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JamesF

Anyone repaired a Maplins XM21X PSU?

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2 hours ago, Oddsocks said:

It's perplexing how often I see this recommended across the web and even written in published manuals.

On the REME sponsored courses I did in Germany back in the late 70's early 80's we were told never to use any type of contact cleaner or solvent on skeleton carbon presets as it softens the carbon bonding agent on the track and within a very short while the preset would fail completely. The general recommendation was to replace a defective preset if possible but if desperate we were instructed to gently lift the wiper from the track and slide a piece of typewriter paper under the wiper, release the wiper back on to the paper and wiggle the preset back and forth a few times to clean the wiper contact, then slide the paper away.

We were told that contact cleaner is only for metals, never for carbon or carbon composites, at least, that was the instruction then and I've not seen any changes to the construction of skeleton presets that would suggest it is now acceptable to squirt solvents on to them.

I'm sure you're right Oddsocks. :wink2:

I worked in the BBC (TV Centre Studios) from 1973 and the most widely used cleaner was Colclene TF (in the orange spray can). This wasn't specifically a contact cleaner but was used in that role up until the early 90s or so when the newly applied COSHH regulations effectively banned its use. We kept a few cans hidden in our lockers to use 'privately' as it was better than the substitutes. I used it on skeleton presents a lot, in preference to 'real' contact cleaner which could leave an 'oily' residue which attracted dust.

I used the term 'contact cleaner' in a more general sense as a cleaner and today would try IPA first before 'actual' contact cleaner though I think with the state of James' board anything would be an improvement.

I only changed skeleton presets if colclene didn't solve the problem. Whether the colclene itself caused a failure in the future I couldn't say, but there was no epidemic of failed presets and it was rare than one had to be changed. :smile:

I was quite diplomatic in describing the construction of the power supply, but as others had no such qualms, I have to agree. I couldn't have given something like that to someone for free, let alone charge them to buy it.

Alan

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5 hours ago, symmetal said:

Colclene TF (in the orange spray can)

I remember that Alan.

I think part of the composition was CC14, Carbon Tetrachloride, and the propellent was Freon, both banned substances now. If you used the spray for too long the can froze to your fingers.

I don't think I would try to repair James's PSU myself, too many bad joints, bad practice and failure points. Not worth the risk, and expense of a mains leakage to the output side destroying everything downstream. At the very least there should be big varistors and/or crowbar diodes on the output but I see none in the pictures.

William.

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Having seen the general construction standards vand condition, I recommend earthing.

That is dig a hole in the garden and bury it.

New power supplies aren't that expensive.

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50 minutes ago, Oddsocks said:

I don't think I would try to repair James's PSU myself, too many bad joints, bad practice and failure points. Not worth the risk, and expense of a mains leakage to the output side destroying everything downstream. At the very least there should be big varistors and/or crowbar diodes on the output but I see none in the pictures.

The thing is, unless you open up the box yourself, how do you know that anything you replace it with (the Nevada unit that FLO sell, for instance) is any better?

James

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I think people are overreacting in these days of neat reflow solder surface mount.

That's not the neatest I've seen but not the worst either.

The mains is kept well separated from the low voltage side. The main connections are all insulated with heatshrink, except I can't see where they attached to the transformer but I would hope they are insulated too? It is fused. I am not sure if it is earthed - it

My only serious criticism would be that the fly leads from the transformer should be supported and insulated, ideally with heatshrink because they could short out.  The soldering to the pins on the board could be neater, but it looks secure enough. The wires could be laid out a bit more prettily, but none of them look to be under tension or at risk of coming loose.

If really paranoid, get it PAT tested, but as long as all the mains connections are well insulated and away from the bare chassis that split bobbin construction will be safe as long as the thing isn't actually wet.

 

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3 hours ago, JamesF said:

The thing is, unless you open up the box yourself, how do you know that anything you replace it with (the Nevada unit that FLO sell, for instance) is any better?

Unfortunately you can’t tell when buying power supplies from non-electrical specialists such as astronomy retailers etc.

