Yaesu FL-2100B Amplifier at ki6jjw

I have a Yaesu FL-2100B amplifier (twin 572b's) that I got which was in pretty rough condition when I examined it. The plate current meter cover had been smashed, and an incorrect replacement was glued over the meter itself. It does at least serve to protect the movement but sure won't win any prizes among the restoration crowd. Heh, heh. Watts is watts on the air! I'm definitely a fan of "run what ya brung" and Operating!

Among other ills it had when I aquired it, was a burned out parasitic resistor assembly.

The power supply interlock had been burned up by previous operators.

More signs of heavy heat buildup are a classic cracked high voltage ceramic capacitor.

Happily, the band switch seemed to be ok.

The underside shows to be reasonably clean despite a couple of mods -- the unit was restored to factory design before beginning upgrades.

Repairs and (upgrades... no) redesigns include:

1. W8JI relay/center tap bias mod

2. PA0FRI' style parasitic supressor

3. changed "bubbled" 220pF with 500 pf silver mica on grids

4. Replace 1k pF ceramic in HV with one moved from other circuit

5. Replace moved 1k pF with .01 uF 3kV 

6. Install HV glitch resistor

7. Install 1k pF added bypass at HV glitch resistor

8. Replace 80M HV capacitors in tank (500/300 by 650/140)

9. PA0FRI' rerouted variable capacitor ground path

10. Diode protection on meters

The above is to document what I did to my amp, and is not so good since item eight is wrong, and only done because the parts were cheap. Now I have run 500 watts out into a "Van Gorden G5RV" and a homebrew tuner with the amp on 80 meters so there is that I guess.

In 2013 I installed a 3 element Yagi-Uda for 10 meters at 25 feet altitude (on the home roof). Being in the bedroom on the second story just underneath the antenna was closer to an antenna on HF than I'd been so far. Of course that puts me in the near field and I had a devil of an R.F. problem when I ran the amplifier on 10. I solved it with snap-on ferrites on the mic cord, tranceiver D.C. power cord, and TX out right at the transceiver output (an FT-757GX).

When I did the W8JI center tap mod, I used a salvaged relay out of an outdoor floodlight unit. This was not the best choice for long term functionality. Nothing wrong with the relay, it was just seriously undersized for the amplifier usage. It was being used to switch dual 80 watt lamps at 120 volts, so it should have been good for at least an amp. W8JI's web page claims that a "...low current relay" will be sufficient in this usage. Not to my experience! Cough, if you think about it, the center tap returns the eight amps filament current to ground. Yes, a one amp rated relay WILL burn out with eight amps through it.

I started getting reports of "Your audio has some distortion" then finally the amp was merely 600 watts of QRM (ouch). It seems I burned up the relay contacts in the salvaged relay. I replaced the teeny relay with a Guardian double pole, both poles ganged, rated at ten amps. Fired it up and have the nice 600+ watts out again, with "good audio" reports.
September 2009

FL-2100B Resources

I've found several web sites with good information on amplifiers with two 572b's such as the FL-2100B. In no particular order:

Further Mumbling

There were two "Dentron" 572b's in the unit when I obtained it. One of these had a part spring loose inside and roll around. I've not run that tube since I noticed that. Cough. Fortunately I was able to loot a Cetron '572 out of a homebrew project I also have. It seems to be a sturdy rebuild so far. 2009