Maintaining and troubleshooting HF welders: A common-sense approach for vacuum tube and solid-state machines

The Tube & Pipe Journal January/February 1998
February 19, 2001
By: Ray Cagganello

The basic steps of general preventive maintenance and troubleshooting for both vacuum tube and solid-state high frequency (HF) welders should assist in keeping welders online and producing pipe or tube.

Vee length

Figure 1Typically, the vee length depends on the mill design, but it should not exceed the tube OD.

Since the 1960s, the workhorse of the tube and pipe industry has been the high frequency (HF) vacuum tube welder. Recently, an increasing number of producers have been installing the HF solid-state welder, in part because of its efficiency, compact design, and high power factor.

Many vacuum tube welders are still in use, however, and operators need to be just as knowledgeable about current vacuum tube maintenance and troubleshooting techniques as they are about solid-state procedures.

This article outlines the maintenance and troubleshooting procedures for each type of welder.

Vacuum Tube Welders

A vacuum tube welder has four major sections: the power supply, which converts alternating current (AC) voltage to direct current (DC) voltage; the oscillator, which converts DC to HF power; a cooling system; and a controls and diagnostics package to monitor and control the welder functions.


To maintain a vacuum tube welder, operators should be completely familiar with the system's technology and adhere to all safety procedures (such as Occupational Safety and Health Administration [OSHA] directives, lockout/tagout, etc.).

Maintenance should be performed every six months or at least once a year, depending on the production schedule. After the welder has been locked and tagged out, all exterior walls and panels should be completely wiped down before initiating the following:

  1. Verify all safety devices and door interlocks.
  2. Inspect all door and panel gasketing.
  3. Inspect all interconnecting wiring between the power supply and oscillator and controls.
  4. Inspect all grounds and high-voltage wiring.
  5. Verify all water flow gauges.
  6. Drain, flush, and refill distilled water circuit and repair any leaks.
  7. Inspect all water hoses for wear or discoloration.
  8. Vacuum oscillator and power supply cabinets.
  9. Clean all walls and floors in oscillator and power supply cabinets with clean cotton rags and water only.
  10. Inspect and clean the air-to-water heat exchanger and change all filters.
  11. Verify settings on all spark gaps.
  12. Inspect and clean output radio frequency (RF) transformer and copper bus bars.
  13. Verify all Teflon®insulation.
  14. Disconnect and clean all copper connections in the oscillator and power supply, then reconnect properly.
  15. Inspect RF choke for discoloration.
  16. Inspect SCRs, grid resistor, and feedback circuit for proper resistance value.
  17. Inspect rectifier stacks for shorted diodes.
  18. Inspect ceramic capacitors in the oscillator for leaks or cracks.
  19. Remove oscillator tube(s) for inspection and hipotting.

When maintenance is complete, perform a final visual inspection to ensure the system has been reconfigured properly. Then notify the mill operators to energize the system to verify proper operating conditions.

Preventive maintenance is essential on all vacuum tube welders. If operators keep the distilled water clean, keep the interior of the cabinets clean and dry, and check connections and components regularly, welder downtime should be drastically reduced.

Assessing Problems

Troubleshooting must be performed by fully trained personnel under the guidance of the welder manufacturer. Most welder manufacturers have a service staff available by phone 24 hours a day, 365 days a year.

Operators should never hesitate to contact the manufacturer for help. If a problem cannot be solved over the telephone, the manufacturer will dispatch a field engineer to perform on-site emergency service.

Welder faults fall into several categories: problems outside the welder, in the weld area setup, or in the mechanicals.

If the heat fluctuates without adjustment of the welder controls, the problem could be either impeder saturation or a breathing or rolling vee.

If the impeder is going in and out of saturation, this will be shown as weld current not flowing regularly in the vee and flowing on the inside diameter (ID) of the tube. This typically happens if the impeder does not get enough coolant or the coolant lines become blocked during operation.

