71-2781 inverse first l.o. injection kit

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70-2781 Equipment SpecificationsInverse Is t L.O. Inj ection Kit (21. 7 MHz)

70-153 A/B/C

Inherent to all synthesized radio receivers, is the possibilityof internal interference at certain channel frequencies. Signalsat or near the receiver I.F. may be generated by and radiated fromthe synthesizer and microcomputer circuitry. Strong externalsignals at nearby channels may irritate the problem.Evidence of such spurious interference ranges from weak sensitivityreceiver self-quieting on certain channels, depending on the strengof the internal interfering signal. These symptoms appear onlywhen the transceiver is operating on a channel frequency wheresynthesizer divider ratios are such that harmonics of signalfrequencies generated within synthesizer circuitry are at thefirst I.F. of the receiver (21.6 MHzin standard configurations).Whena non-impaired channel is selected, the synthesizer dividerratios are different and consequently so are the frequencies of itsinternal signals. Therefore, signals radiated from the synthesizerare no longer at receiver I.F.If the transceiver must operate on a susceptible channel frequencyand seLf= Lnter-fer-ence is present, solution is simple: change thefirst I.F. Service shop installable kits are available from MIDLAto replace the first I.F. crystal filters with 21.4 ffilz or 21. 7 'Mfilters (see AUXILIARYPRODUCTSection for part numbers). Therec ei ver 1.F. pr ogramming in th e E2PROMmustal sob e change d asinstructed in the Programmer Operator's Manual.The chart on the following page identifies the susceptible channelfrequencies with respect to each available receiver I.F.

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CHAN NELS SU SCEPT IBLE T O TRAN SCEI VER SELF- INTE RFERE NCE

Fir st 1.F. ofthe Receiver

Susceptible Channel Frequencies (MHz)per Transceiver Frequency BandA-BAND B-BAND C-BAND

21.7 MHz 137.39 to 137.47 148.48 160.53 to 160.63138.24146. 76147.92 to 147.99

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70-2781 21.7 MHz OFFSET IF KIT for 70-153 A/B/C

INSTALLATION INSTRUCTIONS1. Disassemble radio as shown in illustration 1 and 2.2. Remove X201 and replace with the 21.245 MHz Crystal.3. Remove both parts of FL201 and replace them with thenew 21N15BW 21.7 MHz filters.4. Reassemble the unit.

PROGRAMMING

To reprogram your EPROM, first do a complete erase.this mode, press the F and the ERASE or 0 keys. To access

See Manual NO. 70-105000 for complete programming methods.

NOTE: 21.7 MHz must be entered in the IF Frequency mode.

Check radio and realign per the radio manual instructions.

PARTS LISTREFERENCE NO. DESCRIPTION PART NO.FL201X201

21.7 MHz Crystal filter (2)21.245 MHz 2nd Local Crystal

70-17904470-179045

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BATiERYSCREWS RETAININGCLIP . ntIustratio

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J104

Illustration #2

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71-2781 inverse first l.o. injection kit

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