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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 22nd Sep 2016, 8:40 am   #1
cathy_vintage
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Default BBC Computer

Hi from Ash,

I bought one of the 1st BBC's model A when it came out, on advanced pre-release mail order. I still have it in the loft and it has the 'Issue2' early PCB and I subsequently fully upgraded it using a rare ceramic DFS chip etc etc. I co-wrote the book 'A Hardware Guide for the BBC Computer' in 1983. Acorn wouldn't let me include a full circuit diagram, even though they had allowed the authors of another book 'Advanced User Guide' to do so (evidently they were pals of Hermann Hauser). However, when Olivetti took them over I decided to 'publish and be damned' and included an A3 folded copy in later sales. I still have copies of that circuit diagram if anyone needs it but I suspect it's all over the internet now including a 'pdf' free download of my book, which I don't mind but wish they had asked me for permission 1st ! We sold a lot of copies through Farnell but the buyer there got a right roasting when his superiors found out we had included TTL datasheets from RS (with RS's permission). Reprinting (after 2 print runs) was never an option as the (dodgy) printer in Hull told me he had sold the plates for their silver content and taken his wife to a fish restaurant with the proceeds !

Anyway, waffle aside. My Beeb was fitted with a black analogue PSU which was a recall item. I was told by Acorn to take it to a very young lad in Patrington near us to have it replaced with an Astec Switch-Mode PSU, which I did. However, he never had the protection cover for it and never obtained me one. Does anyone have a photo of the cover so that I can find/make one as you could easily poke something in the slots in the case and get a shock.
I think it still has the Watford ROM expansion card in it with several ROMs still fitted. As you can probably gather this thing hasn't beeb-beeped for about 25 years.
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Old 22nd Sep 2016, 12:59 pm   #2
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Default Re: BBC Computer

I have just recently resurrected our BBC B which is fitted with an Astec power supply. I thought it wise to slowly increase the mains voltage via a Variac to reform the electrolytic capacitors, much to the frustration of our 10 year old grandson who was very impatient to experience primeval computing. Fortunately I also decided to run up the PSU while disconnected from the BBC computer. It lasted only a few seconds at full mains voltage before one of the suppressor capacitors exploded in a cloud of acrid smoke - apparently a common experience, e.g.
http://www.retro-kit.co.uk/page.cfm/...X2-capacitors/

You will find other websites with similar information, including other common faults.

I will open up the BBC case and take a photograph of the PSU this evening if noi-one gets there before me.

PMM
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Old 22nd Sep 2016, 3:35 pm   #3
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Default Re: BBC Computer

I have got plenty of 1700V 6n2 caps that were used it the flyback location in CRT computer screens. I have seen them fail and they don't make all that slime that takes so long to clean up. They also run well below there rating on mains voltage and frequency.
They are blue and made by Phillips.

I notice that they used new ones just like the old ones so the BBC will have to come back for another service. I would put a date on it for the day it gets a mains filter capacitor service.
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Old 22nd Sep 2016, 4:09 pm   #4
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Default Re: BBC Computer

Great replies there. I think I will replace that cap as mine has not been operated for a least 25 years and initially wind the mains input voltage up gradually from a Variac. I actually used it for the manuscript for the book using, I think, 'Wordwise' was it? I always loved the red programmable function keys.

How do the Video ULA's fair with age? Mine being early production, was fitted with a large winged aluminium heatsink to keep it cool. My model 'A' cost £235, as I remember in early 1982 'ish.
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Old 22nd Sep 2016, 5:07 pm   #5
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Default Re: BBC Computer

Quote:
How do the Video ULA's fair with age?
As they where 'mask programmed' i.e. the top layer of aluminium was for each design, they don't lose their memory unlike modern flash programmable devices. It will be fine unless it has blown up.
 
Old 22nd Sep 2016, 6:38 pm   #6
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Default Re: BBC Computer

Quote:
Originally Posted by cathy_vintage View Post
I think I will replace that cap as mine has not been operated for a least 25 years and initially wind the mains input voltage up gradually from a Variac.
There is a school of thought which says that it's not really good to wind up the input voltage to a switched-mode power supply slowly, on the grounds that they will strive mightily to produce their specified output wattage into their load across a very wide range of input voltages. The lower the input voltage, the more input current it has to draw in order to maintain the same wattage output to the load. With the input voltage at the extreme low end the PSU will draw a huge amount of current, far more than it would at normal mains operating voltage.

Better designed SMPSUs will have an input undervoltage protection circuit to stop the PSU from trying to do this under mains brown-out conditions, but I don't know whether a typical BBC B PSU has any such protection.

