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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: patrik on December 16, 2003, 03:34:07 PM
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Stumbled upon a page which describes why the sound of the A4000 eventually will die and what to do about it:
Amiga Audio Circuit Capacitor Repair (http://joj.home.texas.net/amigacap.html)
On the same site I also found a page describing a construction miss of the A3640 CPU-card which result in serious damage to the A3640 in time:
Amiga A3640 CPU Board Repair (http://joj.home.texas.net/amiga/amiga00.html)
Happy soldering!
/Patrik
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:nervous:
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odin wrote:
:nervous:
I'd rather get Someone like Dave Hayney to check this out before I start poking my hot iron in my Amiga.
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I'm sure Dave Haynie would tell you a reverse polarised capacitor is a bad thing ;-)
If it's obvious that said capacitors tend to leak after a while, and those same capacitors were reverse mounted, its a no-brainer, really ;-)
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@bloodline:
Measured the voltages supplied these to capacitors of my rev3.1 A3640 and they were indeed reversed as in the article.
/Patrik
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patrik wrote:
@bloodline:
Measured the voltages supplied these to capacitors of my rev3.1 A3640 and they were indeed reversed as in the article.
/Patrik
I have no doubt of that, the A3640 is no masterpeice of engineering... but I'm concerned about the Audio caps... that's got me worried.
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@bloodline:
Inspect the area around them and see if you see any oily substance on the motherboard itself. If there is a substance like that present it should be removed and the capacitors replaced as soon as possible to prevent damage to the motherboard and nearby circuits. The capacitors are the two inside the red circle of the picture below:
(http://www.megaburken.net/~patrik/A4000audio.jpg)
(edit): Propanol (I am not sure if that is also the english word for it) is good to use as pcb cleaner as it will absolutely not damage the protective coating of the pcb.
/Patrik
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Hmmm, where are these audio caps on an A3000? I have a service manual and could look for them. It would help if they got pointed out to me though. I would hate to have to poke in my A3000 again with a soldering iron!. I've done it before though.
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Damn, could that be why my A3640 has died... ?
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I never knew this sure is nice to know. :-) :-o
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so many fantastic benefits of using an "old " computer.
there has been years for people to find ways to fix even the silliest and most crucial of things for it:)
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@Acill:
They should be located near the audio-connectors and they probably have a capacitance of 22uF as that was their rating also in the A500 when I checked now.
Though I think that you are quite safe with the A3000. Those capacitors are in all amigas subjected to these reverse voltages under certain conditions. This is not healthy to any polarised capacitor, but the capacitors in the A4000 seems to be of especially low quality and very prone to leakage.
(edit): It is never a bad thing to have those capacitors replaced with non-polarised capacitors though.
(more edit):
Checked a bit in the A3000 service manual and it should be the capacitors named C433 and C443 (named exactly the same as those in A4000). These two are the ones inside the red circle in the image below:
(http://www.megaburken.net/~patrik/A3000audio.jpg)
@MrZammler:
Not impossible, if you examine the areas of the A3640 near the capacitors for leakage you can see if that was the reason. Not impossible to fix it either. The guide in the link I posted is quite detailed on how to try to fix it.
/Patrik
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Very good stuff!
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I've known all the time about the capacitor issue in A4000 (the #1 cause for bad sound or no sound at all in A4000), however I've been told by a friend who was a huge Amiga geek/repair guy back in the day that they simply dry out. Never heard of any leakage.
edit:
So what I am saying is that as long as you've replaced or removed the Ni-Cd battery from your A4000 motherboard you are probably safe from any further damage. The dry capacitors will not cause any harm, but only make your sound dead, and even this won't happen right away but you will notice it slowly degrading through time, until you replace the capacitors with more advanced quality ones.
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Thanks for the info. I found them on my A3000. They look like they are in good shape still. I have perfect sound, and its mixed through my sound card too. If I ever loos the Paula sound I know what to do and look for.
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Hmm.... I have a dead 3640 card. (From my backup A4000) I never bothered to try to track down the cause of death. Maybe it's an easy one. I'll check it out. Thanks for the info!
It's also good to know what causes A4000s to lose sound. It's not a problem mine has, but I know they are prone to it. (I always just assumed it was caused by people mixing sound output by splicing wires together. ;-) )
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x56h34:
I guess it turns out different in different A4000's. In some cases there evidently have been leakage from those two capacitors as there are goo-gaa around them on the motherboard, but them drying out sounds very sensible in many cases, they are after all electrolyte capacitors.
/Patrik
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@patrik:
I guess the right time to replace them would be as soon as you notice something funny going on with your Paula sound, just to make sure.
