Help finding noise source on old 386 motherboard - Page 1
Forum section: electronics / repair

author: humidbeing
author_rank: Regular Contributor
timestamp: September 22, 2020, 11:54:50 pm
content: Hi gang!I'm trying to fix an old Packard Bell 386 motherboard. It's gives all sorts of random errors. But I think when I find the noise generator I will find the true fault.This board emits so much noise when turned on that it will jam all FM radios in the house. I have a similar board and it does not do this.Things I have tried:Alternate power suppliesrecapping board (actually made noise a little worse)removing RTC chipremoving keyboard controller chipremoving everything from board but vga (on motherboard), ram, and cpuThe noise also shows up on the 5 volt power rail on the motherboard, and it shows up in the vga output of the onboard video chipset.I'm a noob when it comes to RF. I have a good voltmeter and a scope at my disposal.Any tips on how I can track down which component is generating all this noise?

author: bob91343
author_rank: Super Contributor
timestamp: September 23, 2020, 12:43:54 am
content: First try to analyze the noise. Is it random, or are there definite repeatable signals? If you can identify the frequency that could give a clue.Another approach would be to wave the scope probe around to see where the radiated noise is strongest.Perhaps it's power supply related. You could substitute a different one. Or try ferrites around the cables from the power supply.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 23, 2020, 12:54:34 am
content: Tried with two known good power supplies. No change in noise output.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 23, 2020, 01:00:11 am
content: Quote from: bob91343 on September 23, 2020, 12:43:54 amI tried this without much luck. What scope settings should I use? I assume I want ac coupling?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2020, 05:04:01 am
content: Does it show up on ground as well?A lot of vintage PCs shipped with a shield over the motherboard. Do you know if that was the case here? It may simply not be designed for low emissions any other way.Tim

author: james_s
author_rank: Super Contributor
timestamp: September 23, 2020, 05:17:20 am
content: There's a reason these things were called Packard Hell back in the day. We had one when I was a kid and it had all kinds of problems down the road when we tried to upgrade the hard drive. I knew lots of other people who had them too, they were very common because they were cheap. It may just be bad design.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 23, 2020, 06:27:27 am
content: Quote from: T3sl4co1l on September 23, 2020, 05:04:01 amNo shield. This is definitely not a normal level of noise. It makes the vga output almost unusable, and the waveforms on the ISA bus look terrible and glitchy.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 23, 2020, 06:28:10 am
content: Quote from: james_s on September 23, 2020, 05:17:20 amI specialize in restoring packard bells. I have never had one make interference like this.

author: james_s
author_rank: Super Contributor
timestamp: September 23, 2020, 06:32:02 am
content: Does it use buck regulators to provide any of the voltages on the motherboard? I don't recall when that became common. Severe jitter in one of the clock sources?

author: humidbeing
author_rank: Regular Contributor
timestamp: September 23, 2020, 06:40:15 am
content: Nope. No regulation on the board itself. I checked all the crystals with my scope and they look good.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 23, 2020, 06:42:27 am
content: here's a photo of the board, if that helpshttps://imgur.com/a/OEdOiwH

author: Twoflower
author_rank: Frequent Contributor
timestamp: September 23, 2020, 07:54:17 am
content: You said you have a scope. Have you tried to create a loop (maybe few windings) with the scope-probe and scan the board with that loop? Or even just add a short wire to the scope tip and scan over the board. Maybe it can point you to the noise source.And put a sticker over the EEPROM before it gets a sunburn ;-)Edit: While removing the keyboard controller it seems you damaged the resistor array between U16 and U21. It might be still OK, but worth a check. And remove the debris located at the array.Also I noticed north of the ISA-slot there is U18. It looks a bit dis-colored. Could be a reflection.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2020, 08:59:16 am
content: Looks to be built with internal ground plane, and grounded screw holes. And EMI strip along the connectors. It's made for a metal enclosure. Are you just testing it out in the open?Tim

author: james_s
author_rank: Super Contributor
timestamp: September 23, 2020, 08:14:47 pm
content: That takes me back. Looks very much like the motherboard in the PB 386sx-16 my family had back in the early 90s. It had a single ISA slot on the board with a vertical riser that held 4 cards horizontally. That machine had the weirdest issue I've ever seen, it worked fine with the original 40MB WD IDE drive but none of the 3 or 4 larger drives we tried later would work. I could partition and format the drive, copy software to it, move files around but the instant I tried executing any program from the hard drive the machine would hard lock. The same happened while booting, it would hard lock at "Starting MS-DOS". Put the original drive back in and it was fine.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 23, 2020, 11:15:44 pm
content: Quote from: T3sl4co1l on September 23, 2020, 08:59:16 amYes. But it does the same thing whether it's in the chassis or not.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 23, 2020, 11:16:24 pm
content: Here's how the noise manifests itself coming through the VGA signal

