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The following rough notes
concern my beloved Amp Supply
Co LK550 HF amplifier. I
bought the amp second hand
from a white stick op in the
UK during the early 1990s and
was my faithful companion for
20 years until Elecraft
brought out the KPA500 and I found an affordable Alpha 87a. The LK550 has 3 x 3-500z valves which, with sufficient drive (more than 100W I believe), makes it capable of kicking out rather more than the 1kW we are allowed. I’m not one to push the red line constantly though. For DXing and contesting, I’m much more concerned about having a clean signal and reliable equipment, which means deliberately under-running stuff. Normally the KPA500 is sufficient.
Over the years, I’ve
made a few repairs and mods
to the LK550, usually with
help from other knowledgeable
hams. This page is my attempt
to give something back.
Warning - lethal high voltages inside
The LK550 HT runs at about 3kV off-load, and dips to about 2.6kV on load.
3kV at
up to an amp or more will fry
and most likely kill you in
an instant, with no second
chances. This
rates alongside skydiving,
motor racing, coal mining,
electric chairs, urban search
and rescue, looking down the
barrel, and playing with
gunpowder and matches in
terms of personal danger.
Even with the main switch
turned off but the mains lead
still in the wall receptacle,
the full mains voltage is
exposed on the main switch,
and that can certainly kill
you too. If you
don’t know exactly what
you’re doing with high
voltages, don’t risk it
- go find a competent amateur friend or professional who builds and repairs valve amps instead. Even if you do know what you’re doing, be extra careful with extra high tension if you value your life and don’t want to be nominated for a Darwin award.
If you ever need to take the
lid off the amp and you have
been using it lately, first
check that the front panel
plate voltage meter is
working by turning the meter
knob to VOLT. It should read
about 3kV with the power on.
Now turn off the mains supply
and unplug it from the wall.
Watch the HT voltage using
the front panel meter until
it has bled away, ideally to
an indicated zero: mine shows
over 500 volts after the
first minute, and still over
100 volts after two minutes
so I recommend waiting AT
LEAST three minutes,
especially as your amp may
behave differently to mine.
Double-check that the correct
mains plug is out of the wall
and remove the Jones-type HV
connector to the HT
transformer from the rear of
the amp. Only now is it safe
to remove the screws and
carefully lift off the lid,
and even so there may be
sufficient charge left in the
PSU capacitors to give you a
nasty surprise: once the lid
is off, use a “chicken
stick” (an insulated
wand with a wire tip
connected to the chassis) or
a good quality insulated
screwdriver to short between
the chassis and the anode
caps. If you hear a distinct
crack, you were a bit too
keen to get the lid off but
be thankful that you heard
anything because at least it
means you are probably still
alive (so far). You may have
just damaged your smoothing
capacitors with that
discharge though, so
don’t make a habit of
it.
LK550 repairs
High voltage power supply capacitors
According to my trusty notebook,
the HV PSU electrolytic
capacitors in my LK550 let
out the smoke with a pop one
day in 1994, blowing the
mains fuse. The originals
were 7 x 450V 160uF
capacitors which I
couldn’t source at the
time, so my friend Ross
GW3NWS advised me to replace
them with 400V 220uF
“computer grade”
units from RS Components. I
added two extra capacitors to
the earthy end of the string
on a little daughter board to
give a bit more headroom for
the normal 3kV HT: the new
set should be OK up to 3.6kV
and in practice has worked
just fine ever since.
Be careful about the
capacitors you use in this
application. I invested in
new “computer
grade” electrolytics.
“New old stock”
electrolytic capacitors
should be carefully re-formed
before being subject to high
voltages, and they may still
fail anyway. Poor quality
capacitors may not have
sufficient ripple current
ratings and so may overheat
and fail prematurely.
Capacitors that burst open
make a real mess and stink
so, trust me, this is best
avoided.
I
checked the original
equalizing resistors, adding
another 100 kOhm 2W resistor
across each of the additional
electrolytics. I also checked
all 32x 1N5408 diodes in the
bridge circuit and they were
all OK, despite the blown
capacitors. Guess I was lucky.
Antenna change-over relay
The little Jennings 24V
vacuum relay (used to switch
the antenna between a bypass
line to the amplifier input
connector used on receive and
the amplifier output on
transmit) doesn’t last
forever. The obvious symptom
of something amiss is that
the rig goes deaf after
switching from transmit back
to receive. A sharp thump on
the amp box or stamp on the
PTT footswitch usually brings
the receiver instantly back
to life, but within a month
or so even those tricks no
longer work and it’s
definitely time to swap out
the failing relay.
