Chasing the hum - Page 1
Forum section: electronics / projects

author: marinedalek
author_rank: Contributor
timestamp: October 31, 2012, 04:34:42 am
content: Last year I bought a Yamaha Electone organ for the princely sum of £20 to rob some attractive lever-type potentiometers from it. That didn't quite go to plan and I ended up keeping it to play and buying another to tear apart later. I just couldn't bring myself to dismantle it when I saw its almost pristine condition, plus being a musician I couldn't resist playing with it and enjoying the bloopy rhythm section and analogue string sounds.It works pretty well considering its age, although upon first power-up the 250V mains filter cap expressed its displeasure at being dried up by spewing its guts over the PSU board, but that was easily replaced. However when on there is a very noticeable mains hum which the later one (that I took apart) did not exhibit. Unfortunately the second organ was a lesser model with no compatible parts so a simple swap isn't going to work.I've stripped down the power supply and recapped it, so I'm confident that that's working OK. The hum is independent of the volume control and is present on both amplifier channels (one main speaker, one tremolo speaker). I've been able to make the hum go away by shorting the output op-amp inputs to ground, which suggests that the hum is somewhere in the signal path. My main suspicion at the moment is capacitors. The PCBs basically look as if someone emptied a sack of capacitors onto them so there's plenty of scope for one to go wrong somewhere. Here's a picture of some of the instrument filtering circuitry and to the top left are some dodgy-looking caps. (warning: large image)http://farm8.staticflickr.com/7144/6482334705_04a6656577_o.jpgThere's a clear blue resin-like buildup with some small bubbles in it on the top of nearly all of that type of cap (Nichicon) throughout the organ and I was wondering if that could be electrolyte. I've desoldered a few and checked their capacity and all seems well, however I don't currently have an ESR meter to check that. They're also present in large quantities on the main board which deals with sound generation, digital logic and the summing and switching of the audio signals prior to returning to the amp outputs. That's where I'm currently guessing the problem may lie...Also can anyone identify the off-white caps? I believe they're polyester film but not 100% certain and if they're likely to have degraded over 30 years I could end up replacing them if I can't avoid it.I've given the organ plenty of time to run in and settle down over the past 12 months and the 50Hz hum is still very obvious so it's not going to go away on its own. Any suggestions or advice anyone could offer would be much appreciated!

author: poptones
author_rank: Frequent Contributor
timestamp: October 31, 2012, 05:04:55 am
content: There's one large blue cap on there that looks from here like it has a bulging top.You don't say what sort of organ this is. Is it all electronic or is it reed? If it's reed it could be coming from the reed section as well. Otherwise, I'd start replacing big electros.That thing must be full of 4558s. Do you know what "vintage" 4558s go for? If you should ever scrap it or "upgrade" it, make sure you strip the boards of those. I see 30 bucks in just the picture you showed here.http://www.ebay.com/itm/RC4558JG-TI-8-Pin-CERDIP-OP-AMP-VINTAGE-4558J-/370594517420?pt=LH_DefaultDomain_0&hash=item56492791acHell, rip out the 4558s, sell'em, and put in 5532s...

author: marinedalek
author_rank: Contributor
timestamp: October 31, 2012, 05:38:38 am
content: Sound generation is all electronic courtesy of Yamaha's PASS (Pulse Analogue Synthesis System) architecture. Yeah, those blue caps are the ones I was concerned about. They're 470uF/16V if I remember correctly and in person they don't seem to be bulging particularly, but if you look carefully on the picture you can see the top is tinted a light blue colour. That's this odd hard clear resin-like substance which doesn't look like any electrolyte I've seen before, though who knows what they were using in the late 70s...You're right, the thing has loads of 4558s, as did the one I dismantled. I desoldered them from that one and there are six of them sitting in one of my component drawers at the moment. These are all PDIPs marked 4558DV JRC with a 9309 datecode, (possibly 1979?). Can't see any of that particular type and vintage on ebay, so perhaps they're not the "special" onesEDIT: I've just found some more photos that I took a while ago.This is one of those caps, showing the blue... whatever it is on top:https://dl.dropbox.com/u/69960/OrganElectro.jpgAnd this is the mainboard. The buffer section is top right, and you can see all of the large electros surrounding it.https://dl.dropbox.com/u/69960/OrganMainboard.jpg

author: JuKu
author_rank: Frequent Contributor
timestamp: October 31, 2012, 06:51:31 am
content: Electrolytics ave a "best before" date, and those a far beyond. The power amp supply is supposed to be smoothed by a bunch of big capacitors. The electrolytic on those has dried years ago, and your supply now has mains voltage ripple on it.It needs more work than money; since you plan to keep it, replace all of them, it does not make sense to fix it only partially.

