Repairing a dead A310 power supply

[Zarchos]

I wonder if there's such a thing as a list of common, well known, parts in the A310 power supplies to look for in priority when they're dead ...
I've got 3 of these, dead, and I'd like to repair them.
I don't know much about testing capacitors except sometimes you can spot the ones which burst or have a big belly, so any assistance welcome.
Yes, I'm an ignorant

[steve3000]

How do you know they are dead? What symptoms do they have?

[Zarchos]

How do you know they are dead? What symptoms do they have?

Ah yes : no voltage whatsoever when testing the connectors you plug into the motherboard.
True for the 3 PSUs when they are in their repective Archies, normally connected to the motherboard and the diskette drive.

I know it could be a good idea to connect something anyway that draws some current (some psus work this way) so tomorrow I'll connect a hard drive and I'll redo the tests (as I don't know if the Archie motherboards are ok or not).

( edited to stop slaughtering the English https://www.google.fr/search?q=images+a ... d=0CCAQsAQ )

[steve3000]

Yes, that's exactly what I was thinking - these are switched mode psu and so need some load to work. When removed from the computer, my A5000 psu does not start up if the internal fan is disconnected and it will not power a single HDD with no other load - but it works fine with 2 HDDs attached.

You could try them in a different Arc? Or try a working psu in the suspect Arc?

Also, do you hear any quiet buzzing from the psu when you try it?

[Zarchos]

Yes, that's exactly what I was thinking - these are switched mode psu and so need some load to work. When removed from the computer, my A5000 psu does not start up if the internal fan is disconnected and it will not power a single HDD with no other load - but it works fine with 2 HDDs attached.

You could try them in a different Arc? Or try a working psu in the suspect Arc?

Also, do you hear any quiet buzzing from the psu when you try it?

Yes good idea but you know how long it takes to remove everyting properly with Archies crammed with podules are mine are ... but you're right it's the best way to proceed. No cheating allowed

And no, there's no buzzing... what would it imply if there were some, for the record ?

[1024MAK]

Light bulbs

That is, use suitably rated car/truck/motorcycle filament type lamps as test loads for the power supplies.

Just keep in mind that you want to have just over the minimum required load (unless there is any published data, this is a bit of a guess, typically 20%), but not larger than 75% of the full rated load (again, unless there is any published data, maybe a bit of a guess).

Use 6V lamps for the 5V outputs, 12V for the 12V.

Of course, for ease of lamp renewal, you also need matching lamp holders (if you cannot get suitable new, try a scrapyard). Failing that, solder wires directly to the lamp caps.

Mark

[Zarchos]

Good afternoon.

I've checked my PSUs with a hard drive.
They're all ok.
I suspect the Archie motherboards are faulty in some way, and in priority I suspect the yellow 'water drop shape' capacitors are faulty (100µF 16V).
If I want to change them, do I have to use the same type 'water drop shape' or can I use the standard type (but yes with the same values) ?
I refer to this type of capacitors when I improperly write 'standard type':
http://www.google.fr/imgres?imgurl=http ... CDcQrQMwBw
as I've got a lot at home with the right values.

Thanks for your help.

[flaxcottage]

The 'water-drop' capacitors will probably be Tantalum capacitors across the 5v power lines. Conventional types may work here but would be bigger and may not fit properly.

Electrolytics also show a greater leakage current and have a poor high frequency response, which would not be good for decoupling IC power lines.

Personally I should replace all suspected Tantalum capacitors with those of a similar rating. 16v will be OK for the 5v lines but the 12v lines would need 26v rated ones.

Make sure you get the polarity of the replacement Tantalum capacitors correct; they do not like to be revere-biased.

[1024MAK]

The problem with changing the type of capacitor used is the ESR of electrolytic capacitors is not good (more details here). So I recommend replacing like with like. But if you have a choice, use higher voltage rated components (this increases the life span of the components).

If you are still going to use electrolytic capacitors, use low ESR types rated at 105 degrees C. And fit a 100nF to 220nF ceramic type in parallel (across the terminals).

