Yaesu FLDX 400 NL 6S

Discussion in '"Boat Anchor" & Classic Equipment' started by K7GLD, Oct 21, 2017.

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  1. K7GLD

    K7GLD Ham Member QRZ Page

    I'm restoring an old Yaesu FLDX 400 transmitter, but under the chassis is a bayonet base that is empty, and according to the schematic and parts list, there's supposed to be a NL6S bulb there - I believe that is a neon type device for voltage regulation, but online search has come up empty as to spec for that bulb. The transmitter seems to work OK without that bulb, but substituting a NE57 I had dropped power output to about half of the 175 watts without that NE57 - and the NE57 was pretty bright, so suspect the proper neon-type light has a higher operating voltage - sure would appreciate some help as to spec for a NL 6S, or possible source?
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Firstly, the PA is basically the same as in the FT101 series . . . so you should only be getting a maximum of 260 watts input, which gives 130 watts output.

    Where in the circuit is this neon bulb? The only one I recall is used in the VOX circuit (as a kind of coupler to level out the audio level) - so it won't affect the transmit signal in any way !

    Roger G3YRO
  3. K7GLD

    K7GLD Ham Member QRZ Page

    "Firstly", I can only tell you what my own power meter reads as to output without the missing NL 6S - which seems totally equal to my own FT 101E, as well as what other owners report the output of their FLDX 400's.

    My question is directed to (hopefully) someone with DIRECT working knowledge of the Yaesu FLDX 400 - NOT meaningless speculation or guesswork from someone not specifically experienced or knowledgeable with this transmitter. If all I wanted was random guesswork, I can do my own. ;)

    And finally, the basic question posed in this thread IS: "sure would appreciate some help as to spec for a NL 6S, or possible source?". But, thanks for the try...
    Last edited: Oct 21, 2017
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Well I repaired dozens of FL400s back when they were a current model . . . when I worked for the Yaesu importer here in the UK. (so I doubt if you can find someone more experienced . . . but guess you don't want my help)

    And if your Power Meter is showing more than 130 watts output from an FL400 or an FT101E, you should throw it in the bin.

    Roger G3YRO
    Last edited: Oct 21, 2017
    W4NNF likes this.
  5. K7GLD

    K7GLD Ham Member QRZ Page

  6. HAMHOCK75

    HAMHOCK75 QRZ Member

    I have no experience with this rig but for fun just analyzed the circuit containing the NL-6S. I assume you are not an engineer so I will try to keep this simple. It is possible to figure out the characteristics of the NL-6S just from the data in the service manual.

    This is no ordinary neon bulb and the NE-57 cannot be substituted. I will try to explain.

    The NL-6S and the 6BM8 form a voltage regulator. The pentode part of the 6BM8 on the right side acts as a cathode follower. It drops the unregulated +300VDC at its plate ( pin 6 ) to a regulated +150VDC at its cathode ( pin 2) to supply the bias for the screen grid of the finals. ( the FT101 does not do this. It has a winding on the transformer to create +160VDC directly. ) Adjustment potentiometer VR9 reduces the +150VDC to around +50VDC which goes to the grid of the triode portion of the 6BM8. The voltage at the cathode of the triode ( pin 8 ) is +50VDC. You can find this voltage listed in the voltage chart of the service manual. The triode compares the divided down voltage from VR9 to the +50VDC of the NL-6S at its cathode and amplifies it to adjust the grid voltage of pentode half to maintain a constant +150VDC if it tries to change.

    So the NL-6S is a very special neon bulb designed to have a very accurate 50VDC maintenance voltage. In other words it acts as a 50 volt Zener. You can substitute a Zener in its place. The NE-57 has an unspecified maintenance voltage so no one knows what voltage it will create at the cathode, but you can measure it with a voltmeter. It is kind of like replacing a Zener with an ordinary diode with unknown reverse breakdown voltage. The NL-6S has no polarity but a Zener does so beware if you use a Zener.

    This same circuit is used in the FL200B transmitter except the manual for the FL200B actually describes how this circuit works better. If you are into restoration as I am and want to find an actual NL-6S I would start scrounging for parts from this rig and the FL200B.
    Last edited: Oct 25, 2017
  7. K7GLD

    K7GLD Ham Member QRZ Page

    An amazing reply, and I sincerely thank you - looks like the 50 volt Zener is the best and easiest substitute, and your reply explains why the NE57 behaved as it did in relation to final output power of the rig, as well as the obvious component differences between the final circuitry for the 6JS6C's in the same tube usage in the FLDX400, and the FT101E.

    I'll wait a bit in case an actual NL-6S turms up, but if not, the Zener will undoubtedly get the job done - only remaining question is whether the supply side voltage for common 50 volt Zeners is too high compared to the NL-6S, or might need a series dropping resistor - the NE57 I substituted was ignited quite brightly, so assume the input side voltage might be pretty high for direct substitution of a 50 volt Zener? Yes - I can check specs for 50 volt Zeners, and get the input/output info, but you might have specific experience or advice...

    Thanks again, and any additional suggestions will be greatly appreciated!
  8. HAMHOCK75

    HAMHOCK75 QRZ Member

    I did do a brief analysis of what power would be dissipated in the Zener under worse case conditions. Supposing that the triode shorted between its plate and cathode, then +300VDC unregulated would flow across the 470K R78 to the Zener. The current would be ( 300V - 50V (Zener)) / 470K = 0.53 mA. The power dissipated would be P = E *I = 50V(Zener) * 0.53 mA = 27 mW. Basically, any Zener would be safe because there is already a dropping resistor ( R78 ).

    Some neon bulbs will be brighter than others because some are optimized to be display bulbs, others like the NL-6S are optimized for stability. As I recall the voltage can change if the bulb is exposed to light. The effect is called the "dark effect". This is a description of that effect and how they compensate for it.

    From https://www.vcclite.com/neon-application-information/
  9. K7GLD

    K7GLD Ham Member QRZ Page

    Great info - thanks again!
  10. K3RU

    K3RU Ham Member QRZ Page

    This thread makes me want to restore my old Yaesu FLDX400 xmtr. I also have the "matching" FRDX400 rcvr and spkr which I used extensively back in the 1970's and 80's. I don't think very many of those Yaesu "pairs" were sold as transceivers were fast "taking over".

    Haven't laid eyes on either of them since I put them in storage probably 35 YEARS AGO !!

    I feel so old.

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