Smoking Mark IV (in a bad way)

[trem]

Ok, I was playing the amp for a few minutes, then I put it on stand by as I had to ease myself. When I returned there was smoke coming out of the chassis. The LEDs on the facia were blinking, and there was a loud hum coming out from the step down transformer (I'm using 220V and the amp is 110V). My suspicion is there was a voltage spike.

Took it to a tech who is a fellow board member that afternoon. Opened her up and visual inspection revealed nothing. Caps check good (was meaning to get those replaced). No burn marks on the pcb. OT's not shorting. Screen resistors were fine. The fuse installed was a normal 5A instead of 3A sloblo (yeah I should've checked, stupid me), and it was intact. Didn't check tubes for red plating, as I was afraid to turn on the amp again. Tubes are Winged C 6L6GCs from Doug. It was getting late so I had to go.

If you guys have any suggestions about what else we should check, please please chime in! Didn't take any internal pic to show you guys though. Thanks a lot in advance.

~trem

 

[Buddy]

Use your schnozolla.

Usually the smell can tip you off , a burned resistor or speaker has a
pretty definitive aroma . A foil trace, more acrid and stenchy .
When a cap vents , you know it if you smell it . Oily and cloying .

With the cooling fan in these amps a lot of smoke has to be liberated to even see it .
I would wack some components with a chopstick , A resistor can smoke with little visible damage , then crumble before your eyes with the application of bamboo .
Luck to you .

 

[trem]

Thanks for the tip buddy. Will try it. Anyone else?

 

[ANIMATED SUSPENSION]

Have you checked the step down transformer too ?? If that was humming it sounds suspicious to me.

 

[boogietone]

I had a MKIV start smoking due to a power tube going out. I was there to witness the event though. Loud noise and an arc of blue. That unmistakable smell of burning electrical components. Then nothing really. Mine was just a bad power tube. Couldn't find anything at all any where else that was bad. I am sending it to M.B. to have it checked out though. This was a while ago and the thing works great after replacing the tube. Anyways, don't know if this helps but thought I'd share just in case. Also, I thought the MKIV A version had a 3 amp slowblow fuse and the MKIV B revision had a 4 amp slowblow. I'm not near my amps to verify this but I certainly don't recall a 5 amp fuse. :?

 

[trem]

Have you checked the step down transformer too ?? If that was humming it sounds suspicious to me.

No we haven't, it's in the agenda. It may be the culprit, but I'm pretty, well partly, sure the smoke came from the chassis. Thanks Animated.

I had a MKIV start smoking due to a power tube going out. I was there to witness the event though. Loud noise and an arc of blue. That unmistakable smell of burning electrical components. Then nothing really. Mine was just a bad power tube. Couldn't find anything at all any where else that was bad. I am sending it to M.B. to have it checked out though. This was a while ago and the thing works great after replacing the tube. Anyways, don't know if this helps but thought I'd share just in case. Also, I thought the MKIV A version had a 3 amp slowblow fuse and the MKIV B revision had a 4 amp slowblow. I'm not near my amps to verify this but I certainly don't recall a 5 amp fuse. :?

I really do hope it was just tubes. Too bad, it was sounding really good before this episode. I had a few RCA 12AX7As in there, and I managed to dial in a great R2 and lead sound. About the fuse, yeah it should have been 3A. I didn't check when I got it from the previous owner. Thanks boogietone.

 

[lento]

As for the humming from the step-down, it could be the laminations.
The steel alloy sheets, on which copper wire coils are wound, can get noisy over time.

Lyn