Disassembling Tek P5050 probe

03.09.2014 18:59

We have a big and noisy 4-channel Tektronix TDS 5000 oscilloscope at work that is used in our lab and around the department. Recently, one of its 500 MHz probes stopped working for an unknown reason, as it's bound to happen to any equipment that is used daily by a diverse group of people.

Tektronix P5050 oscilloscope probe.

This is an old Tektronix P5050 voltage probe. You can't buy new ones any more and a similar probe will apparently set the tax payers back around $500. So it seemed reasonable to spend some time looking into fixing it before ordering a replacement.

This specimen doesn't appear to be getting any signal to the scope. The cable is probably fine, since I can see some resistance between the tip and the BNC connector on the other end. My guess is that the problem is most likely in the compensation circuit inside the box at the oscilloscope end.


So, how does one disassemble it? It's not like you want to apply the usual remove-the-labels-and-jam-the-screwdriver-under-the-plastic to this thing. I couldn't find any documentation on the web, so here's a quick guide. It's nothing complicated, but when working with delicate, expensive gadgets (that are not even mine in the first place) I usually feel much better if I see someone else has managed to open it before me.

First step is to remove the metal cable strain relief and the BNC connector. I used a wrench to unscrew washer at the back of the BNC connector while the strain relief was loose enough to remove by hand.

Tek P5050 probe partially disassembled.

The circuit case consists of two plastic halves on the outside and two metal shield halves on the inside that also carry the front and aft windings for the strain relief and the BNC connector. There are no screws or glue. The plastic halves firmly latch into grooves on the two broad sides of the metal ground shield (you can see one of the grooves on the photo above).

You can pry off the plastic shell by carefully lifting the sides. I found that it's best to start at the holes for the windings. Bracing a flat screwdriver against the metal at that point allows you to lift the plastic parts with minimal damage.

After you remove the plastic, the metal parts should come apart by themselves. The cable is not removable without soldering.

Circuit board inside Tek P5050 probe.

Finally, here's the small circuit that is hidden inside the box. Vias suggest that it's a two-sided board. Unfortunately you can't remove it from the shield without cutting off the metal rivets.

The trimmer capacitor in the center is user-accessible through the hole in the casing. The two potentiometers on the side appear to be factory set. From a quick series of pokes with a multimeter it appears one of the ceramic capacitors is shorted, however I want to study this a bit more before I put a soldering iron to it.

Posted by Tomaž | Categories: Life

Comments

Jaz bi prvo elko preveril. Keramicni kondi mogoce ima tuljavo odspodaj. Lepo je videti sondo odprto :-)

The cable is removable. just lift slightly and pull. It will slide off of the pin that is soldered to the board.

What did you find was wrong with the cable? I have several that display a large (6 or 7 VAC) AC signal on the scope when reading a DC voltage. We're going through all our probes now to sort them out. I hope to be able to repair them instead of buying replacements.

Kelly, unfortunately other things intervened and I didn't investigate this further.

Posted by Tomaž

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