Question on Bogen DB130 damping

[anynameyouwi]

I recently picked up 3 Bogen DB130 amplifiers. Two will go back into service replacing the Heathkit A7s I rebuilt. They work fine but don’t match the room.

Besides the usual questionable wiring practices by previous owners that I’m fixing I came across this.

There is a 100ohm pot, 0.47 ohm resistor and what was a 1 ohm resistor as part of the damping factor circuit. I’m not familiar with this arrangement.

Given that the 0.47ohm resistor has been replaced as well as the pot, I’m wondering what might have happened to cause the 1ohm resistor to become a sparkler.

The pot is not OEM and it is 47k rather than 100ohm so I’m not sure if that would have caused the failure, beside the aluminum foil on the fuse and bypass jumper haphazardly soldered around the fuse.

Either way I’ll continue to clean up the mess and get it working again.

For your amusement I’ve added two other unrelated pictures of the capacitor tree from one of the other units.

 

[TriodeLuvr]

What a mess! Difficult to imagine how a 1Ω resistor that size could be damaged unless the amplifier's output power was directed into it somehow. That would imply the current was drawn through the output transformer secondary (and therefore also through the primary and the output tubes).



Jack

 

[anynameyouwi]

That’s what I’m worried about. I haven’t gotten to checking the transformer yet but it looks “OK”. Granted this wouldn’t have been a sustained high current event so there may not be physical evidence on the transformer.

The tubes are fine but they have a better chance of survival.

 

[TriodeLuvr]

Looking at the schematic, the connections being made at the Impedance and Speaker selector switches isn't clear to me. However, I think that 1Ω resistor could be overpowered if improper connections were made at the speaker output terminals. IOW, the damage might have occurred long before a previous owner went into the circuit and made changes. On a good note, the DCR of the transformer secondary should be a fraction of 1Ω, so likely the resistor itself took the brunt of the abuse. Good luck with the restoration!

Jack

 

[gadget73]

I'm impressed that amp could make enough power to smoke a 1 ohm 2w resistor like that. I wonder if those aren't really skid marks from the pot vaporizing. Those tend to be much lower rated, though under normal conditions it shouldn't be possible to develop enough power to cause that sort of damage.

I might be tempted to just hot wire the thing to "DF Off" to dispense with all this myself, but the 47k pot is definitely not going to make it work as expected.

 

[anynameyouwi]

The 1ohm is definitely smoked. Waiting on parts to arrive.