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Do NOT burnish or file contacts. Not ever. |
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Skip To Correcting Receive Problems
There is a tendency to blame the relay (and switches) straight away for all sorts or amplifier problems, from high input SWR to intermittent output.
Relays (and switches) very rarely cause input SWR issues or intermittent operation on transmit in an amplifier. Despite the relative rarity of contact problems stopping transmission or causing high SWR, one of the most common things people do is "clean contacts". Cleaning contacts generally should be one of the last things done, if at all, in transmitters.
This isn't to say a badly pitted contact, or relay that has been physically or electrically abused or damaged, won't ever have problems severe enough to stop an amplifier from transmitting. This failure just very rarely occurs in something that has not subjected to some extraordinary physical or electrical abuse, like lightning.
There are rare cases where a relay or switch contact can cause intermittent transmitting, but such cases generally mean the relay or switch has been so severely damaged replacement is a better solution.
Cleaning often leads to life and reliability problems. Cleaners themselves can contaminate ceramics in high power switches, reducing voltage breakdown. Misplaced lubricants also catch and hold dust and dirt, reducing voltage breakdown.
Intermittent connections in any pressure contact, from relays to large switches, is almost always aggravated by low or zero current. These poor connections almost always heal at the first application of RF power. It is actually very rare for switches and relays to open while carrying high power. It is very common for them to develop open or poor connections with very low voltages and currents, such as when receiving signal or panel meter currents pass through them.
One way to clear a bad connection on receive is to "bump" the path with a little power. If the receive drops down, or drops out, from a bad connection in an amplifier relay, place the relay on standby and bump the system with exciter RF. Another way is by application of a dc "wiping" current while bumping the relay. This will often restore receiving for a while. You'll see why below.
Order of Relay Problem Occurrences
The following data is from my 30+ years in manufacturing engineering, 45 years in repair and service, and 48 years building equipment. Much of my experience comes from companies like Ameritron, who have sold more remote antenna switches and amplifiers than any other company by far. For example, while I was general manager of Ameritron at Prime Instruments, we sold about 100 antenna relay systems and over 50 amplifiers per month. More than 500 RF switching relays went out every month. While I cannot disclose current numbers and products, I currently have access to data on relays numbering well into the thousands every month. This gives a good database.
In order of failure, properly sequenced and non-abused relays have the following issues:
1.) Lack of continuity on receive. This is the largest failure by a very considerable amount, accounting for around 90% of all relay field problems (based on service and warranty history).
2.) Welding or contact pitting from lightning, tube arcs, or hot switching. This is around 9 or 10 percent. This number has gone up with the proliferation of Chinese tubes, which have an unusually high arcing problem. At times it is customer induced (switching an antenna switch while transmitting) or lightning induced.
3.) Mechanically induced problems from mechanical shock (dropping the item or device) or mechanical abuse (installing a cabinet screw that pushes into the relay). This is a very small number, requiring carelessness or human error.
4.) Open coils. This is very rare, but it happens.
5.) Contamination of pole pieces with oxides or deposits that causes the moving armature piece to lodge against the pole and stick. This has happened a few times with the rotor on my large tower. The cure is a shot of WD-40 on the pole (not the contacts) and drawing a thin hard cardboard back and forth between the pole and armature while forcing the armature down by hand. I initially thought this was a magnetized pole, because the pole appeared "sticky" or to capture and hold the armature
By far the most common relay problems or outright failures are lack of receive, or high resistance connections. Now let's look at a few common claims or causes I have never seen:
1.) Weakening of beryllium copper contact carriers by flexing. I have seen this when excessive current heats the beryllium copper to the point of oxidation and discoloration. I have found a few relays particularly sensitive to RF heating of contact carrier bars. The relay used in Ameritron power line transfer makes a very poor RF relay at very high power, despite its robust appearance. At about 8-9 amperes on steady 10 MHz carrier for five minutes, the contact bars in the AC power relay will overheat. This same relay is fine at 30 amperes 60 Hz AC (the actual application) yet when operated at higher radio frequencies current or duty cycle must be substantially reduced. Why the poor RF performance? This particular power relay is designed for high switching currents with high contact pressures, not RF conductivity.
