Help with non-charging capacitor off of a bridge rectifier - Page 1
Forum section: electronics / repair

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 08:03:41 am
content: Help with non-charging capacitor off of a bridge rectifier fed by a torrodial transformerI have 2 brand new caps on a powered subwoofer circuit. I am getting good voltages across the rectifiers, then the circuit charges 2 caps. One caps charges to the 40v+ desired, but the other cap never charges past -1.5v or at least shows this voltage on the meter.Can anyone think of a reason the other cap wont charge? The cap is not giving me a bad reading on the surface that I can see so far, and its new.What culprit might explain this set of behaviors?

author: inse
author_rank: Super Contributor
timestamp: April 16, 2022, 08:51:09 am
content: Photos from both sides, please.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2022, 10:16:10 am
content: I'd bet that if it doesn't have an inrush surge prevention circuit, something (or a connection) has gone high resistance 'upstream' of the capacitor, as anything that could drag down the output voltage by over 96%, without an upstream fault, would be drawing so much current it would be blowing fuses or even tripping the breaker on the mains circuit feeding the amp.In addition to inse's request, provide the exact make & model number, or if its a home-built project, provide (a link to) the schematic.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 10:51:24 am
content: Here is a photo of the board.One of the photos shows the star ground and the location of the cap that is not charging.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 10:54:00 am
content: Quote from: Ian.M on April 16, 2022, 10:16:10 amIt has 2 fuses which were blown when I received the sub.Unfortunately, no schematic.There doesn't appear to be anything at between the rectifier and the cap.I am going to go back to the variac and measure voltages again so I can share what values are around the rectifier.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 11:13:43 am
content: Please see the attached. These were taken with the variac on at 80v.I also wanted to note the resistors under the rectifier.With the 2nd handwritten note, I am trying to point out that I am seeing 40v at stripe side of D8 and D9, and -40v if I reverse the leads.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2022, 11:41:47 am
content: Pull the fuses to isolate the input side of the bridge rectifier and use the diode check range on your DMM (or a low ohms range on an analog meter) to check each diode in the bridge, in circuit. If all are good, their Vf should all be very similar. You wont be able to get a reverse reading as the other diode in each pair will conduct, passing current through the load circuit.If you get significantly differing Vf readings, desolder the cathode ends of D8 and D9 to disconnect the positive side of the bridge and repeat your diode tests. Now the load circuit is disconnected, you can test for reverse leakage of individual diodes and be certain that the Vf and reverse leakage you are seeing applies to the diode the test leads are on.While D8 and D9 cathodes are disconnected, take the opportunity to check continuity from the bridge + and - pads to their reservoir capacitor terminals.I suspect its either got a cracked track or the diodes D10 and D11 feeding the negative reservoir capacitor have gone high resistance.

author: inse
author_rank: Super Contributor
timestamp: April 16, 2022, 01:06:57 pm
content: Where are the fuses FS1 and FS2?Those are ceramic capacitors under the diodes.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 01:16:11 pm
content: Quote from: Ian.M on April 16, 2022, 11:41:47 amI have done this previous, they all check out. I was intending to share the voltages I am seeing in circuit.Quote from: Ian.M on April 16, 2022, 11:41:47 amI have performed most of these checks but not in the exact manner you prescribe. Let me give that a try and see what I read.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 01:18:28 pm
content: Quote from: inse on April 16, 2022, 01:06:57 pmI removed them and wired in glass fuses so I could see if something blew. I have replacements ready once the unit is sorted again.Should those caps below the diodes be a worry? I didn't think they should be.

author: StillTrying
author_rank: Super Contributor
timestamp: April 16, 2022, 01:55:12 pm
content: Have you measured theACvoltages fron the star point to FS1, and from the star point to FS2, they should be about the same.And checked that's not a short.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 02:02:21 pm
content: Quote from: StillTrying on April 16, 2022, 01:55:12 pmI cleaned that up to no effect.The AC voltages are just about identical at both points.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 02:25:35 pm
content: Quote from: Ian.M on April 16, 2022, 11:41:47 amI disconnected one lead of each of the fuses so no power is in circuit and I am getting similar readings in the low mv range for each diode of about 200 to 300 mvThe continuity check for the diodes shows resistance in the Mohm range testing all 4 cap pads against both sides of the diode.

