Excalibur works fine one day than not the next...

SSB uses AM. Maybe it is switched to FM.

If your receiver was operating previously but now the SSB option is not working, very likely the sideband detector has failed. I'm not directly familiar with your Yaesu unit but, in most Yaesu transceivers, the SSB chip and control circuit (including diodes) is identified in print on the main board.

check voltage going to final

I cannot say for sure with that particular model, however I did have a radio that would do the same thing, and found that the wiring was shorting out at the terminal end causing the cw key to key itself multiple times. you might check that, or make sure that the wiring is correct for that particular radio.

I HAVE THE SAME PROB WITH ALL THE STATIC , %50 OF THE MEETER MOST OF THE TIME, HARD TO HERE EVEN LOAD STATIONS, CAN ANY ONE EXPLAIN , IT IS ONLY ON THE 618 , NOT MY PRESIDENT ,,

hELLO THERE
hERE IS SOME MODIFICATIONS FOR THIS RADIO
Galaxy Mods DX 2527 - VR 1....AM/FM S-Meter, VR 2....SSB S-Meter, VR 3....SSB Squelch Range, VR 4....AM/FM Squelch Range, VR 7....SSB Carrier Balance, VR 8....RF Meter, VR11...Driver Bias, VR12...SSB ALC, VR13...Hi Power, VR14...AMC, VR15...AM Low Power, VR16...SSB Low Power, VC 1....10.240, VC 2....10.240. DX 2517- VR 1....AM/FM S-Meter, VR 2....SSB S-Meter, VR 3....SSB Squelch Range, VR 4....AM/FM Squelch Range, VR 5....FM Deviation, VR 6....SSB Carrier Balance, VR 7....TX Offset, VR 8....CW Beep Level, VR 9....RF Meter, VR10...Final Bias, VR11...Final Bias, VR12...Driver Bias, VR13...SSB ALC, VR14...AM/FM High Power, VR15...AM Modulation Meter, VR16...AM Modulation, VR17...SSB Power, VR18...AM/FM Low Power, VR601..13.8 Volt Power Supply. DX 22B- VR 1....AM/FM S-Meter, VR 2....SSB S-Meter, VR 3....SSB Squelch Range, VR 4....AM/FM Squelch Range, VR 7....SSB Carrier Balance, VR 8....RF Meter, VR11...Driver Bias, VR12...SSB ALC, VR13...High Power, VR14...AMC, VR15...AM Low Power, VR16...SSB Low Power, VR21...TX Offset, VC 1....10.240, VC 2....10.240. DX 11B- VR 1....AM/FM S-Meter, VR 2....SSB/CW S-Meter, VR 3....SSB Squelch Range, VR 4....AM/FM Squelch Range, VR 5.....FM Deviation, VR 7....SSB Carrier Balance, VR 8....RF Meter, VR10...Final Bias, VR11...Driver Bias, VR12...SSB ALC, VR13...AM/FM High Power, VR14...AM Modulation, VR16...CW Tone Level, VR20...Final Bias, VR21...TX Offset, VR601..13.8 Volt Power Supply, VR803..AM/FM Low Power. DX 77HML, DX 88HL, and DX 99V- VR 1....AM/FM S-Meter, VR 2....SSB S-Meter, VR 3....SSB Squelch Range, VR 4....AM/FM Squelch Range, VR 5.....FM Deviation, VR 7....SSB Carrier Balance, VR 8....RF Meter, VR10...Final Bias, VR11...Driver Bias, VR12...SSB ALC, VR13...AM/FM High Power, VR14...AM Modulation, VR16...AM/FM Low Power, VR20...Final Bias, VR21...TX Offset.
Unmarked VR in R189..AM/FM Medium Power for DX 77HML only. DX 33HML, DX 44V, DX 55V, DX 66V, and DX 73V- VR 1....AM/FM S-Meter, VR 4....AM/FM Squelch Range, VR 5.....FM Deviation, VR 8....RF Meter, VR12...Low Power AM Modulation, VR13...AM/FM High Power, VR14...High Power AM Modulation, VR16...AM/FM Low Power.
Unmarked VR in R188..AM/FM Medium Power (DX 33HML only)
Unmarked VR in C139..Med Power AM modulation (DX 33HML only) DX 949,DX 959- VR 1....AM S-Meter, VR 2....SSB S-Meter, VR 3....SSB Squelch Range, VR 4....AM Squelch Range, VR 6.....SSB Carrier Balance, VR 7....TX Offset, VR 9....RF Meter, VR10...Final Bias, VR12...Driver Bias, VR13...SSB ALC, VR14...AM High Power, VR15...AM Modulation Meter, VR16...AM Modulation, VR17...SSB Power, VR18...AM Low Power, VC1....10.240. DX 2517 and DX 2527-
Symptom:Buzzing or motorboating sound in either the transmit or receive.
Problem: TR602 is oscillating
Cure: Replace C602 with a .01uF cap {Cap is located in the power supply board}.
RG 66,99,DX2517 Replace Q54 2SB754 60watt 7A with a higher power 2SB817 100 Watt 12A. Derating of this component will cool operation, provide better voltage stability,decrease wobble, allow voltage increase to 14.5 volts comfortably. (replace 10 volt caps in this section of the radio to a minimum of 16 volts before increasing supply voltage to this level)and provide faster swing rise time, Replace Q37 2SA473 to a 2SA1012-0 or ECG153 this will improve the clarity and volume of the modulation and it will also run a lot cooler.