The relay is located right
behind the SO239 connectors
on the rear panel. It is
mounted on the QSK board,
with three RF connections on
one side and two relay coil
connections on the other.
Replacement relays are
available from various HV and
amp parts suppliers but shop
around as prices vary
markedly. Second-hand relays
pulled from commercial
equipment may be cheaper but
probably won’t last as
long. Similar relays are made
by other companies such as
Siemens.
I
made a rubber gasket to
reduce the loud click when
the relay fires. It does
helps make the changeover
quieter but wrapping some
sort of sound insulation
material around the body of
the relay, or buying a relay
already encased in silicone
rubber, would probably be
worthwhile as well.
[See the mods section below for ideas to extend the life of vacuum relays.]
Coupling & decoupling capacitors
In 2008, something in the amp
let out the smoke with a loud
bang. Turns out a decoupling
capacitor in the PA
compartment had blown a chunk
of itself out. The capacitor
decouples any RF that gets
past the anode choke to
ground before it has a chance
to get into the HT power
supply. This was an easy fix
to make - the only hard part
being to find a source of
suitable HV capacitors.
A
pair of coupling capacitors
feed RF from the anodes to
the tank compartment. I
changed those at the same
time.
Padder capacitors
On the low bands, the air-spaced variable capacitors don’t have enough capacitance to resonate
the tank circuit so the bandswitch adds in some additional fixed capacitance.
-
On 160m, the padder consists of a ‘ladder’ of several paralleled HV ceramic capacitors right
beside the band switch (soldered between the two vertical wires on the far left of the whole
amp shot below - I had removed the old ones at this stage). The dark colour and waxy
deposit visible on the photo above shows that these were suffering due to age and heat.
-
On 80m and 40m, the padders are tucked underneath the LF coil to the left of the band
switch on a pair of solder tag strips bolted to the cross wall/chassis.
I replaced the padders at the same time as I replaced the blown coupling capacitor, figuring that
these were probably to blame for the amp needing more and more load and plate capacitance to
load up properly on the low bands.
On 160m, unfortunately, the replacement padder capacitors caused a strange drifting effect: on
key down, the output would climb gradually to the normal level over a few second but would drop
back down again between overs. I guess that it was a thermal effect so replaced the 160m padder
capacitors again with a larger HV capacitor from Russia, but that didn’t work properly either. I’m
still looking for the right type of padder capacitors for 160m ...
Output valves (tubes)
I replaced a faulty 3-500z once due to a cathode-grid short caused by mechanical damage while
transporting the amp between houses. I also replaced all three valves with a matched triplet of
Chinese ones from Taylor as the output seemed to be declining, and I wanted to have some spare
valves on hand. Changing the valves is easy - just treat them gently, take care to seat them
properly in the bases and replace the anode caps properly. Personally I try to avoid touching the
glass with sweaty fingers and leaving fingerprints that will probably go brown later, using pieces or
clean rag or tissues to hold them, but that’s not strictly necessary.
Thoriated tungsten filaments (heaters) in valves such as the 3-500 need to run hot for a few hours
to reach full emission. The suggestion is to turn on the amp after fitting new valves and leave it on
standby for a few hours before transmitting (though I can hardly wait!).
It is also recommended to run the valves hard (so the anode fins glow orange) every so often to
heat the zirconium getter compound on the anode and so destroy any gas that may have leaked
past the seals. In my case, this means running key down QRO at least once a year, into a QRO
dummy load. And by the way, it’s best to run any spare 3-500s through this procedure at the
same time since leaving them sitting in storage for too long increases the chance of a catastrophic
flash-over when they are eventually put into service. Alternatively, I guess it’s possible to use a
variac to reduce the HT, but only if you separate the HT from the heater and LV control wiring
which should remain at the normal supply voltage.
Main supply switch
A distinct thud
when turning on the amp suggests that there is a sizeable surge as the HT
capacitors charge up and the cold tube filaments suck up the juice. Turning off the amp doesn’t
make the same thud but there’s a good chance there is a pulse of back-EMF from the
transformers, and quite possibly an arc in the mains switch. That probably explains why I have had
to replace the main switch at least two or three times already. For the past few years, I have been
using a high current mains switch (32A at 250v AC) intended for electric oven supplies, which
meant resizing the panel cutout with a file but was otherwise quite simple to fit. That switch has
lasted longer than previous ones.
[A more permanent solution would be a step-start circuit - see the mods below.]
Anode choke
 During the past year or so, the anode choke on my
LK550 has gone dark about a third up from the lower
HT supply end, and the turns have become even more
uneven there (I’d noticed some loose turns before).