2.) Residual magnetism in pole pieces. If this happens, it must be rare. I've only heard of this from one person or source. I haven't ever seen this, nor have service techs recalled this as a problem. If residual magnetism is a problem with a relay, it should be a problem from the first relay operation. To be magnetized, the wrong iron would have to be used in the relay. Proper magnetically-soft irons do not harden over time. The material is either a magnetically soft iron incapable of supporting much residual field after excitation is removed, or a magnetically hard material that retains magnetism.
Even the proposed solution is strange, reversing coil leads. If we reverse coil leads, unless we apply a reverse field above the magnetic material's coercivity, nothing changes. If we did change magnetism, it would simply reverse the field.
Removing a magnetic field (without heating or hammering) requires exposure to a gradually decreasing alternating field! TV sets with CRT's use a decreasing alternating field to degauss the CRT mask. As a rule demagnetization progressively occurs only when the magnet is exposed to cyclic fields sufficient to move the core away from the linear part of the magnetic B-H curve, gradually walking the alternating field down to zero.
In the 1980's I managed a meter manufacturing operation. We manufactured and calibrated meter movements. I designed devices to "charge" magnets to controlled levels, allowing us to calibrate meter movements and meters. When we went past the desired level of magnetization, we had to erase the field magnet to low levels with AC and start over, gradually stepping up the charge. DC fields up, and AC fields down. Of course there are other tricks to remove magnetism, like heating or physical shock, but none would work with an electromagnet.
Contact Materials
Many people think silver makes the best low-pressure contact material. The
basis for this misconception is that silver oxide is a conductor. Unfortunately,
pure silver or silver flash is poor choice for dry or low current switching! Silver and silver alloys have terrible sulfidation problems, especially in urban
areas. With shelf times as short as a few weeks, a clean silver contact can
contaminate with a thin layer of sulfides. Silver low-pressure
connections do not have long low-voltage (receiving) life or reliability!
The best receiving or low power transmitting contacts have a very thin gold flash. While the
gold flash solves receiving
return problems and low-current low-voltage connection problems, it also creates a
new problem. Gold flash is thin and soft, and does not take well to sanding,
filing, rubbing, heat or
arcing. Gold flash
should not be burnished, filed (no contact should be filed), or cleaned with
anything abrasive. Burnishing a gold flashed contact can quickly take the
contact back to the raw base contact alloy. This reduces shelf and service life,
increasing surface resistance and receiving connection problems.
Pretty much any material that reduces pitting (arcing during switching) makes receiving contact problems more of a problem. Conversely any
material that makes low voltage low pressure connections better is damaged
easily by hot switching or arcs.
Failure to return to receive is often mistakenly assumed to be a "sticky relay". This is because bumping the relay or lifting the contact carrier generally fully restores receiving. Receiving connection failures are common, because the contact is in a near-zero current and voltage contact operation. The real problem is a very thin film builds up on contacts. Without enough voltage to punch through the insulating layer and enough current to "clean" the film away, we have only mechanical wiping and pressure to break through the thin (generally just a few molecules thick) insulating layer that builds up over time.
The largest single problem with amplifier relays is caused by "dry" switching. Dry switching is where a relay switches with virtually no contact current. The lack of current lets the relay contact build up a very light non-conductive film, often just a few molecules thick. This has been a problem since the days the first relays were used. Telephone companies applied a small "wetting current" to relays to increase reliability.
High contact resistance is by far the single most common amplifier and antenna switch relay issue. Large relays suitable for transmitter power have a large contact. For a given return spring tension, a larger contact has less pressure per unit contact area. This means less mechanical pressure to push through non-conductive surface contaminants. High current contacts often use materials that resist pitting and withstand arcs, and that generally means the materials is wrong for low current or dry switching applications.