The Yaesu FT-8900 is a quad band FM transceiver only. It can not be modified to receive or transmit AM, SSB, RTTY or CW. It works on all or portions of 70 centimeter, 2 meter, 6 meter and 10 meter bands. The 26.805 Mhz frequency is 11 meters, so it is out of range of the 10 meter band in the radio, it is close. Even if able expand the transceiver, the AM / SSB mode commonly found in this band makes it impossible to use. Further, the radio is not FCC Type Accepted for use in this band.

Note: The following was taken from a web site as follows:
http://www.kb2ljj.com/data/yaesu/ft-857.htm

WARNING: changing these will reset all the memories.
I want to warn you do not change these values unless you are sure of what you are doing. To get them turn transceiver off. Press and hold the A,B,C keys; while holding them in, press and hold in the [PWR] switch for 1/2 second to turn the tranceiver On. Now let go of all keys. Attention, for menu NO-017. Input voltage 13.8V = 138 to calibre voltmeter.
Press F key to finish.
Function Setting in my radio
============================================================= Adjust-No Function Setting Mode Frequency ============================================================= NO-001 HF1-RXG 118 CW 1.800.00 NO-002 HF2-RXG 91 CW 7.068.19 NO-003 HF3-RXG 133 CW 21.225.13 NO-004 50M-RXG 106 CW 50.000.00 NO-005 VHF-RXG 77 CW 145.437.50 NO-006 UHF-RXG 103 CW 438.900.00 NO-007 SSB-S9 61 CW 21.225.13 NO-008 SSB-FS 54 CW 21.225.13 NO-009 FM-S1 68 FM 145.437.50 NO-010 FM-FS 99 FM 145.437.50 NO-011 DISC-L 50 FM 145.437.50 NO-012 DISC-H 79 FM 145.437.50 NO-013 FM-TH1 100 FM 145.437.50 NO-014 FM-TH2 100 FM 145.437.50 NO-015 FM-TI1 10 FM 145.437.50 NO-016 FM-TI2 10 FM 145.437.50 NO-017 VCC 138 FM 145.437.50 NO-018 HF1-IC 83 CW 1.800.00 NO-019 HF2-IC 80 CW 7.068.19 NO-020 HF3-IC 87 CW 21.225.13 NO-021 50M-IC 84 CW 50.000.00 NO-022 VHF-IC 72 CW 145.437.50 NO-023 UHF-IC 74 CW 438.900.00 NO-024 HF1-PO-MAX 165 CW 1.800.00 NO-025 HF1-PO-MID2 105 CW 1.800.00 NO-026 HF1-PO-MID1 31 CW 1.800.00 NO-027 HF1-PO-MIN 13 CW 1.800.00 NO-028 HF2-PO-MAX 159 CW 7.068.19 NO-029 HF2-PO-MID2 102 CW 7.068.19 NO-030 HF2-PO-MID1 29 CW 7.068.19 NO-031 HF2-PO-MIN 