Ashdown MAG 250 - blowing fuses

[Jazz P Bass]

Start over.

If the one output transistor emitter resistor had no or an intermittant contact, then it's paralleled brother had to do all of the work.

Look for blown outputs.

[audiopete]

What path do think would have to open up (short out) for R10 to burn? TR7 and TR8 to ? I guess I'll find out with the ohmmeter. This never happened before with this amp. Here's the power amp schem again.

[Jazz P Bass]

Schematic.

ASHDOWN_Mag250 Pwr Amp.pdf

[audiopete]

That's weird, the schematic did post. I still have it open on another tab from my own post. Thanks for that.

I looked again and I bridged the collector and emitter of TR7 when I retouched all the solders So I blew TR6, R10 and popped C7 for a nice path to ground through the relay (NC). I checked around and there doesn't seem to be any shorts on other transistors, including the outputs. When I pulled TR6, R10 and finally C7, the negative rail to ground continuity on the board disappeared, so that's good. Now I know why R10 burned and now I recognize the little pop I heard right before R10 started smoking - C7 going short (and bulgy). I am pretty confident it will be fine when I replace those parts and I am pretty sure those cracked and cold solders (especially on the emitter ballast) were the cause of the fuses blowing intermittently - the original problem here. I'll update once I get it all sorted and tested. Cheers and thanks, sorry for the big waste of time.

[audiopete]

This amp has been running fine the last couple days, so I think I can finally close the book on it. Thank you substandard Chinese manufacturing.

[JohnMagBass]

Good day members, I am John from Cape Town South Africa. I recently purchased an Ashdown Mag300 4x10 of which the transformer was dead. The primary coils were burnt. I carefully removed the Mylar tape insulation and the coils. I joined the wires and using heat shrink to insulate the soldering joints. I successfully rewound the primary and secondaries, it tested perfectly with the limiter lamp in place, giving the desired voltages 50-0-50 and 16-0-16. Upon reconnecting with the amp, it played for a while and then suddenly popped the rear fuse. Limiter lamp connected, it stays bright. After checking every single component on the board, everything seems to be fine, but something is causing a dead short. Yesterday I replaced the output transistors, new 4700uf Caps, bridge rectifier, Polyester Caps, new diodes, all small transistors on the input stage, as well as the C4382 and A1668 transistors, together with the 4 big 0.33ohm resistors. I had the lamp connected, swithed on, it came on bright briefly and then slowly dimmed. After a while I placed the head into the cab, connected the speaker, plugged my bass guitar in, it work quite well. The EQ controls were set halfway. A few minutes into play, I heard a buzzing sound, something exploded inside and the amp died. Upon inspection I found resistor R12 470ohm popped as well as the fuse on the right side of the rectifier. This morning I connected it with the lamp again, new fuse, without replacing the resistor, the lamp came on brightly. A few moments later i saw a spark at TR3 A1294, then TR8 A1668 burnt. I am thinking of putting back the old diodes, small transistors and the polyester caps, replace the burnt transistors and resistor and see how it goes. If anyone in this group might have a similar experience, please do assist. I also have another Ashdown Mag300 4x10 that is working perfectly, the PCB's and components are exactly the same. I can't for the life if me get my head around this problem. Please note that I am nowhere close to be an electronic specialist, just a hobbyist who loves taking on new challenges. I am looking forward to the replies. Thank you so much, greetings from a sunny Cape Town, South Africa.