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2020, 06:44:51 am
content: Interesting. Well, I'd have to have a close look at it, with instruments.Tim

author: Haenk
author_rank: Super Contributor
timestamp: September 24, 2020, 07:54:26 am
content: There are several quartzes on the board, so a lot of frequencies are generated at the same time. Scoping around really might show something, you might be ableto match the frequency to a quartz.

author: Ian.M
author_rank: Super Contributor
timestamp: September 24, 2020, 08:43:59 am
content: Back in the 90's Packard Bell PCs had a reputation for being very low quality, (usually worse than what the average back-street cash only clone builder of the era was building) and were cost conscious to the point of getting sued several times for using recycled parts sold as new!I'm seeing a board full of ICs with Q1 1992 date codes. Although its too early to have been from the 'capacitor plague' era, all the electrolytic capacitors on that board are twenty eight years old, and were 'economy grade' when new. Odds are many or all of them have dried up, have high ESR and aren't doing much decoupling.If you've still got the noise problem after fully recapping the board, you may have to dig deeper, though if one of the custom chips is bad, its probably BER.

author: drussell
author_rank: Super Contributor
timestamp: September 24, 2020, 09:05:41 am
content: Indeed, those things were absolute junk right from the factory.You need to replace every decoupling capacitor for a start, if you want it to work right.Personally, I would just throw anything from that era with the Packard Bell name on it as far as possible and run away, then pull out a real motherboard, but then again, I have dozens of proper, decent quality 386 boards here.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 24, 2020, 10:47:12 am
content: -are you sure the noise causes the problems, and not the other way around?A gate in a gate array left with floating input (from ESD, faulty chip etc.) could concievably oscillate and create lots of noise.I agree with the others, though. Unless the potential noise source is findable with the tools at hand, I'd toss the motherboard. They can't be expensive to replace via Ebay, if needed.I was going to say 'get one of those cheap chinese 35-4400 MHz spectrum analyzers off of ebay' but for a one-off problem that's probably more expensive than replacing the board.

author: james_s
author_rank: Super Contributor
timestamp: September 24, 2020, 05:53:54 pm
content: Quote from: ChristofferB on September 24, 2020, 10:47:12 amI guess it depends on your definition of "expensive". I haven't shopped for 386 boards but I've sold a couple of untested as-is 486 boards for ~$75 over the past couple of years and those are easier to find than 386 boards. PCs of this era are pretty rare these days, especially the more proprietary stuff that people often junked instead of fixing or upgrading.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 24, 2020, 10:21:23 pm
content: Quote from: ChristofferB on September 24, 2020, 10:47:12 amthese motherboards are actually very rare and worth a good bit of money

author: humidbeing
author_rank: Regular Contributor
timestamp: September 24, 2020, 10:22:17 pm
content: Quote from: Ian.M on September 24, 2020, 08:43:59 amno offense, but if you would have read my first post you would have seen I replaced all the caps already. It actually exacerbated the noise slightly.

author: Ian.M
author_rank: Super Contributor
timestamp: September 24, 2020, 10:39:57 pm
content: Sorry I missed that.How much experience do you have recapping motherboards? Was there any evidence of electrolyte leakage?Use the wrong caps (e.g. too high ESR, or insufficient voltage rating), or damage a PTH barrel's connection to an inner trace and you can make the problem worse.I never was a fan of Oak Technology VGA cards. Do you have enough info to know how to disable on-board video, (which I'd bet was by jumpers if its possible), so you can try a known good VGA card?

