Kenwood TM-2550A Service Manual

Also see for TM-2530A: Instruction manual Instruction manual Instruction manual Instruction manual Instruction manual

TM-2550A/E,2530ACIRCUIT DESECRIPTIONTM-2550A (K1,M1,M3) TM-2550E (T,W) TM-2530A (K2,M2,M4)FINAL UNIT X45- 1440-10 X45- 1440-10 X45- 1440-11PLL UNIT X50-2030-10 X50- 2030-51 X50-2030-11X53-1440-10 (K1,M1)CONTROL UNIT X53-1440-21 (M3)X53- 1440-51 (T)X53- 1440-61 (W)X53- 1440-10 (K2,M2)X53- 1440-21 (M4)DISPLAY UNIT X54-1860-10 X54- 1860-10 X54-1860-10COMPOUND UNIT X60- 1280-10 X60- 1280-51 X60- 1280-10Table 1 TM-2550A/E, TM-2530A PC board chartRX SECTIONSignals from the antenna are applied to the FINAL unit(X60-1280-XX) transmit receive swithing circuit, D2 andD4. In receive, diodes D2 and D4 are reverse biased forcingthe incoming receive signal down thru L3 to the RA (Re-ceive Antenna) terminal. There the incoming receive signalis then coupled to the COMPOUND Unit {X60-1280-XX).The signal is filtered by a low pass filter and amplified byQ1 the first RF amplifier, a GaAs FET : 3SK129(S). Theamplified RF signal is then mixed with the LR (ReceiverLocal Oscillator) signal in the first mixer Q2 : 3SK74(L) toobtain the First iF frequency of 10.695MHz. This IF signalis filtered by Helical Resonator, L4, and a two stage MCF(Monolithic Crystal Filter), L8, which combine to provideexcellent two-signal characteristics and sensitivity.The First IF signal from the MCF is then amplified by theFirst 1F amplifier Q3 : 2SC2668(Y} and applied to the FMiF Amplitier/Mixer/Detector, (C2 : TA7761P. This signalIC mixes the IF signal with the 10.24MHz 2nd Locai Oscil-lator signal (L14} to obtain the 2nd IF frequency of455kHz, then amplifies, routes the signal thru externalfilter L16, and finally convertes it into the received audiosignal with the internal quadrature detector. The 2nd LocalOscillator signal from L14 and IC2 is also used by the PLLunit as a Reference signal.The detected audio output from |C2 is applied to theSQUELCH CONTROL (X59-1040-10), and to the de-emphasis circuit R56 and C99. The (de-emphasized) audiois amplified by Audio preamplifier Q9 : 2SC2458(Y) andthen applied thru the AF GAIN contro! to the Audio Amp-lifier [C1 ; ¢PC1241H which drives the speaker.The noise component of the detected audio is filtered by aband-pass filter, in order to obtain a frequency of appro-ximately 30kHz (which is well outside the normal voicebandwidth) and then amplified by the noise amplifierscontained inside 1C2. This amplified noise is rectified bydiodes D6 and D5 and applied to O1 of the Squeich Switch.Q1 is used to control the conduction of O2, which controlsAF Preamp Q9.A center-stop tuning circuit is provided to ensure that theincoming signal is actually on frequency before the squelchis opened. A portion of the 455kHz IF signal is coupledthru the secondary of L17 and applied to CENTER STOPunit (X59-1030-10) where Ceramic Discriminator L12, andWindow comparator IC1 : NJM4558M are used to deter-mine if the incoming signal is actually the channel center.When the detected signal applied to the window compara-tor reaches a predetermined level, Q5 of the Squeich Switchwill be turned OFF or ON. The Squeich Switch determinesif there is an incoming signal at the same time. If the in-coming signal is at the proper level, and on frequency, thesquelch will open and the C.TUNE indicator in the LCDdisplay will turn ON. This indicator is controlled by IC2,the main microprocessor on ithe CONTROL unit(X53-1440-XX), thru inputs supplied from the COM-POUND unit via the BD line. During Open Channel Scan-ning of the DCL system Q3, Q1, and Q2 of the SquelchSwitch are controlled by the main microprocessor via theSOS line. During DCL operations open channel search thre-shold levels are controlled by VR3 of the Compound ass’y.Transistor O6 of the Squelch Switch, is used to prevent theCenter Stop circuit from activating during Open ChannelScan Operation.Note 1:The Center Stop circuit is not activated during Open Chan-nel Scan.VvWindow comparator frequencybandwidth.o— = fPw Frewuency of desired Open ChannelChannelsInterference such as IMDfrom strong adjacent channelsmakes the use of the Center Stop function erratic duringOpen channel scan. Fig. 1Audio Preamplifier Q9 is controlled by Q1 of the Alert,Vacant CH. unit (X59-1020-10) during Alert, code Squelchand Open Channel operations.