Procedures to test the Main Meter and make adjustments.  Document has schematic to build your own circuit if you don’t have an All In One II test and calibration unit.

This is a passive device that can correct line meter errors of plus or minus 5 volts making it possible to save the old meter. New ones are difficult to find and expensive if you do. You can keep your tester looking original with this modification.

Physically it takes advantage of two unused posts on the resistor board plus one added terminal to mount the components.  Late model 539C’s will require 3 posts to be added.  A simple and easy conversion that has a minimal effect on the Transconductance, but a dramatic effect on the accuracy between testers.

This application of ohms law gives us a simple way to measure Plate Current without breaking the circuit to insert a Current Meter. You only need to connect the multimeter when testing for Plate Current, since the resistor inside the tester completes the Plate Circuit.

For reasons unknown Hickok omitted the Bias Fuse Bulb in these two models. It was used in models before and again after in both the 752 and 580 models. Many others have installed their own version or had a repair facility do it for them.

Operation of the Hickok 539B & 539C Tube Tester with Digital Main Meter.

Operation of the Hickok 539B & 539C Tube Tester with Analog Main Meter.

There is between 7 and 15 ohms difference between the 2 plate windings of the power transformer.  This unbalance is created by the layering of the windings and the need to balance the AC output.  As you can see this does not create a significant error until the plate current gets to around 40 mA.

It has been common practice to replace the two electrolytic capacitors C1 and C4 when servicing these tube testers along with the other capacitors. The following outlines a negative result that can occur when replacing C4.

Tube testers had to operate over a wide range of line voltages and loads. This was accomplished by using transformers with lower voltage primaries. By placing a rheostat is series between the line voltage and the primary of the transformer it could be set on the meter to an established value. Of course this resistance in the rheostat could cause a sag with the load of the tube being tested. Various methods were used to compensate for this, but it had an overall effect on the accuracy of the tester.

The Bridge Circuit consists of two 40 ohm and two 60 ohm precision spool resistors in these units. Over time many of these spool resistors have changed value or failed. Today these resistor values are no longer available in the precision and wattage required.

When Hickok designed the 539C to replace the 539B new sockets had been introduced, which created issues fitting them into the faceplate. A compromise was made with the 9 pin miniature tube socket to accommodate the 10 pin nixie socket. Since the added pin was in the middle of the socket the retainer machine screw could not be used to secure the 9 pin socket saver.

This conversion replaces the type 83 and 5Y3GT tubes with solid state components. It provides the correct voltage drop over the necessary range of test current. No wiring changes are necessary as it plugs into the existing tube sockets. For the later model 539C’s the tube socket assy. must be turned over.