D.W. Fearn VT-1 Instruções de Operação descarregar pdf (Página 37)

D.W. FEARN

VT-1 &

VT-2 Microphone Preamplifers

VU Meter Calibration

The meter am

plifier 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:

. 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 alw

ays be a high-impedance bridging termination of 20k ohms or

mor

3. Apply A

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

45 dBm into the VT-2 input.

4. Measur

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

ence is desir

ed).

5. Set t

he 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 am

plifier printed cir

cuit 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. R

eplace t

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

back).

Troubleshooting

Most pr

oblems 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

-2 to t

he factory for repair. Call first, however, for shipping information.

Fig. 5 VU Meter Amplifier PCB

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D.W. Fearn VT-1 Instruções de Operação Página 37