Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com
 

I wanted to title this page "How many men does it take to change a lamp in an HP 59307A VHF Switch?" but decided not to. But, that's the point of this page - what should be a one or two minute task, to change a power-on lamp, instead requires a half hour or longer disassembly of the instrument.

In the early days of the HPIB instrument control bus, Hewlett Packard sold a series of small accessory devices to assist automating HPIB controlled test setups. HP called them "HPIB Modules." One such device is the 59307A, which is a remote controlled coaxial switch, with two independent single pole, 4-throw switches. Each switch (actually a relay) is good through 500 MHz and can be controlled either by a front panel push button, or remotely over the HPIB interface. I'll continue to call it the HPIB interface, as that's how it was known when the 59307A was released, but the more prevalent identifier is GPIB or IEEE-488 bus, after its specification number.

HP 59307A's are reasonably plentiful on E-bay and are not all that expensive, and in fact are a real bargain as a general purpose RF switch to use around the radio room. The switch should not be used for transmitting, however.

 I use a 59307A to switch an HP 3586B selective voltmeter amongst various antennas to compare signal strength levels when working on active antenna designs. This combination allows signal strength data to be collected over several days at various frequencies to provide a good view of the relative performance of different antenna systems.

The 59307A has a number of instrument lamps.  The eight push buttons each have lamps; when depressed the selected button illuminates. These lamps are small flange base lamps, Chicago Miniature number 7333, that in theory at least can be replaced by pulling the switch cap out without disassembling the instrument. I found that difficult as the small latch projection that retains the switch cap in place didn't easily come loose but with a bit of work it is possible.

There are two additional lamps not so easy to change, one indicating power and one indicating the 59307A is under remote control over the HPIB bus.

I have three 59307A's in the basement shop and two had both the ON and REMOTE lamps inoperative. Replacing the lamps has been on my list of projects for some time and I finally got around to it today.

HP used small 6v wire lead incandescent lamps, Chicago Miniature 1730, a 6V, 40 mA lamp with a rated life of 10,000 hours. The lamps operate at 5 volts, which should multiply  the rated life many times, but these instruments are 25 years or more old and long does not mean infinite life.

Rather than use another pair of incandescent  lamps, I replaced both with similar size LEDs, green for power and white for remote.

The first step is to break out a Pozidrive screwdriver. HP hardware is generally Posidrive head and a standard cross-head Philips style screwdriver is a close, but not perfect fit. Pozidrive tools are not commonly available at local hardware stores but can be found at a good industrial supply house. Mine are made by Wera in Germany and are high quality tools.

Pozidrive screws are identified by the extra four lines offset 45 degrees from the drive  recesses, as can be seen in the photo.

Remove eight  flat head screws (3 top, 5 bottom)  from the front frame and four screws that secure the front frame to the plastic side housing. Also remove the  grounding strap screw connected to the front frame.

So far we have removed 17 screws. Two more screws holding a center support bracket must also be removed in order to slide the frame away from the circuit board. This brings the total number of screws removed to 19.

The lamps in question are housed in the three opening holder at the left center of the photograph. The ON lamp is in the leftmost opening and the REMOTE lamp is in the rightmost opening.

The photo below shows most of the parts that must be removed to change the lamps. Front housing, top  trim strip, top and bottom covers, brackets, screws, lamp housing and bracket are shown. The front dress panel is not shown.
 

After unsoldering, the lamps can be removed from the housing.

Since the lamps are rated at 6V,  40 mA, they appear, for loading purposes, as roughly a 150 ohm resistor. In selecting the LED series current limiting resistor, we'll need to keep this value in mind. The particular LEDs I used provide good brightness with 10 to 20 mA current, and the series resistors are 130 ohms for the green LED and 300 ohms for the white LED. (Since the LED has a couple of volts drop across it, 130 ohms limits the current to about 20 mA, half the level of the incandescent lamp. The white LED has less than 10 mA current.)

I insulated the common leads of the two LEDs with a short length of Teflon tubing and connected the remaining leads to their associated current limiting resistors.

The point of  this page is not to show how clever I am in replacing a couple of incandescent lamps with LEDs, but rather to make the point that electronic designers should consider maintenance and repair access when designing the equipment. HP's custom lamp holder is quite elegant and even though the 59307A was a relatively inexpensive instrument, it is built with HP's traditional quality and attention to detail.

But, it should not be necessary to remove 22 screws, partially disassemble the instrument, and use the following tools to change two panel indicator lamps:

• Pozidrive screwdriver
• Angle grippers to hold screws for starting in awkward places
• Long nose pliers for wire holding during soldering
• Flush cutters
• Soldering iron

The lamp replacement process required about 45 minutes, but with practice I could probably do it in 30 minutes. I have no idea what Agilent charges for shop time, but I suspect its in the $ 150 - 200 / hour range, in which case a customer might expect to receive a bill for well in excess of $100 for labor, plus $1.50  for two model 1730 lamps, should the instrument be sent in for repair services involving just lamp replacement. (And, yes I know the 59307A is long out of support by Agilent, but I'm illustrating a point about designing for maintainability.)

This is a case where the HP engineers got carried away, at least in my opinion. It's not as bad as some General Motors vehicles requiring loosening motor mounts and  tilting the engine to change certain spark plugs, but it's the same concept.