New to me Mark ii B blowing fuses

[brion]

Bummer. I received a MarkIIB today and it blows the fuse after 3 seconds with only the power switch being turned on. I removed the chassis and power tubes, replaced the fuse and plugged it into the speaker. I turned the power switch on and a 220 Ohm 1/2 watt resistor smoked on the Board with the filter caps on it. I didn't leave it on long enough to blow the fuse as I turned it off after the smoke. This resistor looked good before I turned it back on. I sent a message to the ebay seller and I'm waiting to hear back. I could take it to the local Boogie authorized shop or send it to boogie. I'm thinking on sending it to boogie to insure a complete fix. I've recapped some old fenders, but just don't want to tackle this one as there are caps on top and on the bottom of the board as well as the silicon diodes and a handful of resistors. Anyone have any suggestions other than send it in for service?

 

[McBarry]

In no particular order and this is not an exhaustive list.., look for shorts on boards (pre and PSU), looks for loose leads shorting to chassis.
But, more specific to your symptom, does yours have GEQ - my diag shows the 220R feeding -56v to the EQ. If this is correct for you, should be an easy diagnosis.
I'd look at the transistors, then the 10uf and both 220uf electrolytics.
download the schematic.
Goes without saying be careful inside there, nasty surprises for the unwary..
DB

 

[brion]

I have the 220 feeding the GEQ too. It is a DRG. I have experience with tube amps such as changing filter caps on my Vibrolux and building a 5E3 circuit successfully. I use one of those Weber 'sticks' to drain the residual charge from the amp. I notice that the silicon diode connected to the 220 ohm resistor reads 0 ohms in both directions. I didn't check the 10uF or the transistors on the GEQ. I'll check it out tomorrow if I get the time. Thanks.

 

[brion]

I found one of the small metal bodied capacitors on the back of the EQ .15mF touching a trace that the rest of the similar capacitors were arched over. This trace is connected to a grey or light blue wire that connects to Q3, a 10pF cap and a 3.3K resistor on the EQ circuit board on the side of the chassis. I'm not sure if this was the root cause of the resistor burning up, but it seemed out of place. Now I need to get a 220 Ohm resistor, replace it and see what happens I guess. I'll hit radio shack tomorrow and see if I can try to get this amp running.

 

[McBarry]

Hi, pics would be helpful,
get a schematic if uUhaven't already, I'd be careful with the transistors also just in case they're hard to find..
I'm not sure exactly which cap was touching what, and what part of the cap was touching.. Any burn or heat marks visible?

 

[brion]

I have the schematic. The cap is the second one to the left from the EQ foot switch on the schematic, the .15mF one. The metal casing of the capacitor was touching a trace that runs under the row of capacitors. All other capacitors were arched over that trace, but this one was pushed down onto the trace. The trace connects to a light blue or possibly grey wire that travels to the other side of the amp and connects to the EQ board with the transistors on it. I do not see a burn mark on the capacitor or the trace, but there is very little space to see under the cap that was touching. If the case is attached to ground and it grounded out the -30.6V on that trace, maybe that is why the 220 resistor burnt?

 

[brion]

Called Boogie today and I'm going to send it in for repair, general maintenence and to see about getting the effects loop mod that is frequently discussed here on the forum.

 

[McBarry]

Sound good, pls let us all know what the problem was - I'm interested to hear..
DB

 

[brion]

Will do. I'm planning on shipping it out by Saturday.

 

[brion]

Mike B found a Filter cap under the board touching and shorting the 220K resistor that feeds the EQ. I had him replace the Filter caps, do a general cleaning, replace the Power tube sockets as one was messed up, he found a few pots to "rebuild" whatever that means and he lubed the EQ sliders. I should have it back next week. I'll report back once I get it.