author: humidbeing
author_rank: Regular Contributor
timestamp: September 24, 2020, 11:29:34 pm
content: Quote from: Ian.M on September 24, 2020, 10:39:57 pmPTH barrel?I agree oak vga sucks. I went so far as removing the oak vga flatpack from the board with a hot air station. No change. Add in vga cards also show the noise on their outputs as well. It's omnipresent on this board.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 24, 2020, 11:39:16 pm
content: I"m about to fire up the hot air station and just start pulling chips until the noise stops lol

author: Ian.M
author_rank: Super Contributor
timestamp: September 24, 2020, 11:49:26 pm
content: PTH=plated through hole. If you are slightly too rough with it desoldering something, especially if the hole wasn't properly de-smeared before plating, or the resin component of the substrate composite is of low quality, you can break the connection from the hole plating to an interior trace, without it being obvious that you have done so.Have you tried running the board properly screwed down to a chassis baseplate,so every mounting point is grounded, with the baseplate grounded to the PSU caseing either through case metalwork, or if loose on your bench, via a short copper braid strap?C87 which you fitted is 22uF 50V. That's more than a bit unusual on a motherboard with a maximum supply rail of +12V. It appears to be in the middle of the digital side of the VGA subsystem, so there cant be any conceivable need for that rating there (unlike round RS-232 transceivers, where its *remotely* possible one might need a cap from +12V to -12V with a min. 35V rating, although a pair of 16V or 25V caps, from each rail to ground would be far more likely).What was the working voltage rating of the cap you removed from that location? Did you replace all other caps with ones of similar voltage ratings to the originals?

author: humidbeing
author_rank: Regular Contributor
timestamp: September 24, 2020, 11:56:41 pm
content: Quote from: Ian.M on September 24, 2020, 11:49:26 pmYep I've ran it mounted in the chassis. Same issue. Believe it or not the original caps were rated 50 volts.

author: Ian.M
author_rank: Super Contributor
timestamp: September 25, 2020, 12:24:16 am
content: Quote from: humidbeing on September 24, 2020, 11:56:41 pmFair enough. P.B's assembly contractor must have got a deal on some cheap and nasty 50V ones. If you compare like for like, higher voltage caps of the same can size and value often have somewhat higher ESRs, so I'm always a bit wary of increasing the voltage rating excessively.Sorry, I'm almost out of ideas except try probing around the board with your scope using a x10 probe with a short spring clip for ground rather than the usual 3" clip-on croc-clip lead, looking for a higher than elsewhere noise level on supply rails at decoupling caps or the supply pins of any chips you can find pinouts for.Also, have you got access to a thermal camera? You mentioned you had a very similar board that's not noisy. Its a long shot, but you could use thermal imaging to see if the noisy board is over-heating anywhere compared to the good board, running side by side, as that could point to a chip with ESD damage or a floating input, drawing excessive and pulsed supply current

author: drvtech
author_rank: Regular Contributor
timestamp: September 25, 2020, 12:25:26 am
content: I think a clue might be that the noise got worse when you replaced the caps - did you use absolutely the best (meaning lowest ESR) caps available?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 25, 2020, 12:56:12 am
content: Low ESR might make things worse; it was designed for whatever caps they used. Better to replace with a similar grade.Assuming it was designed at all, I suppose. I haven't seen any reason so far to believe it would've behaved any different when brand new.Don't discount your monitor or other hardware either; perhaps they're just more susceptible?Tim

author: drussell
author_rank: Super Contributor
timestamp: September 25, 2020, 02:25:04 am
content: Quote from: drvtech on September 25, 2020, 12:25:26 amYou don't need extremely low ESR, high-ripple-current-capable capacitors on boards like this. This is well before there were switching regulators on-board. This is for local decoupling of the processor and logic only.

author: james_s
author_rank: Super Contributor
timestamp: September 25, 2020, 02:31:14 am
content: Sorry if you already covered this, but are there any other symptoms besides the video noise? If it's just the video then it's possible that there's a fault in the video circuitry. The fact that it got worse after replacing capacitors makes me wonder though.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 25, 2020, 05:08:55 am
content: Quote from: T3sl4co1l on September 25, 2020, 12:56:12 amI've refurbed quite a few of these machines and never seen one with a noise problem like this. Nothing with my test bench setup has changed recently.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 25, 2020, 05:09:51 am
content: Quote from: james_s on September 25, 2020, 02:31:14 amIt emits enough RFI to jam an FM radio in the same room. Puts noise on the ISA bus data lines and the 5V power rail on the mobo.

author: humidbeing
author_rank: Regular Contributor
timestamp: September 25, 2020, 05:10:27 am
content: Welp. I just ordered a spectrum analyzer and a near field antenna. We're getting to the bottom of this.

