bobbyonions
AK Member
Hi folks, I recently picked up an early version Fisher 400 serial # 16289E. It has cleaned up very nicely and I want to start a rebuild.
I've found alot of usefull info here concerning rebuilding the 400. I was hoping some of you could give me some support if I get stuck or confused as I go along. I've read some conflicting info here, partially because there are a few different schematics with different info. I have yet to find a schematic for the units under 20000.
Here's the basic background on this unit:
It has the early-style wood cab in nice shape (missing the metal grill and wooden rail-type feet). It has all 9 w. german 12ax7's, six of them diamond bottoms, and the three in the mpx section don't have diamonds( still w. germany). Unit came with only three good 7868's various brands. This is the version with the selenium bridge and the 5.6k resistor in the bias supply.
I've done a slow power with a variac, and verified good output on both channels. I noticed a little red plating which doesn't surprise me. I've had it powered up for max of 5 minutes or so, and don't plan to power up again until I rebuild the bias supply.
I have the new bridge rectifier and the two 1000uf caps and various resistor values to begin the work. So far I read that I need to replace the bridge, the two 1000uf caps. I’ve read to change the 5.6k resistor to 2.7K. I also read a post saying to change the 15ohm power resister to 20ohm (something to do with adjusting voltages on the heater to the phono section 12ax7’s). I’m not sure about the resistor changes.
Any help or advise will be greatly appreciated. I will update as I go, and post photos if anyone is interested. Thanks.
I've found alot of usefull info here concerning rebuilding the 400. I was hoping some of you could give me some support if I get stuck or confused as I go along. I've read some conflicting info here, partially because there are a few different schematics with different info. I have yet to find a schematic for the units under 20000.
Here's the basic background on this unit:
It has the early-style wood cab in nice shape (missing the metal grill and wooden rail-type feet). It has all 9 w. german 12ax7's, six of them diamond bottoms, and the three in the mpx section don't have diamonds( still w. germany). Unit came with only three good 7868's various brands. This is the version with the selenium bridge and the 5.6k resistor in the bias supply.
I've done a slow power with a variac, and verified good output on both channels. I noticed a little red plating which doesn't surprise me. I've had it powered up for max of 5 minutes or so, and don't plan to power up again until I rebuild the bias supply.
I have the new bridge rectifier and the two 1000uf caps and various resistor values to begin the work. So far I read that I need to replace the bridge, the two 1000uf caps. I’ve read to change the 5.6k resistor to 2.7K. I also read a post saying to change the 15ohm power resister to 20ohm (something to do with adjusting voltages on the heater to the phono section 12ax7’s). I’m not sure about the resistor changes.
Any help or advise will be greatly appreciated. I will update as I go, and post photos if anyone is interested. Thanks.