Bridge rectifier overheating

[RuneGuitars]

Just testing the waters to see if anyone has seen this issue before. I have a 2005 dual Rec that has some toasty bridge rectifiers.

We’ve since swapped out the rectifiers, reran some scorched trace lines and when bringing the amp up to full power they started over heating again.

Seems like something is pulling the +3 vdc rail down and causing a short..

Anyone come across this issue have any ideas on what it could be?

 

[electric mayhem]

I had a green stripe that fried a carbon comp right at the bridge on the board iirc. I believe I just replaced the CC but it's been a while and the layout is likely different.

 

[bandit2013]

I would almost suspect it is one of the electrolytic caps. Depending on the model as I am not versed in solo heads, 2CH or 3CH, the +3/-3 volt DC rail is different between the two. One has several capacitors (3CH since it has to feed that 6V to the relays) and the other only has one. Something in the circuit may be breached to circuit ground instead of using the -3 as its low point.

 

[RuneGuitars]

I would almost suspect it is one of the electrolytic caps. Depending on the model as I am not versed in solo heads, 2CH or 3CH, the +3/-3 volt DC rail is different between the two. One has several capacitors (3CH since it has to feed that 6V to the relays) and the other only has one. Something in the circuit may be breached to circuit ground instead of using the -3 as its low point.

Yeah starting with the caps and looking at traces. The circuit is simple enough which is why it’s a head scratcher. Should be something obvious but it doesn’t seem to be jumping out at us.

 

[John Micka]

Do you have a thermal gun, or Thermal Camera to follow the heat.

 

[bandit2013]

If it is a 3CH, there is a zener diode in that circuit, used to drop the +3V to +1.5V or something like that. That voltage is used to enable an opto-isolator for the FX selection.

Since it is probably out of warranty, I would still inquire with Mesa Boogie technical support on what may be causing the issue. They should be able to provide guidance through email on the subject. You may get more than you asked for and that would be a good thing.

I would also measure the AC voltage on the two green wires, it should measure 6.3VAC rms.
You can also measure across the bridge circuit to confirm the voltage is at 6VDC. It would also measure 3VDC from + to the chassis ground or -3VDC from the - part on the bridge to the chassis ground. The V1 tube heater is driven by the 6VDC circuit (+3/-3) and the rest of the preamp tubes and power tubes are fed by the 6VAC part before the bridge circuit.

The 1N4007 diode rectifiers are rated for a maximum forward average current of 1A. Having a 1000V reverse breakdown voltage. If you had damaged traces, it is possible some of the vias were damaged as well or some traces on the other side of the PC were also taken out. You may have to pull the PCB out of the chassis to give it a good look over on both sides.