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AuthorTopic: Amiga 1000 Power Supply repair?  (Read 2001 times)

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Offline yorgle

Amiga 1000 Power Supply repair?
« on: October 25, 2013, 06:18:30 AM »
Hey all.  My Amiga 1000's power supply (dated October 1985) has just failed.  It was working last night, but this evening, I went to turn it on to listen to some mods, and i was left sad instead.

It's a late 85/early 86 NTSC Amiga 1000

Here are the symptoms:
- Power cord is functional
- Dust blown out/off the board (fairly clean for a 28 year old machine)
- switch on the side only turns on the fan (110v fan)
- Continuity test on the fuse shows that the fuse is fine
- Voltage tests on the pinouts of the output cable show 0v
- No obvious burnout marks on the board, resistors, capacitors

I'd like to repair it if I can, rather than just replacing it with a switcher.

are there common fail points in these?
is there anything I should look for?
Is there a repair manual anywhere on line (with test point voltages listed, etc)

Any help at all would be awesome
 

Offline TCMSLP

Re: Amiga 1000 Power Supply repair?
« Reply #1 on: October 25, 2013, 02:17:47 PM »
I was hoping this would include PSU schematics but sadly it just says "if volrages are incorrect, replace PSU" - but worth checking all the same:-

http://www.devili.iki.fi/mirrors/4x4.hopto.org/A1000_ALR.zip

Several sites suggest A1000 PSU schematics have been available but all downloads fail.
A1200 50MHz 68030 16Mb, PCMCIA Ethernet, Indivision AGA MkIIcr
http://www.coherer.net Coherer: Electro!
 

Offline TCMSLP

Re: Amiga 1000 Power Supply repair?
« Reply #2 on: October 25, 2013, 02:27:28 PM »
This site offers a reconditioned A1000 PSU for $29: http://www.oldsoftware.com/Amiga.html

However, if it's an old fashioned linear PSU it should be quite easy to diagnose and fix.   The form factor makes me think it's a SMPS though?

Switched PSUs typically suffer from a few common modes of failure.  Without trying to teach you to suck eggs, I've found this guide to be very useful in the past:-

http://www.repairfaq.org/sam/smpsfaq.htm

I have to admit though, the last SMPS I tried to diagnose I ended up simply replacing.

If you do make any progress I'm sure your experiences (positive or negative) would be useful to others.
A1200 50MHz 68030 16Mb, PCMCIA Ethernet, Indivision AGA MkIIcr
http://www.coherer.net Coherer: Electro!
 

Offline JimS

Re: Amiga 1000 Power Supply repair?
« Reply #3 on: October 25, 2013, 02:39:42 PM »
Make sure you don't have a shorted drive or motherboard loading down the supply and putting it into shutdown.
Obsolescence is futile. You will be emulated. - Amigus of Borg
 

Offline yorgle

Re: Amiga 1000 Power Supply repair?
« Reply #4 on: October 25, 2013, 03:50:52 PM »
JimS: I've tried it in and out of the chassis.  There's nothing coming out of any of the voltage outputs.  I'll be doing more extensive testing and repair work soon.

TCMSLP: Thanks for the suggestions.  The $29 original supply looks like a good solution if I can't repair this one. I feel like it's a linear supply, but yeah, the parts in it look more like a switcher. I can't find any details about it anywhere.

If i completely fail, and decide to go modern with it, I may put into the case, something like the PicoPSU (if it supplies enough power, otherwise, i'll make an A500-like box housing an ATX supply)  along with the "Big Box ATX Adapter to provide the other voltages and tick signal...

http://www.ianstedman.co.uk/Amiga/designs/Amiga_ATX_Adaptor/amiga_atx_adaptor.html
 

Offline Castellen

Re: Amiga 1000 Power Supply repair?
« Reply #5 on: October 25, 2013, 09:17:07 PM »
Quote from: TCMSLP;751093

Several sites suggest A1000 PSU schematics have been available but all downloads fail.