author: StillTrying
author_rank: Super Contributor
timestamp: April 16, 2022, 02:45:27 pm
content: "The AC voltages are just about identical at both points."That's a good start.Measuredfrom the startpoint, if the -Ve DC voltage was only around 0.7 of the +Ve DC voltage it would mean there's a no connection between the -Ve diodes and the -Ve DC smoothing cap., I think.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2022, 03:11:12 pm
content: That's far too low a Vf for those diodes to be good. I can make out enough of the part numbers in your photo to determine they must be 1N5402, which is an ordinary 200V 3A silicon rectifier diode.[Vishay Datasheet]Vf should be in the 600mV to 700mV region and even at a very low test currents shouldn't drop below 500mV.Either your DMM is faulty, or something is shunting the diodes, or at least two of them are bad and they are shunting each other. Confirm it isn't your DMM by testing an ordinary silicon diode (*NOT* a schottky one) or a transistor junction out of circuit, then desolder D10 and D11 (my suspects for the bad ones) and check them out of circuit. You can also then check D8 and D9 in circuit as the negative side of the bridge will be disconnected. All should have Vf in the range above and no measurable reverse leakage.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 06:43:44 pm
content: Quote from: Ian.M on April 16, 2022, 03:11:12 pmI didn't think it mattered to mention, but the transistors have been pulled from circuit to eliminate a root cause.They are 1N5402 diodes, I have some 5404s here I can swap them for and retest the circuit.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 07:30:01 pm
content: Quote from: StillTrying on April 16, 2022, 02:45:27 pmI reconnected the fuses and turned the voltage to 120v.Here are my readings, I have not changed any diodes or put the transistors back in.I left the black lead on star ground and measured AC and DC across both sides of each diodeAt D8 and D9 striped side, the meter cycles thru looking for a reading and then just beeps, it never actually reads anything for AC.

author: StillTrying
author_rank: Super Contributor
timestamp: April 16, 2022, 08:54:57 pm
content: At D8 and D9 striped side, the meter cycles thru looking for a reading and then just beeps, it never actually reads anything for AC.That's probably correct, with no load on the +40V there'll be almost 0 AC ripple.The 22 VAC and -17 DC on the -Ve of D10 and D11 are about the AC and DC the voltages you'd get for half wave unsmoothed. Which mught mean there's 2 faults, one of D10 or D11 is blown, plus there's no connection to the smoothing cap.Which is similar to.I suspect its either got a cracked track or the diodes D10 and D11 feeding the negative reservoir capacitor have gone high resistance.So check the resistance from the -Ve diodes to the cap.I can't see every connection in the photos, but can't see how it's not just this.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 16, 2022, 10:11:50 pm
content: Quote from: StillTrying on April 16, 2022, 08:54:57 pmThanks for the response.I changed D10 and D11 and this is the updated voltages. It looks like the rectifier is working, but voltage is off on one side.The cap still takes no voltage, but I wanted to address this first as it does seem like 2 problemsAny thoughts towards the imbalance? Note I changed the diode to a 1n5404 as its all I had but it shouldnt matter for this. And other than the imbalance, this looks fairly normal correct?

author: StillTrying
author_rank: Super Contributor
timestamp: April 16, 2022, 11:16:20 pm
content: I would think the -35 VDC will go up to -40 VDC when/if you find out why the -Ve smoothing cap is not working.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 17, 2022, 11:27:05 am
content: I'll dive into that. On the surface, it seems the bridge rectifier is now working unless someone still sees an issue.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 17, 2022, 01:07:54 pm
content: I pulled the cap that was not charging from circuit and connected it directly via the leads as photographed here.I went from the - voltage I read (about 25v) to the neg lead of the cap, from the + lead of the cap to the star ground.No cap worked, I tried all of them including a brand new one out of the box. Nothing will charge past about .5v.Unless I am doing something wrong with the way I am leading it, I dont think so, since the behavior has not changed. With the cap directly wired in, I still get the same charge.After seeing the new voltage readings before, I thought perhaps this would expose a bad connection between the diode and the cap but it doesnt seem to.

author: Ian.M
author_rank: Super Contributor
timestamp: April 17, 2022, 01:13:29 pm
content: Do you have a bench PSU with an adjustable output and current limiting? If so, pull the suspect diodes and test them at at least 300mA. If not, test them on whatever power supply you have with a low voltage filament bulb in series. I suspect the lead to die joint has cracked and they are barely passing any current.

