically somewhere that I don't know about, any amp I've had works fine leaving the speaker connected all the time.

I think what he meant was that amps tilt for a short period of time when turning on and off but normally they recover from that. Mine doesn't
And yes - there are special circuits that try to mute the amp on power ups and power downs.

[J M Fahey]

Sorry, I was answering at an earlier post.
I meant "with the bulb limiter still connected", which is how the OP was testing it.
Most amps tilt towards one rail, until capacitors charge and they can get back to normal.
If not loaded, it takes a second.
If loaded, it causes such a huge voltage loss (thanks to the bulb) that they do not have the "muscle" to recover .

[crane]

I'm kind of embaressed now... It turned out that the problem was in power transformer which I had recently upgraded (this is rated 600VA). It blew my flat's fuse even with no load connected at all. I didn't know that I have to design a soft start for a transformer bigger than 300VA...
Made a soft start circuit in primary winding and it's fine again.
Thanks everybody for help and comments.

[joecool85]

I'm kind of embaressed now... It turned out that the problem was in power transformer which I had recently upgraded (this is rated 600VA). It blew my flat's fuse even with no load connected at all. I didn't know that I have to design a soft start for a transformer bigger than 300VA...
Made a soft start circuit in primary winding and it's fine again.
Thanks everybody for help and comments.

I didn't realize that either.  Could you share what you did to make it a "soft start"?