FL2100 Modifications and Repairs |
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Oscillation or Instability
The FL2100Z was revised in an attempt to fix the inherent stability problems. The FL2100Z:
Bias design errorIf you look at my page on amplifier cathode bias, you'll notice the best practice is to let high mu tubes float up to cutoff bias by opening the cathode return path while the control grid is either grounded or biased negative The FL2100 only applies ~15 volts negative bias to the 572B grids for cutoff. 15 volts is not adequate for some 572B tubes, especially those with slightly lower mu like Svetlana tubes.
Yaesu obviously learned standby bias was not high enough. Later versions of the FL2100, the FL2100Z, used a voltage tripler in the bias. While this is a great improvement the best method is still to let tubes seek their own bias by opening the cathode path.
The FL2100Z has an overly-complicated three voltage power supply for control and bias. Cut-off bias Diodes D301, 302, and 303 along with capacitors C301 through C301 form a negative voltage tripler for tube bias. Approximately -45 volts bias is supplied through R303. This is adequate to cut off any tube, regardless of slightly lower production mu. Operating Bias On transmit, a divider consisting of R301,302, and R4 is connected. R301 and 302 connect to a "tap point" on the tripler where -15 volts is available. This divider supplies the operating bias. Bias is slightly unstable with grid current, and decreases the IM performance of the FL2100Z. Control Voltage +15 volts is supplied from D304. D304 is a half-wave rectifier filtered by C304, supplying positive "12 volts" for the antenna relay RL301. Had cathode bias been used, only one supply would have been required!
An inexpensive 12V low current relay corrects the bias issue without complicated circuit or wiring changes. This circuit is normally not required in the FL2100Z, but it is absolutely necessary in the FL2100.
Feedback Design ErrorThe FL2100 design uses a capacitor from the OUTPUT of the pi network back to the input of the tubes. A good design would NEVER do something like this. A pi-network transforms phase along with load impedance. As the tuning and loading controls are moved, and as the antenna load impedance changes, the phase of the feedback voltage changes. The output tuning and loading as well as the antenna impedance greatly affects feedback voltage.
This is because the load impedance as well as the setting of the plate and load capacitors will vary feedback phase! Feedback phase should be a stable 180-degrees, but in this case it can range from -80 to -150 degrees depending on band and pi-network control adjustments.
Effect of
Yaesu FL2100 design
errors
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| C215 | C211,216 | C212,217 | C213 | C218 | C214 | C219 |
| 100 pF | 200 pF (180 or 220 pF OK) | 250 pF (220 or 270 pF OK) | 350 pF (330 or 360 pF OK) | 400 pF (390 pF OK) | 500 pF (470 or 510 pF OK) | 1000 pF |
Any silver mica over 500 volts dc rating is fine for these capacitors. I do not have any idea why Yaesu used such unreliable high-voltage capacitor. Voltage across each capacitor is only 150 volts peak.
Capacitors, if both are changed on any band, should be resized in the same direction for the same band. In other words if you substitute a 270 pF for C217, C213 (same band) should be subbed up to a 360 pF.
I have a full set of suitable reliable replacement capacitors available for $28. This includes priority mail. We do FL2100 upgrades.