author: greenpossum
author_rank: Frequent Contributor
timestamp: September 25, 2020, 05:37:35 am
content: This is better than any whodunnit.

author: james_s
author_rank: Super Contributor
timestamp: September 25, 2020, 05:59:42 am
content: Never hurts to have an excuse to get more toys.

author: Rasz
author_rank: Super Contributor
timestamp: September 25, 2020, 07:01:08 am
content: boot it with no crystals

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: September 25, 2020, 10:29:07 am
content: just a shot in the dark... in the good old days, when i tried to max out the performance of an octek hippo VL, i went a tad too aggressive on motherborad ram timings and all over sudden had "snow" on the vga. so, maybe the register settings of the chipset get screwed on boot-up? in other words, could a re-flash of the bios do the trick?

author: drussell
author_rank: Super Contributor
timestamp: September 25, 2020, 10:35:21 am
content: Quote from: Le_Bassiste on September 25, 2020, 10:29:07 amThat doesn't explain it causing so much interference that it knocks out all nearby FM radio reception, nor the fact that it got worse when the OP changed the decoupling capacitors.

author: Labrat101
author_rank: Unknown
timestamp: September 25, 2020, 11:17:07 am
content: Hi ,That's an oldie I have not worked on one of those Dells for Yonks.I noticed in your Photo that the sticker on the BIOS is missing . cover the hole or the internal Program will get damaged.This not the noise problem But will cause you a headache if it gets erased .Have you tried running the board from a UPS . so that it is isolated from the mains running just on UPS ?If this solves the problem .The source of the noise could be from a local transmission source , ie Police . Ham Radio , CB the worse as they send outa lot of harmonics that are close to the boards operating frequencySo your old 386 is running like a receiver .So I would recommend Running from an isolated source ie UPS .Then if you have an oscilloscope make a loop of a few turns between the tip of the probe to the ground . and sweep theprobe over the entire board to find the strongest signal .The outside signal will get reproduced on your board and sent back .I had some thing similar about 15 years back it was an army base and a bad aerial with a high SWRFrom all what you said It sounds like its an outside signal .Baby monitors are also good for transmitting harmonicsBTW.. I guess you checked the monitor you are using an old CRT EGA/VGA 405line This could also be the noiseproblem that the HF stage has a dodgy componentsIf I had that in my Lab the first thing I would do. To hitch it up to a different monitor.as there is not lot on that mother board that has the Power to Oscillate at FM .But the monitor has ..Have you tried booting it up and then unplug the Monitor and remove the VGA & mains from it .Have fun you have to do some detective work

author: humidbeing
author_rank: Regular Contributor
timestamp: October 01, 2020, 01:41:08 am
content: Well, after playing around with the handheld spectrum analyzer and the near field antenna I find the strongest output on this side of the chipset flatpack. Peaks around 65 and 100MHz. -24dBm!I removed the 3 oscillators next to the chipset and the noise is still present.https://imgur.com/a/SVSoikM

author: Labrat101
author_rank: Unknown
timestamp: October 01, 2020, 12:21:54 pm
content: Quote from: humidbeing on October 01, 2020, 01:41:08 amThat about sums it up "" Well, after playing around ""That chip runs @ 25Mhz .. So your TOY will see the 4thHarmonicIF only you had read previous post above .. The answer is obvious .You just need an excuse to buy some more TOYs that will not Help you. But look CoolThe solution is simple .But I am sorry to say You will never solve it .

author: Haenk
author_rank: Super Contributor
timestamp: October 01, 2020, 01:24:19 pm
content: Quote from: Labrat101 on October 01, 2020, 12:21:54 pmYou didn't look at the RF spectrum, did you?There are harmonics of about 32MHz, so that 25MHz has nothing to do with it. No sign of 25MHz harmonics. So it's rather "look for stuff that runs at 32MHz".

author: Labrat101
author_rank: Unknown
timestamp: October 01, 2020, 01:33:07 pm
content: Quote from: Haenk on October 01, 2020, 01:24:19 pmDer OF course its not the motherboardYour Crappy pictures That don't show any thing .. You should look Again at you toy and learnto read better the things you are testing ..And Put up some better photo's OF THE whole set up and not just small locationsthat are of no use what's so ever .I dare you put up some real photos of the complete setup every thing including close ups .including how its connected SMPS leads to MB Monitor Connections etc.up them Here on this site ..

