FM Transmitter — Complete Analysis & Repair Guide

MC145151P2 PLL · POS-150 VCO · 2SC1970 Driver · 2SC1971 PA · 8.400 MHz XTAL

⚠ PA transistor: 2SC1971 (confirmed "C1971 06AR")Must test out-of-circuit before re-powering. Heat damage visible on PA sub-board.
⚠ DIP switch: all OFF (Image 5)All 8 switches in OFF position. Frequency may be out-of-band. Set correct N before testing.
✓ Driver: 2SC1970 confirmed ("C1970 99AA")On main board — separate from PA transistor. Both transistors now identified.
DIP Switch Channel Calculator — 8.400 MHz XTAL · R=42 · f_ref = 200 kHz per FM step
500
1 11110100
Switch positions — SW8=N7(128), SW7=N6(64), SW6=N5(32), SW5=N4(16), SW4=N3(8), SW3=N2(4), SW2=N1(2), SW1=N0(1) · N8 likely hard-wired HIGH (+256)
f_out = N × 200 kHz. Switches 1-8 control N bits 0-7. ON = LOW = bit included. Adjust switches to match the green ON positions above for your desired frequency.
8.400 MHz HC-49 crystal MC145151P2 PLL · CTGD0404 DIP sw — ALL OFF now err V LF351N Active loop filter tune voltage → N÷ fb POS-150 VCO 100–150 MHz 2SC1970 Driver · main board 2SC1971 PA · ⚠ DAMAGED 7-el LPF 3 coils + 4×27pF 50Ω confirmed IDs: 2SC1971: "C1971 06AR" TO-39 (PA) 2SC1970: "C1970 99AA" TO-92 (drv) POS-150: "MCL POS 150" VCO mod XTAL: "8.4000" HC-49 can LM2940CT-12 (+12V LDO) · Red LED = power indicator · 5V reg (still to locate) 470µF 10V bulk filter confirmed · 13.8V in → +12V RF chain · +5V logic (must find)
↑ Click any block above for component details and test measurements.
Measurements / Test Points

    

  







  Complete component inventory
  

    
PartMarking / ValueFunctionStatus

    
      
PLL synthesizerMC145151P2 · CTGD040428-DIP CMOS PLL, R÷, phase det, N÷, LD✓ Confirmed

      
Crystal reference8.4000 MHz HC-49Sets 200 kHz FM step with R=42✓ Confirmed

      
VCO moduleMini-Circuits POS-150100–150 MHz, Vtune 1–12V, +7dBm✓ Confirmed

      
Loop filter op-ampLF351N (NSC)JFET active integrator, tune voltage✓ Confirmed

      
Voltage regulatorLM2940CT-12 (JM35AG)+12V LDO, 1A, TO-220✓ Confirmed

      
5V logic regulatorunknown — locate!+5V for MC145151P2⚠ Not yet found

      
Driver transistor2SC1970 · C1970 99AATO-92 VHF NPN, ~50mW, main board✓ Confirmed (Img 1)

      
PA transistor2SC1971 · C1971 06ARTO-39 VHF NPN, 2W, PA sub-board⚠ Damaged — test/replace

      
7-element LPF3× air-core coil + 4× 27pFHarmonic suppression, FM band✓ Confirmed (Img 3)

      
DIP switch8-pos, all currently OFFN-counter frequency programming⚠ Set before use

      
Electrolytic caps22µF 35V (Philips/ICON)Decoupling throughout✓ Confirmed

      
Bulk cap470µF 10VSupply rail filter✓ Confirmed

      
Red LED5mm red LEDPower-on indicator✓ Confirmed (Img 1)

    
  







  Repair bill of materials
  

    
PriorityPartValue / SpecSourceNotes

    
      
Critical2SC1971 RF transistorTO-39, Vceo 12V, 2W, 350MHzeBay NOS, Mouser, DigiKeySub: 2SC2078, 2N3866, NTE107

      
CriticalPA board burnt passivesMatch from color codes on boardMouser/DigiKeyReplace all discolored resistors + caps

      
ImportantLocate 5V regulator78L05 or 7805 TO-92/TO-220Already on boardTest output; replace if needed

      
ImportantSet DIP switchPer channel calculator aboveNo purchase neededAll-OFF = likely out-of-band

      
RecommendedIPA board cleaning90%+ isopropyl alcoholHardware storeBoth boards before re-powering

      
Recommended50Ω dummy load ≥5WPer FRB PDF designBuild or buyNever test PA into antenna directly

      
RecommendedThermal paste refreshNon-conductive compoundElectronics storeBetween 2SC1971 case and chassis

      
OptionalPOS-150 spare moduleMini-Circuits POS-150eBay NOS ~$15-30Source now in case module is damaged

    
  







  Step-by-step repair procedure
  
    
    
  1. Do NOT power on until steps 2–5 are complete
    2. 
    
  2. Remove PA sub-board; clean with IPA; identify all scorched components under magnification
    3. 
    
  3. Desolder 2SC1971 from PA board; test out-of-circuit with DMM — any low-resistance C-E or B-C in reverse = failed transistor
    4. 
    
  4. Replace all burnt/discolored resistors and ceramic caps on PA board; re-tin cold coil solder joints
    5. 
    
  5. On main board: find the 5V regulator (TO-92 near MC145151P2); confirm +5V output with DMM
    6. 
    
  6. Set DIP switch using calculator above. Example: 100.1 MHz → N=500 → SW8=ON, SW7=ON, SW6=ON, SW5=ON, SW4=OFF, SW3=ON, SW2=OFF, SW1=OFF (if N8 hard-wired HIGH=256, lower 8 bits = 244 = 11110100)
    7. 
    
  7. Connect ONLY 50Ω dummy load to LPF output — NOT antenna
    8. 
    
  8. Power on; LM2940 output should read +12.0V; 5V rail should read +5.0V
    9. 
    
  9. Scope crystal pins: 8.4 MHz sinewave at 0.5–1.5Vpp should be present
    10. 
    
  10. MC145151P2 pin 1 (LD) should go HIGH within ~100ms — if stuck LOW, PLL is not locking
    11. 
    
  11. LF351N pin 6 should settle to mid-rail DC (~4–7V) when locked
    12. 
    
  12. Scope POS-150 output: RF signal at ~100 MHz?
    13. 
    
  13. Check 2SC1970 driver output: amplified RF present?
    14. 
    
  14. If PA transistor replaced: measure idle supply current <50mA; keyed current 150–400mA
    15. 
    
  15. Confirm output power into dummy load with RF power meter (expect 1–2W)
    16. 
    
  16. Verify on FM receiver at your target frequency — only then switch to antenna via LPF
    17.