BBC B Micro Power Supply - okay if run unloaded?

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robbyp
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BBC B Micro Power Supply - okay if run unloaded?

Post by robbyp » Fri Mar 16, 2018 5:28 pm

Hi,

I've treated myself to a brand new BBC B, after stupidly giving away mine in the 1990s. I've got the PSU apart and am replacing the caps. My question is, is the PSU okay to run unloaded? Be nice to check the volatages before i plug it back into the motherboard.

Thanks

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vanekp
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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by vanekp » Fri Mar 16, 2018 6:32 pm

Does not say anything in the service manual about testing it off load, but in my experience you need to put some sort of load on it as with no load the voltages would be way out of tolerance.
Peter.

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by Boydie » Fri Mar 16, 2018 11:12 pm

Not sure if this counts as sage advice or just an example of luck triumphing over stupidity...
Whenever I re-cap a PSU, I always test it off load. The voltages are always what they should be, and nothing bad’s happened. Yet.
Maybe the few seconds it takes to test isn’t long enough to make me see the error of my ways.

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by 1024MAK » Sat Mar 17, 2018 1:24 am

If it was going to blow, it would only take 3 to 5 seconds... :lol:

It's normally not dangerous to run a PSU off-load. But as hinted above, some of the output voltages may go outside the specifications. With the Beeb PSU, the 5V rail should be okay-ish, as that is the regulated rail.

A 6V motorcycle lamp (any reasonable wattage, say 4W to 10W) will do as a test load for the 5V rail. Or use a USB test load (search eBay).

Mark

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by marcusjambler » Sat Mar 17, 2018 1:27 am

I've repaired over 20 PSUs ( 3 caps and reflow wound components ) and tested with no load short term ( approx 1 min ) and they have not failed

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by Bat » Sat Mar 17, 2018 6:54 am

Hi,
Has anyone come across this when repairing a PSU?
IMG_20180316_201455.jpg
Is it just a failure of C19 or is it the result of a fault elsewhere?
Cheers,
Gavin :)

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by vanekp » Sat Mar 17, 2018 8:57 am

Electrolytic capacitors with age do tend to leak and in your case seems to have corroded the one leg completely.
Its always a good idea to give all the electrolytic capacitors a look over for bulging on the top and or leaks at the bottom of the caps as in this photo.
Peter.

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by Bat » Sat Mar 17, 2018 10:01 am

Hi,
Thanks Peter, I was hoping it would just be an age related problem.
Would that cap arcing cause damaging spikes in the output to the main board?
Cheers,
Gavin :)

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by vanekp » Sat Mar 17, 2018 10:06 am

it is possible as I see that C19 is the main smoothing capacitor for the +5v.
Why does the BBC not boot up anymore?
I would replace the cap check the outputs for the correct 5V out and check on AC to see if there is not too much AC ripple on the voltages.
Peter.

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by daveejhitchins » Sat Mar 17, 2018 10:08 am

vanekp wrote:Why does the BBC not boot up anymore?
There may have been enough capacitance left in the leaky cap. for everything still to work OK!!

Dave H :D
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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by Bat » Sat Mar 17, 2018 10:54 am

Hi,
It doesn't boot due to a RAM fault.
Several RAM chips have already been socketed so I wondered if this cap had caused the failures?
Cheers,
Gavin :)

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1024MAK
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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by 1024MAK » Sat Mar 17, 2018 11:24 am

DRAM chip failure is a very common fault in 1980s built computers. Single supply DRAM chips (as used in the Beeb) are slightly more reliable compared to triple supply DRAM (like 4116 used in other machines). But there are plenty of failures where a DRAM chip has failed on it's own.

Having said that, having an erratic +5V supply would not have helped... Digital circuitry does not like large amounts of ripple. The computer would not run if the ripple is too high.

Mark

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by cmorley » Sat Mar 17, 2018 1:01 pm

The SCR crowbar on the 5v output should protect the 5v circuitry against a lot of PSU failure modes.

I've fixed a few Model B PSUs and I test them unloaded first. I have a 150W programmable load (fairly cheap one from eBay) and then load it up to ratings with that & check the voltage and ripple + leave it on full load for an hour or so.

I think it is fairly common to get machines with multiple faults... perhaps the machine was put in the loft because it was broken (e.g. memory fault) and while in the loft that finishes off the PSU. They are not necessarily related faults just coincidence & age.

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by Bat » Sun Mar 18, 2018 1:43 pm

Hi,
I think most retro computing items are things that are pulled out of a loft! Be it your own or someone elses :lol:
Cheers :)

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by robbyp » Mon Mar 19, 2018 8:07 pm

Thanks for the replies. I've got the new caps so am going to replace them this week. I'll think i will give it 5s off load just to check, i'd rather do something to the PSU rather than the motherboard!

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by 1024MAK » Tue Mar 20, 2018 2:19 pm

robbyp wrote:Thanks for the replies. I've got the new caps so am going to replace them this week. I'll think i will give it 5s off load just to check, i'd rather do something to the PSU rather than the motherboard!
It takes some multimeters that long to actually present a meaningful reading :lol:

Don't worry about how long you have it on for, a working Beeb PSU (with no faults) won't go bang with no load.

Mark

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by robbyp » Mon Mar 26, 2018 7:59 pm

All went well, Beeb is working fine. Thanks for all your help.

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Re: BBC B Micro Power Supply - okay if run unloaded?

Post by vanekp » Mon Mar 26, 2018 8:04 pm

Glad you got it up and running again, well done :)
Peter.

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