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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 5th Jul 2016, 6:46 pm   #1
itmustbesid
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Default BBC Micro and US Power Supplies

Greetings .. I an expat currently living in the US and have been looking at various BBC microcomputers for sale on eBay: many of these claim to have been refurbished with new "switching" power supplies. My question is, will this be a problem for me? I know that a step-up converter is needed (and I have one, which will convert my 110V input into 220-240V, and has a socket for a UK plug), but I don't really know what a switching PSU is compared to a non-switching PSU.
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Old 5th Jul 2016, 8:45 pm   #2
cmjones01
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Default Re: BBC Micro and US Power Supplies

The 'switching' power supply is simply the one which the vast majority of BBC micros were fitted with. A very few early BBC micros were fitted with a linear power supply which runs rather hot and can't power disc drives, but the 'switching' (more usually called switch-mode) power supply is the one you want. They're generally very reliable.

The difference between a linear PSU and a switch mode one is that a linear one uses a standard transformer and rectifier running at mains frequency (50 or 60Hz) followed by regulators which give a stable output by disposing of excess power as heat. A switch-mode supply uses special electronics to run a transformer at a higher frequency (usually tens or hundreds of kHz) and regulates its output in a more efficient way. They're generally lighter and cooler-running than linear supplies but at the cost of greater complexity.

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Old 5th Jul 2016, 8:56 pm   #3
TonyDuell
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Default Re: BBC Micro and US Power Supplies

The original (and now very rare) linear PSU for the Beeb ran so hot it was reputed to take the varnish off the table.

You may find there is a link on the switch-mode power supply board to set it for 115V mains. If so, set that, and it will run fine in the States. Or you can use any step-up transformer of sufficient power rating.

The video output will still be UK specification (50Hz vertical, 15625Hz horizontal, and PAL colour encoding if you use that output rather than the RGB one). I have seen references to a US model of the BBC micro with 60Hz vertical and NTSC colour, but I have never seen one.
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Old 6th Jul 2016, 12:36 am   #4
itmustbesid
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Default Re: BBC Micro and US Power Supplies

Thanks for the responses. Sounds like the PSU thing will be fine. My step-up converter is rated for 500W so if the PSU hasn't got the 115V switch, I should be OK. As for video, I was hoping to combine an RGB-to-SCART cable with a SCART-to-HDMI converter (both easily obtained online) and then use any old monitor from the back of the cupboard for now ... but of course, a good old CRT monitor like the Cubs used in schools would be the goal eventually
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Old 6th Jul 2016, 2:09 am   #5
Ed_Dinning
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Default Re: BBC Micro and US Power Supplies

Hi, I'm not absolutely certain about this SMPS, but many modern ones have automatic voltage regulation from about 90v to 250v. Some may have low range / high range voltage selector, so your step up transformer may not be necessary.

Ed
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Old 6th Jul 2016, 10:13 am   #6
mhennessy
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Default Re: BBC Micro and US Power Supplies

The service manual for the BBC Micro is here:

http://chrisacorns.computinghistory....Oct85_Sec1.pdf

On page 89 of the PDF, there is the PSU schematic, which clearly shows the 115V link. It's a standard setup, which makes 2 diodes in the bridge act as a voltage doubler.

If you change this, it would be wise to attach a prominent label or sticker to the case. If you accidentally give it 230V when set to 115V, you can do a *lot* of damage

While in there, replace any RIFA class X capacitors (making sure you buy replacements carefully). You might also want to check the ESR of the electrolytics, and check that R2 and R3 haven't risen in value.
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Old 6th Jul 2016, 10:38 am   #7
MrBungle
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Default Re: BBC Micro and US Power Supplies

There are a few repair kits available for these supplies on eBay as well. I used one a couple of years ago after a friend's one let out a rather large plume of smoke. Worth doing preventative maintenance for sure.
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Old 6th Jul 2016, 11:14 am   #8
winston_1
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Default Re: BBC Micro and US Power Supplies

Quote:
Originally Posted by itmustbesid View Post
and I have one, which will convert my 110V input into 220-240V, and has a socket for a UK plug),
US mains is 120V plus or minus 5% so that is what your input will be not 110V.
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Old 6th Jul 2016, 12:35 pm   #9
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Default Re: BBC Micro and US Power Supplies

My understanding is that the principle of operation of switch mode power supplies means that they are inherently indifferent to significant variations in mains supply voltage. Modern laptop power supplies seem to be specified for use between 100V and 250V.