Over the years I've looked at dozens of ways to run a "wetting current". None
have been satisfactory for many reasons. The general overriding problem is
"wetting currents" cause a "pop" or voltage-spike when going back to receive.
The system also requires either isolating or "grounding" the load and source
ports for dc, and
that adds a new set of reliability or component problems.
If receive drops out when going from TX to RX, it almost certainly is not the
relay "sticking", or a
magnetized relay pole. The problem is, nearly all the time, caused by larger
relay contacts
running at zero current.
Cleaning a Relay
There are two methods of cleaning relay contacts.
Physical Cleaning
If your amplifier has an open frame relay, wet a piece of solid glossy paper with cleaner and draw it between the closed contacts. Proper physical cleaning involves drawing a hard glossy paper, soaked in a cleaner, back-and-forth between the contacts. DO NOT soak the relay ever.
Electrically Cleaning
Electrical cleaning can be just as effective, if not more effective, then physically cleaning a contact. If you have a few parts and some ingenuity and electrical aptitude, electrical cleaning can be one of the fastest and safest cleaning methods to restore weak signal or dry switching. Some relays are enclosed, giving us no choice but to electrically clean. (Either electrically clean them, or replace sealed relays.)
Trace through the schematic or circuit wiring to verify the center pins on the input and output RF connectors have a direct dc path. If they have a dc path, your system is a candidate for electrical cleaning.
I recommend using a 12-volt supply of around 2-amperes or more as a power source. Connecting the supply through a 10-ohm power resistor, or 12-volt ~one ampere light bulb (like an automotive incandescent brake or turn-signal bulb), to the exciter (radio) input connector of the amplifier or antenna switch. Ground the amplifier antenna connector center pin, or return the center pin by wire connection, to the opposite terminal of the power supply.
Turn the amplifier on, and cycle the antenna relay in and out of transmit a few dozen times. This will almost always completely burn off any film on the contacts without damaging contact plating.
All Ameritron amplifiers are suitable for electrical cleaning, without removing the cover, so long as they do not have an internal PIN diode switch. Other suitable amplifiers are the Heathkit line, RL Drake (L4 and L7 series), and many others.
RF Cleaning
If you are brave, have an amplifier with a low-pass or band-pass tuned input, and have an SWR protected transceiver, it is sometime possible to RF-clean relay contacts. Place your amplifier on 160-meters, and your radio on 40 meters. Go in the FM or RTTY mode, and with the amplifier on standby, adjust for about 10-20 watts. With a carrier on, cycle the standby-operate switch over-and-over dozens of times.
I do this to clean remote antenna relays, but ONLY with ten watts or so.
Mechanical Failures
A less-common failure is contact welding, pitting, or pocking. These types of problems can occur from hot switching, tube arcs, or lightning. in severe cases the relay can weld and stick, and in other cases the surface is damaged resulting in unreliable connections. In any case, even if the relay "un-sticks", the contact surface and delicate thin layer of flashing that allows dry switching is damaged or destroyed. This results in an unreliable relay.
While filing, tinning, burnishing, and other harsh repair methods might temporarily restore operation, plan on replacing the relay.
A second cause of mechanical failure is physical damage. This can be designer error aggravated by carelessness, such as locating a relay where an excessive length cabinet screw can push into the relay. Often times people lose screws, and someone just grabs a random length screw to bolt a foot or cover back on. The designer, through poor hole or parts placement, in essence set the system up to fail. Several commercial amps have screws entering directly in-line with the relay, and just fractions of an inch away. Before closing up any cabinet or changing hardware, look to see what components the screws might hit.
Accidents also happen, so try not to mechanically shock the relay by dropping
equipment. Pack well for shipping, with at least 2 to 4 inches of proper
density closed cell foam supporting any amplifier you ship.