Yaesu FLDX-400 VFO removal

Discussion in '"Boat Anchor" & Classic Equipment' started by AF5XF, Jul 4, 2021.

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  1. AF5XF

    AF5XF Ham Member QRZ Page


    So I recently bought a FLDX-400 transmitter as-is. It came without the cabinet or the final tubes.

    there are at least two different versions of this radio. One has a 12AU7 vfo, the other is transistorized.
    this is the transistorized version.

    so I fixed a few minor obvious problems, and bought a set of finals.
    I also brought it up on a variac.

    at this point I found that there was no screen voltage on the finals. backtracking I found that the 150v line feeding the screens had only 7.5k ohms resistance to ground when there should be 50k ohms.

    I have traced the 150v bus back through the radio and isolated the problem as being after the dropping resistor that drops the 150v to a zener regulated 9v for the vfo.

    i.e disconnecting the zener side of the resistor restores the 50k ohm test condition.

    so the problem is inside the vfo housing. perhaps a shorted zener, shorted transistor or capacitor, I don't yet know.

    SO how is the best way to get the vfo out of the radio to work on it?

    NOTE: while working on finding the problem I removed the 3 section can capacitor to restuff it, so I cannot power up the radio at this time.

    Thanks for any help provided.

    Matt, AF5XF

    P.S. attached the manual for the transistorized version. this is from Fox Tango. it has a lot of scan issues but it's the only one I've found.

    I also will be at work tomorrow so don't expect any replies from me before 5-6 PM CDT.

    Attached Files:

  2. AF5XF

    AF5XF Ham Member QRZ Page

    SO, I got the vfo out. here is the procedure for anyone else who may need this info.

    you will have to unsolder the voltage feedin(a blue wire connected to a large wirewound in my radio)
    and unsolder the vfo coax output.( attached to solder lug on vfo cabinet and to the vfo trap. this is a delicate cable be very careful.)

    take the vfo knob and dial assembly off from the front of the radio.
    remove the trim that is around the front panel, there are 4 small screws on the bottom and then the trim will slide off the front panel.
    reach inside from the bottom and loosen the 2 setscrews holding the vfo assembly to the reduction drive.
    remove the 2 screws holding the reduction drive to the front of the vfo and remove.

    there are 3 screws on the bottom of the vfo that hold it in place, remove these and then place radio right side up.

    the dial lamp will need to be removed from the socket and the socket will need to be unscrewed from the front panel.
    just scoot it over out of the way.

    at this point the vfo will slide straight up and out of the radio. you may need to flex the front panel a tiny bit to get it to clear

    Good Luck. Matt, AF5XF
  3. AF5XF

    AF5XF Ham Member QRZ Page

    now that I'm inside the vfo I'm finding more abnormalities. there is no zener diode!!!????

    removing the 2 FETs and the BJT restored resistance values to what i would expect to see.

    any suggestions??????????
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Without a Zener Diode the voltage to the VFO would be all over the place . . . and probably very high (which would probably blow the transistors in it)

    Has somebody removed it ? (it sounds like the rig has been messed with)

    And there are about 5 different versions of that rig, during its production run !

    Roger G3YRO (former Yaesu engineer)
  5. AF5XF

    AF5XF Ham Member QRZ Page

    sorry for the slow response, been busy working and haven't checked the forums lately.

    this rig has Serial # 2N314063 (I'm thinking Dec 1972?).

    about what I thought regarding the voltages. could not determine if it has been messed with, I only found one solder joint in the whole rig that looked like it had been redone.
    (I installed a zener at the proper location)

    5 different versions(and only manuals to be found for one or two)!!!

    I have VISUALLY compared the radio to the schematic over the course of about a week. so far most everything seems to be in order, the only things I have found is

    V5(carrier oscillator) is not wired anything like shown on schematic.the manual states that the carrier osc portion of the tube uses the pentode half while my schematic shows the it using the triode half.

    The mechanical filter is not grounded on one side but has a 10k across the output terminals in addition to the 10k to ground on pin 2 of V2 and is capacitively coupled to pins
    2 and 7 of V2(this looks to me like a rf short of both output terminals to one electrode?)
    pin 7 doesn't have a 470p to ground either.

    V2 shows to have Pin 3 and Pin 8 connected to a single 1K resistor to ground and side-band oscillator coupled in capacitively.
    however my radio has 3 and 8 separate with their own resistors(820 ohm) and two capacitors joined like a Y
    (one end of one cap connected to 3, one end of other cap connected to 8, remaining two leads tied together and connected to the side-band oscillator injection coupling cap.)

    I'm currently waiting for a set of PCB drills so I can make a new VFO PCB, I lifted some traces removing the old transistors, but its a simple board not too hard to create another one just like it.
    when I get the vfo PCB rebuilt I can test fire it on the bench outside of the radio before installation.

    I bought a set of 6js6B's for the final but cannot get them to cutoff at the proper bias voltage.
    the manual contradicts itself showing 140v on V9 pin 2, and 150v on V13/14 Pin 3/11.
    with 140v on Pin 3/11 I have -55v in operate mode to set the bias at 50ma,
    and -65v in standby which allows the tubes to draw up to 20ma while siting still.(manual says -45/-55)

    its a head-scratcher to me about these differences from an otherwise ordinary radio vs. schematic.

    I'm wondering if I should rebuild these areas to match the schematic I have in hand?
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Well you might have a later rig with improvements ! So why change it?

    What does your VFO knob & Skirt look like . . . is it the same as on the FT101 ?

    Roger G3YRO
  7. AF5XF

    AF5XF Ham Member QRZ Page

    the vfo knob & skirt are identical to the FT101.

    my test gear is limited(dvm, 20Mhz scope, homebrew freq counter currently needing a battery, and uncalibrated output signal generator) but last night I verified that the 455kc osc is oscillating and the lower sideband osc is oscillating, the upper sideband osc has a bad xtal(i can hear something rattling inside the can).

    don't yet know if any of the band crystals are working, but two are missing(one I can't read the numbers on, and the other is a 33.020 xtal from a FT101)

    many thanks for any help you can provide me with.
    Matt, AF5XF
  8. AF5XF

    AF5XF Ham Member QRZ Page

    well I found out AFTER opening the USB Xtal to see if it could be salvaged that it was intact on the inside, so I put it back in the radio and decided after awhile to tweak the trimmer cap for that Xtal to see if that would change anything. the trimmer still had the factory paint on it and didn't look like it had been adjusted but it was screwed all the way down.
    backing it out 1/2 to 1 turn caused it to begin oscillating but with only half the amplitude of the LSB Xtal and its off freq.
    (with peak amplitude about 8.5 kc low. backing trimmer screw all the way out will get about 3.5kc movement in the right direction)

    I tried placing a 10p cap in series with the trimmer but with negligible effects.

    so what to do now? did I damage it(likely) while looking at the inside of the Xtal?

    Matt, AF5XF
  9. AF5XF

    AF5XF Ham Member QRZ Page

    finally got my drills in and created a new VFO board, testing on the bench I had a nice 5Mhz signal. got it installed and put everything back together. so far I have got it working on CW. I have 120w output on 80 and 40m, 65w on 20 and 15m, and 10-20w on 10m. Don't have a mic to drive it with so I haven't tested it on SSB yet.

    it drifts considerably.

    end notes: making progress.

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