Converting FT-101 Rigs From Sweep Tubes to 6146B Type Finals                 

Important Note:  Before attempting this conversion, know that there are dangerous and potentially lethal voltages involved.  Unless you are absolutely certain that you know what you are doing and have taken the proper  precautions, do not perform this conversion.  
I cannot be - nor  will  be - responsible for any injuries or damages incurred as a result of your reading this website. 


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Introduction

Previously Documented Conversions
 - QST Article - Roger Davis - ZS1J
-  Bob Goodrich - K7KMQ

Most Recent Conversion - Estimated Costs  
 
        -  Conversion Overview
        -  PA Tube Selection
        - Candidate Radios for Conversion
        - Starting the Conversion
        - Replacing the HV Filter Capacitors
        - Reassembling the Radio
        - Neutralization Capacitor
        - Higher Power Conversion
        - Final Alignment Steps

Hints and Kinks:

        - Electrolytic Capacitor Replacements
        - Pi-Net Coil Burned Connections
        - Low Drive / Low Power Output
        - VFO Re-Alignment
        - Low Sensitivity / 'S Meter' Indications
        - Regulator Board Improvements
       - Transmit / Receive Frequency Tracking

1. Introduction

    In the 1970's Yeasu produced the FT-101 series which were well received by the amateur community. Using easy to repair or modify plug-in boards. and the (then) low cost sweep tubes found in many TV sets, they were capable of RF outputs of 130 watts on most amateur bands.

   Yaesu designed the FT-101 / B  / E / EE / EX and F series around the 6JS6C tubes.  However, their engineers subsequently opted for the 6146 tubes in their FT-101ZD series. Kenwood did the same in their hybrid TS-520 / 820 radios.  The deliberate design shift from 'sweeps' to genuine transmitting tubes was most because sweep tubes quickly became obsolescent as televisions became increasingly solid state.

    There are many FT-101's that could be pressed back into amateur service if only the proper 6JS6C sweep tubes could be found. Regrettably, perfectly fine radios are currently being systematically 'cannabalized' and parted out.

2. First Documented 6146B Conversion

    In 1999, Roger Davis - ZS1J - wrote a QST article describing how 6146B tubes could be used in an FT-101B radio.  An addendum followed improving the neutralizing circuit. This conversion worked for some amateurs, but others reported 'whistles, pops and cracks' on the higher frequencies while receiving. These complaints - and negative comments made on various user groups - tempered subsequent conversion enthusiasm.

3. Second Documented Conversion

    Bob Goodrich - K7KMQ - updated the QST article with one of his own.  In it, he shows how he ingeneously solved the neutralization problem with a 400 pf mica capacitor (instead of the 2000 pf recommended in QST).  His website article provides some very nice pictorials of his conversion.

4.  Third Documented Conversion - Schematic - Parts Listing

    This follows the work done by the two 'pioneers', offeing an overall description of the conversion, a parts listing and a step-by-step conversion process.  Interested parties are invited to improve upon this effort and document their own progress, either here, or elsewhere.   You may email me here.

   Note: With new parts, the conversion costs (including the tubes) are about $65 - less for those with a well stocked junk box.

    a) How Does the Conversion Work - a Low and High Power Option
    If the user is satisfied with an 'honest' 70 watts output (maybe a bit more) on 160, 80, 40 and 30 / 20 meters (with about 40 / 45 watts on the higher frequencies), the conversion is finished, except for the installation of the 10 meter trimmer, setting the bias, neutralization, etc.  

    But if the user wants around 95 watts on the lower frequencies, additional parts (a 5 watt wirewound resistor, three zener diodes, three ceramic capacitors and another small diode) needs to be added on one terminal strip. Two (regulator board) capacitors also need upgrading. Other more involved steps (described later) can likewise be taken to boost the output power to close to 115 watts on 160 meters, etc.

    b) Which Final Amplifier Tubes to Use?

    Either the 6146B or its 12 volt equivalent - the 6883B tube - can be used.  The 6883B is a better choice as it is less expensive and can often be found as NOS (New Old Stock).  The schematic drawing that accompanies this article depicts wiring for both 6 and 12 volt filament tubes.