author: LateLesley
author_rank: Frequent Contributor
timestamp: October 01, 2020, 01:49:24 pm
content: Quote from: humidbeing on September 23, 2020, 11:16:24 pmI've seen that before. Try ad different, heavier VGA cable. I had noise like this before, and it was cable related.

author: humidbeing
author_rank: Regular Contributor
timestamp: October 01, 2020, 11:25:45 pm
content: Quote from: Labrat101 on October 01, 2020, 12:21:54 pmwow you're an unhelpful goon, aren't you.not all of us can afford $5000 spectrum analyzersand the harmonics are 33MHz not 25and I removed all the crystal oscillators from the board yet it still emits this rfi

author: Labrat101
author_rank: Unknown
timestamp: October 02, 2020, 08:52:54 am
content: I was trying to get you to use your brain in the right direction so you would learn.But that seems to have failed .First your spectrum analyser will not help you solve this problem Nor would a a $5000 scope .You never gave enough info . of the Complete set up that means every thing .I also noticed from you poor photo you uped to the other site were it says keyboard controller on the PCB there was some damagetracking .And the VGA plug sits at a funny angle .. maybe due to bent pin or just not pushed in all the way ..Was the hard Drive connected when the noise was present ??Is the SMPS the original ??What is the monitor .You are asking for Help but you have not given enough info .The screen video you manage to remove 50% of the picture .So asking for Help and only giving 10% of the story .You are assuming we all have crystal balls and can see what's on your DeskSo if you want help . give full info and then there is a chance that some one May help you .If You are unable photo and supply all the Info .Make it into a novel WHO Dunn-it . By IBrokit .QuoteYES you ARE

author: Haenk
author_rank: Super Contributor
timestamp: October 02, 2020, 09:42:42 am
content: (sidenote: I have 10+ exactly same mainboards and power supplies, running as memory testers - one board is showing similar distorted VGA symptoms from day one, but I didn't bother, because it works fine otherwise, and no, it's not in the cables, the PSU, the monitor or radar or whatever - they all sit next to each other...)(edit: to clarify, these are *other*, much newer mainboards - still the symptoms of one board are the same)

author: james_s
author_rank: Super Contributor
timestamp: October 02, 2020, 09:26:30 pm
content: Now I'm curious, what are you doing that requires testing memory in 10 386 motherboards?I have a very handy DRAM tester made by Darkhorse Industries, I think I paid $50 for it on ebay years ago and it has been indispensable for testing and identifying old SIMMs, it will even measure the actual access time. I've seen one other like it that was listed for about the same price but I don't know if it ever sold. I wish I could get more of the adapter boards for things like laptop memory but I think the company is long gone and it's hard to even find much info.

author: humidbeing
author_rank: Regular Contributor
timestamp: October 03, 2020, 01:26:50 am
content: Quote from: Haenk on October 02, 2020, 09:42:42 amYou wouldn't be open to selling one of them, would you? This is my childhood machine I'm trying to restore.

author: Labrat101
author_rank: Unknown
timestamp: October 03, 2020, 08:07:13 am
content: Hi ,Just to put you out of your misery .Your floppy drive system is MFM Which spelt out is ..Modified frequency modulation (MFM) is a run-length limited (RLL) coding scheme.also known as FM encoding, delay coding and Miller coding for encoding data onsingle-density floppy disks and some early hard disk drives.Recommendations Put it all back together in the box and enjoy it as is .This system was made obsolete for many obvious reasons .There were batches of this MB with a modifactions to limit the interference .Your lucky your MB is not one of them ..

author: humidbeing
author_rank: Regular Contributor
timestamp: October 03, 2020, 08:28:50 am
content: Quote from: Labrat101 on October 03, 2020, 08:07:13 amit's got nothing to do with the floppy drive. the drive unpowered, disconnected, and the super i/o chip with the floppy controller has been removed from the board along with its oscillator.

author: james_s
author_rank: Super Contributor
timestamp: October 03, 2020, 04:58:03 pm
content: By coincidence, one of those memory tester things like I have popped up on ebay. I have no affiliation with this seller, it just came up on my search since I've been watching for more of the test fixtures.https://www.ebay.com/itm/Darkhorse-Systems-Inc-Sigma-LC-Memory-Test-Unit/133535713229