B&K Precision 9131B - Blown 5W resistor on power input - Page 1
Forum section: electronics / projects

author: amaschas
author_rank: Regular Contributor
timestamp: August 27, 2025, 04:39:58 am
content: Hey all, I recently acquired a B&K Precision 9131B that wouldn't turn on when I hit the power. I took a look inside and immediately noticed a blown up ceramic 5W resistor near the power input. The component got hot enough to burn the board. I've ordered a replacement resistor and I'm going to dremel out the carbonized parts of the board (with breathing protection of course), but I'm somewhat concerned about the ultimate cause of the problem. My thought is that once I replace the resistor, I can power the board and watch to see what heats up, but I'm wondering if anyone has any experience with this type of problem on this particular model, and can point me to any likely culprits for this kind of failure.The AC power comes in through the connector in the lower right corner of the board. I assume that the resistor is part of a noise filter of some kind?

author: Sorama
author_rank: Frequent Contributor
timestamp: August 27, 2025, 06:10:50 am
content: Does it turn on after removing this burnt component?Ain’t it part of a switched PS?Although I don’t see the switcher.

author: H.O
author_rank: Frequent Contributor
timestamp: August 27, 2025, 07:16:55 am
content: No experinece with the model so this is just a guess:Being a linear power supply capable of >350W the resistor could be part of a softstart circuit for the main transformer, the nearby relay bypasses the resistor after a short delay. If relay or its driver circuitry is faulty the resistor will be in circuit indefinetly and overheat.A little bit of reverse engineering of the input circuitry will help figuring out if that's the case or not.

author: amaschas
author_rank: Regular Contributor
timestamp: August 28, 2025, 02:36:37 am
content: This is the schematic of the input section between the line/neutral connectors and the power switch. Downstream from this is a 120/220V selector, and some connectors that route to a huge toroidal transformer and a variety of other places including a few separate boards. The resistor that blew is R1, and as you can see it appears to lie across the terminals of a relay that is controlled via a bunch of logic that I was having difficulty tracing. So I suppose it's intended as a sink for the stored energy in the relay coils when it turns off. Not sure what would lead to it exploding though.

author: H.O
author_rank: Frequent Contributor
timestamp: August 28, 2025, 04:45:24 am
content: That doesn't look right...When the switch is closed its connections 1&2 would create a direct short between line and neutral.Where does is bugger off to the voltage selector and transformer?The fact that the relay contact(s), when closed, bypasses the resistor matches my theory of a soft-start circuit but not much else there makes sense to me at the moment.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 28, 2025, 10:53:14 am
content: Quote from: H.O on August 28, 2025, 04:45:24 amIndeed. You probably need to rotate the switch 90 degrees.For the rest. It's already been mentioned that it's common to have such a resistor as an inrush limiter, and that it gets shorted by the relay after a few seconds. In such applications, it is also very common to put 5x or more of the power rating of the resistor into it, because it only works for a few seconds, and it takes time before it would overheat. So indeed, the resistor failing is probably the result of the relay not switching (which has also already been mentioned).I am a bit surprised by the capacitor in series with the resistor, but it's also plausible that it is indeed in that place.

author: amaschas
author_rank: Regular Contributor
timestamp: August 29, 2025, 06:29:25 pm
content: I think the suggestion that this is a soft-start circuit is correct, and that the relay is failing to switch. I replaced the resistor and as expected it started to heat up. I've tested the relay and it does in fact activate, so a likely culprit seems to be something in the timer circuit that activates the relay. I'm going to start digging into that, but I'm wondering if it might make sense to just pull the resistor and put an external soft start in before the power input just to see if I can get the thing to start up? I'm thinking something like this, that is designed for toroidal transformers like the on in this device:https://www.antekinc.com/ss-12-soft-start-module/One thing that has me confused about this circuit—and I definitely drew it wrong so I'm going to have to revise—is that as far as I know these types of relay-driven soft starts generally require two resistors, and I don't see a second one anyone in the vicinity.I'm also thinking that from looking at other soft start circuits that a 1k resistor is probably too high of a value in this case (I assumed that's what it was because there are two other 1k resistors of the same type elsewhere in the supply). From what I've seen a resistor value < 10ohms is much more common. The resistor in question was destroyed to such an extent that measuring the resistance or reading off the resistance from the case is impossible. If anyone has a B&K supply in this series and would be willing to take a peak at that resistor value for me it would be much appreciated. Added my updated understanding of the circuit.

