Teac 4010S Bias Trap

SCompRacer

The Teac manuals says I need a bias trap to perform some calibrations on the 4010S r2r deck. There are unanswered questions in forums about this bias trap, so I figured I'd build one and put it out there. Kind of surprising due to its age. Ken gave me alternatives to perform the adjustments without the trap, but the bias trap is one stop shopping. Basically, the trap uses inductors, AKA chokes, to block some frequencies while allowing just the clean calibration frequency to reach the scope unmolested.

The manual diagram. Note how they have a channel selector switch. Most AC millivolt/dB meters are single channel. Instead of swapping L and R leads while calibrating, you just flip a switch.



This is my plan. Use DPDT switches that I already have, to switch the 100K load and bias trap out of the circuit. I'll add a 10K resistor for loading my Teac A-6300 when I calibrate that by switching out the 100K and trap. I could not locate a 5-15mh inductor so you see two in series.



I found these 3~10mh chokes. The internet says I can series them provided I space them at least one inductor apart or in the case of these, mount at right angles. The total inductance in series is the sum of the individual inductances, so I'll be at 6mh minimum instead of 5 with more than enough range to get to 15mh. I do not know how much I need until actual calibration.



The plan...



This a 20-turn inductor. It only takes 15 to get the max mh. (The meter I use to test will be shown later). If you continue to turn the slug, the mh reduces until it drops out of circuit. I made a "stop" using a plastic rod cut to length, dropped it inside the inductor tube and glued a piece of plastic at the end. I'll add 4-40 screws in the PCB as a solid back stop to prevent the slug from turning beyond its maximum mh. The second inductor will have a stop for its minimum setting. Again, it you back the slug out too far the inductor drops out.



This PCB will hang from top of metal project box. Holes will be drilled to allow the non-metallic 2.5mm adjusting tool for adjusting the chokes.




Even though we are dealing with millivolts, I made shields for the chokes in an attempt to block their flux fields. Waiting for parts and got some more testing to do. If they interfere with signal the bias trap may have to be external of the channel switch/load box. That is not mentioned in the manual.

SCompRacer

My Sencore LC53 also tests inductance. With the shields I fabricated in place, I lose a little MH, .14. No idea if that presents a problem so I made a wrap around.



Individual shields, -.14MH.



This one doesn't affect MH. Maybe I'll make another that looks better. I do not have a bending brake.



Shield off, 6.34MH,,,,



Wrap around shield on, 6.34MH.



So my range is 5.78MH minimum...



to 12.9MH with choke number 1 adjusted to max.



A few turns of choke number 2 gets me to the goal of 15MH.



I lucked out when I got my Z meter. The guy had all the accessories including the little button pusher, or button holder. It fits between the button and handle allowing you both hands to adjust inductor or swap caps out. You gotta be careful with leads when performing capacitor leakage tests as voltage is applied when leakage button is pushed, and you can select up to 600V.



I started assembly. I mounted the channel selector switch, load and bias trap switches and RCA's. I installed load resistors, a 100K and 10K, to the ON/ON DPDT switches. On one way and you get the value across signal and ground. Opposite way, out of circuit.

I soldered what I could and tested for shorts, excess resistance, proper load values. All good. Waiting for one 250pf capacitor and I can connect, solder and install the inductor board. The inductors, when switched in, go across output signal and ground. With all switches set to out, it's just a channel selector switch. Everything mounted to top makes it easier to connect everything.