Help with blown resistors and what can I replace the with?  - Page 1
Forum section: electronics / beginners

author: Crambone
author_rank: Frequent Contributor
timestamp: April 25, 2022, 03:38:28 am
content: Hello I’m looking for the following resistor but can’t find any data on it? Listed as: S.RESISTOR ERJ1WYJ4R7U (4.7 Ω)Is the S.Resistor referring to current sense???If you look at the schematic R38, 39 & 40 blew I’m not sure what could have caused it, it’s on a PA board of and ICOM IC-756 pro2 transceiver. You can also see the trace has been damaged also.

author: golden_labels
author_rank: Super Contributor
timestamp: April 25, 2022, 04:15:45 am
content: I see the schematic has such paralleled components in many places, with no obvious reason other than sharing load. Also BOM optimization: re-using 4.7Ω instead of having one more reel just to put a single 1.5Ω. So I assume that it’s the same with those resistors and they’re in no way special.They may be of series similar tothat, but I suppose any 4.7Ω will work.edit:In fact Digikey has ERJ-1W series too, just not 4.7Ω.ERJ-1W datasheetconfirms it’s just’s a plain SMD 1W 5% 4.7Ω resistor.

author: CaptDon
author_rank: Super Contributor
timestamp: April 26, 2022, 01:11:24 pm
content: I wonder if the inductor had bad solder connections?? Those resistors appear to be parallel across an inductor. You will have to probably use an Xacto Knife to scrape away the carbonized portions of the FR4 board material and be careful not to cut into any sub-terrainian layers (if there are any). Usually when I see resistors in a final PA stage in that configuration they are there to stop parasitic oscillations. Was the unit perhaps transmitting into an open circuit and the circulating voltages and currents got very high?

author: Crambone
author_rank: Frequent Contributor
timestamp: April 26, 2022, 01:49:59 pm
content: It is possible, the unit was in tuning when it went poof. I wasn’t the user at the time the unit is being given to me and the previous owner took the pic for me. I am scheduled to receive the unit today and will re-solder the inductor. My plan is to replace the 3ea 4.7 ohm resistors and try the unit on low power into a 50 ohm load and see how it reacts. I’m going to check a few voltages first prior to trying also.Andrew

author: Crambone
author_rank: Frequent Contributor
timestamp: April 26, 2022, 09:59:25 pm
content: Radio was dropped off today and the true story came out. The tuner stopped functioning properly and the repair tried failed. So the tuner consists of 2ea stepper motors and one wasn’t functioning so he swapped them and and the one that wasn’t working still didn’t work. A replacement motor was ordered but the plug was different. He installed the plug from the old motor onto the new but never checked any data sheets or referred to the radios schematic and poof went the three capacitors.I ordered 2 new motors they are only $6 each at Mousser and a few 4.7ohm sma resistors. My hope is that I retire the one motor correctly, install the 3 new resistors and nothing else went bad. I will check the resistor pads for shorts to ground also.So hopefully the 3 Parallel resistors took the hit.He did say after the resistors let out the smoke he tried the radio quickly into a load and it showed full power out. Not something I would have suggested but it was a bit of good news that the PA wasn’t destroyed. We shall see.Andrew