Transistor Heat Sink Fatigue

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Some of the older PHI electronic units have a type of heat sink with a built-in spring tab (shown in the picture below) that forces the back of a transistor into the heat sink.  Over time, the spring tab can lose tension (most likely due to heat induced metal fatigue) and then the transistor no longer connects to the heat sink, eventually resulting in the transistor failing due to overheating.

For units such as the 32-100 Electron Multiplier Supply, overheated transistors are often the cause of multiplier voltage output problems. 

For this blog post we will look at a 32-100 electron multiplier supply with no output on the CMA high voltage output. The problem was isolated to a bad TIP120 transistor which shorted out and melted because the back of the transistor separated from the heat sink over time.

In addition to replacing the TIP120 transistor, we also modified the heat sink to ensure a good contact with the transistor. 

To modify the heat sink, you need to first remove the defective transistor and then remove the heat sink.  You will need a hot soldering iron as the heat sink has enough mass that it will drain away some of the heat from the soldering iron.   You can use a solder sucker or some solder braid to remove the solder from the heat sink contacts.

Once the heat sink as been removed, break off the spring tab.

Next, drill a small hole in the back of the heat sink where the indent is located. We used a 9/64″ drill bit since we needed to clear a 6-32 screw and lock nut.

Put some heat sink compound or conductive tape on the back of the replacement transistor.  This is necessary to ensure good thermal transfer from the transistor to the heat sink.

Use a screw and lock nut to attach the transistor to the heat sink.  Make sure that the transistor is centered in the heat sink.   In this case we also added another small heat sink to the back of the original heat sink to add some additional cooling for the transistor.

Next, insert the transistor leads into the holes on the board and insert the heat sink into the larger holes in the board.   Solder the heat sink and the transistor leads.  Cut the excess leads from the transistor and remove any excess flux from the board.

Now that we have replaced the transistor and improved the transistor to heat sink contact, the 32-100 should perform well for many years.

Since we were replacing the one defective TIP120 transistor, we also replaced the one for the SED supply as well (and modified its heat sink) as a preventive measure.  In this case we could not add the extra small heat sink due to a tight clearance to the nearby transformer. Even so, the improved contact to the heat sink will provide improved heat transfer from the transistor and result in improved reliability.

Need help with your older PHI (Physical Electronics) surface analysis system electronics (or optics)? Contact us at

Actuel 1.8 Adds New Auto-Save and Graphing Features to the 9103 Picoammeter

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Actuel 1.8 adds new features to assist with saving and graphing data. As always, updates are free and can be found here.

Set the Auto-Save Interval

Prior to Actuel 1.8, the auto-save interval for recorded graph data was every 5 minutes. The latest version gives you the option to save the data every half-minute to every 30 minutes.

Option to Show / Hide Graph

You now have the option to show or hide the graph display. This is useful if you only want to record data and not view the real-time graph information. Large data sets and fast sampling rates can cause some performance issues on slower PCs, and this option mitigates that.

Set a Fixed Range for the Graph Y-axis

By default, the Y-axis scale automatically adjusts to the range of the incoming data, with the options to display dual or single polarity, and to zero the baseline.

9103 Actuel Y-axis Default Range
9103 Actuel Y-axis Default Range

The fixed-range option for the Y-axis is especially useful, as it serves as a Y-axis zoom (independent of the X-axis) and also allows you to more easily compare data between multiple units or sessions.
You can set the min and max for the range as well as the units.

9103 Actuel Y-axis Default Range
9103 Actuel Y-axis Default Range

There have been some minor changes to the user interface to accommodate the new options. The Data window is now (slightly) larger, and the formatting options have been moved to the Data Options group.

For more information on RBD Instruments’ 9103 USB picoammeter, visit our website here –

11-065 Raster Size Modification

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The raster size potentiometers that are used in the 11-065 ion gun control are coarse single turn potentiometers that make it difficult to accurately reproduce raster sizes.

By replacing the single turn potentiometers with 10-turn 1% potentiometers, the accuracy of the raster sizes is greatly improved. This will improve the repeatability of your sputter rates when changing raster sizes. One turn of the raster size potentiometer will now equal 1 mm instead of 10 mm.

When you replace the 25 kΩ 1-turn deflection potentiometers in the 11-065 with 25 kΩ 10-turn potentiometers, note that CCW = black and CW = solid brown. The other wires are the wipers.

You will also need to replace the one turn raster size knobs with a 10 turn vernier dial.

TIP: When removing the old potentiometers you can simply cut the tabs off the old ones and then insert and solder the tabs into the new potentiometer. That is much easier than un-soldering the wires from the old potentiometers.

It is very important that the CW, CCW and wipers wires match up with the old potentiometers. Otherwise the raster sizes might be backwards (0 = full raster and 10 = zero raster.

If your 11-065 controller ever needs repair and you send it to RBD Instruments, we will ask you if you would like us to add this update to your 11-065 as part of the repair.