Oops, that would probably be my fault.  Helps when the HTML link matches the file it's linking to.  The A1000 PSU schematic download will work now:
http://amiga.serveftp.net/schematics.html
But that's for the 220V model, the American/110V one might be totally different, not sure.

Yes, of course it's a switch mode power supply, and a fairly old and simple one at that.  I've written a few notes on repairing A3000 power supply common faults here which might help also.

Power transistor or bridge rectifier failures are common in these supplies.  Though when they fail, it's usually in a short-circuit condition which blows the input fuse, but it sounds if this is OK.  Would be worth checking them anyway, easy enough to do with a multimeter on diode test range.  Also check low value series resistors (under about 47 Ohms) for being high resistance or open circuit.  During power on, the high inrush current through these resistors can damage them.  As stupid as it may sound, I've seen this many times in practice.
 

Offline yorgle

Re: Amiga 1000 Power Supply repair?
« Reply #6 on: October 26, 2013, 01:59:17 AM »
Excellent! Thanks for the pointers, Castellen.

So, I just cracked it all back open, powered it on, and probed at it with my multimeter.  The bridge rectifier seemed to behave as I'd expect four diodes to behave.  Everything worked in the right directions.  I powered it on, checked voltages on it, and everything looked reasonable.  I've never worked on such a thing, so the +80 and -80 volts coming out of it (with 110v AC going in to it) seemed somewhat reasonable.  I had expected it to be +-60 volts, but whatever.  

There's a line printed on the board by the smallish transformer there, so I assumed that on one side it's AC, and the other side is DC... I checked voltage from ground to the heat sinks on the two transistors on the AC side, and was getting 80v, which was interesting...

I did notice one of the ICs on the DC side had a bit of dust on it, that I neglected to remove last night, so I did that.  powered it back on, and continued probing.  I had the black lead of my multimeter jammed into the black (ground) pin on the motherboard connector (not connected to the motherboard), and I started probing around the DC side.  I was getting 0v on most of the chips, then i tried one, and got 5v.  then i checked the cable, and I was getting all expected voltages again.  +5, +12, -12, ~3v (the TICK pin).  I plugged it into the motherboard, turned it on, and it powered up.

I reassembled the whole thing, turned on the switch and.... nothing.  No red LED anymore, nothing.  I was getting frustrated but then I just decided (for whatever reason) to leave it on for a few minutes, to see what happened.  After a minute or so, it came to life.  That was about an hour ago, and it's still on without rebooting or powering off.   I recopied the Kickstart floppy that failed a moment earlier (I think I put my magnetic screwdriver on it. oops).  As an A1000 owner, you can never have too many backups of Kickstart floppies!

I'll check those resistors in the future (I meant to do it, but then it came to life and I forgot) for the next time this happens.  I'm getting really quick with opening this thing.

So... do I trust it?  Probably not.  I'm still looking into my options, but being that I didn't really have the cash to do anything about it, it's nice that I got a bit more (admittedly, borrowed) time on it.
 

Offline Castellen

Re: Amiga 1000 Power Supply repair?
« Reply #7 on: October 26, 2013, 02:37:43 AM »
Quote from: yorgle;751137

So... do I trust it?  Probably not.  I'm still looking into my options, but being that I didn't really have the cash to do anything about it, it's nice that I got a bit more (admittedly, borrowed) time on it.



So it's obviously an intermittent problem then.  Agreed, it may stop working at any time.

Check all solder joints, particularly on larger components and components that dissipate significant heat.  Resolder anything that looks cracked or suspicious.

These older SMPS designs usually work around a self-oscillating principle as opposed to newer designs that have a dedicated controller IC.  There are always capacitors involved in the oscillation network and often these are aluminium electrolytic types.  As these components age, the internal liquid chemical electrolyte gradually dries out and the capacitence decreases as a result.  So what you may be seeing is that certain capacitors have degraded to the point where the oscillation is not self-starting reliably.  The solution is of course to replace all of the aging electrolytic capacitors.  Not expensive to do, but somewhat time consuming in ordering and replacing them.  But in doing so, the power supply is likely to be good for another 25+ years.
 