There are pictures of tantalum capacitors here

Mark

[Zarchos]

Yes, 'tantalum', that's much better
Found this :
http://www.conrad.fr/ce/fr/product/4470 ... archDetail
sounds incredibly expensive to me, is it a normal price for such a component ?

[danielj]

Nope...

http://cpc.farnell.com/_/cb1c107m2mcb/c ... dp/CA05688

d.

[cmjones01]

Tantalum capacitors are, in general, trouble. Though they have very high performance - low ESR and small size for their capacity - they've always been prone to turning into little fireworks at the slightest provocation, especially when connected across power lines at anything close to their rated voltage. It's been a problem for at least 40 years and still is, if my experience of modern surface-mount tantalum capacitors is anything to go by.

Fortunately modern electrolytic capacitors are miles better than they used to be, in both ESR and physical size, so unless the application was really demanding (an Archimedes motherboard definitely isn't) I'd have no hesitation in replacing tantalums with good quality electrolytics. I'm in the habit of using the Panasonic FC and FM series in new designs and they seem to work well. If you really want to push the boat out, check out organic electrolytic capacitors like the Sanyo OS-CONs, but they're generally only relevant for squeezing the last ounce of performance out of tiny switch-mode power supplies. If you really, really need low ESR, modern ceramic capacitors are ideal, and definitely more reliable than tantatums. Putting a 10uF ceramic capacitor (they're only available surface-mount) on the underside of the PCB across anything you replace with an electrolytic should do the trick, although for something like an Arc motherboard I'd be surprised if it was necessary.

I hope that's helpful
Chris

[Zarchos]

I'm ignorant but these capacitors are here for the 5V to 12 V output(ted ?) by the Archie power supply ... so if it's 16V I don't think it's really near the max supplied (although I know there's a difference between theory and what you actually get when switching on the PSU, but well here I doubt the Archie PSU isn't stabilised).

Oh dear, talking to electricians in electronics is like talking to economists
Ask 2 of them and you'll get at least 3 answers

[steve3000]

Search for my thread from about 2 years ago on this issue. There is one specific tantalum capacitor across the 12v rail which tends to fail on the A310 and short out, leading to an apparently 'dead' computer.

I suspect you have this problem. As the A310 doesn't really need a clean 12v rail to function (12v is only for the fan and HDDs, unless you have lots of podules...), you can remove the capacitor simply by snapping it off and the computer will work

For the long term, you should find a replacement, I found a good price for 5 x identical replacements from here and I may still have one spare, will have a look tonight.

[steve3000]

In case you can't find via the search button, here's the thread: viewtopic.php?f=16&t=5933

It's C37 which is the problem tantalum cap. Try unplugging the 12v lead as I suggested back then, to see if this fixes the computer, before removing any capacitors...

See the pictures in the thread.

[Zarchos]

In case you can't find via the search button, here's the thread: viewtopic.php?f=16&t=5933

It's C37 which is the problem tantalum cap. Try unplugging the 12v lead as I suggested back then, to see if this fixes the computer, before removing any capacitors...

See the pictures in the thread.

Thanks Steve.
Capacitors ordered, cheap enough to store them even if not guilty in the Archie.
I had forgotten this thread about C37.
Btw, if it's always this one (more or less) that gets faulty, isn't it because Acorn should have used another one, like for example 20 V 100µF ?

[cmjones01]

Oh dear, talking to electricians in electronics is like talking to economists
Ask 2 of them and you'll get at least 3 answers

It's an analogue world out there, even inside an Archimedes

[flaxcottage]

Btw, if it's always this one (more or less) that gets faulty, isn't it because Acorn should have used another one, like for example 20 V 100µF ?

From experience the Tantalum rated voltage should be at least twice the rail voltage. So for a 12v rail you would need 25v Tantalum capacitors.

[steve3000]

Yes, they probably should have put a higher rated one there as this failure is well known. But these computers are 25+ years old now, which also might have something to do with the failure...

[1024MAK]

I think tantalums have got themselves a bit of a bad name. Party due to the specified (as printed on each one) voltage rating not being as helpful as it at first looks.

