Carvin X100B series 4 problem

[geet19]

I've got an X100B that I bought new about 7 or 8 years ago. Recently it had a sort of meltdown. Not sure exactly what failed, but it popped the main power fuse inside of the power cord connector. I replaced the rectifier diodes and the first filter cap and jumpered across the burnt trace with a 14 awg wire, and have managed to get it to power back on without blowing the fuse or anything else failing thus far. The issue now is that, when it's turned to Operate with a cabinet hooked up to the output. It just makes a loud hum with or without any tubes installed. I don't know if I should be looking at the Output transformer or someplace else for this issue. That's also without any input plugged in. I Ohm'd the output transformer and I've got 37 ohms and 34 ohms respectively between each plate and the center tap. The output side has 0 ohms between any of the 4 connections, so I'm kind of leaning towards that as a potential issue. Not sure if that's the problem or not. It's not shorted across the primary and secondary.

Any and all help would be greatly appreciated. Thank you.

[alexradium]

OT secondary is not 0 ohms,usually you have the various taps from 0,3 to around 1 ohm,best way to see if it works is isolate the primary (just leave out power tubes),then feed a low ac voltage on secondary,and measure primary plate to plate voltage,do the math.
I highly doubt a fault there.

[xtian]

Did you clean up that circuit board? It’s hard to see from the photo what is actually burned, and what is just superficial smoke stained. Be warned that oxidized (burned, even just darkened) PCB is conductive, and will weak havoc with your circuit. You have to drill/cut/Dremel out the burned portions of the PCB and reconstruct the traces.

[geet19]

I haven't tried cutting out the portion that got damaged yet. It melted an entire section of the trace off the board entirely. I'll try and cut out the section where the board was gouged by the initial failure and see if it makes any difference.

[J M Fahey]

And as of transformer secondaries *near* zero ohm resistance is the norm, many multimeters will show plain zero.

[alexradium]

edit

[geet19]

So I got it working momentarily. Turns out that when I repaired the trace, I connected the wire to 1 too many locations and tied it into a different trace on the board. Re-did the repair and no more buzzing. Added all of the pre-amp tubes and turned it on. No odd noises or irregularities. Added 1 power tube and, I have guitar sound again! Turned it off, added a second power tube and BOOM! A Cap close to the socket that second tube was in discharges, burning the trace off the board. Blowing the main power cord fuse once again. Completely different location from the initial failure, but exact same type of failure. A weird crackle noise and then a big spark. I don't know if this is the same tube that was in the socket closest to the initial failure or not. My question is, could a power tube be bad in such a way as to cause this type of issue, or do I have some other major issue wreaking havoc on my rig? I have a hard time believing that I have a series of different types of capacitors all going bad in exactly the same way one after another.

[xtian]

Yes, when power tubes fail, they often short, I.e., plate to heater, so you get 400v ripping thru your heater wiring to ground. As an example.

[geet19]

It's fixed!!!

Thank you all for the information and assistance. The amp is successfully repaired and back in great working order. Sounds as good as the day I first got it. I don't think I could've figured this out without your help! You guys rock!