    You might be able to use 6146A's, but the output power will be less and the screen grid voltages should be reduced..  No thought has been given to using the Chinese equivalents as their neutralization requirements might be different.  Let's buy American if we have a choice!

    c) Get A Working, 'Known Good' Radio, If Possible

    The radio chosen for conversion would best be one whose history is known, perhaps one's own radio whose finals have weakened or a radio purchased from a friend.   While suitable radios may be found on eBay, the buyer is taking a chance that the seller's description is accurate and truthful.  

    Before starting, ensure that there are no open or shorted power transformer windings.  These transformers are tough to find and a nuisance to replace.  If the radio receives properly with an antenna connected, and if it will produce some output either at the antenna or via the transverter jacks, then you may start.

    If you have a nice, but non working radio to convert, if the power transformer is good, and if all boards are present, you can repair it using the FT-101 Service Manual.  Depending upon the nature of the problem(s) and the time you have available, you may need a signal generator and a 'scope.

d) Starting the Conversion - Here's the Schematic

    Get a container for the screws and, with ith the radio unplugged, remove the top and bottom panels from the radio, followed by the wrap around cover.  

Note:  This is a good time to repaint the case, sanding it down and applying two coats of automotive primer.  Give the primer a day or two to dry, wet sanding as needed.  The finish coat paint is Rustoleum Painter's Touch, Ultra Cover -  Gloss - Winter Gray, an excellent match -  Home Depot.  Let the finished coated pieces bake in the sun for a couple of days to give the finish coat ample time to harden.
e) Replacing the HV Filter Capacitors with Low Profile Units

    This is a good time to replace C77 & C78 as failure here could damage the power transformer.  If you are NOT replacing them, continue here.
f) Reassembling the Radio
Note: For the High Power conversion, Regulator Board C7 and C8 (22 mf) should be changed to 350 volt units to provide an adequate safety margin for the increased screen grid voltage. These capacitors 'stiffen' the voltage and minimize voltage swings and IMD problems.
g) Neutralization Capacitor

    A new 390 pf 1KV mica capacitor is installed on the driver's output side for neutralization. Failure to do so may cause premature PA tube failure.
 h) Padding Capacitor - If you don't plan to work on 10 meters, skip this step.

    Due to the reduced interelectrode capacitance of the new tubes, a small trimmer capacitor must be added across T103.
    If satisfied with the 70 watt Low Power option, do the final alignment.  For close to 100 watt output on the lower frequencies, then continue below:

i) Higher Power Conversion

    The FT-101 power transformer supports the 6JS6C tube.  While not built for RF service, this tube can generate more RF that the 6146 and at lesser voltages.  However, 'sweeps' can be easily be damaged by improper tuning, over driving and higher than designed AM output levels.

    When wired with the 117 volt taps, the power transformer will supply about 625 volts to the plates and 165 volts to the screen grids in the 'no load' state.  The screen grid tap on the power transformer also supplies final amplifier bias.  If the screen grid voltage were higher, the FT-101 would be able to produce 80 to 90 watts on 80 meters.  But unfortunately, it doesn't.

    About 220 volts is needed on the screen grids of the finals to help them develop maximum output power with the existing power transformer.    The goals are to maintain relative voltage stability (to minimize IMD) without exposing the screen grids to more than 250 volts (as specified in the datasheet).
    So, a  5 watt 5.6K resistor connects to a zener diode series arrangement limiting the screen grid voltage to around 230 volts.  This circuit also contains an  isolation diode (protects the power transformer from a plate to grid short - a valuable suggestion by N4ML).  All this fits on a 7 lug terminal strip.  

Note: Once 'warmed up', this dropping resistor / zener diode arrangement draws about 18 ma. while receiving and 11 ma. when transmitting.  There doesn't appear to be any over heating problem especially since the 6883 / 6146B's draw less filament current than the 6JS6C's.  

Admittedly, the resistor zener diode combination will generate some under-the-chassis heat even when the filaments are turned off.  This has not proven to be a problem in the radios that I've converted.  However, if the user would like to eliminate this heat, a supplemental relay may be installed below the chassis as described on this link.  

