Voltage Regulator drops voltage when an IC is inserted - Page 1
Forum section: electronics / repair

author: Midphase
author_rank: Regular Contributor
timestamp: February 05, 2025, 12:11:26 am
content: I am working on this circuit board. I have a 7815 and 7915 voltage regulators as you can see from the schematic. The 7915 gives me a perfect -15v and the 7815 gives me a near perfect 14.98v. Having said that, when I insert a TL082 into either U4 (and sometimes U1) the 7815 voltage output drops to .5v. Is this normal?Assuming it isn't, is this indicative of a short somewhere? My soldering is clean and the PCB is relatively straightforward. I am getting correct -15v's on Pin 4 of the TL082's sockets.I'm at a bit of a loss, but needless to say the circuit is not working properly.

author: wraper
author_rank: Unknown
timestamp: February 05, 2025, 12:15:45 am
content: Try adding diodes to the output like on this schematic

author: Midphase
author_rank: Regular Contributor
timestamp: February 05, 2025, 01:21:55 am
content: So that's different than the Schottky diodes that are already in the schematic?Would I place them before or after C3 and C4?

author: wraper
author_rank: Unknown
timestamp: February 05, 2025, 01:28:39 am
content: Quote from: Midphase on February 05, 2025, 01:21:55 amThose rectify AC, diodes I suggested protect Vregs from reverse polarity on the output during power on.QuoteThey're in parallel with those capacitors, what before/after you're talking about?

author: ledtester
author_rank: Super Contributor
timestamp: February 05, 2025, 02:11:56 am
content: This mod might help troubleshoot things...if you remove the two 2SK30 transistors and resistor R75 you will cut all connections to the sound modulation circuitry and the audio input should appear on both outputs.You will need to have U9 and U10 populated, of course. See if the voltage rails are good in this reduced circuit.btw, is this the project?https://github.com/gligli/juno-chorus-clonehttps://gliglisynth.blogspot.com/2014/05/new-diy-project-standalone-juno-60.html

author: Midphase
author_rank: Regular Contributor
timestamp: February 05, 2025, 03:08:35 am
content: Yeah, that's the project from GliGli.I'll give it a try tomorrow. Honestly this project has been a massive PITA starting from the generated Gerber files being flagged by JLCPCB to some of the components being more challenging to locate than expected to now the circuit not working at all despite my methodical process and trying to be as accurate as possible.If I can get it to work properly, I'm planning on re-designing the schematic a bit and improving on a few elements.But yeah, I'm probing 7815 at the output and it should be delivering 15-ish volts no matter what, so I suspect there's an issue somewhere.This guy had mentioned having issues with the 2SK30's and having to go through several components before finding a good pair. I wonder if perhaps I should try another JFET transistor type since apparently the 2SK30's are temperamental.https://www.reddit.com/r/synthdiy/comments/mt22fu/diy_juno_chorus_clone_is_finally_complete/?rdt=51424

author: Midphase
author_rank: Regular Contributor
timestamp: February 05, 2025, 07:48:27 pm
content: Ok, so I removed the two 2SK30's and R75 as you suggested, I do get a signal to pass all the way thru to the outputs and the voltage reading on the 7815 is spot on.However as I start repopulating the sockets, my positive voltage disappears once more in seemingly random ways, what I mean is that sometime repopulating U9 causes the voltage to drop, sometimes repopulating U4 does. Sometimes having U9 populated but not U1 works, sometimes it doesn't. Could this be a bunch of faulty TL082's, but if so why are they (seemingly) working just fine in certain sockets, while in others they cause a drop in the positive voltage rail?

author: fzabkar
author_rank: Super Contributor
timestamp: February 05, 2025, 08:41:02 pm
content: Did you add the two diodes that @wraper suggested? Your PCB's symptom is something I've seen in a Sinclair device. In that case the fix was to add these diodes. IMHO it would be better to use Schottky diodes rather than 1N4001. National's databook suggests 1N4720 for high current designs.Edit:https://www.badcaps.net/forum/troubleshooting-hardware-devices-and-electronics-theory/troubleshooting-power-supplies-and-power-supply-design/41853-please-help-me-understand-fix-this-discrete-linear-power-supply?p=914484#post914484Quote

author: Midphase
author_rank: Regular Contributor
timestamp: February 05, 2025, 09:57:47 pm
content: I'll give that a shot, so you think there might be a reversing of the current somewhere?Just to be clear, this is how I should do it?

author: wraper
author_rank: Unknown
timestamp: February 05, 2025, 11:03:41 pm
content: Quote from: Midphase on February 05, 2025, 09:57:47 pmDuring power-on, vregs do not start with exactly the same speed. So negative vreg may pull positive vreg output to negative voltage through the load connected between them. Which may result in IC latch-up. If a latch-up happens, IC won't start working properly until power is removed, or even may irreversibly fail.https://www.ti.com/lit/an/slya014a/slya014a.pdfQuoteYes

author: Midphase
author_rank: Regular Contributor
timestamp: February 05, 2025, 11:26:05 pm
content: I do have some 1n4001's handy but the only Schottky's I have are 1N5817's....would those work or should I stick to the 4001's?

author: wraper
author_rank: Unknown
timestamp: February 05, 2025, 11:53:02 pm
content: Quote from: Midphase on February 05, 2025, 11:26:05 pm1N5817 will work, although there is very little maximum voltage margin, but certainly good enough for testing. You don't need Schottky in this circuit, though. BTW, if you used 1N5817 instead of 1N5819 for rectification, you should replace them with something rated for higher voltage.

