Ashdown MAG 250 - blowing fuses

[audiopete]

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

Kinda clever - short both power tube grids together to cancel out the AC signal.

[audiopete]

Ashdown contacted me - suggested I change the relay and replace TR1, TR2, TR4, TR5. They said I could just remove the relay for now. I did all that and fired it up and same fault. Waiting for next suggestion.

[audiopete]

So I finally spoke to the owner of the amp and we cleared up the entire situation. Bad communication between parties (owner and borrower) and nothing to do with me. He spoke to some others who vouched for me 100%, so he is cool with me servicing the amp and on we go. I did help him believe the borrower of the amp had nothing to do with causing this fault. It did actually blow the fuses prior to amp being borrowed but because it's intermittent, the amp worked again for awhile after the fuses were replaced. Everything is much better now for all involved. Whew! C'mon on Ashdown, let's fix this thing!

[audiopete]

Ashdown told me, and I quote, "It would appear you have one that will not play ball for some reason" and advised me to get a new power board from KorgUSA. I guess that's it then...

[audiopete]

Obviously I am not satisfied with this and Ashdown is down a few notches in my opinion. To think the service tech's best answer is "it's broken for some reason, buy a new board" doesn't sit well with me. I think I am going to swap some more transistors and see what happens. KorgUSA hasn't gotten back to me yet., and a new board probably isn't cheap. I already have new Ecaps in this board and I have the transistors in stock so I am thinking I'll roll the dice and start subbing parts.

Question - where would you start? Output transistors/VAS/dif input or the other way around? Think I will swap the thermally coupled bias transistor for sure.

[J M Fahey]

I think you are chasing your tail / boxing your shadow / chasing ghosts.

1) you should have posted your problem here.

Saying "what I wrote on another thread" and having to go back and forth just to see what are we trying to solve is sort of annoying.

So here it is:

Hi everyone, I would like to add to this merry little thread.....unfortunately....

I have a Mag 300 that is doing the same thing. When I got it both T5A internal fuses were blown. I pulled the power amp board and there was a burn mark on the chassis underneath from the negative leg of C9 (I will have to look at the board and confirm that). The cap leg was getting close enough to the chassis to arc.

Ok, that's problem #1.
And what the customer brought it for.
Looks like you solved it.
Next!!!

I replaced all electrolytics.

Why?

and fired it up on no load on the limiter and it scoped perfect with signal.

Good .

I put it on 8 ohm load on the limiter and it immediately pulled limit with DC on the output. I cycled the power and it came up fine and showed nice AC on the load when I plucked a bass. I re-cycled and it would randomly come up good or limit out.

That's not actually "a problem"
Many amps "wake up stupid" and if both loaded and through a limiter they never self center but stick to one rail.
Of course all that DC through the speaker pulls 1 or both rails down, lamp stays ON, etc.
Personally I care to correct that in my amps, but many famous brands (including SWR) don't seem to bother.
Oh well.

That is where I left it yesterday. I'm going to pull and test TR1-4 and do some more investigating. I also want to look at the rectifier and C17-19.

If amp blocks only as described above, meaning with load, through limiter, don't worry, just pack it and write your bill.
FWIW it's not a relay problem (although that hard muting system might have helped hide it somewhat) and a new board will behave exactly the same ... unless they redesigned it which I doubt.

As of the low level 17kHz oscillations they seem to have been there all the time; personally don't like that but if amp worked that way for ages and customer never complained about that, don't worry.
At least Ashdown doesn't either

I'd suggest you play the amp a little, load it with 8 and 4 ohms speakers, leave it on for a couple hours , so you are certain it works, and if so, deliver it.

I pulled each AC PT secondary from the power amp board and it limits out with either secondary attached. It also limits with the center tap pulled. Once I disconnect the power board, the preamp and fan come up fine.

See above.

[audiopete]

I changed Ecaps (on the power board only) because one of the 2200uF that was arcing to the chassis was testing bad. I did all the caps because they are cheap and I don't know how they were affected by the arcing and just to be safe.

Are you saying that it is going into fault because it is loaded AND on the limiter? It seems to behave that way. If I power it up on regular AC outlet with a speaker cab it should come up okay? So the arcing cap was blowing the AC secondary fuses and the limiter is causing this amp to malfunction on the bench. I never considered this scenario. Now that the cap acring problem is fixed, it should be working. I will check it out.

My apologies for not posting the relevant info at the start of this thread. I can see how that would be a pain.

[Jazz P Bass]

Lamp limiters have a 'limited' purpose. (pun intended )
They are best used when powering up an amp in an unknown state of repair.

If there is not excessive current draw, it's time to remove them.
Especially on solid state equipment.

In the other MAG repair post, one condition that was odd was the fact that the amp would power up fine without a load.
With a load, sometimes it would stick to a rail.

Without a load & powered up fine, if the load was then connected & all was good.
Hmm. There's that goofy relay circuit again.

[audiopete]

The amp has been working fine on a regular outlet with 8 ohm cab and sounding great all afternoon. It makes sense now when it's all laid out, but I'm no EE, nor do I have the experience with solid state amp repair/design that some here have. So glad they are here and sharing/teaching. Thanks to all who helped. I owe you all a drink of your choice!

I see how you can get spun around now. The amp was blowing fuses from the arcing, which also damaged the cap. The new caps (with shorter legs) fixed it, but it looked like there was still a fault because the limiter was causing improper power up. From now on i'll power up with the variac after a repair.

[g1]

Unfortunately, you can get similar problems with the variac.

[Jazz P Bass]

Yeah, you have to know when to separate the tool from the crutch.

[audiopete]

Guess I just risk the fuses....I've got more You guys are the best.

[audiopete]

Amp is rocking and Ashdown followed up with me again so all is good with the customer service from them.

[audiopete]

Amp is back with the same problem. Two blown fuses on the power board.

I pulled the board and flipped it and my eye went right to what looks like a cold solder on emitter resistor R20. What would happen if one emitter resistor opened up under load? Would that cause the power amp to go unstable and blow the fuses?

It's funny how I never noticed that cold solder before, but it was literally the first thing my eye went to.

[audiopete]

I touched up all the solder joints under magnification (there were a few bad ones. One of the AC lugs to attach the transformer leads to the board was wiggling in the joints), replaced the fuses, and R10 burned up. I'm certain I didn't short anything with solder but I will double check. So why would R10 burn up? Could this be the intermittent fault finally gone bad that was causing the fuses to blow before?