Offline Castellen

Re: Amiga 1000 Power Supply repair?
« Reply #8 on: October 26, 2013, 02:51:02 AM »
Quote from: yorgle;751137
I've never worked on such a thing, so the +80 and -80 volts coming out of it (with 110v AC going in to it) seemed somewhat reasonable.  I had expected it to be +-60 volts, but whatever.



I forgot to mention....
The incoming AC mains is full wave rectified to DC by the bridge rectifier, and on the DC output of the bridge rectifier there will be one large electrolytic capacitor, or two in series.

The voltage across this reservoir capacitor will be the mains AC waveform peak voltage, less the forward voltage drop of two silicon diodes in the bridge rectifier.

i.e.
Vcapacitor = Vpeak - bridge rectifier fwd drop
Vcapacitor = Vrms / SIN(45) - (2 x 0.7V)
      = 110V / SIN(45) - (2 x 0.7V)
      = 155.6V - (2 x 0.7)
      = 154V approx. across the reservior capacitor

If there are two reservior capacitors in series, there will be about 77V across each one.  Hope that makes sense.
 

Offline Plaz

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Re: Amiga 1000 Power Supply repair?
« Reply #9 on: October 26, 2013, 03:45:41 AM »
Look closely for cold cracked solder joints, replace all eletrolytic caps (normally only good 15-20 years). Bad caps will cause other parts like bridge recifiers diodes and power transistors to burn also, so check those for shorts before power up. Some you may have to remove from the  board to check accurately. You might get lucky with just some cracked solder, but when I go in I get all the stuff that might go later. Good hunting.

Plaz
(Reanimator of thousands of power supplies over the years. :) )
 

Offline yorgle

Re: Amiga 1000 Power Supply repair?
« Reply #10 on: October 26, 2013, 04:31:13 AM »
You know.. I should have made the connection that it would be a capacitor thing.  I've been doing cap-kits on monitors for video games, I should have made the connection.  Feh to me.

Okay. I guess it's time to do an inventory of everything in there, and put together a mouser/digikey order!
 

Offline yorgle

Re: Amiga 1000 Power Supply repair?
« Reply #11 on: October 26, 2013, 05:10:59 PM »
as a quick side note; I had to desolder one of the 12(?) electrolytic caps get its value. Out of curiosity, I snipped it open, and sure enough, the paper in it was damp only on about 25% of it. :)
 

Offline Amiga_CDTV

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Re: Amiga 1000 Power Supply repair?
« Reply #12 on: October 26, 2013, 05:23:20 PM »
If you have a list of the cap values in the PSU, could you post it here? I have been thinking of recapping mine some day.
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Offline yorgle

Re: Amiga 1000 Power Supply repair?
« Reply #13 on: October 26, 2013, 09:22:03 PM »
I have to get some time to triple-check this, but I believe this is correct for all of the electrolytic capacitors on a 110v NTSC Amiga 1000 Power Supply:

C11     2200uF  16V    
C12     2200uF  16V    
C13     2200uF  16V    

C15     100uF   25v    
C17     100uF   25v    
C19     100uF   25v    

C50     470uF   220V

C09     4.7uF   50v    
C10     1uF     50v    
C16     470uF   35v    
C18     330uF   35v
C49     10uF    85v    


C50 runs around $6.50, while the others go for $0.50-$1 US.  Priced out at Mouser, it runs about $13.50.  Here's the Mouser project for the above parts: http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=69b972847a

Remember to preserve polarity while installing them!
 

Offline yorgle

NTSC A1000 Capacitor list
« Reply #14 on: October 28, 2013, 03:40:43 AM »
Related:  The list of electrolytic capacitors needed for an NTSC Amiga 1000 (I don't have the board revision)

All are rated at 16v

C75 100uF (hidden near the side D9 ports)
C78 100uF
C79 100uF
C80 100uF

C49 22uF
C50 22uF
C63 22uF
C64 22uF
C65 22uF
C66 22uF

C92 47uF
C93 220uF
C95 470uF

Daughtercard:
100uF (two of them, no noticeable part number indications on the board)