Model 99 ABS Current Preamplifier no signal problem

The Model 99 Current Preamplifier is used on the older PHI (Physical Electronics) 660 scanning Auger electron spectrometers.

99 ABS preamp

There is a fairly common problem with this design of preamplifier where the input JFET can get damaged from sample arcing which results in a total loss of ABS (absorbed current) image.

The most common solution to this issue is to replace QN2 which is a 2N5546 dual JFET. These parts are obsolete but readily available on Ebay. You can also find equivalent replacement JFETs such as the EXR461.

To replace QN2, first remove the cover from the 99 ABS preamp.

The picture below shows the location of QN2.

Use a tweezers to pull up each leg of QN2 as you heat it with a soldering iron and then use a solder sucker or copper braid to remove all the solder from the holes on the circuit board.

Note the location of the tab that is on the circuit board and then bend the leads on the replacement 2N5546 so that the leads line up with the holes in the board. Press the 2N5546 down so that the pins are firmly in the holes, then remove the 2N5546 and cut the ends of the leads so that they are shorter. You only want about 1/8″ of the leads going into to the holes as otherwise the leads might short to the chassis.

Insert the 2N5546 into the holes and solder the lead wires to the board.

Replacing QN2 will likely restore the ABS image.

The schematic below shows that QN2 is the first thing that sees the input signal and so it makes sense that QN2 would likely be damaged from an arc. Also, JFETs are susceptible to arc damage more than a lower impedance device such as an op amp.

For help with repairs or maintenance on your PHI 660 (or other older PHI surface analysis systems) please contact RBD Instruments here –

Replacing the stage brakes on a 15-630 Specimen stage

The 15-630 Specimen Stage is used on Perkin Elmer Physical Electronics 600 and 660 scanning Auger systems.  The 15-630 is a precision sample stage that has micrometers for the X, Y, Z, tilt and rotation axis.  The gear ratio is high in order to have precise small movements. To lock the positions into place, spring loaded .120” diameter high temperature plastic rods are employed. With use, these brakes wear down and the micrometers no longer hold the stage axis in place which in turn causes drift at higher magnifications.This blog post will show you how to replace the stage brakes.

Stage brakes for 15-630 specimen stage

The PHI (Physical Electronics) part number for the stage brakes is 605857 and as of this posting they go for $6.00 each.

To determine whether or not the stage brakes need to be replaced on a micrometer just move the micrometer a turn and see if it feels loose or slightly snug. If you can easily turn the knob back and forth then the brakes are worn out and need to be replaced. Usually only one or two micrometers need to have new stage brakes. But if you have never checked this before you may find that they are all loose and need to be replaced.

You will need to remove the knurled knob on the micrometer in order to replace the stage brakes. Before you attempt to remove the knurled knob, you will need a 183-6 Bristol wrench. You can get those from Bristol at this link –

But, since the original Bristol was sold a few years ago, the new Bristol sells the wrenches in packs of 10 minimum. So if you do not have a 183-6 Bristol wrench handy you can also use a 5/32″ Allen wrench.

Hold the knob firmly and press it towards the center of the stage as you turn the cap head screw that is inside the center of the knurled knob CCW until the screw is loose. Then, very slowly, pull the knurled knob off.

Z axis micrometer

There are some springs inside two small holes that provide pressure on the two stage brakes. If you pull the knurled knob off quickly then the stage brake and possibly the spring will go flying.

Remove knurled knob

Remove the worn stage breaks and insert the new ones into the holes.

Remove old stage brakes

Replace the knurled knob and insert the cap head screw. Hold the knurled knob firmly and press it in towards the center of the stage as you tighten the cap head screw.

Repeat this process for all of the micrometers that need new stage breaks.

Adjust indicator position

Each micrometer has an indicator that shows where the micrometer is in relation to the min and max movement for that micrometer. For example the Z is from +5mm to – 15mm.

To change the indicator, with the knurled knob off remove the shaft gear.

remove shaft gear

Next, spin the indicator gear until you have the indicator to the desired position. Then replace the center gear.

spin indicator gear

The Z axis indicator may need to be adjusted once in a while as it can slip. You can sent the actual Z position of the stage to as low as it will go and then set the indicator to -15mm. You will know that the Z indicator is off if you are having trouble loading your sample or getting the stage close enough for an elastic peak.

Ion pump elements – Flip the plates part II

Our previous blog post explained how to flip the ion pump Tantalum and Titanium plates in order to provide fresh surface area for the sputtering process that occurs as part of the ionization / pumping process.

But what happens if you flip the plates and then years later the ion pumps are worn out again? That is, what if both sides of the plates have been sputtered?

This blog post will explain how you can modify the plates to expose a fresh surface area which functionally is the same thing as replacing the plates.

Like most things, ion pump elements have gone up in price post COVID.  A set of 8 elements (there are 8 elements in a 220 l/s ion pump) can cost $5,000 or more.    Rather than replacing the elements, you can replace just the ceramics for a lot less and flip the plates.   If the plates have already been flipped once, then both sides of the plates will have sputtered craters. 

By elongating the mounting holes in the plates by 1.5 mm the sputter area on the plates would also shift by 1.5 mm onto a fresh area that has not been sputtered, at least not sputtered very much.

The photos below show the sputtered craters and the location of where the new sputtering will occur.

Sputtered crater
Sputtered crater
Offset by 1.5mm
Offset by 1.5mm
elongated hole
Elongated hole
new sputter area
new sputter area

In this case I was lucky enough to be at a university with a machine shop and the machinist was able to punch the holes without using any oil.  If you do not have a machine shop, you could use a Dremel cutting bit to elongate the holes.

dremel cutting bit

When you reassemble the elements, you want to make sure that you offset both the Tantalum and the Titanium plates in the same direction.  You will be able to tell that the Tantalum plate is offset by the 1.5mm gap with respect to the stainless-steel backing plate.  Then make sure that the Titanium plate lines up with the location of the Tantalum plate.  You can confirm that the Titanium plate is offset correctly by looking at the distance on the mounting tabs on the back of the Titanium plate.

Titanium plate offset
Titanium plate offset

After performing this procedure on the plates shown in this blog post the ion pump elements performed as well as new elements.  The increase in pumping capacity was noticeable.

If you suspect that your ion pumps may need to be inspected (slow pumping, arcing, or shorted) I recommend that you buy the ion pump ceramics before you vent so that you have them on hand if needed.  RBD Instruments provides ion pump ceramics.  Contact us for more information.