600 Ohm Terminator Build
When doing work on older audio gear it is sometimes necessary to maintain a 600 ohm impedance from, for example, a signal generator to a DUT or a piece of test gear. An easy way to do this is with a 600 ohm terminator. The article will document how I built such a terminator.
First I want to acknowledge the work of ElPaso TubeAmps (David Beard) who covered this very subject several years ago:
50 ohm and 75 ohm BNC terminators are relatively easy to obtain, but 600 ohm terminators harder to come by. The solution is to build one as follows:
- Obtain a suitable 50 or 75 ohm BNC terminator . You need the type which can be opened. One clue is that it may have flats on the end of the barrel which were used during assembly.
- The 50 ohm terminators I obtained were of good quality (Pomona 3840-50, as surplus) and could be opened. However the threaded end of the barrel was fastened tight and soldered in place. I therefore had to heat it up and open it while the it was hot.
- Disassemble the the terminator and remove the termination resistor.
- Obtain a suitable 600 ohm resistor or combination and/or center pin
- Determine the correct assembled length of the new resistor + center pin by comparing it with the old one
- Solder the center pin to the new resistor the resistor to the terminator cup
- Check the resistance again before final assembly
- Reassemble the terminator and (optionally) solder the threaded end to the body
The original 50 ohm termination resistor was crimped to the center pin and the resistor was soldered to the cup. Therefore, I could not re-use the center pin.
I used a selected pair of resistors in parallel (1000 and 1500 ohms) to obtain the 600 ohm value.
I measured and trimmed the new pin/resistor combination to match the old one.
Finally, the measured value of the assembled terminator is about 600 ohms:
Thanks for reading.
Shaun Merrigan