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@x56h34:
Absolutely! :)
/Patrik
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All electronic components have a limited life. An aluminium electrolytic cap that lasts for ten years is doing very well.
The claim that the capacitor should be non-polarised is nonsense. In that circuit, a polarised capacitor (which can go in either way if there's no voltage across it) or non-polarised will function equally well. If some have failed, it's just old age, not because they should have been non-polarised types.
In all other respects, they are good articles. In particular, the reverse-biased electros on the 3640 are a real worry. Although they are unlikely to burst after all this time, if they dry out through overheating and go open circuit, they aren't going to do their job.
Probably the excessive heat inside the case of an A4000D is the worst cause of component failures.
tony
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Creepy. As I was reading this topic my A4000 died. That is REALLY creepy. I looked at my CPU board and it looks to have this problem. I've tried to replace the caps and the problem is still there and that may owe to my high skillset for surface mount repairs. I think it's time to just get an accelerator card for the machine. Any recommendations on a decent one that wont break the bank?
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I had to replace the caps in my A4000. The sound, however, wasn't slowly degrading . I just turned it on one day and nothing out of the right channel.
I went with what I could find though, which were polarised caps (non-smt.) Haven't had a problem since.
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Hmm...
The thought of opening my A4k up and removing everything isn't that swell.. but I guess it might be a good idea.
See if I find time to do that someday.
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Had a look at that article, and it is incorrect about using a non-polarised capacitor.
It's perfectly normal to use the polarised capacitor Commodore fitted. In fact you could use either, but since the non polarised ones are usually bigger, more expensive and there were exsisting polarised capacitors in the design, it made sense to do what C= did.
The negative side of the capacitor is biased to ground, so providing the bias on the op-amp's input doesn't get messed up, the circuit is fine.
The facts are that it would appear to be a very large bad batch of electrolytics, or more likely just a bad quality capacitor. Apparently various manufacturers have had electrolytic chemical problems over the years.
I've repaired a lot of A4000T, A4000D, CD32 and A1200 with this problem. In almost all cases it's only the 22µF capacitors, and not just the ones in the audio output section.
From memory there are around 5 of these 22µF capacitors in the A4000D, which I have all seen with leaking problems.
Some of the other SMD electrolytics can have the fault too, but from experience, the 22µF ones are definately the worst.
The problem should not occur on the A3000 as these are all through hole components. Only the surface mount capacitors in the A4000D/T, A1200, A600 and CD32 would appear to suffer from this leaking problem. Leaded capacitors usually just tend to "dry out" rather than leak.
The biggest cause for concern in almost all Amiga models is the RTC battery failing.
These batteries are generally rated for a life of around 6 years, but some of the ones out there now will be double that age.
There's a very high chance of the battery leaking corrosive material onto the motherboard which can cause a number of faults.
I'm actually in the process of writing a guide on this, my work so far is online at:
http://amiga.serveftp.net
under the battery replacement section if anyone wants a look.
Lots more technical info to be added as I get time to write it :-P
Anthony.
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OK, just read the other section of that website regarding the A3640 repair.
Most of the information is pretty good, other than the removal procedure of the capacitors.
I would definately NOT suggest "twisting" them off the PCB!! That's a very good way to rip off solder pads.
The correct way is to use a hot air SMD toldering tool, but at home, two small soldering irons can be used, one in each hand.
Another way is to heat one side, lift it slightly, heat the other, lift that slightly, continuing until it's removed. Not really ideal either as it's just as eay to lift pads doing it that way.
As for the advice of never "poking around in the computer" while powered up...
Geez, I'd never get anything repaired!
There's possibly a slim chance of killing yourself with 5V if you had an unusually low body resistance and you layed in a bathtub full of water, then tightly grabbed a large contact area with the 5V supply on it. As far as I know, the lowest voltage causing death was about 32V or something.
Ohm's law = current = V/resistance. Takes at least 20mA though your chest in the right place to cause problems, meaning you need a body resistance as low as 250 ohms!!
Normally it's 100k ohms or so.
Of course power supplies are a different story. The voltages inside them have a very high chance of doing you damage!
That's today's lesson in electrical safety :-)
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Hi Castellen,
you wrote:
"As for the advice of never "poking around in the computer" while powered up...
Geez, I'd never get anything repaired!"
i 100% agree :-) but remember to remove your metallic bracelet before . . . .erm!!!! beleive me :-( (a VCR is gone here, some years ago)
Remeber also, that the 90% of the premature dead of many "consumer level" boards, is due the bad quality of the elctrolitic capacitors.
