Adcom gtp 750 service manual

Page 1

SERVICE MANUAL

ADCOM®

details you can hear

DTS/AC3 TUNER PRE-AMPLIFIER

GTP-750

TABLE OF CONTENTS

Alignment Procedures ................... 2-3
Parts List ............................ 4-22
IC Block Diagram ..................... 23-36
Wiring Diagram ...................... 37-38
PCB Layout ......................... 39-50
Schematic Diagram ................... 51-64
Technical Specification ................. 65-69

ADCOM

10 TIMBER LANE

MARLBORD

N.J. 07746, USA
PHONE; (732) 683-2356
FAX: (732) 683-2358
WEB: http://www.adcom.com

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ALIGNMENT PROCEDURES GTP-750

FM SECTION

AF Modulation: 1 kHz, 75kHZ Deviation, MONO Mode

Store Frequency: 98 MHZ, 87.5 MHZ, 108 MHZ, 90 MHZ, 106 MHZ
RF Level: 75-ohm Impedance, Open Circuit

OSCILLATION TUNING VOLTAGE
Connect DVM between M211 & GND.
For A16 Front-end Module:
Tune to 87.5 MHZ, adjust FE4 (osc coil) to read 3 i 0.5 V.
Tune to 108 MHZ, adjust FE4 (osc coil) to read 20.5 i 0.5 V.
For G55 or G58 Front-end Module:
Tune to 87.5 MHZ, adjust FE4 (osc coil) to read 1.6 i 0.5 V.
Tune to 108 MHZ, adjust FE4 (osc coil) to read 8.0 i 0.5 V.

I.F.

Connect DVM between M213 & M214.

Apply 10.7 MHZ, 90 dBm via 1 kW resistor to M212.
Adjust L208 for 0 i 50mV reading on DVM.

Adjust L209 for minimum distortion (THD).

Repeat adjustment until no further improvement.

SYNTHESIZER I.F. TRACKING

Maintain connection of DVM across M213 & M214.
Disconnect 10.7 MHZ tap to M212.

Apply 98 MHZ, 60 dBm to antenna input.

Tune to 98 MHz.

Adjust C212 FOR 0 i 20mV reading on DVM.

Fine adjust L209 for minimum distortion.

Repeat adjustment until no further improvement.

R.F.

Apply 98 MHZ, 7 dBm for AH, or 21 dBm for C, to antenna input.

Check THD.

If THD > 3%, adjust FE1, FE2 & FE3 in the front-end module with non-metallic tool for minimum THD.
Check THD at 90 MHZ & 106 MHZ with 8 dBm input for AH, or 22 dBm for C.

FM Stereo: 1 kHZ, 67.5 kHz devi., 60 dBmV, Pilot signal 19 kHz, 7.5 kHz devi.

STEREO SEPARATION & PILOT SUPPRESSION

Set modulated signal to Left only.

Adjust R263 for minimum output at right channel.

Set modulated signal to Right only.

Adjust R263 for minimum output at left channel.

Repeat until readings are the same.

Turn off modulating signal, leaving the pilot tone.

Adjust L211 & L212 for minimum outputs on right and left channels respectively.

AM SECTION

AF Modulation: 400 HZ, 30%.

For AH version, store frequencies 603, 999, 1404 kHz.

For C & B versions, store frequencies 600, 1000, 1400 kHz.

Connect 22 pF ceramic to capacitor between signal generator and antenna terminal.