1993 386 Computer Generating Lots of Electrical Noise -- Spectrum Analyzer - Page 1
Forum section: electronics / repair

author: humidbeing
author_rank: Regular Contributor
timestamp: January 15, 2024, 04:56:32 am
content: I'm repairing a 1993 Packard Bell 386sx @ 25MHz PC. No battery damage or corrosion on board. Uses a 50MHz crystal for the CPU.Symptoms:Turning on PC completely jams all local FM radio stations.VGA video output is very noisy and wavyMachine is unstable and randomly goes into reboot loopsWhat I've tried:Powered it with a modern known good ATX power supply -- No changeReplaced main power filter caps next to the AT power connector -- No changeRemoved everything I can from the board like the extra ram Simms and ISA cards -- No changeI have a TinySA and I notice some pretty big spikes in the 250MHz to 350MHz range. According to the TinySA, these spikes are about 29db higher than my local radio stations. They completely disappear when the machine is off. Strangely, I don't really see anything in the 25MHz to 50MHz range. But I assume these higher frequencies are harmonics? I'm a digital guy and am very weak on RF.Could this be a red herring or am I on to something?I have an H-Field probe after seeing Dave use them in a video years ago. But no preamp. Should I give that a go? Right now I'm using the telescoping antenna held about 1-2 feet from the PC.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 15, 2024, 05:44:21 am
content: I don't think 'Spread Spectrum' was a BIOS setting back in the days of 386, so all you can do is put the motherboard in a closed metallic PC case, and hope this will reduce the spurious enough. Unless that PC is for a demo in a museum, then it doesn't worth the electricity it consumes.What for do you want it restored?

author: humidbeing
author_rank: Regular Contributor
timestamp: January 15, 2024, 05:53:00 am
content: Quote from: RoGeorge on January 15, 2024, 05:44:21 amFirst, I didn't ask for opinions on how I should spend my time. You're not the judge of what deserves to be repaired or not.Second, the noise is still a problem with the case on, because it's infiltrating the video signal path inside the case.If you're going to be an arrogant moron, you could at least read the entire post.Edit by gnif: Please stop reporting this, read on and you will see a moderator has already responded.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 15, 2024, 06:28:31 am
content: Quote from: humidbeing on January 15, 2024, 05:53:00 amYou OK?- You are seeing harmonics. 'Spread spectrum' as a BIOS setting was introduced exactly to mitigate that. Spread Spectrum enabled "wiggles" the clock frequency, such that the radiated energy spreads in less sharp RF peaks, so 'Spread Spectrum' enable should help reducing the spurious interference.- The 386SX is half the memory bandwidth of a normal 386, so in terms of computing power it's less than a normal 386. They do consume a lot of power reported to the computing power, if you benchmark that.- And if you are restoring it to run a particular program from back then, there are all kind of emulators and virtualization ways, so old programs can be run on current hardware. That was why I was asking the 'what for' question.Read the forum rules, and don't call me names for trying to help. That was very rude from you, and for no reason. It is not my fault you felt diminished, I was talking about that 386 SX performance, not about you. I'll assume you have had a bad day or something.Quotehttps://www.eevblog.com/forum/chat/forum-rules-please-read/

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2024, 06:36:34 am
content: Its a Packard-Hell. Nuff said!Those of us who worked on them back in that era have tried hard to forget, though your list of symptoms wouldn't have surprised me if it had crossed my bench 30 years ago . . .

author: Andy Chee
author_rank: Super Contributor
timestamp: January 15, 2024, 06:36:47 am
content: Quote from: RoGeorge on January 15, 2024, 05:44:21 amSuch a comment does nothing helpful.Quote from: humidbeing on January 15, 2024, 04:56:32 amDo you have a POST card? You need to investigate what data activity (if any) is occurring on the ISA bus.https://en.wikipedia.org/wiki/POST_card

author: selcuk
author_rank: Frequent Contributor
timestamp: January 15, 2024, 06:46:48 am
content: You can find the probable sources with the near field probe. It is usable without amplifier. I guess your telescopic antenna doesn't have a flat response on the whole band. So those frequencies might be in some of its resonance bands and SA shows higher than other signals.You can remove the cables one by one to see the effect. Since you are capturing with an antenna, it may be a signal radiating from the cables not from the board itself due to the smaller antenna like structures on board than cables.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 15, 2024, 07:28:22 am
content: Quote from: Andy Chee on January 15, 2024, 06:36:47 amThat was to make the OP aware that, unless it's for a museum or for sentimental reasons restoring, that PC doesn't worth the electricity it burns. Not everybody know SX were the cheap flavor of 386. To make it cost less, they used half the width for the memory bus, that's what SX means. They were not rare either.Now tell me, how is your comment helpful?And how a POST card can reduce EMI?

author: humidbeing
author_rank: Regular Contributor
timestamp: January 15, 2024, 10:08:20 am
content: Quote from: Ian.M on January 15, 2024, 06:36:34 amI've been working on them since the 90s and that's patently false. Once again, another ignorant, unhelpful comment.This forum is fucking worthless.

author: humidbeing
author_rank: Regular Contributor
timestamp: January 15, 2024, 10:13:20 am
content: Quote from: RoGeorge on January 15, 2024, 07:28:22 amIt's quite something how useless people from insignificant countries think they can tell others what to do.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 15, 2024, 10:17:49 am
content: Quote from: RoGeorge on January 15, 2024, 07:28:22 amThe POST card was to investigate the random reboots, not the EMI.The problems could certainly be related, but one step at a time.

