Ashdown MAG 250 - blowing fuses

[Jazz P Bass]

Looks good.
Now you have to figure out why it goes stupid.

I would disable that crazy relay circuit on the base of C7 to see if that is the cause.

[olddawg]

Looks good.
Now you have to figure out why it goes stupid.

I would disable that crazy relay circuit on the base of C7 to see if that is the cause.

Freeze mist and a heat gun? They were always my first choice in isolating intermittent problems. That and the "Enzo whack method", lol. Just do it with no load probably.

[audiopete]

I'll pull C4 and C7 and continue from there.

[audiopete]

Pulled C4 and C7, fired up with 8ohm load and it came up fail.

Pulled the load and the amp returned to working.

Powered down, plugged in the load and the amp powered up working.

Cycled power, came came up fail.

Cycle power - fail.

Power down, pull load, power up - working.

It continues like this seemingly randomly.

Another observation is the heatsink for TR16/TR17 get warm while heatsink for TR3/TR11 remains cold when in fail mode.

I see a low freq (200 - 400 hz about) sine wave on the output when in fail mode.

Do you see this as a transistor issue, because I can change them all and be done with it? The owner is getting antsy. If it's something else then obviously changing transistors is a waste of time and money.

[audiopete]

It never fails with no load.

Freeze mist and a heat gun? They were always my first choice in isolating intermittent problems. That and the "Enzo whack method", lol. Just do it with no load probably.

[Jazz P Bass]

Have you read this full thread?

Same issue.

[audiopete]

Have you read this full thread?

Same issue.

No link

[Jazz P Bass]

http://music-electronics-forum.com/t28046/#post368670

[audiopete]

I did read that and actually there's a post from me there before I started this thread. Doesn't seem to be a real solution, or a cause determined, although the suggestion of checking the ground is one I will investigate. Those symptoms sound exactly like mine, right down to the positive supply output transistors passing big current and the negative side not passing.

What could set up the oscillating? Maybe that's the result of one side of the amp taking a dump. Could the bias circuit be causing this to happen somehow?

I may have to contact Ashdown to see if they have figured out what is up with this.

[audiopete]

I put in an email to Ashdown, hopefully they have a solution.

[Jazz P Bass]

It seems to me that monkeying with the On timing of the base voltage of TR8 did the trick.

Maybe have Ashdown 'explain' that relay circuit.

[audiopete]

I removed C7 and C4 so I don't see how that circuit is affecting the amp module now. The other poster put a 22uF at C5 to delay the FET from turning on longer, but still, that relay isn't even in circuit anymore.

On a side note I'm not sure if I will be continuing on this amp anymore. I got it from a client who borrowed it from someone else. The owner called me out of the blue last night angry and belligerent that I am in possession of stolen property. I laughed and explained the scenario but he didn't seem to listen. He also tried to blame my client for breaking it, but already admitted that the amp blew the fuses on him once before he lent it out, and he just replaced the fuses and said nothing. I had to call the Police after his threatening phone call and today I don't know what is up. I want to fix it and get paid, but on the other hand.....

[Jazz P Bass]

The circuit needs C7 & C4 to properly turn on TR8.
Don't know why.

[J M Fahey]

Really?
That's weird.
C4 is a small 100pF cap which is there only for stability purposes and C7 is used to ground drive to driver and power transistors, working with the relay as a mute/standby switch.

When you turn amp on, it's muted for a few seconds to minimize turn on thump and is muted again if thermal switch opens.

As crude and backward thinking as that Silvertone "standby" which only killed power amp drive signal.

Wait a second, Mesa also uses something like that, so it must be a good thing
I bet they even patented it

FWIW I much prefer a real Macho relay at the output, which if needed can keep DC away from the speakers.

[audiopete]

That's what I figured. The 10uF cap attenuates signal to it's filter cutoff point to minimize thump. Only AC, not DC. Low enough frequencies would pass by that cap on to the base of TR8. The thermal switch cools because of signal attenuation and closes and the relay opens and attenuation ceases. I figured C4 is to kill switch noise from bouncing contacts.

This mystery may never be solved. The person who dropped off the amp is getting it back for the irate owner. He is paying me for work performed already. Bummer for the amp, but what can you do? Can't even figure out why the owner is upset if he is getting his amp fixed and doesn't have to pay for it.

Thanks again everyone. If I do happen to chat with Ashdown and get more info, i'll post an update.