MaxiBox Tuning Hints

The prototype MaxiBox uses an enhanced tuning system which allows the user to eliminate some of the dead spots and have a more even audio stream. This is accomplished by adding two additional controls; sweep gain and sweep offset. The end result is the ability to set the tuning to the most active portion of the AM band.

The standard MiniBox is fixed-tuned to sweep from 550 to 1600 KHz. This spread covers the full AM band. However, many locations have only a few stations, and the top or bottom of the AM band may be vacant. In this situation the sweep tuning yields gaps in the audio, when the tuner moves through the empty areas.

The MaxiBox addresses this problem in two ways. First, it allows the user to set the bottom and top of the band. This is the purpose of SWEEP GAIN. Second, the entire tuning range can be shifted up or down. This is the purpose of SWEEP OFFSET.

The simple diagrams below show how these work.

550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

|---|----|----|----|----|----|----|----|----|----|----|

<---------------------------------------------------------> GAIN MAXIMUM

550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

|---|----|----|----|----|----|----|----|----|----|----|

<-------------------> GAIN MINIMUM

As you can see, sweep gain determines the amount of the AM band that is covered

550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

|---|----|----|----|----|----|----|----|----|----|----| GAIN MID-RANGE

<---------------------------------------> OFFSET MID-RANGE

550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

|---|----|----|----|----|----|----|----|----|----|----| GAIN MID-RANGE

<---------------------------------------> OFFSET LOW

550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

|---|----|----|----|----|----|----|----|----|----|----| GAIN MID-RANGE

<---------------------------------------> OFFSET HIGH

The sweep offset control sets where the sweep begins. The sweep gain control sets how wide the sweep is.

By using both controls you can set the sweep in the most active portion of the AM band, which gives the voices more raw material to work with.

Because the controls allow tuning well outside the AM band, there are indicators that will help you determine how the tuning is set. These indicators show whenever the tuning is outside acceptable limits. It does no harm to operate out of limits but sensitivity may be affected adversely.

These indicators are in the form of two LEDs; High LIMIT and LOW LIMIT

Whenever the sweep is outside either of these limits, the appropriate LED will illuminate. A properly tuned BOX that uses the full normal sweep (550 to 1600KHz) will cause each of these indicators to flicker briefly. When in doubt, tune for a brief flicker of each LED. This gives full standard AM sweep.

If one end of the sweep is dead, increase or decrease the offset in a way that eliminates that dead spot, then tune the SWEEP GAIN to reduce the overall sweep width. Note that all the sweep controls interact with each other, including the sweep rate, so making these adjustments takes practice and a bit of musicianship.

See the diagrams below to understand how the indicators work.

The Heartbeat LED uses intensity to indicate where the sweep is. The LED is dark at the beginning of the sweep (low end) and gains brightness as the sweep level increases (high end). Then as the sweep reverses, the LED will get dimmer until the sweep is at low level again.

(DIM HB) 550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 (Bright HB)

|---|----|----|----|----|----|----|----|----|----|----|

<---------------------------------------> OFFSET

In this case, neither LED illuminates

(DIM HB) 550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 (Bright HB)

|---|----|----|----|----|----|----|----|----|----|----|

<---------------------------------------> OFFSET

The LOW LIMIT LED will flicker briefly

(DIM HB) 550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 (Bright HB)

|---|----|----|----|----|----|----|----|----|----|----|

<---------------------------------------> OFFSET

The LOW LIMIT LED will stay on longer