Electronics > Repair

Rohde & Schwarz FSH 3 bad resistor in battery circuit

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I recently got a working FSH 3 spectrum analyzer from a government auction.  I noticed that the battery wasn't holding a charge well, so I bought another one on-line.  I didn't have any problem disassembling the FSH3, but when I plugged the new battery in I released a small amount of the magic smoke, and it would no longer run on battery.  It still runs on the AC adaptor, though. 

Looking at it, I found a burned 0.15 ohm (150 mOhm) resistor (outlined in RED in attached picture) that is connected to the ground of the battery connector.  It appears as though it's a wirewound surface mount resistor - I can't find any of these in stock anyplace, so I've ordered some metal film resistors to replace it.

I'm concerned that I don't know know what blew the resistor in the first place. I've checked the wiring on the battery, and it all seems right. I'm thinking that maybe when I was installing the battery, I didn't get all of the pins connected quickly enough, or I somehow shorted pins 1 and 2 together and put voltage on pin 2?  IDK.  I've attached a screen shot of that portion of the schematic (from a YouTube video - I don't have the schematic), and I don't see what could have caused it.

Any thoughts?  I bought 10 replacement resistors, but I don't want to damage anything else in the FSH3.

It would take a lot of current to damage that resistor unless it was already faulty.

After replacing it I'd check the mosfet V4102 before anything else, then check for charging current with the device turned off. From the service manual https://userequip.com/files/specs/5046/Service_Manual_FSH.pdf the current should be about 90mA.

It turns out that the bumps on the sides of the resistor is adhesive.  You can see the same sort of thing under the capacitor just below it and the R330 resistor to the right of it, in addition to the FETs to the left.  Although it looks as though the adhesive is very close to, if not touching, the connection just below the resistor, I don't think that it was touching anything conductive on the bottom of the resistor.

The resistor came apart while I was unsoldering it from the board.  I've scraped away the adhesive, just in case it turned conductive.

By the way, the left end of the resistor is ground, the right side is battery minus. 

Plus side of battery isn‘t shorted to ground somehow?

Two things stand out. This was working with the old battery prior to replacement, with the caveat that charging was suspect. And the issue happened when a new battery was installed.

I'd measure the new battery and verify that its in good condition, and check for charging current after replacing the resistor.


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