• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

NFJ & FX-audio tube amp repair

Hi
I have a NFJ & FX-audio tube amp, the one with a cd and phono input
One day I was trying some new power tubes and fried two resistors right next to the tube sockets
Can anyone help me find a schematic showing all the resistors I can’t find one and all the pictures are fine don’t show any of the resistors that are right next to the tube sockets
Here’s a pic of the amp. Later I’ll take some pics of the fried parts
IMG_4971.JPG IMG_4967.JPG IMG_4957.JPG IMG_3073.JPG IMG_3072.JPG IMG_3075.JPG
 
Both R looks to be connected to pin 9 of the 6P1P. Pins 9 is G2. The value of that resistor is less critical. How many wires does the output transformer preliminary side has (the wire that are not connected to any speaker terminals). If it is has three wires I would put in 500R to 1K. If 2 wires I would consider 1K to 2K. If you have no idea, try 1K.
 

pcan

Member
Paid Member
2015-12-31 4:57 pm
I have the same amplifier on the bench with another kind of issue (loud background hiss that starts and stops randomly - maybe a HF oscillation). I attach the picture I made when I disassembled it for the preliminary inspection. I will resume the repair next week, at that time I will open the amplifier again and I will measure the resistors while tracing the schematic.
I suggest to be careful on tube rolling on this amplifier. It does have minimal protection on the switched mode power supply and the microprocessor may be damaged easily.

Edit: I checked your picture more carefully, and I now have a confirmation of the hypotesis I made during the disassembly: on my specimen, the value of R52 is wrong. They fitted a resistor that should have been on the motherboard instead. There are 3 QC passed stamps on this amplifier, I wonder what kind of quality control they do.
 

Attachments

  • IMG_4228.jpg
    IMG_4228.jpg
    448.1 KB · Views: 149
Last edited:

pcan

Member
Paid Member
2015-12-31 4:57 pm
The value of R35 and R73 on my board is 2k.
I exchanged the wrong R52 resistor but I still get the hissing noise. Could you please take a picture of the board (maybe splitted on right half - left half) with good lighting so I can read the value of all resistors? Maybe R52 is not the only wrong one on my specimen.