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Old 8th Feb 2014, 11:38 am   #1
WalterEgo
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Default BBC Micro PSU blowing fuses

Hi

I have a BBC Micro which was working fine, but has now popped the internal power supply fuse (and its replacement). The PSU is not the one that you usually see with the dodgy X2 cap - it's a "Wong's" version. Same schematic, I think.

Any pointers as to where to start looking?

Thanks
John
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Old 8th Feb 2014, 1:05 pm   #2
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Default Re: BBC Micro PSU blowing fuses

There are three capacitors that can fail, one of which is an electrolytic....


Although my money is on the bridge rectifier having a few diodes blown, It's the black rectangle, just below and slightly to the left of the heatsink that the switching transistor is bolted to.
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Old 8th Feb 2014, 3:20 pm   #3
WalterEgo
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Default Re: BBC Micro PSU blowing fuses

Categorically not the same as the oft-seen schematic. Should have checked before I confidently stated that it was. It would be handy to have the proper schematic.

I too am leaning towards the bridge rectifier, having dabbed around with a multimeter - I'll take it out and check it.

Regards
John
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Old 8th Feb 2014, 4:50 pm   #4
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Default Re: BBC Micro PSU blowing fuses

No. Not the bridge rectifier. Other paths in the circuit were fooling me, but on its own it's fine.

John
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Old 9th Feb 2014, 1:59 pm   #5
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Default Re: BBC Micro PSU blowing fuses

This unit failed in my Beeb micro many years ago. I seem to remember that the switching transistor had failed short circuit and simply replacing it sorted everything out.

Steve.
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Old 10th Feb 2014, 12:03 am   #6
WalterEgo
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Default Re: BBC Micro PSU blowing fuses

Hi

My suspicions fell on that next - when I unsoldered it, the voltage drops were exactly what I'd expect, unfortunately!

Regards
John
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Old 10th Feb 2014, 3:56 am   #7
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Default Re: BBC Micro PSU blowing fuses

Hi John,

Did the fuse blow violently, ie blackened with a loud crack, or did it go gently into the night?

Did it squeak before the fuse blew?

I'm assuming this is switched mode PSU?

A photo of the board would be nice. Haven't worked on one of these since the nineties!

Rob.
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Old 10th Feb 2014, 5:28 pm   #8
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Default Re: BBC Micro PSU blowing fuses

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Originally Posted by WalterEgo View Post
My suspicions fell on that next - when I unsoldered it, the voltage drops were exactly what I'd expect, unfortunately!
Oh... If all appears relatively correct on the high voltage side then I guess it has to be worth a few checks on the low side for shorts - rectifiers, regs etc?? I guess the one thing you don't want to find is a transformer problem.

Steve.
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Old 10th Feb 2014, 7:26 pm   #9
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Default Re: BBC Micro PSU blowing fuses

During all this testing, have the PSU outputs remained connected to the main board of the BBC B? If so, try first marking and then unplugging all the spade connectors from the main PCB, and see if the fuse still blows.

It could just be protecting itself from a genuine overload caused by a fault on the main board, so if you haven't already, try it without a load or, more ideally, with a nominal load across the 5V output.
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Old 11th Feb 2014, 11:00 pm   #10
WalterEgo
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Default Re: BBC Micro PSU blowing fuses

Hi

I've pulled all the electrolytic caps off the board and ordered replacements, just to exclude that possibility. As it happens, they all tested within spec for capacitance - I don't have an ESR meter.

The PSU is disconnected from the main board of the BBC, which powers up fine and draws ~2.1A on 5V.

I replaced the 500mA fuse with a 2A one (tut, tut) and connected the mains in series with a DMM recording peak current consumption. Oddly, it never seems to peak very high - must be a very short transient. Normally it's only drawing ~1mA! There's nothing on the low voltage side at all - absolute blank - and the base of the big power transistor doesn't have any kind of high frequency going on - just wabbling around at 50Hz.

When I get the caps in, I'll power it up again.