The solution is to verify that impeder coolant is flowing properly; if it is, then the strip presentation to the weld point should be examined. The strip must address the weld point in a stable manner (the vee length must remain stable). If it varies, the weld current will vary, causing a noticeable heat fluctuation in the weld.

A similar problem is insufficient heat generated into the weld vee, especially on small-diameter tubing. This can happen because no impeder was used, or it was too small or, depending on the tubing size, the vee length is well beyond the norm for that particular tube outside diameter (OD).

The rule of thumb for HF induction welding is that the impeder should occupy 75 percent of the ID of the tube and should extend .125 inch beyond the apex of the weld rolls, extending upstream through the coil by one coil width.

The more impeder an operator can fit inside the tube without any mechanical interference, the more efficient the welding operation will be. The impeder is the most easily overlooked component but perhaps the most important for welder efficiency.

The vee length should be kept to a minimum. Typically, its length depends on the mill design, but it should not exceed the tube OD (see Figure 1).

Another kind of problem is a short circuit in the welding system, usually noticed by a fault registered by the diagnostics.

When a fault is registered, first visually inspect the system with power off. Check the oscillator, output station, and power supply to identify anything unusual, such as water leaks, burning, arc marks, or damaged or cracked components.

If no obvious problems are found, the system needs to be separated and troubleshooting should begin.

Troubleshooting Vacuum Tube Welders
  1. Separate the output station from the oscillator to validate the RF output transformer and associated leads and insulation by installing a test coil in place of the output station and then energizing the welder at low power.
  2. If the welder stays on line, the problem is in the output station, usually the RF transformer or Teflon insulation on the output leads. If the welder registers the same fault, the output station is all right and the problem lies elsewhere.
  3. Reconnect the output station to determine if the fault is in the power supply or oscillator cabinet. Isolate and disconnect the high voltage (HV) DC connection to the oscillator (more commonly known as the b+ connection). Energize the power supply to full DC voltage with regulation.
  4. If this is accomplished, the problem is in the oscillator; if a fault is registered, the problem is in the power supply. Assuming the power supply is working properly, begin troubleshooting in the oscillator cabinet.
  5. Validate the working condition of the oscillator tube(s). All vacuum tubes have a finite life, and users should have reliable spare tubes as backups. Trained personnel are needed to change the tube. The tube is fragile, and all tube manufacturers' warnings and specifications must be adhered to.
  6. Once the spare tube is installed and preheated, re-energize the system. If no fault is registered, the problem has been solved; if a fault occurs, continue troubleshooting.
  7. Investigate for a shorted tank capacitor by isolating one tank capacitor at a time on each side of the tank circuit; then energize the system at low power for each set of capacitors until the faulty component is identified.
  8. If a bad tank capacitor is not located, move the diagnostics onto the grid circuit, where many problems can be discovered. The grid circuit of most vacuum tube welders consists of many capacitors, chokes, and resistors. Each component now must receive a thorough examination by being removed, one at a time, and hipotted or checked for proper resistance value to the manufacturer's specification.
  9. Once the oscillator has been diagnosed as problem-free, assume that the power supply is not able to achieve full DC voltage without a fault occurring: recall earlier that this happened when the HV DC was disconnected from the oscillator. This means a component is breaking down under load.
  10. If so, keep the DC disconnected from the HF oscillator and, with power off, again visually inspect the low- and high-voltage sections of the power supply.
  11. Once the visual check is performed and no problem is found, begin diagnosing the power supply. Start at the b+ filter network and work toward the incoming AC line. After disconnecting the input to the filter, energize the power supply and determine if a fault will register with the filter out of the circuit, checking to see if the capacitors or iron core choke of the filter network are breaking down under load.
  12. The strategy is to isolate each piece of the power supply until the faulty component is found. After the filter inspection, disconnect the rectifier stacks, the plate transformer, and finally the SCRs.

Solid-State Welders

Although their use is increasing, HF solid-state welders have a downside because of their infancy compared to vacuum tube welders, which have been around for almost 40 years.