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2024, 10:34:59 am
content: Patently false?https://www.sfgate.com/business/article/packard-bell-settles-suit-over-used-parts-2967237.phpYou've got a Packard Bell branded motherboard, and in that era they were notorious for muntzing everything they could, so unless you've tested it screwed down to a chassis with *ALL* the mounting holes with ground pads properly grounded, you've been wasting your time.

author: Halcyon
author_rank: Unknown
timestamp: January 15, 2024, 10:36:06 am
content: How about you calm down a little humidbeing. Whilst you might consider the comments of others "ignorant" and "unhelpful", the same can be said for your hostile language. It achieves nothing other than getting others off-side.

author: SeanB
author_rank: Super Contributor
timestamp: January 15, 2024, 11:30:10 am
content: Most common sourse is radiated EMI from the HDD cable. Grab a 66MHz UDMA compatible cable, which is backwards compatible with IDE, but has better shielding on the cables. Ferrite beads clamped around all wires off the board, and when placing on the backplane of the case use every single screw that has a hole in the board, with the right brass standoff and a screw that makes contact with the grounding ring on the hole. Then case use all the screws, and also check power supply actually has a RF filter in it, many were cost cutted out, so add in the common mode choke, the 10n class Y capacitors to ground, and the 470n class X capacitors either side of the common mode choke as well. If they are there and are RIFA, and crazed, change them before smoke time. Old AT supplies also you need to put a ferrite clamp on choke on the internal power switch leads, as it is otherwise a very effective antenna.On the board, all those black electrolytics on the power rails, probably around 100-220uF 10V, replace the lot, using low ESR ones, they likely are either close to dead, or dried up totally. You should see a good reduction, provided you use a metal case with metal front, and put in all the case screws.I would send you a SX33 board, as I have an old one, but shipping to the USA is prohibitively expensive, last time it was $300 just for shipping.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2024, 11:54:56 am
content: Computers of that age often had EMI problems. So there may be no easy way to make the old computer run without interferening with radio reception.With contruction with an extra riser bord for the extension also does not make EMI easier.If the vidio card is producing a poor signal also a bad video card is possible - I don't hink this board has on board video.The 387SX is a bit of an oddity (odd that such a thing even exists) - so maybe a museum piece for this reason.Given the age and problems the way to recycling is a very real option and not such a bad advice.

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2024, 12:25:05 pm
content: Its got on-board VGA. You can see the Oak Technology OTI066/067/069 VGA chipset, with what appears to be 512K RAM, and the sticker for the OTIVGA video BIOS, which is presumably combined with the Phoenix bios in a single ROM chip. I can see a jumper 'JVGA8' or something similar next to the Oak OTI067 TQFP-144 IC that *may* disable it to allow an alternative graphics card to be used.There's also 2MB of soldered in FPM DRAM on-board, so the SIMMs are optional.If our O.P's got a PLCC puller it would be worth reseating the i387SX as its closely coupled to the CPU bus and tarnish on its control pins could result in bus contention.

author: Stray Electron
author_rank: Super Contributor
timestamp: January 15, 2024, 12:52:13 pm
content: Quote from: humidbeing on January 15, 2024, 10:08:20 amI predict that you're not going to be on this forum very long and that with your pit-bull attitude very few people are going to try to help you. As far as you having worked on PCs since the 90s, I've worked on PCs since the early 1970s (two Altairs, an SWTPC, Heathkit H-100s, a NorthStar, Apple II, Apple III, and countless IBM-PCs) and I've worked on mainframe computers since about 1968. Other's on here have similar experience and have told you what the problem is, we can't help it if you don't like their answers.