Thanks
John
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Old 11th Feb 2014, 11:33 pm   #11
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Default Re: BBC Micro PSU blowing fuses

must be something fairly fundamental to blow the fuse. Normally a secondary overload will just shut the psu down with minimal fuss. Brave to risk a DMM on a fuse-blowing PSU!
If you've check the chopper transistor, rectifier, I'd quickly meter the secondary rectifiers, main bus capacitors (make sure they are actually discharged first!!) including tracking underneath and then the small semi's on the primary side. A dry joint on the primary side may cause fuse blowing so a blanket resolder won't go amiss especially on the transformer. You're already changing the electrolytics which of course are suspect in any SMSPU.
What's with the track connecting to the transistor heatsink? Is the heatsink live or does it have a mica? If so, check that, I'm sure in my dim past I had a washer break down caused by a tiny ball of rust under the transistor punching the mica..though it might have been on the old Astec-built PSU's.

PS The chopper tr is a BU208A (usually unmarked, may have equivalent fitted).
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Old 23rd Feb 2014, 10:39 pm   #12
WalterEgo
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Default Re: BBC Micro PSU blowing fuses

Damn it. I hoped that changing the electrolytic caps would miraculously fix it - sadly, not to be.

I've checked everywhere for shorts - none that I can see. I've double-checked the power transistor - that works. The BBC micro is working fine on an ATX power supply - draws about 2A, which is normal.

But no oscillation - so something must be wrong somewhere! I could just start replacing components, but there must be a more scientific way!

(McMurdo - the transistor is isolated from the heatsink.)

Any ideas at all welcomed.

John
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Old 24th Feb 2014, 2:09 pm   #13
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Default Re: BBC Micro PSU blowing fuses

Does the fuse blow with the load is disconnected?
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Old 24th Feb 2014, 5:12 pm   #14
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Default Re: BBC Micro PSU blowing fuses

Do you want a Beeb PSU?
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Old 25th Feb 2014, 12:57 pm   #15
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Default Re: BBC Micro PSU blowing fuses

Hello.

Sorry, hopefully a stupid question but you are replacing the fuse with an antisurge version?

Regards

James
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Old 26th Feb 2014, 12:16 am   #16
WalterEgo
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Default Re: BBC Micro PSU blowing fuses

Hi

Threeseven: Yes, it blows with no load - I've fudged up an ATX power supply to power the Beeb in the meantime and it works fine.

JimmyDeath: There are no stupid questions; yes, I replaced the fuse with an antisurge. I discovered that there's a 1R 1W resistor that had failed open circuit. I spent some time scratching my noggin trying to draw a schematic, but I'm stumped with the primaries on the transformer. I have no idea what they're doing, nor why there seem to be six of them, including one which is connected to earth but seemingly nothing else - it could be melted, or it could be some weird design I don't understand.

Courtney Louise: Oh lord, yes! If you have a working PSU, or even one that needed some TLC that didn't say "Wongs Elect Co Ltd" on it, we need to talk!

Regards
John
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Old 26th Feb 2014, 7:51 am   #17
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Default Re: BBC Micro PSU blowing fuses

I do have a spare, PM replied to.
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Old 26th Feb 2014, 8:23 pm   #18
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Default Re: BBC Micro PSU blowing fuses

Quote:
Originally Posted by WalterEgo View Post
including one which is connected to earth but seemingly nothing else - it could be melted, or it could be some weird design I don't understand.
The winding with only one connection connected to earth will be a Faraday shield for rejection of common mode noise.
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Old 26th Feb 2014, 9:42 pm   #19
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Default Re: BBC Micro PSU blowing fuses

It also offers some (rather crude) protection against insulation breakdown, by ensuring any short on the primary winding goes to Earth and not to the computer. That could be the problem with your transformer If so, it will need to be rewound, or the whole PSU replaced. You could find out, if you have access to a "Megger" type insulation tester, by unsoldering it and Meggering from each primary terminal in turn, to the shield.
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Old 28th Feb 2014, 11:52 pm   #20
WalterEgo
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Default Re: BBC Micro PSU blowing fuses

Quote:
Originally Posted by wave solder View Post
The winding with only one connection connected to earth will be a Faraday shield for rejection of common mode noise.
Ah! That does make sense. But then that leaves five connections to the primary side, all linked together with gradually increasing resistance (but only to about 9 Ohms end to end).

I've bought a spare PSU off another member, but I'd still like to work out what's wrong with the damned thing, so if you can shed any further light on how it (ought to) work...

Cheers
John
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