In its factory-stock form, the TL-922 has the following
problems:
1. The filament inrush-current is 48 peak amperes. This exceeds Eimac®'s
maximum allowable rating for the 3-500Z.
2. When operated from a stiff 240vac source, the inrush-current through
the on/off switch is so great that it will eventually cause switch failure.
.
3. The filament-voltage is typically 5.3V measured between pins 1 and 5
on the 3-500Z sockets when the amplifier is operated from 120V/240V, 60Hz.
5.3V exceeds Eimac®'s maximum filament-voltage rating. According to
Eimac®, each 3% increase in filament-voltage, above what is needed to
achieve full power output, will reduce the useful emission life of a directly
heated amplifier-tube by one-half. The bottom-line is that at 5.3V the useful
emission life of the 3-500Zs will be reduced to about one-sixth of what
could be realized if the filaments were operated at 4.8V.
4. The noisy RF relay is so slow that it causes the amplifier to hot switch
with many modern transceivers.
5. Burned or vapourized output-bandswitch contacts; shorted zener bias diode;
arcing/pitting of tune-capacitor's plates; filament-to-grid shorted 3-500Z
or 3-500Zs. These problems are caused by an intermittent VHF parasitic-oscillation
at 130MHz. See "Parasitics Revisited" in the September and October,
1990 issues of QST magazine, for a discussion of this subject.
Fixes
Fix for problem 1: Install a stepstart circuit for the entire amplifier.
All that's needed is a 100vdc, or 12vdc, or 24vdc-coil; dpdt relay with
10A contacts, and a pair of 25 Ohm, 7 - 10 watt resistors. If a 12v or 24v
coil relay is used, a rectifier-capacitor filter circuit is added to rectify
the 11vac that is available from the filament-transformer. A fullwave bridge
circuit is used for 12vdc coil relays. A fullwave voltage doubler circuit
is used for 24vdc coils. Use a series R between the coil and the source
of DC voltage to control the relay pull-in threshold. Adjust the R so that
the relay pulls in when the HV reaches around the 60% level.
Fix for problem 2: This problem is taken care of by the fix for problem
1.
Fix for problem 3: Add resistance between the filament-transformer and the
filament-choke. The easiest way to accomplish this is to replace the stock
heavy-gauge wires between the filament-transformer and the filament-choke
with smaller-diameter, lossier wires. If your line-V is 240V or 120V, a
slightly longer run of 22-gauge, >105ºC-rated, stranded hookup wires
will bring the filament-voltage down to the optimum 4.8V. 22-gauge wires
will run comfortably warm to the touch in this application.
Fix for problem 4: Add high-speed RF relays and an electronic cathode bias
switch to the amplifier. This is not a difficult modification. The circuit
is shown in the January 1994 QST on page 33. The stock relay in the 922
makes an excellent step-start relay. It can be mounted in the roof of the
terminal box at the rear of the amplifier. There is plenty of room for the
step-start resistors. Mounting holes for a perfboard are inside the terminal
box. The resistors are mounted on the perfboard.
Fix for problem 5: Remove the stock high VHF-Q parasitic-suppressors and
install low VHF-Q parasitic-suppressors. Repair any damage that was caused
by the intermittent VHF parasitic oscillation. Items to check: the Zener
diode front to back ratio; the plate current meter shunt resistance; and
the grid to ground RFCs are prime candidates.
Note: Fixing the TL-922's designed-in problems will definitely void the
warranty. The good news is that most warranty returns seem to be caused
by the designed-in problems. If the problems are fixed up-front, the TL-922
will very likely perform reliably for many years. >>>Modified TL-922s
should never be returned to "Factory Service." Such amplifiers
should only be touched by knowledgeable persons.<<<
If you need help repairing or modifying a TL-922, please feel free to telephone
me at [805]386-3734. If you need a parasitic suppressor retrofit-kit, I
sell them. They have a money back guarantee. If you would like to discuss
the step-by-step process of modifying a TL-922, try Dario, N5QVF, [915]-691-9808
in Abilene, TX.
This is the short version of the circuit improvements section in "QSK
for the TL-922 and SB-220
With Circuit Improvements for the TL-922"--which is available on the
home page.
End