D.W. Fearn VT-1 Operating instructions manual - page 37

D.W. FEARN 

VT-1 & VT-2 Microphone Preamplifers

37

VU Meter Calibration

The meter amplifier circuit is stable and re-adjustment of the meter calibration is not normal-

ly required. If the output reference level needs to be changed, here is the procedure:

1. Remove the VT-2 top cover (see above).

2. Terminate the output in the same impedance as the VT-2 will normally be feeding.

This will almost always be a high-impedance bridging termination of 20k ohms or

more.

3. Apply AC power. Allow at least a fifteen minute warm-up. Feed a 1 kc tone at about -

45 dBm into the VT-2 input.

4. Measure the output and set the Attenuation control for +4 dBm (or whatever refer-

ence is desired).

5. Set the Meter Calibration trimmer potentiometer for 0 VU on the VT-2 meter(s). Be

sure the meter is turned on. This control is a 25-turn pot so the adjustment is easy. A

small screwdriver or alignment tool is necessary. The controls are located on the

meter amplifier printed circuit board on the top of the chassis and they are the only

trim pot(s) on the VT-1/VT-2. See Figure 5 on page 38.

6. Replace the top cover. Be sure the ventillation slots are over the tubes (towards the

back).

Troubleshooting

Most problems will be traced to defective vacuum tubes. However, if normal tests do not eas-

ily reveal the problem, feel free to call the factory for assistance. If you lack access to a qual-

ified service technician with vacuum tube equipment repair experience, you may return the

VT-2 to the factory for repair. Call first, however, for shipping information.

Fig. 5 VU Meter Amplifier PCB