author: marinedalek
author_rank: Contributor
timestamp: October 31, 2012, 04:17:34 pm
content: Ah sorry, I forgot to mention that the recap of the PSU included the big reservoir electrolytics for precisely the reason you mention, so those have already been taken care of with no reduction in hum.

author: SeanB
author_rank: Super Contributor
timestamp: October 31, 2012, 04:30:14 pm
content: The blue is a sealer applied by the Yamaha factory to the caps, it just makes them look nice, and keeps them shiny. I would look at all the caps and replace any that have bulging aluminium tops or bottom rubbers, as this is more likely to be a fault. Check as well by unplugging, spraying with contact cleaner and then replugging twice all cables onto the boards, as this will often fix hum problems caused by poor common connections between the boards. As well check the shielding foils are still connected to ground in the case and by the electronics, surprising just how often the wire falls off or is never replaced after servicing.

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 31, 2012, 04:51:15 pm
content: Also, you can use an ESR meter if you have access to one. That will help identify the bad caps.

author: M0BSW
author_rank: Frequent Contributor
timestamp: October 31, 2012, 04:55:33 pm
content: Quote from: poptones on October 31, 2012, 05:04:55 amThink i'd change both of them, shame you haven't got an ESR meter, you could go over the lot in one evening, I checked some a radio I was repairing and found quite a few , not all of them were bulging either.

author: Thor-Arne
author_rank: Unknown
timestamp: October 31, 2012, 08:03:30 pm
content: Quote from: marinedalek on October 31, 2012, 04:34:42 amYes, they are polyester film.Don't know the brand, anyone? Haven't seen those for a few years.As for degrading you really need the manufacturer's datasheet.Most of the poly caps is listed as long-life, but not a word about what is considered long life.Also, temperature, humidity and operating temperature seems to affect the life-time.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 31, 2012, 08:27:02 pm
content: Normally hum is not introduced by a bad cap other than in the power supply, so changing caps on the other pcb's is probably not going to fix it.It's either something in the power supply (a bad cap there is the best shot), something else is drawing too much power or something is picking it up because of bad shielding (unlikely) or a bad earth connection. Since it is independent of the volume control it is introduced after that, in the final amplifier. A scope on the output of the power supply should tell you a lot more.

author: David_AVD
author_rank: Super Contributor
timestamp: October 31, 2012, 09:08:08 pm
content: I'd clean all of the plug-in connectors first. Sometimes you get hum introduced when an 0V pin for a board is high resistance. The signal ref ends up going through a different 0V and gets contaminated.

author: ptricks
author_rank: Frequent Contributor
timestamp: November 01, 2012, 08:48:54 pm
content: Did you clean the board that contained the bad cap that burst ? I had a situation recently where I replaced a burst cap and the circuit still failed to work properly, after cleaning and removing all traces of the electrolyte from the board the circuit worked fine again. I didn't really see any trace of the electrolyte on the circuit but I could see a faint image of it on the board so I didn't clean the board right away.

author: marinedalek
author_rank: Contributor
timestamp: November 02, 2012, 10:44:44 pm
content: Thanks for all the helpful replies! I'll give the connectors a squirt of contact cleaner at the weekend and go from there. Yamaha were quite good and most of the boards have multiple earth connections but you never know. @ptricks I did clean the whole power supply board when I recapped it, which got rid of the awful 70s fishy flux too.@SeanB thanks for the verdict on the caps. I didn't recognise it as electrolyte but you never know.On the ESR meter side of things, does anyone know a cost-effective place to get a Blue ESR Meter kit in the UK? I can only find one shipped from the States on eBay with massive shipping costs plus the VAT when it arrives, so if there's anywhere here that stocks it that'd be much preferable

author: notsob
author_rank: Frequent Contributor
timestamp: November 02, 2012, 11:24:40 pm
content: If you want to - make your own ESR - here are the details - it's also available as a kit from Aztronics in Adelade, Australiahttp://www.users.on.net/~endsodds/esr.htm

author: Kremmen
author_rank: Super Contributor
timestamp: November 03, 2012, 07:32:42 am
content: Those blue caps (and the smaller ones also, if i saw correctly) are made by Nichicon, considered to be a quality manufacturer. I would not bother replacing the caps since they will not fix this problem.What was a plausible cause in the PSU is not so in the signal path. The hum must already be there, thes caps (if bad) would not put it there. So look elsewhere.Are you sure all the wiring inside the thing is in original state? No broken shield braids, no loosely re-fit connectors?P.S. sorry seem to repeat hints here - didn't read through all the replies.

author: brainwash
author_rank: Frequent Contributor
timestamp: November 06, 2012, 10:43:07 pm
content: Guaranteed way to get rid of the hum:Sorry for somewhat trolling