Ciao
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Good point, and especially around car batteries.
Only 12V, but several hundred amps through your metal watch strap and your wrist is going to get extremely hot extremely quickly!
The biggest failure I usually see in day to day repairs is the failure of high power devices (transistors, etc) and sometimes dry joints on the legs of through hole components subject to a lot of thermal stress.
Leaking/dry electrolytics is still a big problem, mainly with older equipment. Also it's common to get "resistive" tantalum and ceramic capacitors from time to time. These typically cause bias upsets in audio circuits.
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Hi
I have a 4000T that is starting to produce noise from the audio output. Is this the problem described in the article? Does information for 4000D also apply to 4000T?
@framiga: do you think that Casonato would be able to repair this problem? How can I contact him?
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The A4000T is electrically similar, but the output amplifier and those 22µF coupling capacitors in question are fitted to the audio/video module which unplugs from the motherboard.
I have repaired these before due to leaking capacitors causing audio problems, so you may well have the same problem. There are some 22µF capacitors on the motherboard as well, so would be worth replacing those at the same time.
A4000T schematics (and of the module) are here:
http://amiga.serveftp.net
Look in the schematics section.
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Hi darkcoder
yes sure! he is the only one, in Italy that can do all that you want (if the spare part is available).
Contact him by phone before and despach him the mobo only.
He can do a full check of the Amiga mobo, with its own hardw/softw diagnostic gear.
Ciao
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@Castellan
If I got your argument, I could simply replace the video/audio moduule, right?
I think Vesalia sells some spare modules
@framiga
ok, when I'll be ready I'll ask you the adress / phone number of Cassonato
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I have the same problem and posted a similar thread to the forums. My A4k is going for repair right after the Christmas holidays. :-)
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@Castellan
The audio of my 4000T is making noise. Do you think that in order to fix, it would be enough to replace the Video/audio module?
Merry Xmas to everyone!
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Well my problem was the CPU board. It is this issue as well that was preventing it from booting. The caps were in backwards and had indeed failed. I have verified this by testing voltages under power. I actually replaced all 5 capacitors and the board works beautifully.
My board revision is 3.1. In addition I did verify that the silkscreen on the board was also backwards for the three caps in question.
Many thanks for posting this article, and many thanks to Anthony at http://amiga.serveftp.net (http://amiga.serveftp.net) for his assistance in providing the info on the site and in the many emails we exchanged.
:-D <-- My A4000D now.
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Thanks Dalamar :-)
@darkcoder:
Yes, you could replace the audio/video module. But a new replacement module would NOT be cheap!
I saw one for sale on either A.org classified or Amibench a few months ago for about £5.
Or you could chuck up a wanted add.
Though the module itself is not very complicated, so repair is quite simple if you can SMD solder.
Or send it to me in NZ for repair if you're desperate, would still be way cheaper than a new one though.
Anthony.
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Electrolytic capacitors carry the major advantage that you can obtain a high capacitance in a relatively small package, and this is why they are used extensively by manufacturers. It is a property of the Electrolytic capacitor itself that makes it polorised.
Failure of Electrolytic Capacitors due to drying out or leaking is common, and is the main disadvantage using them. Look at any product and you'll usually find that at least a couple of electrolytics are favourites for leaking or drying out. These devices normally fall down in high stress areas such as power supplies or in our case Audio Amp stages! It's not poor design on behalf of Commodore, it's just a property of the electrolytic capacitors themselves that they have a limited life expectancy in high stress areas.
Castellen was right to offer the advice of cleaning the surrounding PCB if an electrolytic capacitor is ever found to be leaking - the substance is corrosive and over time can cause problems with the PCB surface, although normally it is easily cleaned off as the leak occurs topside not copper side.
For removal of surface mount devices like the type you describe, I find it is actually easier to leave the solder on them because it aids in the removal. Get yourself a precision pair of tweezers, pull gently on the component and then heat both ends alternately. You'll find the component will lift off fairly easily that way because the solder stays molten at both ends giving you enough time to free the component :) This tactic changes completley when manufacturers are kind enough to physically GLUE components down though, although this method is only normally practised with surface mount ICs - which require a different methof of removal anyway.
Brian
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Castellen,
This link you posted:
"A4000T schematics (and of the module) are here:
http://amiga.serveftp.net"
Is VERY useful! I'm downloading the A1200 manual now, more for pure interest than anything else. I tell you what, I've ALWAYS been after a service manual or schematic for the Blizzard PPC Accelerator range, although I believe Phase5 kept these under wraps?
Brian