The Nevada unit is a mass market general purpose power supply used primarily to power devices in the home or workshop that would otherwise be used in a car or truck, such as a CB radio or portable short wave receiver, coolbox, air jack or tyre inflator etc.

I’m always going to be biased after a working life in medical engineering where we have some of the toughest regulations regarding power supplies, particularly in operating theatre environments where as little as 6 microamps of leakage via an intravenous catheter will stop a beating heart.

Of course, the power supplies we used in that environment, roughly equivalent in output to the Nevada, cost in the region of €2,000 each and though they still fail occasionally they never fail in a way that is dangerous to the equipment they are powering or the patient.

It’s really a question of risk and how much you may be prepared to lose in the event of a failure.

I’m currently running three QSI cameras and at nearly £5,000 each plus nearly as much again for the Atlas digital focusers, I would not risk powering them with the Nevada, or an equivalent. I don’t even use the switch mode power supplies that were provided by the manufacturer but I bought ITE & Medical grade rated, 12V 12.5A switch mode PSU’s from Farnell Element14 that cost close to £80 each and for me that was the minimum level of risk I was prepared to take.

With a full specification and data sheet provided plus all the relevant UK and EU safety markings I am as sure as I can be that the power supplies I am using offer the best chance of avoiding catastrophic failure and effectively the end of astronomy for me as now I am retired I could not afford to replace any of my cameras or other equipment should the worst happen.

When you look at the Nevada unit, for example, you have to ask where is the specification sheet for it and what UK/EU standard markings does it carry?

As you say, without opening it up yourself how do you know how well it is built? and the answer is that you can’t and therefore you have to look at where you are buying from, the data sheets provided and the national-international markings it carries. 

For something that is really quite critical, even though rather boring, I would only be looking at specialist suppliers with full accountability, such as RS or Farnell Element 14 and I would expect to see a full data sheet/product specification.

If your observatory is going to be housing something in the region of ~£10,000 - £15,000 worth of equipment all linked to a single £40 power supply, well, do you really feel that is worth the risk or maybe spend up-to four times that for something that is fully compliant with current standards and the higher level of protection that is implied by those standards?

As they say, “You pays your money and you takes your chances” (sic). 

 

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I agree entirely - it isn't worth scrimping on power supplies.

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Posted (edited)

For heaven's sake don't connect your gear to your laptop with it's $10 PSU then...

 

The irony is that it's a LOT harder for a transformer-base power supply to produce a damaging over-voltage than a SMPSU as it will be designed so the unregulated voltage is reasonably close to the design voltage.

SMPSUs are also inherently noisy with voltage spikes all over the place and need a lot of careful design if these are to be minimised.

If I was worried about protecting the equipment at all costs above all other considerations, I would choose a  transformer and linear regulator solution over an SMPSU every time. If it fails, it's more likely to fail safe.

 

I strongly recommend reading this:

http://sound.whsites.net/articles/power-supplies2.htm

Quote

 

Conclusion

Decisions, decisions. The main purpose of this article is to provide some general information about small power supplies, regulation, their application and potential dangers. There is no doubt that the traditional transformer based supply is the safest. It is extremely easy to ensure that no live connections are accessible, often needing nothing more than some heatshrink tubing to insulate joined wires. Note that if possible, two layers of heatshrink should be used to provide reinforced insulation over joined wiring.

A transformer has full galvanic isolation and requires little or no EMI filtering, leakage current is extremely low, and a well made transformer based supply is so reliable that it will almost certainly outlive any equipment into which it is installed. While certainly not the cheapest option, a transformer provides a reasonable attenuation of common mode mains noise, and the final supply can be made to be extremely quiet, with virtually no hum or noise whatsoever.

The next best option is a modified plug-pack SMPS or a purpose built chassis mounting SMPS. These are useful where high efficiency is needed, along with very low standby power requirements. They are rather noisy though, and the full range of voltages is not available. There are few (if any) ±15V SMPS available for example, so powering preamps and other low power audio equipment will be easier, quieter and ultimately cheaper with a transformer.