11 CW 7.068.19 NO-032 HF3-PO-MAX 158 CW 21.225.13 NO-033 HF3-PO-MID2 101 CW 21.225.13 NO-034 HF3-PO-MID1 29 CW 21.225.13 NO-035 HF3-PO-MIN 11 CW 21.225.13 NO-036 50M-PO-MAX 145 CW 50.000.00 NO-037 50M-PO-MID2 92 CW 50.000.00 NO-038 50M-PO-MID1 47 CW 50.000.00 NO-039 50M-PO-MIN 8 CW 50.000.00 NO-040 VHF-PO-MAX 87 CW 145.437.50 NO-041 VHF-PO-MID 43 CW 145.437.50 NO-042 VHF-PO-MIN 7 CW 145.437.50 NO-043 UHF-PO-MAX 112 CW 438.900.00 NO-044 UHF-PO-MIN 16 CW 438.900.00 NO-045 HF1-TXG 48 USB 1.800.00 NO-046 HF2-TXG 38 USB 7.068.19 NO-047 HF3-TXG 43 USB 21.225.13 NO-048 50M-TXG 40 USB 50.000.00 NO-049 VHF-TXG 47 USB 145.437.50 NO-050 UHF-TXG 49 USB 438.900.00 NO-051 ALC1-M 203 USB 21.225.13 NO-052 ALC-M 85 USB 21.225.13 NO-053 HF1-REV-ALC 61 CW 1.800.00 NO-054 HF2-REV-ALC 56 CW 7.068.19 NO-055 HF3-REV-ALC 50 CW 21.225.13 NO-056 50M-REV-ALC 47 CW 50.000.00 NO-057 VHF-REV-ALC 62 CW 145.437.50 NO-058 UHF-REV-ALC 57 CW 438.900.00 NO-059 CW-CAR-LEVEL 144 CW 21.225.13 NO-060 AM-CAR-LEVEL 125 AM 21.225.13 NO-061 DEV-W 216 FM 145.437.50 NO-062 DEV-N 110 FM 145.437.50 NO-063 MOD-MTR 200 FM 145.437.50 NO-064 DTMF-DEV 10 FM 145.437.50 NO-065 CTCSS-DEV 233 FM 145.437.50 NO-066 DCS-DEV 168 FM 145.437.50 NO-067 LSB-CAR-POINT -7 LSB 21.225.13 NO-068 USB-CAR-POINT +5 USB 21.225.13 NO-069 VSWR2 at 10W 17 CW 14.257.90 NO-070 VSWR3 at 10W 42 CW 14.257.90 NO-071 ATAS-TEST LSB 14.257.90 NO-072 AMTR-TEST LSB 14.257.90 NO-073 HTEMP-THRESHOLD 38 LSB 14.257.90 NO-074 FTEMP-THRESHOLD 102 LSB 14.257.90 Hope thia helped........kevin KC8IDG

Might be worth checking the microphone cable on the transceiver, specially near the plug, or the push to talk switch. I saw this umpteen years ago with a CB that was intermittent.

High voltage capacitors are a bit hard to find but look on the net for vintage radio suppliers. While you have the tube radio open, look for resistors that gone high, this can put higher voltages than design on the caps and of course can quickly wreck tubes that were otherwise OK.

If the swr does not deflect even you press the PTT it might be the power amplifier has the problem,pls check also your power supply it should be regulated nor constant voltage output....