Radiosurgical Unit (RSU) repair/part identification - Page 1
Forum section: electronics / repair

author: bahnss
author_rank: Newbie
timestamp: January 23, 2025, 09:07:24 pm
content: I was asked to look at this RSU when it began to blow fuses. I have provided a few pictures to hopefully help me identify the faulty part. Picture 01 is of the circuit board, picture 02 is a close up of the part in question. Picture 03 is of the faulty part. Picture 04 shows the value in ohms of the part. I had access to a second fully functioning unit (pic 04 ). I believe the part I'm looking at is an RF choke, but I'm not sure if that is correct. I cannot find any descriptors or values on any of the four parts in question (there are two per unit). Whatever the part happens to be, how do I go about ordering the correct replacement part? the unit is an Ellman RSU FFPF.besides testing resistance, is there any other values I should be checking, or are there any resources you recommend?

author: indeterminate
author_rank: Regular Contributor
timestamp: January 24, 2025, 09:49:27 am
content: In this circuit you could call it a RF choke , in general it is an inductor just a coil of wire.it will have a low ohms reading you need a LCR meter to read its inductance in Henry'shttps://en.wikipedia.org/wiki/Henry_(unit)

author: Gyro
author_rank: Super Contributor
timestamp: January 24, 2025, 10:22:06 am
content: Welcome to the forum.Yes, it looks like an RF inductor or choke. From the position, it appears to be for suppressing interference back onto the mains input. It shouldn't be playing a major role in the RF oscillator, the inductor for that is the large one, bottom right of the tube in your last photo. There appears to be an identical suppression inductor on the other side of the matrix board, so it ought to be possible to compare the two, firstly for DC resistance and secondly, with an LCR meter or bridge, to measure the required inductance value.What makes you think that the inductor is faulty? Overheating or high DC resistance? If it is the latter, check the soldering at the ends of the winding.In terms of a replacement. Looking at the end of the winding, it looks like a wave-wound winding, for lower parasitic capacitance. These are almost impossible to find these days, but substituting an RF interference suppressor of similar value should hopefully be sufficient.EDIT: I should probably add that, if the unit is actually going to be used for medical purposes, you really shouldn't be messing with it, and referring the repair to the manufacturer or appropriately qualified maintenance company.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 24, 2025, 11:50:04 am
content: I second Gyro's comment about medical equipment. I am most familiar with laboratory equipment that is FDA approved or cleared. It cannot be modified in the least without manufacturer approval or you can get dinged on the next inspection. FDA inspects some parts of the lab directly, like the blood bank. Hospitals are JCAHO inspected at a minimum. That includes surgical units, but I don't know how rigorous JCAHO is on equipment.* It's worth checking on, since a malfunction or poor outcome from a surgery could lead to investigation. I am not familiar with inspection requirements for free-standing physician offices . If it is used in a veterinary clinic that concern may not be relevant.*JCAHO evolved from quality efforts by surgeons and nursing. It does not seem to focus on equipment as much as the FDA does.

author: CaptDon
author_rank: Super Contributor
timestamp: January 24, 2025, 01:29:23 pm
content: That is the very typical '6146' or '8950' vacuum tube power oscillator. Dentsply was famous for these as well as some other major medical companies. If it blows fuses chances are something may be shorted in the power supply section. Next suspect would be a bad vacuum tube. The RFC is mainly there to keep RF out of the B+ high voltage supply. These units typically had around 350 to 400VDC on the plate supply. It is a self oscillating 'power oscillator' and the circuit is not complex at all. You could try running it without the tube and see if it still blows fuses. You could also observe the tube visually to see if it arcs or sparks internally when you energize the unit. Depending on the circuit topology you could get two very different outcomes if the oscillator fails to start oscillating, the first is that the tube will draw large current, the plate may turn cherry red or even orange and the tube will shortly self destruct, and the heavy current flow could take out the fuse. The second is that the tube draws very little current and has no power output in which case the fuse blowing signifies something shorted like the bridge rectifiers or even the power transformer itself. These old tube units are still common and the tube is readily available however most newer units are all solid state and operate at a precise crystal controlled frequency. The old units worked 'approximately' on a standard medical frequency. There were two frequencies often seen, one was around 13MHz the other was around 27MHz.

author: Hr_Satch
author_rank: Contributor
timestamp: January 24, 2025, 06:37:16 pm
content: On that last picture, in the background there are some blue rectifier caps.Jumping fuses, in older instruments would make me think of capacitors surge currents,when charging at power-on.Put 220V or 120V mains lamp bulb in series, and see what happens ?Try replace big rectifier caps out of the equation ?sheers, Harry

author: bahnss
author_rank: Newbie
timestamp: January 24, 2025, 07:58:12 pm
content: Thank you to everyone who has responded so far. Once repaired, this unit will be used medically again. This unit will get repaired by a certified lab.I was asked to look at it, and having access to a second unit that is fully functional, the "RF choke" was the outlier for everything I could test. This RSU was interesting for me to look at and learn a few things, but at the end of the day patient safety is top priority. As simple as it appears to repair, I don't have the correct instrumentation, and knowledge to verify the systems safety. With that being said, I will seldom pass up the chance to learn something new. The unit will get sent off and hopefully, the lab will share what the problem was. I appreciate the feedback, caution, and general knowledge shared. Assuming I am able to find out the Problem(s) I will be happy to post the findings if anyone is interested.