The TS-440S Control unit supplies band information to four circuits in the transceiver. The circuits are the receiver front end Band Pass Filter (BPF), the antenna tuner (AT), the transmitter Low Pass Filter (LPF), and the VCO circuits on the RF board that are part of PLL #1.
There are ten receiver front end BPFs that are diode switched into operation. If one of the diodes (D4-D23) becomes shorted or leaky, the band information is felt at two BPFs at one time. The voltage going through the bad diode will loop back to the VCO circuit, antenna tuner circuit, and LPF circuit.
The unwanted voltage will then create one or more of the following symptoms:
- PLL unlock (the transceiver only displays decimals).
- No antenna tuner operation.
- Low receiver sensitivity.
- Low or no transmit power output.
- Distorted transmit audio.
- RF feedback.
The most common symptom is the PLL unlock. To test the diodes, remove the large shield on the RF board to expose the BPF circuits. Reset the transceiver by holding in the A=B button as power is turned on. Even if the display only shows decimals, only the BPF for 14 MHz should be turned on. Using a DC voltmeter, measure the voltage at coils L11, L15, L21, L28, L34, L40, L46, L52, L58, and L62. Only coil L52 should measure 8 Vdc. The other coils should read 0 Vdc. If one reads 0.5 - 8 Vdc, replace the two diodes in that BPF circuit.
Coil/Diode/frequency range relationship:
L11 D4 D5 0 - 0.5 MHz L15 D6 D7 0.5 - 1.6 MHz
L21 D8 D9 1.6 - 2.5 MHz L28 D10 D11 2.5 - 4 MHz
L34 D12 D13 4 - 6 MHz L40 D14 D15 6 - 7.5 MHz
L46 D16 D17 7.5 - 10.5 MHz L52 D18 D19 10.5 - 14.5 MHz
L58 D20 D21 14.5 - 22 MHz L62 D22 D23 22 - 30 MHz
Replacement diode part number
1S2588 or 1SS91S: D4, 6, 8, 10, 12, 14, 16, 18, 20, 22
BA282 D5, 7, 9, 11, 13, 15, 17, 19, 21, 23
Time required to perform this modification is 1 hour or less.