FL2100 Modifications and Repairs

 Jump to high FL2100 Input SWR

Oscillation or Instability

The reason Svetlana tubes oscillate in the FL2100 is two-fold: 

1. FL2100 Bias is inadequate for Svetlana tubes. Svetlana tubes have LOWER mu (not higher as some people claim) and draw more current for a given amount of bias. 
2. The FL2100 has no neutralization and is not stable at HF under all load conditions. This is true with any tube used in this amplifier.

The FL2100Z was revised in an attempt to fix the inherent stability problems. The FL2100Z:

1. The FL2100Z has an improved (but still unusual) bias system.
2. The FL2100Z has a very poor attempt at neutralization or feedback.

Bias design error

If 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 Error

The 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.

A good design would never neutralize or add feedback from the OUTPUT of a tank circuit with a varying load back to the input.

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

When the antenna relay open, the LOAD is removed from the pi-network. This INCREASES the amount of feedback by several times. The tubes, having less mu and requiring more bias, are no longer in cutoff with the relay open and draw current. This allows the tubes to amplify, and the greatly increased feedback causes the FL2100 to oscillate on whatever frequency the tank is tuned to. With no load, the voltage soars to many kV until the bandswitch arcs.

Changing tube brands in even a reasonably design amplifier is no problem at all. Changing tube brands (especially when a brand does not have the same characteristics) in an amplifier with multiple major design errors can be a big problem.

The Svetlana "fix" of increasing FL2100 bias is really not a " fix". It is a marginal patch.

Corrections

Feedback

The most important "fix" would involve removing the capacitor from the output of the tank back to the tube input. This would disable the very poor feedback system. Neutralization page.

If the amp is unstable at upper HF, a neutralization system similar to that used in the AL-811H or AL-572B Ameritron could be used. 

Bias

The FL2100 bias system should be rewired so the relay opens the filament center-tap return. Normal operating bias should be left on the grids. This would FULLY cut-off any brand of tube.

Another common failure, High Input SWR.

The tuned input circuit in the FL2100 series has capacitors that are very prone to failure. The problem is the wire bond between the internal foil and capacitor leads open, resulting in an "open-circuit" capacitor.

The "problem capacitors" are C211 through  C219.

 

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.