BBC/Master Power Supply Adjustment

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ask_nz
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BBC/Master Power Supply Adjustment

Post by ask_nz » Thu May 24, 2018 8:13 pm

Hi all,

I just noticed that my Master is delivering nearly 6v on the 5v rail of the power supply - which might explain why it seems to be killing eproms fairly regularly. :(

Does anyone got any advice for adjusting the power down to the correct level?

Cheers

Ade
Ade

RobC
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Re: BBC/Master Power Supply Adjustment

Post by RobC » Thu May 24, 2018 8:36 pm

There's a pot that adjusts the voltage on the side of the PSU. You can see it in a dismantled PSU here. The pot is the round black thing with a slot for a cross-head screwdriver. It's worth carefully checking that you have the right size screwdriver and have it in the right place before turning the PSU on.

I've adjusted the voltage in the past by unplugging one set of the 5V connections from the motherboard (red and black leads) and putting a voltmeter across them to measure the output. Then just adjusted the pot as required.

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Re: BBC/Master Power Supply Adjustment

Post by dp11 » Thu May 24, 2018 9:04 pm

I'd see if the output capacitors in the PSU are starting to fail. Have you a scope you can look at the +5v rail with ?

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Re: BBC/Master Power Supply Adjustment

Post by CMcDougall » Thu May 24, 2018 9:16 pm

^ over time caps change their values, usually 10 years depending on the bad (Alba LOL) or good (Dubilier / Rubycon) make of them.

just turn the pot to 5.09v! 8)
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Re: BBC/Master Power Supply Adjustment

Post by tricky » Fri May 25, 2018 8:37 am

I adjusted mine a few days ago.
I wasn't sure if they were susceptible to giving out random voltages, like you hear if you turn an old volume control after many years of disuse.
With the power off, I twiddled the pos about a bit, leaving it turned down slightly from where it was before turning it back on to do the final adjustment.
I have no idea if this was the right thing to do, but it seemed safe and turned out OK.
Now I suppose I have to turn it up from 5.00v to 5.09v.

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Re: BBC/Master Power Supply Adjustment

Post by 1024MAK » Fri May 25, 2018 10:48 am

The tolerance of most 5V TTL and compatible chips is at worst, 4.75V to 5.25V. So in practice, you should measure the voltage on the +5V rail at the point on the motherboard that is furthest from the power supply connection point. The voltage here should be at least 4.8V. At the power supply connection point on the motherboard, the +5V rail should be no greater than 5.2V. Of course, ideally both should be as close to 5.00V as possible.

Note that the voltages may vary if expansion are added, or removed. So set the PSU up with no expansions connected. Then recheck the voltages with the expansion(s) connected.

Despite most 5V TTL and compatible chips having an operational range of 4.75V to 5.25V, many will survive up to 6V. Sometimes up to 7V. But you may be at the maximum allowable value at these voltages.

EPROM (and EEPROM, E2PROM and Flash memory) are internally analogue storage cell devices. Voltage comparators actually sense the charge on the storage cell for each bit. So an out of specification supply voltage can cause misreading of the stored data. So once you have your PSU 5V voltage correctly within specification, you may find that they now work correctly. If not, try reprogramming them.

Mark

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1024MAK
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Re: BBC/Master Power Supply Adjustment

Post by 1024MAK » Fri May 25, 2018 11:05 am

tricky wrote:I wasn't sure if they were susceptible to giving out random voltages, like you hear if you turn an old volume control after many years of disuse.
There are about three different types of potentiometers (variable resistor user controls): wirewound, carbon film, "plastic" film.
The carbon film type (which are the cheapest and most common) can become noisy and erratic due to dirt and dust getting between the wiper (the moveable contact) and the conductive carbon film track. Or the carbon film track becoming worn from use. 'Plastic' film may also suffer from the same problems, but are supposed to be a bit higher quality. Wirewound types are more reliable in that there is no track to wear out. But dirt can be a problem.

The 'preset' variable resistors (like used for the voltage adjustment in the BBC Master PSU) are normally carbon film. But types with 'plastic' tracks are also available. They come in open frame types and in enclosed types. The enclosed types are supposed to prevent dirt and dust from getting in.

