Mark III broke down

[arnek85]

Hello guys,

Last wednesday my all time favourite MK III blue stripe suddenly began to hum, then lots of smoke passed out of the eq-slots and finally it completely died.
So the next day I opened it and saw that a few resistors were burned. Now I just want to change the damaged things, but it wont't make sense, when I'm not gonna find out the reason, I guess.
Maybe somebody has allready made experiences with those kind of problems?

The last 10 years the boogie made a so reliable job. It's a fully loaded blue stripe with 4 6l6´s. 3 years ago I changed the inner tubes to TAD tubes. The outer tubes are all original Boogie tubes and I guess the rest should be also completely stock.

Anyone an idea?

Cheers!

 

[MK IIB]

Possibly a tube has blown and taken the fuse with it too.
Have you looked at the fuse to see if it's blown or at the power tubes to see if there is any damage to any?
You could try replacing the fuse(with the correct value)and remove all the power tubes and unplug the speaker and then power the amp up.This will eliminate the power transformer and the electrolytic caps.
If the amp powers up and stays on with the power tubes removed then it's probably a tube related problem
If one of the tubes looks blown remove it and try the amp with just the one matched pair installed.

 

[McBarry]

Those resistors/diodes come off the 65v transformer lead, and look like the bias supply - given your symptoms and:
solder on the middle resistor/diode pads is burnt but solder on the eq resistor/diode is OK.
heat from bias resistor cooked the eq resistor..and heated the LDR supply resistor nicely too..

What to do ??
It's tech time to get on a bench and start hunting the cause - something drew a ton of current...
It's likely the eq resistor failed - hopefully didn't take out anything downstream in the eq.
Also, hopefully the big 680/1W baby which feeds the eq switching, treble shift and other LDR's didn't fail and damage any of these parts.
Access to them is a ***** and if I was doing the job I'd pull the board..

Sorry to be bearer of bad news..
May well be a simple repair, but pulling the board will take a bit of time(read labor cost...)
Cheers, Dave B

 

[arnek85]

Thank you both for your informative comments!
I'll bring the amp to my tech next week, in the hope that he won't need to pull the board.

Those resistors/diodes come off the 65v transformer lead, and look like the bias supply - given your symptoms and:
solder on the middle resistor/diode pads is burnt but solder on the eq resistor/diode is OK.
heat from bias resistor cooked the eq resistor..and heated the LDR supply resistor nicely too..

What to do ??
It's tech time to get on a bench and start hunting the cause - something drew a ton of current...
It's likely the eq resistor failed - hopefully didn't take out anything downstream in the eq.
Also, hopefully the big 680/1W baby which feeds the eq switching, treble shift and other LDR's didn't fail and damage any of these parts.
Access to them is a ***** and if I was doing the job I'd pull the board..

Sorry to be bearer of bad news..
May well be a simple repair, but pulling the board will take a bit of time(read labor cost...)
Cheers, Dave B

 

[McBarry]

Respectfully, I'd caution saving $'s by not pulling board..

Those eyelets will be loose courtesy of being cooked.. they'll pull up unless U retain them with a shroud.

Sure, a dentist mirror will tell if there's tracks underneath.
But pulling the board is the only way to effect a quality repair.

Spend the bit extra $'s.. it's worth it..!

My 0.02worth only of course, Cheers, DB

 

[fifa17pointer]

Thank you for sharing, very helpful to me, thank you.

 

[qbju]

Great work.


_______________________________________
Lucky Patcher v5.6.8 Apk [LATEST Version]

 

[McBarry]

Hi Arnek85 - what news of the repair ??

Regs, Dave

 

[arnek85]

Hi Arnek85 - what news of the repair ??

Regs, Dave

Hi Dave and thank you!

sorry, didn't recognize your last post.
I changed the burnt resistors and diodes, the fuse and all power tubes. Didn't pull the board. For a while it seemed working fine. Played some shows with great tone. But sometimes at powering on, the amp immediately blows a fuse. I made another post:

http://forum.grailtone.com/viewtopic.php?f=27&t=74338

I guess the problem hasn't really been solved. But now I'm using fuses with lower value. Didn't remember the value of the fuse when the amp was burnt. But maybe the value was too high, what led to a way bigger problem.

Cheers,
Arne

 

[darkbluemurder]

The correct fuse value for the mains fuse in Germany with a 230V mains line should be 2A slow blow. For a 60W model 1A would be sufficient.

Cheers Stephan

 

[arnek85]

The correct fuse value for the mains fuse in Germany with a 230V mains line should be 2A slow blow. For a 60W model 1A would be sufficient.

Cheers Stephan

Hi Stephan,

the rear panel of my Boogie says 1.5A. I'm using 1.6A slow blow fuses from my electronic dealer.


Within next days I'm going to change the filter caps.

 

[darkbluemurder]

1.6 A should be fine in that case - you can keep it.

 

[McBarry]

G'Day Arne,
Stephan nailed it - SLOW-BLOW fuses should be your first priority..
Theory being they can absorb a short burst over the rating..

If it still happens, it's bench time to record what's going on and begin diagnosing..

Good luck - you're def on the right path..

Cheers, Dave