    To prepare the terminal strip before installing it, first:
  1. mount the zener diodes (D2*, D3*, D4*), add the
  2. zener diode noise suppression capacitors C9* ,C10*, C11*; add the
  3. 1N4007 screen grid isolation diode D1* ,  and add
  4. R4*, the 5 watt, 5.6K wire wound resistor.
    The terminal strip may be soldered directly to the chassis. Before soldering, thoroughly sand the areas where the two 'feet' will be mounted.  Use a pair of 'vice grips' to hold one of the lugs flat while you solder the other.  

    Once done, connect the two wires (one from the NEGATIVE side of C77, and the wire removed from the 160 volt rectifier tap), double check your work and turn just the power switch on.  Then, verify that there is approxinately 230 volts on the PA tube screen grids.. Turn on the filament switch, wait for 60 seconds (for the tubes and zeners to warm up), and adjust the bias (below).

Note: Take care that your components will not short out to the bottom plate when it is installed.

Also Note:  I have some terminal strips if anyone needs them - for just the cost of the postage.

j) Final Alignment Steps - Setting the Bias, Neutralizating the Finals and Adjusting the 10 Meter Trimmer Capacitor

i) Setting the Bias   

    With both power switches in the ON position, place the function switch in either the LSB or USB position, operate the MOX switch and adjust the PA bias to 60 ma.  Release the MOX switch, turn both power switches off and unplug the radio.

Important Note:  if you are using the supplemental relay, wait about 15 seconds before setting the bias.  This allows enough time for the zeners to heat up so that the voltage will be at its maximum.

 ii) 'Cold' Neutralization - Thanks to Peter Roberts - G4DJB - Please Review Before Attempting
   
    Neutralization minimizes signal feed thru by coupling a signal back to the driver equal in amplitude but opposite in phase.  Use a non metallic adjusting tool as there are lethal voltages present:
             iii) Adjusting the 10 Meter Trimmer Capacitor
    Before replacing the radio in its case you may want to perform a complete alignment per the service manual as some additional capacity has been added to the T103 circuit..  When finished and if you've selected the High Power option, and if your line voltage is around 117 volts, your radio should put out close to 95 watts on 160, 80, 40 and 30 meters, close to 90 watts on 20 meters, and less on the higher frequencies (which is to be expected).  

    Note: If the line voltage is less, then the RF power will be somewhat less.

     k) Capacitor Replacements

    You may want to replace all the electrolytic capacitors as they can become problematic as they dry out.  There is an eBay vendor who offers a complete capacitor kit with instructions, claiming that the complete conversion can be done in 4 to 6 hours.  I ordered the parts from Mouser and it took me a lot longer, but it was a relaxing activity in its own peculiar way.

    While apart, consider replacing C131 and C13 with 1KV mica units.  These capacitors have been known to fail with sometimes catastrophic results.

    l) Pi-Net Coil Burned Connections

    Inspect the connections underneath the pi net coil for burning.  If the previous owner was a 'Golden Screwdriver' who flogged it on 11 meters, then there's a good chance that the 10 meter coil tap and the one that feeds C25 may both be burned.  This happened on one of the radios that I converted.

    For more information, check out the following W4CLM post which describes the problem / solution in detail: http://groups.yahoo.com/group/FT-101/message/3164 .  Carol does really nice work, doesn't she?

    m) Low Drive / Low Power Output

    If you are unable to get at least 90 watts out on 80 meters with a new set of final amplifier tubes, you may want to check the high voltage.  Rather than opening up the PA compartment (can be dangerous), measure the voltage on the 600 volt 'tap' of the rectifier board beneath the chassis.  Under no load, it should be in the vicinity of 625 volts, and under load - around 590 volts.  If the voltage is significantly less, then one or more of the rectifier diodes may be 'marginal' or leaky, especially if the radio was owned by the bane of the FT-101, the worldwide illegal CB operator.

    It can be difficult to find a maringal diode under load. The best way to solve this problem (if you think you have it) is to replace all 8 diodes with 1N4007's.  The P/C board solder mask shows correct diode orientation.  While you're at it, replace the 8 470K resistors and any other component that may be burned and / or out of tolerance.  In one of my converted radios, R1 - a 5.6 ohm, 2 watt resistor - was severly burned, was still operational, but was also replaced.