Lately, the amplifier has started taking off occasionally
, the key symptoms being no output, off-the-scale
negative grid current, about 400mA of plate current
and a burning smell, all of which continues even if I
stop transmitting and release the PTT. I’m pretty sure
this is due to progressive damage to the anode choke
, itself probably caused by me operating the amp on
30m using the 20m position (the amp’s design pre
-dates ham access to the WARC bands). What I
presume was a choke resonance near 30m made the
amp perform quite inefficiently on that band, needing
about 50W input to squeeze 2-300W out instead of
the normal 25W in for 500W out on the other bands.
The bunching up of turns is probably a sign of
overheating, making the thin enamelled wire sag.
Rather than just rewind the choke and test it for
unwanted resonances using a GDO, in my junk box I
found a suitably meaty HF choke on a larger ceramic
former than the original. Using a coupling loop on my
MFJ259 with the choke shorted out, it appears not to
resonate in any ham band but there is a resonance just above 15m. It is a tight squeeze in the PA
compartment but hopefully the air flow is still sufficient to cool both the rear 3-500’s. It’s a good
thing I’m not running the amp flat out.
In case the surplus choke doesn’t work out, I’ve also bought a choke from Ameritron (now part of
MFJ) that is rated for 4kV at 1.5A (6kW input). Not bad for US$20 + P&P.
LK550 modifications
Extending the useful life of the antenna change over relay
Someone (I forget who) suggested that the reason the main antenna change-over vacuum relay
fails periodically is that, over time, the actuator coil becomes permanently magnetized, just enough
to prevent the normally closed relay contacts closing completely when the relay is powered off.
They suggested changing the polarity of the coil supply, which means swapping over the
connections to the coil. To avoid having to open the box and resolder the connections every time,
I’ve fitted a small DPDT switch to the rear panel of the amp, wiring the coil connections to the
middle pins and the supply leads to the outer pins on one side, with a pair of patch wires connecting
them to the outer pins on the other side but swapped over. I’ve marked the switch positions on
rear panel “EVEN” and “ODD”, intending to flip the switch according to whether it is an even or odd
month of the year. I should be able to tell you if this works in a year or so!
[I’ve also heard it suggested that running a bit of DC through the relay contacts on receive will help
remove any corrosion on the contacts, but I’m not entirely convinced: apart from the need to rig
up a DC supply and fit coupling capacitors in the RF path, the whole idea of using a VACUUM relay is
that there should be no reactive gases in there to cause corrosion. However, it might be a way of
rejuvenating some of the old relays that I have removed and kept, since they are quite expensive
to keep replacing.]
[I keep meaning to confirm that the relay is not hot-switching on QSK as that will definitely shorten
its life. All I need is some time to rig up the oscilloscope, find a way to sample some RF on one
beam (perhaps there’s a suitable contact in my external wattmeter?) and connect the other scope
beam to the keyer. Meanwhile, I just avoid using QSK most of the time, and make sure the K3
allows a few milliseconds’ grace between PTT and transmit.]
Quietening the antenna change over relay
Like most amplifiers, the LK550’s stock antenna change over relay makes a noticeable and
annoying click every time it operates due to the mechanical movement. Inserting a foam rubber
gasket (homebrewed to fit) between the relay body and the flange has quietened it down
significantly, so much so that I sometimes find myself checking the standby/XMIT switch or power
meter to confirm the amp is actually working!
Fan speed control
NB I haven’t made this mod yet - it’s just an idea at this stage.
The LK550 has a two-speed fan, manually selected from a front panel switch. I always try to
remember to flick it to the high position when transmitting but the noise means I prefer to leave it
on the low position when idling. I’m a bit worried about transmitting with it on the low position,
even though at around 500 watts out it will probably be just fine, so I’m planning to dispense with
the separate fan switch but use the transmit-standby switch to turn the fan to high when set to
transmit.
Other suggestions include:
-
Automating the fan speed switch using a temperature sensor in the PA compartment -
perhaps a mechanical thermostat as an electronic one may not work well in the high RF
environment.
-
Using an external electronic thermometer to monitor the PA exhaust flow, ideally with an
alarm if it climbs above, say, 50C.
-
Coupling the PTT control line to the fan speed switch with a relay, such that the fan set to high
whenever the PTT line is earthed.
-
Using a temperature-sensing proportional fan speed controller (perhaps one from a PC) with
sufficient decoupling and shielding to ensure that it works even with lots of RF floating around.
-
Replacing the 4 muffin fans with a set of temperature controlled computer fans (though I
would have to match the CFM ratings of the originals, and ideally the temperature sensors
would have to be placed on the far side of the valves).
-
Replacing the 4 muffin fans with a “proper” squirrel cage blower on the rear panel (probably
too much mechanical work for me, and those blowers are noisy too).
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