FM transmitter teardown and repair

The unit arrived with no RF output. With no service manual available, the first job was reconstructing the schematic from the board itself, stage by stage, before any measurement would mean anything.

What the board turned out to be is a PLL-synthesised exciter along the lines of the Free Radio Berkeley design: an 8.400 MHz crystal reference divided by 42 to give 200 kHz channel steps, an MC145151P2 CMOS synthesiser with its N-counter programmed on an 8-position DIP switch, an LF351N integrating the phase error into a tuning voltage, and a Mini-Circuits POS-150 VCO covering 100–150 MHz — comfortably bracketing the FM band. From there the RF path is a 2SC1970 driver, a 2SC1971 power amplifier, and a 7-element Chebyshev low-pass filter into 50Ω.

Approach

1. Traced the board and redrew the schematic.
2. Verified the supply rails and isolated the regulator fault.
3. Swept each stage to find where gain departed from what the topology predicted.

What the teardown found

The PA stage had taken heat damage — scorching on the sub-board and discoloured passives around the 2SC1971, the signature of sustained keying into a bad load. That is the fault that has to be cleared before anything else is meaningful, because every downstream measurement is taken through it.

Two things were still open when the analysis was written. The MC145151P2 runs on 3–9V logic and cannot be fed from the +12V rail, so a separate 5V regulator must exist on the main board — it had not been located. And the DIP switch was found with all eight positions OFF, which puts the synthesised frequency out of band unless the switch logic is inverted from what it looks like. Which of those two conventions applies is a meter measurement, not a guess, and the analysis page carries a calculator for setting the switch once it is known.

Write-ups

• Complete analysis and repair guide — annotated block diagram, full component inventory, repair bill of materials, step-by-step procedure, and a DIP-switch channel calculator.
• Reconstructed schematic — the circuit as traced from the bare board.
• Repair report (PDF) — 4.4 MB.