Ashdown Mag 300 shutting down

[lowell]

This amp seems to go into thermal shutdown after being played, turned up to about 150 watts of output. I'm trying to wrap my brain around the protection circuit in the amp.

Is TR18 controlling turn-on delay? Maybe TR18 is basically a shorted switch until C13 charges and turns it off. TR17 and TR15 are only turned on when TR18 turns off and the voltage on TR17's base goes up. All of this in turn energized the relay coil which turns the speaker output relay RL1-A on.

I'm thinking SW1 gets hot it opens thus turning the speaker protection relay off.

Just wanna confirm I've got this correct.

[Enzo]

First, identify the problem. Is it really thermal shutdown, or is it just the protect mode coming on? Is the heatsink too hot to touch when it shuts down? Does the amp get really warm just sitting there on but not being played? Is there an indicator light? Is the relay dropping out or is the sound just disappearing? If the relay drops out, scope the output bus inboard of the relay. A convenient spsot is the emitter ballast resistors. Look on one of those, is there a good solid output signal? or is it DC offset or distorted?

If it is a true thermal shut down, then the heatsink will be hot, and the amp will not cycle back on until it cools. This is what freeze spray is for. If thermal is the problem, then a quick squirt of freeze on the temp sensor should bring the amp right back to life. Doesn't solve the underlying problem, but exposes the cause.

Does the fan work? is SW2 present or is it a wire? TH1 is the thermal sensor for the fan, hit it with heat, does the fan speed up?

Monitor the mains current. At idle, I'd expect les than half an amp draw. If it is a lot higher than that, we might have to play with the Tr10 circuit. The bias.

The only protection circuit I see is the speaker relay, and it is thermal only as far as SW1 will drop out the relay if it gets over 100 degree C. 100 degres is the boiling point of water, damned hot. The relay itself could be defective for that matter.

Yes, you have accurately described the transistor control circuit for the power-on delay control of the relay.

[lowell]

Actually the owner says it happens during rehearsal with the band. I had it into my dummy load almost all the way up for a half hour and it had no problem. How would you go about troubleshooting an issue that seems to only happen when the owner uses the amp? My thoughts were speaker issue. I said to bring in the cab.

[MarkusBass]

Have you tried measuring voltage drop on ballast resistors (and calculating the quiescent current)? Is the heatsink hot? I thought that MAG has MOSFETs as output transistors. Is it correct schematic?
You are right asking him to bring the cab. Let him show the problem. How about loose jack in the cab?

Mark

[Enzo]

Or a crappy speaker cord. Or he is using a guitar cord instead of a speaker cord for the cab.


Let me see if I have this straight. The amp works perfectly for you in the shop. The customer reports it shuts down. Customers have a nasty habit of sugesting diagnoses based upon nothing. "Yeah, my amp cuts out, probably a bad transistor." "Yeah, my amp cuts out, probably a bad transformer." and so on. They have no idea. Did the customer suggest a thermal problem? Or did you find it overheating? If we are assuming it really has a thermal problem based on only the customer's notion, that is a wild goose chase.


Not unusual for the customer to mis-identify a problem. "My speaker cuts out." What he really means is the sound stops. It may not be a speaker problem at all. A bad guitar cord can also make the sound stop. But all the customer knows is sound comes out the speaker, and now it don't.

[lowell]

Exactly right. Customers come in ALL day long saying what THEY think is wrong. I've learned to rediagnose myself and then tell them what the real problem is. All I know for this amp is that he says it stops making sound during rehearsal, then it comes back. That's it. I'm at a loss here and all I can tell him is to bring in the cab so I can make sure the speakers are good. I'll also confirm a good SPEAKER cable.

[Enzo]

HAving the whole system is key when diagnosing intermittants. That was we can substitute any portion of the system, and monitor performance as well. And don;t let them get away with bringing in the amp and cab, but leaving the pedal board at home. "I know it isn;t any of my pedals, because it still does it when they are all turned off." Famous last words. Oh, and a personal fave, "It can;t be the cord, it's new."

Ever seen the following? A speaker cord that works anywhere else, tests good. A speaker cab that works anywhere else. But for some reason THIS speaker cord plug does n;t make good contact in THAT speaker jack. Individually they seem to work 100%, but for some reason, the exact point in the jack that touches the exact point on this plug don;t make good connections.

I once had a Crown 300 with 5 fuse holders. Short story is that one of the perfectly good fuses would not work in one of the perfectly good fuse holders. Same fuse worked in the other four holders, and any of the other fuses worked in the one holder. But that particular fuse and that particular holder would not work together.

We always want to listen to the customer report, to learn what we can of the problem. But if the customer throws out the term "thermal shutbown," we can't just focus laser-like on the thermal circuit and ignore the rest.

[lowell]

Wow... sounds like a real hair puller-outer. Yeah I agree sometimes we need the whole system. Unfortunately customers seem to be impatient and irritated when asked to come back in with all their gear. Sometimes I have to reiterate "hey, i'm simply trying to fix the problem for. I'm not trying to inconvenience you."

[Enzo]

Had an SVT through the shop a long time ago. Doesn't really matter what amp it was. It had some issues that I corrected, did mainteneance on it, amp was running great. Guy brings it back a day later - still making noises and cutting out. OK, works for me, but still, I'll go through it again. COUldn;t find anything, played through it fine, played music through it all afternoon. Looks good to me, returned to customer. Next day he's back and steamed, "Do you even LOOK at the amps when you fix them?" Matter of fact I do...

Told him to bring in his axe and cab and cords. Grumble grumble, be he brings in all his stuff.

We quickly identified the problem was loose wires in the guitar - the amp was fine.


At least this guy had the decency to apologize. Usually they don't.


Oh, and the Crown? I called my junior tech over and told him about it. He response was, "That is impossible, it cannot be." SO I demonstrated it for him. He still didn;t believe it.

[lowell]

Sheesh... it really is necessary to put our pride aside in such cases isn't it. Can't be fixing it and saying "told you so." Well actually we could but then we (techs) come off as jerks. I am somewhat of a practicing Buddhist and this job is great because it causes me to practice every day. :-)