Rhode Schwartz SML03 Repair Help

[cgz2001]

All,

I recently came into possession a damaged SML03 which I would like to get back into service.  Like others, I have located both service manual volumes however I have not located schematics.  Powering the device up finds the unit passes calibration except for the items shown in pictures 1 and 2 below.  On what I believe is the 'output voltage card' there seems to be a burnt precision resistor and the board is burned.  See the last three pictures for reference.

I have several questions.  First, can anybody confirm for me the value of this component?  Next, if this isn't a precision resistor (it looks like it to me), what is it?

The board looks damaged but repairable.  If another individual can advise the value of the part in their unit I can order one and get the unit running (once of course I determine the root cause of failure).

Thanks in advance!

[cgz2001]

Further information:

The chip connected to the component under question is a BCP69.  The mystery component is connected to pin 2 / 4 per the datasheet (SOT223).  The other side of the component vias under the board.

https://assets.nexperia.com/documents/data-sheet/BCP69_BC869_BC69PA.pdf

I have a suspicion we are dealing with a susumu component.

https://www.susumu.co.jp/usa/product/series2.php

The size of the part appears to be 4.5 mm long and 3.20 mm wide.

I'll continue exploring this over the coming days.

[cgz2001]

Below are two more pictures which provide more detail than the previous pictures did.

[dah5477]

I have an SML02 at work and can confirm that the "mystery component" is indeed a resistor. The value is 10 Ohms. Unfortunately, I suspect one or both of the HFET amplifier ICs (SOT-89) on the other side of the board may be shorted. If you measure them, keep in mind that it's normal for pins 2 and 3 to have only a couple Ohms or less between them. The ICs on mine are FP2189 - the datasheet can be found here. Luckily, it looks like they're available on AliExpress.

Here's a photo of my board (same area):

[cgz2001]

Hi dah5477, thanks tremendously for sending me this information.

I confirmed with a coworker the board appears repairable.  The resistor was laser trimmed, and there are three pads internally.  We attempted to measure the resistance of the burned resistor but wasn't able to get a reading less than 7k (obviously wrong).  After scratching away some of the material near the edges we could measure the resistance of each internal pad to be 38.6 ohms, 34.7 ohms, and 60.7 ohms, without taking into account the resistance of the probes.  This placed the parallel resistance at a minimum of 14 or 13.7 ohms.  I saw the other resistor on the board was a 12 ohm resistor and so suspected we were looking at that value in reality.  Looking at your board I can tell our strategy got us at least in the ball park.

Googling H2 markings located the GaAs transistor to be SHF-0289.  It originally was made by a company called RFMD but is now Qorvo.  It's obsolete, with some venders indicating stock.  Last thing I did this evening was measure the continuity of these leads, and sure enough one of the components has the gate shorted to the drain / source.  The other GaAs transistor only has the drain and source shorted together which makes sense as this is a depletion part from the datasheet.  It's interesting your markings indicate FP2189 - can you give us a picture of the other side of your board?  I recognized in my search earlier an alternative marking to H2 was to list the part number and some other digits.

[dah5477]

Here are a couple pictures of the other side of my board. From the datasheets, it looks like both FETs are similar in performance/characteristics.