Good morning to the group.
I experienced a grid-filament short in my TL922 while testing
a pair of unused 3-500Z tubes which I had kept in storage for
several years. The reason for trying the unused tubes was that
the amp was giving low output after service in the CQWW SSB.
The grid short took out several small components.
A friend in Finland sent me the service manual for TL922
(thanks OH5BM). I identified the failed and suspect components:
L7 470 uH RF choke grid to chassis.
R22 4.7K 1/2w resistor grid to chassis.
V1 3-500Z grid-filament short (V2 = OK)
D1 diode Relay power supply
C3 33uF 160V Relay power supply
PL1, PL2 Dial lamps out. Unable to replace
I was surprised how dirty the inside of the amp had become after
10 years service. Vacuumed much dust from RF deck, power supply,
and bottom side of cabinet. Dust was inside Plate and Load plates
of capacitors. Sprayed *Contact Cleaner* on relay and band change
switch- and rotate knob.
I was unable to source the RF choke in New Zealand.
After removing the RF choke from grid circuit, I cleaned
off soot and carbon (caused by burned resistor).
One lead ( to inside layer) is still intact.
Other lead is burned up (like fuse). Unwound two turns
from top layer, scraped off insulation from wire
and re-soldered to pigtail of ferrite former.
Checked resistance = 3.0 ohms.
Also checked other RF choke on right-hand tube = 3.0 ohms.
Replaced 4.7k ohm 1/2 watt old style morganite (composition) resistor.
Removed diode from relay (and shut-off bias) power supply.
Replace with 1N4007 (1000v 1A)
Install 100uF 160v filter capacitor.
Checked operation of Zener diode:
Removed connection from Zener diode (type is 7.5v 10W).
Connect 570 ohm load and 15 to 30 volt power supply.
Voltage across zener = 7.2V regulated
Insert 2x 3-500z tubes (original pair used in CQWW SSB)
Top cover is off (no HV as interlock removes T1)
Measure filament voltages with tubes in sockets:
V1 = 5.1V AC
V2 5.1v
across supply = 10.2v
Measure relay supply = 110v DC
Measure shut off bias at cathode = 100v
Insert short circuit RCA plug in relay control.
Select Std-by -> Operate
Relay is activated.
Place multimeter leads between center pin of RF-IN connector
(SO239). Measure resistance at cathode blocking
capacitor = 0.2 ohms. Rotate band change switch from 1.8 to 28 MHz
All positions = 0.2 ohms.
Measure between RF-Tank circuit and RF-OUT connector = 0.2 ohms
Therefore antenna relay and band change switch is all OK.
Replace Plate leads and fasten top cover.
Connect AC cord, switch on power.
HV = 2000V (CW position) - reading on panel multimeter
HV = 3100V (SSB position) - reading on panel multimeter
In Operate position Ia = 140 MA SSB (should be 200ma ???)
Placed TL922 in the line. Using original tunes (as in CQWW SSB).
Apply 80w drive. Ia = 650 mA, Ig = 200mA, Po = 1000w 14 Mhz
Po = 900w 21 MHz
Po = 850w 28 Mhz
SWR between TS680 and TL922 below 1.2:1
No adjustment made to input circuit.
Summary
-------
Still not sure why we had low output before the experiment
with unused but faulty tube (one of two).
But the symptoms of high input SWR and low-output power
have gone, while still using the original tubes.
Thanks to the friends who offered suggestions during this
repair project. My second op Dragan ZL1/YU1UN gave me the idea
of repairing grid RF choke - lucky that the side that had
burned off was from the top of the winding.
Regards,
Martin ZL1ANJ
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