The voltage that a tantalum capacitor can withstand varies and a designer of a circuit must take many factors into account. So often the component has to be derated to take these factors, like temperature and the actual peek voltage spikes (for example) into account. Because otherwise, the lifespan of the tantalum capacitor will be reduced as it is being operated outside of the manufacturers specification.

As not all designers were aware of this, it is hardly surprising then, that these tantalum capacitors die.

It is not helped by the normal failure mode (or at least the failure mode that gets noticed) is the capacitor going short circuit. And if the PSU does not have suitable overload/short circuit protection, yes, tantalum capacitors can go BANG!

Mark

PS the three answers are normally "the right way", "the wrong way" and "my way"

[Zarchos]

And one A310 back from the dead !
It was 'simply' the C37 capacitor to replace.

The A310 has only got 1 Mbyte but hey, I'm happy with that.
RO 2 but well from the size of the ROMs not a surprise.

Not very nice, but I was lazy and didn't want to dismantle the Archie, so here is an elitist capacitor, a young one despising the old ones surrounding it :

The disk drive works and a small test with an Arc Angels demo from 1992 (although it seems I've got the sound on half the channels, weird) :

I think I'll solve the sound issue and the filter issue at the same time with the filter by-pass solution.

When installing back in place the backplane, I'm surprised to read this :

Is it a normal backplane or could I be the happy owner of a protype backplane ?

[steve3000]

Great result. That looks like a nice RISC OS 2 computer!

[Zarchos]

This is very ugly, and I wonder if it could be the culprit for the missing channels :

Has sbdy ever noticed this kind of repairs on an Archie mainboard ?

OK : I see.
Looking more closely I thought there were 2 resistors x2 in parallel, but in fact one of them is alone and has a leg in the air.
Oh dear, poor little Archie.
Let's fix that now.

Not really better even with the fix.
Definitively the 'no-filter' mod is the next step for this week end.
This way I'll see if it's the resistors that are faulty ; or the OA and / or arrrgghhh the VIDC.

[flaxcottage]

Good to see you have got it working again. Well done.

[sirbod]

This is very ugly, and I wonder if it could be the culprit for the missing channels :


Has sbdy ever noticed this kind of repairs on an Archie mainboard ?

OK : I see.
Looking more closely I thought there were 2 resistors x2 in parallel, but in fact one of them is alone and has a leg in the air.

Looks like a diode in parallel. I've just checked my A305 and A310, neither have anything attached to R67.

Your "leg in the air" is also still attached on both of mine.

[steve3000]

Be careful, the were at least two different versions of the A310 PCB, and the early one in particular had lots of engineering/service 'fixes' applied.

My A310 has a diode across R67, but not the 'leg in air' resistor.

Before removing components, check they're not in the full list of fixes in the technical/service manual.

[Zarchos]

Thanks guys.

Yes it's a diode.
I measured the 'resoldered' resistor and it's ok (not cut).
I'll do the filter mod today, I'll get rid of the muffled sound at the same time.

Any idea for the 'PROJECT A' on the backplane ?

[sirbod]

Any idea for the 'PROJECT A' on the backplane ?

Both of mine have the same silkscreen, so it's probably normal.

[paulv]

The piggy backed diode is part of a FCO E011 to reduce noise on the video output. It looks half done as one of the dipped resin caps should have been replaced with an axial electrolytic cap too...

If you look closely at the VIDC Enhancer photo, on the link below, you'll see the diode and replaced cap in my A310.

http://www.retro-kit.co.uk/page.cfm/con ... edes-A310/

Paul

[Zarchos]

The piggy backed diode is part of a FCO E011 to reduce noise on the video output. It looks half done as one of the dipped resin caps should have been replaced with an axial electrolytic cap too...

If you look closely at the VIDC Enhancer photo, on the link below, you'll see the diode and replaced cap in my A310.

http://www.retro-kit.co.uk/page.cfm/con ... edes-A310/

Paul

Spotted !
Thanks.
I'll read the Archie docs to know which parts I need to buy to fix what I can.
I think at the same time I'll replace the 9 pin rgb connector by a 15 pin VGA, as detailed on the Qube web site. (properly ie replacing, not piggy backing it as proposed by Charlie).