author: tggzzz
author_rank: Super Contributor
timestamp: January 15, 2024, 01:02:01 pm
content: Quote from: Stray Electron on January 15, 2024, 12:52:13 pmSeconded.Fortunately since "This forum is fucking worthless" I doubt the OP will be around for long.I note that recent server upgrades have relieved the workload on the servers. I'm using that to more aggressively "ignore thread" and "ignore poster".Unfortunately "ignore poster" still wastes my attention on the threads they start. (Bring back usenet readers)

author: HwAoRrDk
author_rank: Super Contributor
timestamp: January 15, 2024, 01:40:39 pm
content: Quote from: Ian.M on January 15, 2024, 12:25:05 pmI think you may be on to something there.Don't VGA chipsets typically have a RAMDAC that operates in the 100's of MHz?I think it would be worthwhile for OP to see if they can find a separate VGA ISA card and install it, and find whatever BIOS option or jumper disables the on-board graphics.Edit:Here are the jumper settings:https://theretroweb.com/motherboard/image/pb303-schema-651d518a294ba807963733.jpg"JVGA" is definitely to enable/disable the on-board VGA.

author: nali
author_rank: Frequent Contributor
timestamp: January 15, 2024, 01:50:49 pm
content: The only time I can recall noisy VGA was due to "ground bounce", either by bad connection or by ground loops.I wonder if the power GND path is bad and power is returning either thru the motherboard chassis fixings, peripherals or both?Unless there's a direct comparison with a known good system I wouldn't get too hung up on emissions considering the vintage. Not sure about the US but 1993 was about the time CE came in and EMC became "a thing" over here... before that there was some pretty horrendous kit about.

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 15, 2024, 02:02:01 pm
content: Quote from: humidbeing on January 15, 2024, 10:13:20 amUSA all the wayFunny how people seem to forget history. It is people from these insignificant countries that populated the USA in the beginning and made it what it is today.Quote from: HwAoRrDk on January 15, 2024, 01:40:39 pmDepends on the resolution, but yes it can be way up there.

author: Haenk
author_rank: Super Contributor
timestamp: January 15, 2024, 02:12:43 pm
content: At that time, "CE" marking was introduced - which actually meant meeting "emission rules". A friend of mine worked at a PC assembling house - they did check their systems for compliance. None of the systems were able to meet emissions regulations, by far. Their solution was to just put a "CE"-sticker on their systems and call it a day.The hardware just wasn't made to be RF friendly and there just is nothing you could do about it.

author: factory
author_rank: Super Contributor
timestamp: January 15, 2024, 06:52:55 pm
content: Quote from: Ian.M on January 15, 2024, 06:36:34 amI would disagree, it's not a real Packard Bell, they were just another "zombie brand" by the time this PC was sold.Ironically the original Packard Bell made radios (and TVs), exactly the kind of thing this jammer blocks.David

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2024, 07:43:29 pm
content: I've got nothing against the original Packard Bell Corporation, but they got out of the TV business ten years earlier and sold off their computer division ten years before that, so the only thing going for the resurrected name was fading brand recognition in NA. Quality wise, they were generally worse than clones built from ex-auction parts in back ally workshops, and were one of the pioneers of bundling unwanted software to increase perceived value to the less savvy. Combined with riser boards that went intermittent if you looked at them wrong, and the difficulty of finding the right drivers for their OEM hardware, which were often *NOT* the ones on the supposed system restore/drivers media, they truly earned the epithet 'Packard Hell'.

author: brabus
author_rank: Frequent Contributor
timestamp: January 15, 2024, 08:13:13 pm
content: Known issue, I fixed many of these back in the days.Long story short: the quickest way to proceed is just to replace the VGA graphic processor U19. The date code refers to a known bad batch from OAK. You can easily find some NOS starting from 9305 and swap it. Hot air, flux, bottom heater and you're good to go.Thank me later.

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 15, 2024, 08:44:11 pm
content: Someone should have helped him back in 2020.https://www.eevblog.com/forum/repair/help-finding-noise-source-on-old-386-motherboard/I added the picture pointed to on imgur in one of his posts. Looks like the same board, except for the missing Intel 387SX.And there you have the problem. An AMD processor with an Intel co-processor.

author: Halcyon
author_rank: Unknown
timestamp: January 15, 2024, 10:30:35 pm
content: Quote from: Kleinstein on January 15, 2024, 11:54:56 amI wouldn't go as far as saying it's a museum piece just because of this reason.Math coprocessors were pretty common, especially among power users and business machines that needed the extra grunt for applications like CAD. It's just a regular 386SX CPU, with the coprocessor installed along side it. Most motherboards had this socket but it generally wasn't populated by default.The 287, 387DX and 487SX were fairly common variants as well. You can still find these on ebay pretty easily.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 15, 2024, 11:42:35 pm
content: oh remember some stories, had an magus 386 DX25 and this one loved to make artefacts on crt tv's, but such in a perfect way, tons of dots at 1" spacings on the screenand in my case, it was really the case shielding who did the problemi had some flat spring load (sorry for the term) who get pushed between the case parts, gottent flat with time ..

author: amyk
author_rank: Super Contributor
timestamp: January 16, 2024, 03:43:00 am
content: This is pre-capacitor-plague but caps can still dry out. I'd check and replace the ones nearer the GPU too.