As a last resort, a transformerless supply can be used, but only where the current drain is low (typically less than 25mA or so), and only where there is no possibility of contact with any part of the connected circuit. There is no such thing as a 'safe' transformerless power supply, and they are potentially lethal. There are so many limitations and so few advantages to this approach that IMO it is usually a pointless exercise, unless one has a mains powered appliance that needs a low current supply that can remain completely isolated from contact with the outside world.

 

 

https://www.circuitspecialists.com/blog/power-supplies-switch-mode-vs-linear/

Quote

 

In general, a switch-mode power supply is best suited for portable equipment, since it is lighter and more compact. Because the electrical noise is lower and easier to contain, a linear power supply is better suited for powering sensitive analog circuity.

 

 

Now the confession - I've changed to using an SMPSU to power my kit, but I placed it inside a fully insulated secondary, fan-cooled enclosure and chose one that is vastly over-rated. I still feel uneasy about this choice...

Edited by Stub Mandrel
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Posted (edited)

A good quality backup battery permanently connected to supply the equipment with adequate fusing should protect against PSU failure as well as power cuts but I still wouldn't skimp on the PSU.  And yes, there is no need to use switch mode when weight is not important.  A good beefy analogue supply is the way to go.

Edited by Gina

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Well we haven't fixed James' power supply. But we have talked a lot about power supplies in general.

Excellent comments on construction and reliability from other contributors.

As for PAT for reassurance. Be very wary. Often this work is carried out by people who have been trained to push the butons on the test equipment and look at obvious external damage - nicked cables, broken connectors, etc.
They do not have any electrical or electronics knowledge, training, experience, qualifications.

If you are concerned about safety remember this. A PAT test box will check the insulation at high voltage, and ensure the earth bond (if present) is able to provide continuity in case of fault.

It will not tell you about spiders in the box, old components that are at risk of failure, etc.
It will not tell you if the design might fail to put 24V on your 12V kit. 24V won't electrocute you so is of no safety concern.

Part of my job is to supervise or participate in PAT testing the company's equipment.
Basically if the tester doesn't open the 13A plug and remove the cover from the equipment to look inside, they have only done half the job.
 

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Just now, Carbon Brush said:

Well we haven't fixed James' power supply. But we have talked a lot about power supplies in general.

Excellent comments on construction and reliability from other contributors.

Indeed so.  A bit of thread drift has certainly been informative.  I've not had the chance to poke the innards with the meter yet, but looking at the sky I shan't be outside fixing the observatory rail supports for much longer so I may well do it later this afternoon.  Tinkering with electrics after being out in the rain... What could possibly go wrong there? :D

James

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Well, I just went out and gave the trimmer pots a good wiggle about and made an attempt at cleaning them.  A meter on the output now shows a steady 14.5V.  So it looks as though it may just have been them that needed.  I guess the question is what happens when I attempt to reset the voltage back to 13.8V.  Could be that the track is too damaged at that point to give a reliable output.  If I can work out the actual resistance then I may well replace them anyhow.  I can't exactly make the board look a lot worse.

James

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Posted (edited)

Good that it's just the trimpot. As you say being in one position all their life the track contact can corrode or wear at that point so trying to reset it at that same point the fault can reoccur. Replacing with resistors is a good idea or else replace the trimpot with a cermet one. You may find one with the same pinout spacing though they tend to be smaller now.

It would almost certainly be a linear track trimpot and the value should be stamped/printed on it somewhere. The rotational angle of the wiper compared to the end stops should give you a good idea of what resistor values to choose if you want to replace it with resistors.

Alan

Edited by symmetal

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On 01/06/2019 at 15:02, symmetal said:

 

I worked in the BBC (TV Centre Studios) from 1973 and the most widely used cleaner was Colclene TF (in the orange spray can).

Oh, Colclene certainly brings back memories, I was a young BBC VT engineer in the early 80s, it was used to clean tape heads back then, both 2" and 1" VTRs....