Even the old Amstrad PCW, UK models of which which had a specified voltage range of 200-250V, was apparently capable of successful operation at much lower voltages, according to a reader's letter I once saw in a PCW magazine. It was from a Royal Navy Officer who had been using his on board ship without noticing that the socket he was using was actually 110V. On checking to see how low the voltage could go using a Variac, he set it printing a long document to maximize current drain, and found that, although the screen went black at about 50V, printing continued, and normal operation was resumed on winding the voltage back up again.
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Old 6th Jul 2016, 12:49 pm   #10
mhennessy
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Default Re: BBC Micro and US Power Supplies

It just depends on the specific design.

Lower input voltages require a higher duty cycle (assuming simple PWM operation), but the magnetics will be designed for an optimum range of duty cycles, and operation outside of that range might cause problems. If nothing else, the efficiency can go down. A responsible designer will include under-voltage lockout if there is a serious problem (which the bean counters will try to remove!).

Wide supply voltage tolerance can be built in at the design stage, but usually at the cost of efficiency or complexity. For a low-powered unit, this matters less. Move up to bigger units (100s of watts) and things have to be more carefully considered.

Modern supplies use active power-factor correction. This is a boost converter placed between the input bridge and the smoothing capacitors (easily retro-fitted, or made optional for Marketing reasons). This attempts to emulate a resistive load by drawing a current that is proportional to the input voltage, with the result that you have a reasonably constant voltage on the main smoothing capacitor (usually 380V or thereabouts - higher than the usual 325V you get with raw mains rectified). The main converter that follows is now working from a fixed DC input, so can be optimised for that, which improves efficiency. And, as a nice side-effect, the AC input is now "universal".

PFC is used in most modern power supplies these days. The cost amounts to an extra controller IC, a MOS-FET and an inductor. The benefits to the manufacturer - apart from meeting the various regulations - is no more worries about voltage selection, particularly when the user has it set to 115V by mistake in a 230V area.
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Old 6th Jul 2016, 3:14 pm   #11
TonyDuell
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Default Re: BBC Micro and US Power Supplies

I am pretty certain the original BBC micro power supply was not auto-sensing. It was designed to be set (by said link) for either 115V or 230V mains. In the former position it will be happy with 120V of course.

Switch mode PSUs approximate a constant power load, meaning they draw more current as the input voltage decreases. Some units will actually draw enough current to do damage if the input voltage drops too low. This is one reason why it is often a bad idea to run an SMPSU up on a variac when testing it.

As an aside, I have a portable computer (HP Integral) where there is an extra circuit in the PSU which is designed to short-circuit the mains input with a fat triac (thus blowing the fuse) if the machine is set to 115V mains and it is plugged into a 230V supply. HP felt that a portable machine could be used all round the world and that a user might make that mistake.

I also heard of a large organisation in the UK that had both 115V and 230V mains sockets in their buildings. Apparently the first thing they did with a new instrument was to set it for 115V and plug it into 230V mains. If there was any damage beyond a blown fuse, the unit was returned as not being suitable!. No I do not recomend trying this.
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Old 6th Jul 2016, 3:38 pm   #12
mhennessy
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Default Re: BBC Micro and US Power Supplies

There's a high-res picture of the PSU here (click the image to enlarge): http://blog.jgc.org/2011/11/my-bbc-m...-of-acrid.html

It looks like the header hasn't been installed, so to get 115V operation, the OP would have to solder a link directly to the PCB. Not as nice as a switch or a moveable link, but at least it's feasible.

Quote:
Originally Posted by TonyDuell View Post
Switch mode PSUs approximate a constant power load, meaning they draw more current as the input voltage decreases. Some units will actually draw enough current to do damage if the input voltage drops too low.
Yes, I did mention the frequent omission of the under-voltage lockout circuit in an earlier post. A risky strategy, especially given that brown-outs are a fact of life.

Electricity supply companies don't like negative resistances either

Quote:
Originally Posted by TonyDuell View Post
As an aside, I have a portable computer (HP Integral) where there is an extra circuit in the PSU which is designed to short-circuit the mains input with a fat triac (thus blowing the fuse) if the machine is set to 115V mains and it is plugged into a 230V supply.
A variation on that theme (from Tektronix) involves a spark gap that will hopefully cause the input fuse to fail in that scenario.
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