It might be less dangerous to remove the suspect capacitor and subject the capacitor -only- to a stress test from slowly increasing mains voltage. Inside a strong plastic or wooden box, ideally...
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Old 22nd Sep 2016, 6:43 pm   #7
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Default Re: BBC Computer

It's even less dangerous to assume said capacitor will be antisocial and to replace it anyway.
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Old 23rd Sep 2016, 5:39 pm   #8
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Default Re: BBC Computer

Hello

Not sure what you mean by a protection cover. All the switch mode PSU's that I have in my bbcs are open chassis however they are mounted upside down into the BBC bottom case such that you cant get to the parts without removing it first.

regards

James
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Old 23rd Sep 2016, 6:11 pm   #9
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Default Re: BBC Computer

Some BBC micro power supplies do come with a metal plate - but I can't see any obvious reason for it unless it was for RF shielding to pacify the BBC when changing from the original psu to the switched mode version and then quietly dropped later.
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Old 23rd Sep 2016, 7:58 pm   #10
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Default Re: BBC Computer

There are two filter caps that need replacing. Both will let go with various crackling sounds and large quantities of acrid smelling smoke. It's quite interesting when this happens to a Beeb that's otherwise OK, as clouds of smoke appear out the back vents and through the keyboard, yet the Beeb carries working as if nothing had happened. The biggest problem is getting the circuit board out of its' casing. Quite fiddly.

As has been said, don't bother with a variac. Once you've replaced the two caps, shove the plug in the socket and switch it on. The supply is quite happy to start up without a load, so you can check the output voltages before putting it back in your Beeb, ready for another 30 years' service.

Those early video ULAs do run very hot. I would take off the heatsink and re-apply some heat-sink compound as any that is on there now is bound to have dried out.

Mark.
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Old 24th Sep 2016, 2:39 am   #11
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Default Re: BBC Computer

Hmm, so they put metal shielding on the bottom yet the PSU sides are still open? (or do the PSU's with the bottom screen also have sides screening?)

Before this thread I thought it was only the US BBC's that had extra metal work

Regards

James
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Old 24th Sep 2016, 9:39 am   #12
GrahamN
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Default Re: BBC Computer

The PSU from which the bottom screen I have was definately open sided, and I'm 100% sure this was as originally fitted (and is a standard UK machine, fitted with an issue 3 board from memory).
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Old 24th Sep 2016, 5:52 pm   #13
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Default Re: BBC Computer

Hmm .. maybe I am wrong then, it was about 33 years ago but I distinctly remembering the Acorn 'agent' (Who was actually a really young lad in Patrington) telling me that he could fit the supply but he was waiting for the covers to come in. All I know is that the old black linear supply when I first got the computer gave off a smell of paint every time it got hot. And it ran VERY hot. I will order those two caps and fit and not use a Variac
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Old 24th Sep 2016, 7:09 pm   #14
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Default Re: BBC Computer

While you've got the PSU out, it is a good idea to replace C9 at the same time - 220uF 35V 105 deg radial electrolytic normally slots strainght in.
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Old 28th Sep 2016, 10:31 pm   #15
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Default Re: BBC Computer

I have recently replaced the two power supply filter capacitors on four Beeb computers. It’s relatively simple to do, so long as you remove the push-in mains switch before removing the pcb from its chassis. I agree with SiriusHardware, it is not good practice to slowly wind up the input voltage on a switch mode power supply for the reasons he’s given. Ideally, electrolytic capacitors should be rejuvenated using a series limiting resistor but this isn’t practical with components in circuit. Personally, I power up the unit for about a second and leave it off for about a minute to help the rejuvenation and then for 30 seconds and off for another minute and the run as normal.
Regards,
Steve.
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Old 3rd Jan 2017, 5:50 pm   #16
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Default Re: My poor old beeb.

I've got one of these that's been in 'cold storage' for many years. It's complete with original monitor and printer. All in fetching two tone cream and brown. I even have the matching shelved table / trolley with brown square section frame and cream shelving with fitted switched 4 way power sockets on the side, although I've got this in use with other more modern computer gear at the moment.

What's the betting that any of it will still work if I fire it up after all these years - particularly that monitor, and the ribbon on that printer will be well dried up by now!

Quick edit to say that there's a disc drive with matching disc storage box with discs and original manuals around somewhere for it too.
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Old 4th Jan 2017, 1:17 am   #17
JimmyDeath
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Default Re: BBC Computer

Well, I would expect some early 'phoenix' action from the PSU, you could of course pre-empt the filter capacitors early demise by replacing them first, it does save the clean up work that can be required when they fail.
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Old 4th Jan 2017, 2:07 am   #18
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Default Re: BBC Computer

PSU would likely benefit from being gently woken up or recapped, but other than that I'd bet it will still work nicely.

Not sure if the disc drives used a belt drive, if so they might need the belts changing.

Biggest headache is likely to be degradation of the discs themselves, especially if they haven't been ideally stored.

Have to admit to the Beeb having long been "the one that got away" in my collection. Got a fair bit of Acorn gear from the Archimedes era, but a Beeb and a RiscPC are the two really obvious machines I've not tracked down yet.
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Old 4th Jan 2017, 8:29 am   #19
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Default Re: BBC Computer

Yesterday my son decided to get his BBC B out from under the stairs, where it's lain for around 5 years. Within an hour it was producing smoke as one of the 2 Rifa caps in the PSU popped. Both got replaced as described by Mark in Post #10.

If you're bringing one of these computers back to life I'd strongly recommend a pre-emptive replacement of the RIFA capacitors. They all die messily sooner or later and it seems just to be a lottery as to when it happens.

Hugh
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Old 4th Jan 2017, 9:32 am   #20
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Default Re: BBC Computer

Someone used to sell power supply repair kits for them on eBay a few years back. Saved sourcing the parts.
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