Repairing a dead A310 power supply

[Zarchos]

In this A310
http://www.ebay.fr/itm/Acorn-Archimedes ... SS:FR:3160
there's even a missing resistor.

[steve3000]

In this A310
http://www.ebay.fr/itm/Acorn-Archimedes ... SS:FR:3160
there's even a missing resistor.

Check the circuit diagram in that case. It could be another 'fix'...?

[paulv]

It's quite possible. A lot of FCO's were meant to be applied to early Arc's. It's highly likely that the cut resistor which has just been repaired was one such FCO but I don't have the time to go through them all to check ATM.

Paul

[sirbod]

A lot of FCO's were meant to be applied to early Arc's

Are the FCO's available online?

[paulv]

They're in a few places.

Here's some...

http://archive.retro-kit.co.uk/acorn.riscos.com/fco/

http://www.4corn.co.uk/aview.php?sPath= ... //docs/fco

http://www.riscos.com/the_archive/acorn/fco/index.htm

Paul

[Zarchos]

Thanks for this gold mine of infos !

[paulv]

If the resistor you fixed was R39, then it should ideally be removed as part of FCO E009.

Paul

[Zarchos]

If the resistor you fixed was R39, then it should ideally be removed as part of FCO E009.

Paul

Thanks.
I understand why it had a leg in the air : it was on purpose.

[simonm]

Resurrecting this thread in the hope some kind folks could help out a non-hardware-techie type like me with fixing an old A310 I bought on gumtree. The seller wanted to show me it working, but when he did, a bunch of smoke came out of the machine so he offered it for just a tenner - bargain!

So, I'm ok with a soldering iron and a multi-meter, and keen to try my hand at getting it safely back up and running, but I've followed this thread and frankly I'm feeling a bit out of my depth as I dont have the electronics know-how really.

What kind of fault finding repair steps would you guys recommend I follow? I'm guessing the smoke is probably burnt out caps in the PSU similar to beebs? But what kind of caps? Where would I get them from? Are the caps mentioned here also a potential cause of the smoke?

I've hunted around for example process to follow, but this thread is the closest and most relevant one I found. However, some kind of checklist would be amazingly helpful if that's even possible?

Thanks so much!

[daveejhitchins]

I would think the first job would be to find out where the 'smoke' came from and work from there. Switch-Mode PSUs are no more difficult than any other type of circuit to repair - However, like all circuits, you need to understand how they work - with the added PSU caveat: Once the fault has been located - replace ANY component around it that may have been 'stressed' by the failure.

I'm sure there'll be lots of specific help on it's way - Main thing is not to be afraid of PSUs - But be carefull!

Dave H

[1024MAK]

Any electronic or electrical component that is overheating, being overloaded or self destructing can be the cause of smoke.

I recommend that you open it up and carry out a visual inspection by eye (under a good light) looking for any physical trace of the cause of the smoke, such as soot marks, blackened, damaged or distorted component(s).

Post a photo up of anything that looks suspicious.

If you don't know about being safe while working on power supply systems where DC voltages of up to 340V DC may be present (large electrolytic capacitors can hold a charge for many hours after the system is powered down) please ask for further advice.

Mark

[simonm]

Thanks, I think I'm going to journal what I do, in the event its helpful for future hardware beginners like me.

I recommend that you open it up and carry out a visual inspection by eye (under a good light) looking for any physical trace of the cause of the smoke, such as soot marks, blackened, damaged or distorted component(s).

I shall do this first then.

If you don't know about being safe while working on power supply systems where DC voltages of up to 340V DC may be present (large electrolytic capacitors can hold a charge for many hours after the system is powered down) please ask for further advice.

It's a new experience to me, but thanks for the tip - I'm aware of the safety in mind. Many years ago I worked in a TV repair shop where I acquired a healthy respect for electrical equipment! I spotted this link for safe cap discharge procedure.

Will post up as I progress - likely over a few weeks. Thanks for the guidance.

[RobC]

I recently picked up some vintage graphics kit but the PSU in one of them was faulty and was causing the entire circuit to trip. I thought about repairing it but found a brand new Meanwell unit with better spec for less than £15. It was probably cheaper than replacing all the caps and certainly quicker than fault finding.

It might be worth looking into whether there's a suitable replacement for the Arc PSU. (It's a while since I've looked at an Arc PSU but when my R260's PSU died, I ended up using the innards from a spare RiscPC PSU.)

[simonm]

Ok, so I had some time to disassemble a bit more of the beast.
The PSU in situ

The ganky motherboard

This was rattling around inside the PSU - looks like what's left of a capacitor

The PSU - so my question is, how is this PCB disassembled/removed from the outer metal case? I've removed screws on top of case, and it does slide a bit down, but doesn't lift out easily and I didn't want to force it? I can't see any other way to get it out though...

[danielj]

More screws on top? Check under the label? I've not done one of these myself so I'm guessing. Also the plastic grommets in the side will i need to come out if there are any.

[zeem]

It's probably recommended to replace them, but those particular capacitors are not required for the power supply to function. I've often removed them or left the blown remains in place and continued to use the machine in question.

[1024MAK]

Oh, what a surprise, it says RIFA on it

These being an interference suppression / filter paper insulation capacitor that is connected between line and neutral. The old ones do have a nasty habit of either going bang, or going up in smelly smoke (or both).

Mark

[simonm]

Thanks for help so far.
Does anyone know how I get the black grommet loosened? I can't get the PCB out unless I can remove it.

[simonm]

Ok. I decided to get gnarly with it. It pops out if you encourage it upwards with a screwdriver.
Right so now I can see the PCB. There are loads of caps! But which ones will be borked or nearly borked?