Even if a vacuum tube welder is 20 years old and its manufacturer is no longer in business, that welder can be maintained by a long-standing competitor because of the common operational attributes of all vacuum tube welders.

With solid-state welders, however, customers must be careful. Manufacturers do not always have the ability to diagnose competitors' equipment because of the diverse technologies of solid-state welders on the market.

A solid-state welder has the same four major components as a vacuum tube welder, except that an inverter section replaces the oscillator.


All of the safety and documentation reviews must be performed, as with vacuum tube welders, followed by the proper lockout/tagout of the equipment and a thorough exterior cleaning of all cabinets and panels.

An acceptable maintenance schedule is once every 12 to 18 months, depending on production requirements. Maintenance should include the following:

  1. Verify all safety devices and door interlocks.
  2. Inspect all door and panel gasketing.
  3. Inspect all interconnecting wiring between power supply and inverter cabinet and between controls and ground connections.
  4. Drain, flush, and refill distilled water circuit.
  5. Inspect all hoses for wear or discoloration and repair any leaks.
  6. Inspect all flow gauges.
  7. Thoroughly clean all interior walls of power supply and inverter cabinet.
  8. Vacuum interior of cabinets.
  9. Inspect and ohm-out SCRs to manufacturer's specifications.
  10. Inspect all control board connections; proper electrostatic discharge (ESD) procedures must be followed—a critical step of maintenance.
  11. Inspect all internal power cable and copper connections for proper torque.
  12. Inspect all Teflon in the inverter cabinet and bus bars.
  13. Verify all fuses with a continuity check.
  14. Verify settings on all limit switches.
  15. Inspect capacitors in the inverter section.

Troubleshooting Solid State Welders

On solid-state welders, the circuit design, lower voltages, and circuit-board-mounted technology help to increase reliability and mill uptime. When problems do arise, however, fault diagnostics may be needed.

Most solid-state welders today are delivered with a computer graphics diagnostic package. If a fault occurs, the diagnostics package directs operators to the most probable cause. A modem can also be installed to access the system remotely to aid in troubleshooting.

With any welder, the most common fault is a short circuit, caused by an insulation breakdown or component failure.

When a fault does register, the diagnostics should aid in directing operators to the first area to investigate, which is usually outside the welder proper and in the area of the coil or contacts:

  1. Inspect induction coil for overall condition and insulation status; also check coil connections to bus bars.
  2. Inspect contacts for excessive wear; check insulation between contacts.
  3. Inspect bus bar insulation for contamination from the mill area; it should be clean and free of holes or tears.
  4. Inspect the weld area, the weld rolls, the impeder, and general alignment of the coil or contacts for any mechanical interferences.
  5. If no fault is found in the external area, inspect the power supply and inverter cabinet. Look for the obvious — a water leak, or condensation buildup, or any general overheating.
  6. If no problem is found, review the fault array. Most solid-state welders have an internal diagnostic array that will show whether or not levels of the system are functional. This is usually LED-driven and mounted on the assemblies.
  7. In the inverter section, first inspect the fault LED array for proper illumination: HF modules, HF power supplies, auxiliary supplies, and HF control circuitry.
  8. If all the inverter LEDs are normal, inspect the power supply fault array: the SCR assembly, control board, firing board, and 24-volt power supply.


These basic steps of general preventive maintenance and troubleshooting for both vacuum tube and solid-state HF welders should assist in keeping welders online and producing pipe or tube.

To achieve this result, operational personnel must be fully trained on all aspects of safety and equipment. If problems arise that cannot be readily addressed, operators should contact the welder manufacturer for technical assistance.

Ray Cagganello

Contributing Writer

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The Tube & Pipe Journal

The Tube & Pipe Journal

The Tube & Pipe Journal became the first magazine dedicated to serving the metal tube and pipe industry in 1990. Today, it remains the only North American publication devoted to this industry and it has become the most trusted source of information for tube and pipe professionals.

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