author: Midphase
author_rank: Regular Contributor
timestamp: February 06, 2025, 12:06:17 am
content: Ok, I installed the 1n4001's for now.I feel like I'm losing my mind. I will get a correct voltage reading from the 7815 one moment, then I insert one of the NPN's and it stops giving me the right voltage. Then I unplug the power supply, wait a few seconds, plug it back it and it works again with the 15v output.Here's how I soldered the diodes, I think I did it correctly, the 7915 is on the right side and the 7815 is on the left in the photo.I also just had another thought...is it possible that the power supply I'm using has some sort of circuit protection safety and that I'm triggering it with some short or something? That would explain why waiting a few seconds and then plugging it in again seems to bring the voltage back to normal?

author: wraper
author_rank: Unknown
timestamp: February 06, 2025, 12:10:11 am
content: Quote from: Midphase on February 06, 2025, 12:06:17 amLooks fine

author: bdunham7
author_rank: Super Contributor
timestamp: February 06, 2025, 12:11:04 am
content: Quote from: Midphase on February 06, 2025, 12:06:17 amIs either regulator getting hot? They have both current limiting and thermal protection.

author: wraper
author_rank: Unknown
timestamp: February 06, 2025, 12:14:24 am
content: If diodes do not help, you can try 1N5817 for 7815, it will limit negative voltage to lower value. Also I suspect your vreg may be a fake Chinese shit that latches-up if you look at it funny. Post a front side photo (marking).

author: WB8PFZ
author_rank: Regular Contributor
timestamp: February 06, 2025, 12:29:55 am
content: It looks like the diodes on the left side go to the left and middle pins and the one on the right go to the outer pins? Not sure if this is correct?

author: wraper
author_rank: Unknown
timestamp: February 06, 2025, 12:32:38 am
content: Quote from: WB8PFZ on February 06, 2025, 12:29:55 am79xx has different pinout.

author: fzabkar
author_rank: Super Contributor
timestamp: February 06, 2025, 12:34:04 am
content: In that Sinclair thread, the original diode on the positive regulator output was a 1N4148. This was affected by latch-down. Replacing this diode with a 1N5819 fixed the problem.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 06, 2025, 01:01:19 am
content: Quote from: Midphase on February 06, 2025, 12:06:17 amWhat power supply are you using?Try measuring the voltage before the regulators when the fault happens.

author: Midphase
author_rank: Regular Contributor
timestamp: February 06, 2025, 07:03:21 am
content: Thank you all for the great ideas and help, after I spent the past several hours trying all sorts of things, but I finally got it working.It turned out that there were a couple of issues which together made for a bit of a confusing troubleshooting process. The first one is a somewhat "duh" sort of issue in that the barrel socket connector that I was using for the AC power wall wart to the circuit was iffy at best so that it didn't always make full contact which was leading to my wacky voltage readings. After I replaced the barrel socket I got steady voltage readings on both regulators -- whew!Next however I was not getting any sound output, and most definitely not any chorusing effect. Turns out the issue were a couple of malfunctioning 100K potentiometers (one used for the input level and the other for the chorus speed setting). Once I replaced the faulty potentiometers I was finally able to get a proper output from the circuit and so far it does appear that it's working as it should.Now that I have the basic functionality down, I want to make some customizations, the first one should be relatively straightforward which is to have a switchable preamp circuit added so that in addition to the current Line Level input, I can also run a guitar level through the chorus.The next thing is going to be a bit more tricky I think -- at the moment I'm using an AC to AC wall wart transformer, but I would prefer to use a more standard 9v or 12v DC wall wart instead, which means that I will need to create a negative voltage rail and ideally boost the voltage back up to +-15v (although I think the circuit should run on +-12v).Any ideas on what my best options would be to accomplish that?

author: Midphase
author_rank: Regular Contributor
timestamp: February 27, 2025, 07:18:13 am
content: I wanted to re-open this thread since a few weeks have gone by and I'm still running into this bizarre situation. I ended up re-building the entire circuit on a second PCB, and sure enough I'm running into the same exact issue.To recap:- I plug in the 16v AC to AC wall wart into the circuit and I would say about 50% of the time it works perfectly, however the other 50% I get no voltage on the output from the 7815 regulator. Typically unplugging it and plugging it back it a few times gets me the proper 15v out of it.- If I disconnect the regulator output from the circuit, then everything is 100% normal, which tells me that the issue is not between the wall wart through the rectifying diodes and through the electrolytic caps to the regulators, but somewhere between the 7815 output pin and the rest of the circuit.- The soldering is pretty clean, and on my 2nd build I was very meticulous about making sure that I triple checked everything. Components are from Tayda, not the greatest quality supplier, but not the worst either. I haven't had issues with any of my other builds, so this seems an unusual situation that is specific to this circuit.- I beefed up the rectifying diodes with 1n4004's and the caps with ones rated at 50v instead of the recommended 25v...just in case.- The regulators are not overheating. They are barely warm to the touch (and they should be since they're not doing a whole lot of work).- I suspected that perhaps the wall wart might have some issues, and thankfully I have a second one (albeit a slightly lower 12v output, but still hot enough to drive the 7815 and 7915, and I still get the same issue. Works fine about half of the time.- Just to try something else, I also changed up the regulators from 7815 and 7915 to a 7812 and 7912. The circuit works fine at 12v, but even in that scenario the issue still persists.I have spoken with other individuals who have also done this very same build and they have not run into the same issue as me, which leaves me further scratching my head.I have ordered a power supply that should deliver +/- 15v rails allowing me to completely bypass the regulators to see if perhaps that's a more stable solution.Other than that, I'm not quite sure where I am possibly going wrong. The PCB matches the schematic which in turn matches the original schematic that this one was cloned from. Electrically speaking, the circuit should be working quite well.Anyhoo..if anyone has any other ideas, I'm all ears! Many thanks.