Burning Plastic Smell - Master power supply?

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PitfallJones
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Burning Plastic Smell - Master power supply?

Post by PitfallJones » Sun May 12, 2019 11:20 am

Hi There,

So I'd left my Master on all day yesterday and came home to a burning plastic smell !

I opened it up and nothing obvious looked damaged but the smell was coming from the power supply area.

Has anyone got any suggestions what I might do to fix it?

Can you power a BBC from a modern external power brick?

PJ

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Elminster
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Re: Burning Plastic Smell - Master power supply?

Post by Elminster » Sun May 12, 2019 12:23 pm

Dried out caps? Any with bulges etc?

I replaced my faster psu with the one from Simon site https://www.waitingforfriday.com/?p=794, I think I have the old still working master psu somewhere.

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Re: Burning Plastic Smell - Master power supply?

Post by 1024MAK » Sun May 12, 2019 12:37 pm

On the PSU PCB, what type are C1 and C2? If they are Rifa, or there are cracks or breaks in the case of either of these two capacitors, this is the most likely cause.

Other possibilities include, but are not limited to:
  • One or more electrolytic capacitors failing
  • Dust and dirt causing a resistor or the main transistor to run hotter than normal
Mark at RetroClinic offers a replacement service. He will send you a serviced PSU, but will want you to send your old one to him so he can service it so he can continue the exchange system.

Mark

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Re: Burning Plastic Smell - Master power supply?

Post by 1024MAK » Sun May 12, 2019 12:49 pm

PitfallJones wrote:
Sun May 12, 2019 11:20 am
Can you power a BBC from a modern external power brick?
Yes, but!!!

The main board uses +5V and -5V DC supplies. The digital logic only needs the single +5V supply. So the computer will run from a +5V DC regulated supply capable of supplying sufficient current.

However, the cassette system and the sound system analogue circuitry requires the -5V supply in order for it to work correctly.

If you use disk drives or any other expansions that need the +12V supply, then this must also be considered.

Mark

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Re: Burning Plastic Smell - Master power supply?

Post by tricky » Sun May 12, 2019 1:11 pm

I was wondering whether you could use a 5v supply and a "buck converter" to get the -5v.
Maybe they would be too "noisy".
PS You don't need the 12v for a gotek.

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Re: Burning Plastic Smell - Master power supply?

Post by 1024MAK » Sun May 12, 2019 3:07 pm

To get a low power -5V supply from a +5V supply, an isolated DC/DC converter/inverter will do the job.
For example this one: XP Power IK0505SA -  Isolated Board Mount DC/DC Converter, 1 Output, 250 mW, 5 V, 50 mA https://uk.farnell.com/xp-power/ik0505s ... dp/1859046.

Mark

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Re: Burning Plastic Smell - Master power supply?

Post by tricky » Sun May 12, 2019 3:52 pm

This is off topic really, but while you might be getting one of these, would that also be suitable to "jack up" the 5V from the beeb to 10V in an RGB to SCART cable for TVs that want a higher voltage to auto swap to RGB 4:3 instead of 16:9?
Last edited by tricky on Sun May 12, 2019 3:52 pm, edited 1 time in total.

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Re: Burning Plastic Smell - Master power supply?

Post by 1024MAK » Sun May 12, 2019 4:11 pm

tricky wrote:
Sun May 12, 2019 3:52 pm
This is off topic really, but while you might be getting one of these, would that also be suitable to "jack up" the 5V from the beeb to 10V in an RGB to SCART cable for TVs that want a higher voltage to auto swap to RGB 4:3 instead of 16:9?
One of these would be better: RECOM ROE-0512S -  Isolated Board Mount DC/DC Converter, ITE, 1 Output, 1 W, 12 V, 84 mA
https://uk.farnell.com/recom-power/roe- ... dp/2774088.

Mark

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Re: Burning Plastic Smell - Master power supply?

Post by PitfallJones » Sat Jun 01, 2019 9:56 pm

So I opened up my Master up again today and here's a photo:
power.jpg
From sprows site it smells like this: (and it still actually works!)
http://www.sprow.co.uk/bbc/howto.htm#powersupply

Leaking or smoking filter capacitor
During normal operation an acrid burning smell comes from the power supply vents. If left unattended smoke will also be ejected though the power supply will continue to run and the fuse remains intact, as C2 is a UL approved part it doesn't catch fire.

The problem is that the 100nF filter capacitor C2 has perished, usually because of small cracks in the plastic between the layers of foil which when exposed to mains potential break down - melting the plastic. It's counterpart, C1, is also prone to failing but is a 10nF value.

But the supply itself says do not open (on fear of death!)

So I'm thinking I should just get a replacement...

I can buy a MOLEX power supply that has 12v and 5v so I'd just need a way to force that to provide -5v as well - any ideas?
https://www.ebay.com/itm/External-MOLEX ... xy4YdTWwr0

PJ

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Re: Burning Plastic Smell - Master power supply?

Post by Elminster » Sat Jun 01, 2019 10:28 pm

First thing anyone does is swap out the caps, or gets someone to do it, for any retro machine really. Plenty of guides on how to do it. Depends if you feel confident soldering. Everyone has had the magic smoke at some point. Mark at retroclinic will sell a kit contains caps, and think he supplies instructions as well. Done 6 myself I think.

Swapping psu you can also do https://www.waitingforfriday.com/?p=794

People also use replace psu supply service, but most are uk based, so not sure on postage.
Last edited by Elminster on Sat Jun 01, 2019 10:29 pm, edited 1 time in total.

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Re: Burning Plastic Smell - Master power supply?

Post by 1024MAK » Sun Jun 02, 2019 6:49 am

Yeah, every manufacturer puts warnings on consumer items. Danger of this, warning of that. No user serviceable parts inside. Warrantee invalidated if opened, caution: live gremlins inside, etc. etc...

For the lay person, these warnings are to keep them from doing something stupid, like opening the covers and putting their fingers in it when connected up and live [and so that you send your suspect item back to their service centre].

If the PSU has not been powered up for weeks or months, all the capacitors will have discharged by now anyway.

But if you are sensible, are reading this, and know how to solder, we can talk you through the safe way to do the work. As with a lot of things in life, the solution to safety is to control the risk.

With regards to soldering, it’s a skill that can be learned. Use good quality tools, good quality 60/40 lead solder (don’t get it from China) and then practice on either a scrap PCB or on a practice PCB. Once you have got the hang of it and have built up confidence, life will be a dream. Okay, maybe it won’t make your life a dream, but you will have a new skill :mrgreen:

The two capacitors that need renewing and the third capacitor that is recommended to be renewed are only slightly difficult because in most PSUs the leads were bent flat with the PCB before being soldered. But the PCB pads are large and it’s not that difficult to do.

Alternatively, as outlined above, RetroClinic does a servicing/swap arrangement. You pay him money. He sends a serviced PSU to you. You send your old PSU to him. He then services it ready for the next customer :D

You can replace the existing PSU with a more modern PSU. But that will involve some electrical work as well. As it’s not a quick swap out/in.

Mark
Last edited by 1024MAK on Sun Jun 02, 2019 6:54 am, edited 1 time in total.

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