author: StillTrying
author_rank: Super Contributor
timestamp: April 17, 2022, 03:45:00 pm
content: Have you checked all those crock leads, half the ones I've got have gone high resistance, intermittant or open circuit, especially when using them around a power supply that can supply pulses of many Amps.When I've had to remake one I've always used thicker wire.From some of you measurements I think it's unlikely there's a problem with the star point.Have you checked again that the bridge isstillworking, measured from the star point, if the+Vside is around 50V DC the-Veside should be around -30 VDC because there's no smoothing on that side. If the-Veside drops to around -17V DC again one of D10 or D11 would be blown, again.I can't think of anything else ATM.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 17, 2022, 04:29:56 pm
content: Quote from: StillTrying on April 17, 2022, 03:45:00 pmSo the voltage has dropped again to -17.I went ahead and soldered in some leads and soldered the cap to the lead so it was well separated from the board, then checked those for the voltages again. It seems like I am getting a reading across the capacitor now.I want to get back to your request but I want to make sure I am getting a measurement correctly and the capacitor problem was perplexing.I am getting 30v DC now at the points that were showing 54v before. The cap working in circuit is the reason for this now perhaps?Should I replace those 2 diodes again and try once more? Or a different way?I do want to take a moment and thank everyone for their help. It has been YEARS since I touched this stuff and its a mountain to climb re-learning it.

author: Ian.M
author_rank: Super Contributor
timestamp: April 17, 2022, 04:45:32 pm
content: 1N540x diodes are *CHEAP* compared to the time and effort you've put in so far and the value of the boards and risk of damage if you have to keep removing/refitting parts to test them. I'd 'shotgun' it by replacing *ALL* the diodes in the bridge rectifier, checking each track for pad to pad continuity while they are out. When bending the leads of the new diodes, support them next to the body with needle nose pliers so you don't micro-crack the epoxy encapsulation which can lead to immediate or early failure.Its likely that will cure it, but if it doesn't things are going to get *INTERESTING*

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 17, 2022, 10:33:03 pm
content: I've rebuilt the rectifier with new diodes, and soldered new traces to the little caps under the diodes that seemed suspect.I get continuity everywhere, a capacitor that charges, but only partial voltage (about -17) on one side of the aisle, and 50+ volts on the other.At this point, I am wondering if I should rebuild the rectifier off circuit and patch it in, or perhaps introduce a single component rectifier that would do what I need here instead?The more I mess with the board, the more it deteriorates, so its a risk to try and patch this rectifier again unless there is a component we have not discussed that could be influencing the voltage difference?I guess it gets interesting?is not the emo I had hoped for

author: Ian.M
author_rank: Super Contributor
timestamp: April 18, 2022, 01:02:06 am
content: We are all going to be kicking ourselves for not thinking of *whatever* when you finally find the cause.Lashing up a clone of the rectifier + reservoir capacitors circuit on stripboard or tag strip then measuring voltages round the clone would be a good sanity check, though I wouldn't actually use the clone to replace the original as a repair.Its an extreme longshot, but it is remotely possible the fault may be due to poor contact somewhere in the circuit on the AC side of the bridge: A rectifying junction can form between oxidized copper surfaces. Its known that a copper oxide rectifier can have a Zener-like characteristic with a fairly low reverse breakdown voltage (), so its remotely possible if *BOTH* secondary connections have developed such a fault that you could get an asymmetry between the supposedly equal positive and negative bridge rectifier output voltages. I would normally regard this as vanishingly unlikely, but the presence of crimped spade connectors on the transformer wires could provide the conditions required if insufficient crimping pressure to obtain gas-tight contact was used. Continuity checks should reveal any such faults, which will show as the resistance changing when you reverse the test leads. If it is a rectifying contact issue, go out and buy a lottery ticket as inadvertent ones are *RARE* and stable one even more so, and you are lucky enough to have two sitting in front of you!(A single secondary properly connected would still produce the correct output voltages by half-wave rectification, at half the available current and twice the ripple.)