But back to the PSU in question, I'm using the exact same one in my obsy, and have been since 2009, it's not the prettiest, but mine still plods on.

6 hours ago, JamesF said:

Well, I just went out and gave the trimmer pots a good wiggle about

I'd think it's probably just a buildup of crud on the surface of the trimmer pots, doubt very much if there's any lasting damage there. Tweak the voltage set pot back to roughly where it was and everything should hopefully come good, it not give me a PM, I'll take mine to bits so we can compare notes.

 

Huw

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Thanks, Huw.  I'll try to remember to give it a go tomorrow.

James

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3 hours ago, Horwig said:

Oh, Colclene certainly brings back memories, I was a young BBC VT engineer in the early 80s, it was used to clean tape heads back then, both 2" and 1" VTRs....

Small world. We probably passed each other many times, most likely on the way to and from the canteen. :D

Alan

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Hmmm, having started my working carrier at BBC Equipment Dept. (Avenue House late '70s  and moving on to Studio Capital Projects \ Planning & Installation (Television) Dept.'s the one and only product recommended, and what we would use & I still use is Servisol Super 10 (https://www.ebay.co.uk/itm/Servisol-Super-10-Switch-Cleaner-Lubricant-Contact-Cleaner-Mixing-Desk-Studio-PA/202077401067?epid=853371964&hash=item2f0cc06feb:g:8ccAAOSwJ4hY9ehG&frcectupt=true) I still have and use it regularly, except where the component is sealed, even on 'live' equipment.

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1 hour ago, Dr_Ju_ju said:

Hmmm, having started my working carrier at BBC Equipment Dept. (Avenue House late '70s  and moving on to Studio Capital Projects \ Planning & Installation (Television) Dept.'s the one and only product recommended, and what we would use & I still use is Servisol Super 10 (https://www.ebay.co.uk/itm/Servisol-Super-10-Switch-Cleaner-Lubricant-Contact-Cleaner-Mixing-Desk-Studio-PA/202077401067?epid=853371964&hash=item2f0cc06feb:g:8ccAAOSwJ4hY9ehG&frcectupt=true) I still have and use it regularly, except where the component is sealed, even on 'live' equipment.

I last used servisol in the workshop in my dad's shop, maybe 40 years ago. I can still smell it...

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It shows that it has stood the test of time.....

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I've tested the voltage whilst adjusting the trimmer now, and in the area where it needs to be for 13.8V the voltage just won't stabilise, varying from 12V to 14V.  Replacing the trimmer looks like the only option.  That shouldn't be too hard, but I need a magnifying glass to read the resistance.  It looks like it says 5K, but the text is so small I can't be sure.

James

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maybe just remove it and measure it off the board, should be close enough...

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The magnifying glass suggests that the voltage trimmer is 5K and the other is 50K.  I've ordered replacements and I'll see if replacing them sorts everything out.

James

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Well, I am most pleased.

I've just replaced the 5K trimmer with a new one (actually 4.7K, but who's counting?), fired the PSU up and adjusted it to get a very stable 13.8V open-circuit.  I thought I should perhaps set  the voltage with a load on the output, so plugged it into a camera with a cooler, dragging it down from a claimed ambient temperature of about 21C to -1C and the voltage didn't even wobble.

Thanks to every who has helped.  It's nice to have spent a few pence on a repair rather than £35 to £40 on a replacement and perhaps more importantly to have saved throwing something away that clearly can still have a useful life.

James

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there's a lot to be said for the older tech and non-SMD boards, at least there's a fair chance to diagnose and repair. I've a couple switch-mode bricks from SFF HP PC's that are dud now and looks like I'l have to chisel out the compound they glued everything down with just to get to diagnosing what failed. Most likely the big electrolytics but so far I've put this job off, was easier to just buy replacements lol

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4 hours ago, JamesF said:

It's nice to have spent a few pence on a repair rather than £35 to £40 on a replacement and perhaps more importantly to have saved throwing something away that clearly can still have a useful life.

well said, now if everybody did the same..., well we all know the outcome...

Nice one

 

Huw

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