Here's the silver popped one

Yep deffo explodinated

[simonm]

Seems the caps on this board are very similar but not quite to the ones in Retroclinic's BBC cap repair kits.

C1 = X2 0.01uF (RC's are 10nF X2 RIFA)
C9 = 25V 470uF (RC's are 35V 220uF)
C2 = ??? (the popped one has burned so no idea what it is apart from some type of RIFA) (RC's are 100nF X2 RIFA)

[1024MAK]

Thanks for help so far.
Does anyone know how I get the black grommet loosened? I can't get the PCB out unless I can remove it.
IMG_20181127_200049.jpg

Yeah, there is a special tool for it. I actually bought one. But can never find it when I want it!

Mark

[1024MAK]

Seems the caps on this board are very similar but not quite to the ones in Retroclinic's BBC cap repair kits.

C1 = X2 0.01uF (RC's are 10nF X2 RIFA)
C9 = 25V 470uF (RC's are 35V 220uF)
C2 = ??? (the popped one has burned so no idea what it is apart from some type of RIFA) (RC's are 100nF X2 RIFA)

For C1, 0.01uF is a 10nF. Both the same value, just using a different SI multiplier.
For C2, if it physically fits, a 100nF (or 0.1uF) type will be fine. They are interference suppression capacitors, so the value is not critical.
Both C1 and C2 must be X2 rated (for safety) as they are connected across a 230V AC mains supply.

On a BBC B PSU C9 is the “start-up” and “running” reservoir capacitor for the switching transistor. It is recommend to replace this for reliability rather than safety.

I don’t have a A310, so am not sure which one it will be on the A310 PSU. But it will be not too far from the metal cased transistor (mounted on the heatsink).

Mark

[IanS]

Seems the caps on this board are very similar but not quite to the ones in Retroclinic's BBC cap repair kits.

C1 = X2 0.01uF (RC's are 10nF X2 RIFA)
C9 = 25V 470uF (RC's are 35V 220uF)
C2 = ??? (the popped one has burned so no idea what it is apart from some type of RIFA) (RC's are 100nF X2 RIFA)

I'd offer to look in my (unexploded) A310 PSU for the C2 value, but it looks suffiently different to yours, that it may not be helpful.

[simonm]

For C1, 0.01uF is a 10nF. Both the same value, just using a different SI multiplier.
For C2, if it physically fits, a 100nF (or 0.1uF) type will be fine. They are interference suppression capacitors, so the value is not critical.
Both C1 and C2 must be X2 rated (for safety) as they are connected across a 230V AC mains supply.

On a BBC B PSU C9 is the “start-up” and “running” reservoir capacitor for the switching transistor. It is recommend to replace this for reliability rather than safety.

I don’t have a A310, so am not sure which one it will be on the A310 PSU. But it will be not too far from the metal cased transistor (mounted on the heatsink).

Mark

Awesome thanks Mark. Of course - uF / nF ! Doh!

Pics of C9:

C9 from side

C9 from top

[simonm]

I'd offer to look in my (unexploded) A310 PSU for the C2 value, but it looks suffiently different to yours, that it may not be helpful.

Thanks Ian. Crumbs, very different looking psu's! I wonder if my other A310 will be the same psu now!

[1024MAK]

That 470uF 25V capacitor does look like the “start-up” capacitor. Even if it is not, there is no harm in renewing it (it being close to a heatsink and therefore likely aging faster).

Replace it with a high quality 105oC 470uF 25V (or higher voltage) type.

Mark

[daveejhitchins]

Back to the original question of Black plastic grommet - a pair of pliers will do the job. Not as good a the correct tool, but hey! How many times will you want to use it?

Dave H

[Mattmos]

I found this thread really useful when repairing my R140 (~A440) PSU, thanks everyone!

The PSU appears to be a really early model (which screams 'errata' to me ):

The previous owner reported the "typical explosion", and sure enough C2 had experienced a wee catastrophe. He said the machine was then dead (surely just X2/filter caps wouldn't cause the PSU to die?), so I also replaced the suspected-startup-cap-C9 as per this thread.

In case others are searching, I used the following parts:

• C1: X2, 10nF, Mouser p/n 80-PME271E510MR30
  C2: X2, 100nF, Mouser p/n 80-PME271M610MR033G (both of these are made by RIFA, so maybe the PSU'll explode again in 2030...)
  C9: Electrolytic, 25V, 470uF, Mouser p/n 667-EEU-FR1E471

Bizarrely, I also noticed R4 looked explodey:

It looks like the heatshrink's just split, but there's a grainy layer of resistor that's peeled away too. It still measured at 120 ohms, same as in another PSU, so resistance isn't degraded but its power capacity surely was. I replaced that with, guessing, a 2W 120 ohm metal oxide resistor -- on fitting, I realised this was slightly smaller. Maybe the original was 3W? I've run for a few days without issue there, albeit without a load of discs etc.

Finally a tip on the grommet saga: use a pair of flat-nosed pliers. There should be enough slack inside the case to rotate the wires a little so the push-in part lines up with the split in the metal case, which I found made it slightly easier to take out. When re-fitting, make sure you've got the original kink in each wire lined up -- I accidentally shortened the floppy power cable a few mm, and it now doesn't fit without forcing it.

[nicf82]

I am busy trying to repair one of these PSUs and my R4 has also disintegrated, does anyone know the correct part to use here?

I do have one of these ceramic resistors, I am not sure, but think it should be up to the job, ie will be more reliable than the one that i have removed if it will fit?

Cheers!
Nic