author: StillTrying
author_rank: Super Contributor
timestamp: April 18, 2022, 11:54:56 am
content: Have you checked the via in the center of the star point.Is the transformer 35-0-35 VAC.Are you sure it's just this circuit.Does that -Ve track go directly to the -Ve cap.I'm running out of ideas.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 19, 2022, 08:12:15 pm
content: Here is the scope trace. Let me know if there is another view you wish to see.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 19, 2022, 08:14:57 pm
content: Quote from: StillTrying on April 18, 2022, 11:54:56 amYes, the circuit is mostly as you describe, with the small caps under the diodes. I have not added the second cap to the breadboard, I wanted to see this outcome first I described above.I think I damaged the 2 sided-ness of the board somehow, that is my suspicion. Some of the diodes trace on one side, the others, the reverse.I can add the second cap if you wish? I might as well rebuild it at this point

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 19, 2022, 08:17:24 pm
content: My posts are out of order... please treat this as first, then the scope traceI managed to rebuild the rectifier with the diodes I had in place, and the problematic cap that wouldn't charge. I only did this with a single cap.At 80v thru the existing transformer, I am seeing the following output on the neg end of the cap on the scope.This seems to look ok to me as output verifying these parts work, so I am left with something bad on the board, I suppose.I have to attach the scope trace to another post bc of file size.

author: David_AVD
author_rank: Super Contributor
timestamp: April 19, 2022, 09:29:50 pm
content: It's not uncommon for large electros to leak, corroding one leg between the cap case and the PCB. From underneath, the board looks fine but the cap is pretty much O/C.Have you removed those caps to confirm that the leg or a PCB trace under the cap is not corroded?

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 19, 2022, 09:33:03 pm
content: Quote from: David_AVD on April 19, 2022, 09:29:50 pmYes, I did so early on. I have new caps in place at the moment.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 19, 2022, 09:49:10 pm
content: I have added the 2nd cap and the rectifier is behaving exactly as expected on the breadboard.I am getting 40v AC in at the v+ and v- and a +20v and -20v at each of the caps.Does anyone know what the value of the small caps is/typically is? I went in with the microscope and saw no writing on it.

author: StillTrying
author_rank: Super Contributor
timestamp: April 19, 2022, 10:06:11 pm
content: Don't go much over 80V on that cap.What would be more useful is to see the -17 VDC trace, gnd on the star point, signal on the -17VDC, leave the -Ve cap off if it's still off. That should show if there's the full wave -Ve rectification there, with peaks a lot higher than the average of -17V."Does anyone know what the value of the small caps is/typically is?"I think they're usually 1nF to 10nF, they just stop a bit of noise, they're not very important, unless one has gone short.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 19, 2022, 10:25:36 pm
content: Quote from: StillTrying on April 19, 2022, 10:06:11 pmThis is what I tried numerous times before and was never able to get it to go off of the 35v vs -17v thing in my readings.What I am seeing now with the same parts is that they work, so I wonder how I can merge my known working piece back over to the board to test it out?I come from the software world, where this kind of hodge podge is easy, but here, I am not sure if its possible.Would tracing a new lead into the old board at the correct points even work?

author: StillTrying
author_rank: Super Contributor
timestamp: April 19, 2022, 10:55:30 pm
content: You need to find out where/why the -17 VDC was there. I don't think it could be a large load on the -Ve supply because something would be getting quite hot.Smoothed peaks of 53 VDC just happen to be -17 V average half wave unsmoothed, and ~32 VDC full wave unsmoothed, which seems quite a coincidence that you get those values on the -Ve side. I think the -Ve cap isn't charging on the board because it's not connected to the -Ve diode(s) somehow.Can't you scope the -17V/-30V, at the cap. the trace shape should/might give some idea of what's happening.https://en.wikipedia.org/wiki/Rectifier#Half-wave_rectification

author: StillTrying
author_rank: Super Contributor
timestamp: April 24, 2022, 02:57:54 pm
content: You must have found out what's wrong with the -Ve side by now.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 27, 2022, 09:51:16 am
content: Quote from: StillTrying on April 24, 2022, 02:57:54 pmHa ha not yet. Some work has been going on in the house so my lab has been shut down since power is constantly going on and off in the house.I am completely committed to fixing this whether itsoror even a

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2022, 12:45:35 pm
content: I say again:Quote from: Ian.M on April 18, 2022, 01:02:06 am*DON'T* give up - electronics repair is approx. 5% technical knowledge, 5% skill and 90% sheer bloody-minded perseverance!