author: H.O
author_rank: Frequent Contributor
timestamp: August 29, 2025, 08:30:55 pm
content: The schematic still doesn't make sense.Now there will be no current at all until the relay closes and if/when/once it closes the current will go thru the resistor all the time. The resistor should be in series with primary coil of the transformer and the relay contact should be in parallell with the resistor - provided it is indeed a softstart circuit. Looking at the previouse photos of the board the capacitor across the switch seems to be 0.33uF, not 33uF.You're correct that 1k is probably way to high but the value really isn't very critical, figure the transformer primary as a dead short and you want to limit the inrush current to 5A at 230V then resistor needs to be around 47 Ohms. Anything between 10 and 100 Ohms should work just fine.If you're convinced that the purpose of the resistor is indeed softstart then, for testing, you can short it and hope that it doesn't blow your main fuse/circuit breaker. If you have a variac or dim-bulb tester - use that.

author: amaschas
author_rank: Regular Contributor
timestamp: September 04, 2025, 11:59:00 pm
content: Progress! set up a dim bulb tester and shorted across the resistor, which in effect shorts across the relay, and the supply not only started up, but the relay clicked open as designed! I'm thinking maybe the high resistor value was preventing it from opening? I'm going to source a lower value resistor to see if that addresses the startup problem. I'm actually a little confused about how you arrived at the 47Ω value? If it's a 375W device, then at 220V the max draw would be ~1.7A right? I'm actually using the supply from a 120V circuit, if that's useful to know.Also I took a third swing at a schematic, for anyone coming across this thread in the future. Hopefully this will more accurately represent what is going on.

author: H.O
author_rank: Frequent Contributor
timestamp: September 05, 2025, 06:08:48 am
content: Yeah, now the schematic looks more like it.When the instrument is turned off and the relay NOT energised the resistor is in series with the primary winding of the transformer, which at that point can be thought of as a resistive short.Power is applied, the transformer inrush current has to go thru the resistor which limits it to what the value of the resistor dictates.If we think of the primary winding as a dead short then a 47ohm resistor will limit the inrush current to around 2.5A @ 120V and 5A @ 240V which should prevent it from tripping any breakers but still be enough to allow the transformer to magnetise and provide enough power to drive the low power electronics.Then, after a short delay, the relay CLOSES (not opens) bypassing the resistor allowing the transformer to pull whatever current it needs to provide the 375W output power. I can't believe the resistor was actually 1k but if it was I'm not surprised it didn't work properly and instead went up in flames.

author: amaschas
author_rank: Regular Contributor
timestamp: September 16, 2025, 04:44:04 am
content: So just as I thought I was home-free, I started reassembling the device and did one last test, and something started smelling like burning. Took me a while to figure out what had happened since the damage was on the underside of the board, right under the big heat sink.Impressively, the trace here got hot enough to melt the solder.The burnt component is a small transistor. Easy to replace, but I'm worried that the failure was a symptom rather than a cause. A few things I had noticed prior to this failure:1) The fan didn't come on when the unit turned on. I was able to get the fan to come on by hitting the heat sensor with some hot air, so I figured it wasn't a problem.2) Right before the smell of burning I heard the sound of a relay repeatedly activating. I'm guessing in response to the short?3) The fans came on when the short happened.4) This circuit is one of 3 similar circuits arrange around a TL084C op-amp. I'm wondering if they're related to heat sensing in some way, positioned as they are under the heat sink?5) The heat sink will be a total bitch to get off if I need to examine underneath it.6) I poked around and didn't find any obvious shorts in this circuit. +V and -V had a 0.5V drop, which is maybe a little low, but in the range of a diode.7) The trace with the melted solder is a leg of a big transistor. I can't really get to it as its stuck behind a big capacitor. Still seeing diode drops across the pins in one direction and no continuity in the other, though I'm not sure which pins are which. Doesn't seem entirely shorted though.

author: H.O
author_rank: Frequent Contributor
timestamp: September 16, 2025, 06:27:40 am
content: Ouch, that's unfortunate. Certainly in need of some cleanup and more troubleshooting. You should've kept that dim-bulb tester on it :-)Can't really offer much insight other than guesswork...Being a tripple output supply I'd be more inclined to think that the three similar OP-amp circuits has to do with voltage regulation or current sensing or something for the individual channels. If the blown circuit is on channel 1 or 2 you can compare readings between them (since they have the same specs). Channel 3 might be different since it has different output specs.Are you saying you're measuring 0.5V voltage drop with your multimeter in diode mode, across V+ and V- of the OP-amp? That doesn't sound right to me.