The open frame types can suffer from dirt and dust, but as they are rarely moved, they should not normally suffer from wear.

Mark

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Re: BBC/Master Power Supply Adjustment

Post by ask_nz » Fri May 25, 2018 8:18 pm

Thanks all for the great advice (as usual)
1024MAK wrote:
Fri May 25, 2018 10:48 am
The tolerance of most 5V TTL and compatible chips is at worst, 4.75V to 5.25V. So in practice, you should measure the voltage on the +5V rail at the point on the motherboard that is furthest from the power supply connection point. The voltage here should be at least 4.8V. At the power supply connection point on the motherboard, the +5V rail should be no greater than 5.2V. Of course, ideally both should be as close to 5.00V as possible.
Mark
Mark - do you set voltage initially to 5V with everything disconnected from the motherboard? then test/adjust with all the connectors on the board?

Cheers

Ade
Ade

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Re: BBC/Master Power Supply Adjustment

Post by 1024MAK » Fri May 25, 2018 10:38 pm

ask_nz wrote:
Fri May 25, 2018 8:18 pm
Thanks all for the great advice (as usual)
1024MAK wrote:
Fri May 25, 2018 10:48 am
The tolerance of most 5V TTL and compatible chips is at worst, 4.75V to 5.25V. So in practice, you should measure the voltage on the +5V rail at the point on the motherboard that is furthest from the power supply connection point. The voltage here should be at least 4.8V. At the power supply connection point on the motherboard, the +5V rail should be no greater than 5.2V. Of course, ideally both should be as close to 5.00V as possible.
Mark
Mark - do you set voltage initially to 5V with everything disconnected from the motherboard? then test/adjust with all the connectors on the board?

Cheers

Ade
As no repair work has taken place on your PSU, yes, adjust the +5V output to +5.00V or thereabouts. Then test the voltages as I described above. Readjust if needed. Retest the voltages. The idea is to find the best compromise.
Then reconnect any expansions. Then test the voltages again. But don't readjust. The PSU +5V output is regulated, so the output from the PSU should not change very much, but the voltage at the furthest point on the motherboard may drop a little.
If it drops a lot, post details here so we can advise you further.

Oh, and it's worthwhile keeping a record of your voltage results for future reference. If at a later date you find that the output from the PSU has drifted, you can compare to these records.

Mark

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Re: BBC/Master Power Supply Adjustment

Post by ask_nz » Fri May 25, 2018 11:20 pm

Thanks Mark, I just tried that and looks like the power supply is failing.

On gently adjusting the potentiometer, the voltage either goes up to around 7v or suddenly drops from around 6.2V down to 2v and the PSU starts to click, it wont set to 5v at all.

So....any clues to what needs replacing in the PSU?

A.
Ade

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Re: BBC/Master Power Supply Adjustment

Post by 1024MAK » Fri May 25, 2018 11:27 pm

It's very likely that one or more of the electrolytic capacitors are failing.

I'll post more details tomorrow (it's late here).

Mark

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Re: BBC/Master Power Supply Adjustment

Post by ask_nz » Fri May 25, 2018 11:33 pm

:) thanks Mark - have a good sleep.

A.
Ade

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Re: BBC/Master Power Supply Adjustment

Post by ask_nz » Tue May 29, 2018 12:40 am

Yesterday I checked another 2 machines and both were also showing around 6v - so being a little suspicious I splashed out on a new Multimeter - and lo and behold, all the machines are reading around 5v on the new meter. :shock:

Lesson learned, don't always trust old equipment to fix old equipment

A.
Ade

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Re: BBC/Master Power Supply Adjustment

Post by CMcDougall » Tue May 29, 2018 8:26 pm

^ my multimeters (5) are all various ages /cost, being from 99p to £3,000 & they are all only out +/- 0.04volt on all :D
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Re: BBC/Master Power Supply Adjustment

Post by ask_nz » Tue May 29, 2018 9:11 pm

Yer - I’ve been trusting my old digital multimeter for years - 1v out is surprising and very frustrating. Time to fire up my oscilloscope i think.

A.
Ade

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