    C11, a 1KV 200 pf unit on driver plate often becomes 'leaky', reducing drive.  It's impossible to remove C11 as it is buried underneath the bandswitch.  However, by clipping the lead of C11 that connects to the bandswitch at the same point (brown wire) where the new neutralization capacitor is attached, C11 can effectively be replaced with a new unit.

    If you are experiencing a low drive problem when in the TUNE, CW or AM modes, take a look at the setting of TC5 on the modulator board. This capacitor is used to peak the output of Q2, the AM Modulator / CW buffer. The CW signal is used when tuning up.

   Place your scope (or VTVM) probe on the 'hot' side of the Drive control and then adjust TC5 for max signal while in the MOX mode. TC5 can be reached from the top of the chassis if you use a long non metallic adjustment tool. You can also check the setting of the CW trimmer while you are there.

    n) VFO Re-Alignment

    If the top scale of your VFO is out of alignment, it may be realigned by adjusting a trimmer on the top of the VFO (the speech processor vill have to be temporarily removed if your radio is equipped with one).  Facing the radio, adjust only the trimmer on your LEFT.  The trimmer on the RIGHT should NOT be touched as it was calibrated at the Yaesu factory to minimize VFO drift.

  o) Low Sensitivity and  No 'S Meter' Indications on Weak Signals

    If you experience poor sensitivity on all bands, if your S Meter refuses to budge on all but the strongest of signals, and if the marker generator will cause the S meter to properly deflect, check out the little lamp / fuse on the rear panel.  This is a protective device designed to save the RF amplifier circuit from damage if the radio is exposed to a strong RF signal (say, at Field Day).  If the lamp is open, the residual capacity within the bulb will be adequate for the radio to detect weak signals, but the S meter won't move.  For what it's worth, there was a GE #47 bulb in my radio.

p) Regulator Board Improvements - PB-1314

    Remove the bottom cover, let the radio warm up for a bit and measure the voltage on the '6 volt reg' input to the VFO. Set it to 6 volts (VOLT adjustment) and watch it for an hour or so. If the voltage starts to drift with heat, the frequency of the radio will also drift - bad news! The obsolete voltage regulator chip on the regulator board is intermittent - quite common with the older PB-1314 Regulator board.

  To fix it, remove the regulator chip, the associated components and the solder traces. Install an LM-7806 chip with a .33 mf on the input side and a .1 mf cap on the output. This solution was suggested by N9WB and it works great - no more voltage drift! You can also remove the VOLT trimmer as it will no longer be needed. Why confuse the radio's next owner?

q) Transmit and Receive Tracking Adjustments

  There are at least 3 ways to accomplish this:

   1) If you have another SSB transceiver, connect a dummy load to it, send an SSB signal and tune your FT-101 until the voice sounds 'natural'. Place the dummy load on the FT-101, operate PTT and talk into the mike, listening with the other radio. Adjust the ZERO trimmer on the regulator board until your voice sounds natural.

Note: If the ZERO trimmer runs to one extreme or the other WITHOUT producing a 'right on' signal, then perform the CLARIFIER ADJUSTMENT as described in the service manual, and repeat..

  2) All you need is an inexpensive HF SSB receiver.  Pick a band and place your receiver next to the FT-101, and set it to either LSB / USB. Tune your monitor receiver between 8700 and 9200 kHz and listen for the VFO beat tone. Set the receiver fine tuning to give you a tone you find pleasing to your ear. Press the PTT switch and listen to the tone. You will most likely hear a shift. Set the "zero" pot on the regulator board to eliminate the tone difference. You can do this adjustment to get the difference down to about 5 Hz; or whatever your ear can detect. It's simple, and it works. - from W4CLM / K6WWH.

 3) Connect an accurate digital voltmeter to the CLARI input to the VFO. Place the radio in the LSB /USB receive mode and note the voltage. Depress PTT, wait a second or two and then while holding PTT, adjust the ZERO trimmer until the voltage EXACTLY matches the receive mode voltage. Repeat several times to ensure that you hit the 'sweet spot'. If this doesn't work properly, you may have to perform the CLARIFIER ADJUSTMENT and then repeat

DISCLAIMER - - If you follow the steps outlined herein, you do so at your own risk. I cannot, nor will not, be responsible for any possible damage to radio equipment, personal property, to yourself or to others caused by modifications that you may make as a result of your reading this.