Voltage critical

[tony draper]

Power supply brick for Bro Draper's Virgin Broadband Modem has gorn TU,got boxes full of PSU but none at all the 1.2 amp 10 VOLT indicated on the buggad PSU, 9 volt or 12 volt of suitable ampage by the score, will they do any harm?
Lord knows how long it will take for a replacement to turn up,could buy a newun at Maplins but Bro Draper is even more reluctant than me to part wi coin,beside he reckons tiz Virgin Media's duty to replace it.
:confused::rolleyes:

[MacBoero]

I would give the 9V one a try first, but make sure the plug polarity is right before you try, and it obviously needs to have sufficient current capability, and as you are going to be 1V low, as much headroom on the current capability of the supply may help, if it is going to work at all.

[green granite]

My BT router says it wants 9v so yes try the 9v first but as MacBoero says the available current is the most important thing in those circumstances.

[Loose rivets]

And of course, curly amps V straight amps, but you wouldn't be caught out by that, I'm sure.


You could look at the voltage on load. If it sustains 9, and runs, then the power supply's heat will be the next clue.

[mike-wsm]

Mr D - There is no way I would answer your question over the internet. You haven't told us whether the broken power unit is ac or dc, whether it is regulated or unregulated, if the latter what the max oc voltage is, and whether it contains any filtering to avoid interference with ADSL or modem (router?). And you have not told us how you 'know' the fault is in the supply, not the router. As both items are specialist matching units and I guess owned by your isp, I would advise hands off don't touch. :ok:

[Milo Minderbinder]

both items are provided by the ISP and are known to be damned unreliable.

You also run the risk of Virgin getting a bit peed off if by using a charger that they didn't supply you end up sticking a voltage down their network because the modem shorts

[Blues&twos]

As both items are specialist matching units and I guess owned by your isp, I would advise hands off don't touch.

But where's the fun in that? The thrill of the anticipation, the rush of adrenaline, the rollercoaster of emotions as you reach a trembling hand out to the 'on' switch.

That's why I became a controls engineer.

[tony draper]

Bro Draper managed to eventually get through the plethora of phone button presses and robot voices and got to speak to a human at Virgin who agreed to supply him a new power supply plus a new cable modem for some reason,so thanks chaps,he can live without the tinternet until tomorrow.
Thanks chaps.

[mike-wsm]

Mr D - Glad to hear it's resolved. The reason for replacing both is that the fault could be in either. Bro will hopefully receive upgraded and less troublesome hardware. :ok:

[tony draper]

Will tell bro Draper to try and hang on to the old modem as a spare,tiz identical to mine,it was def the PSU,brought it here plugged it in and stuck me meter on it it was still putting out 14.5 volts off load but it got very hot in seconds and started emitting that burning insulation smell.
:uhoh:

[mike-wsm]

Mr D - Maybe so, maybe not. There is such a thing as consequential failure.

[Milo Minderbinder]

you can expect that the modem is also buggered because of the overvoltage

or the fault in the power unit could be due to a short in the modem.

bin it
holding onto it is just a liability waiting to happen
or maybe a fire waiting to happen

[tony draper]

Dunno if it is my digital meter but all power supplies read over the rated voltage when not loaded wi summat apart from the meter, just tried a 12v PSU running a CCTV camera it reads 12 volts dc,disconnected just on the probes it reads 17 volts.
:uhoh:

[mike-wsm]

It depends. Supplies differ in design, some are relatively crude and voltage varies with load. Others are regulated and voltage remains substantially constant. But a regulated supply on absolutely no load at all could read high. It depends what the design engineer (me) has decided to do.

You need to know what you are doing, you need to know what you are testing, and you need to apply appropriate loading to simulate the correct test condition.

[Keef]

Power supplies off their design load can go a long way over nominal voltage.

Years ago I was working on an HF radio in a nice twin-engined aircraft, with the external PSU connected to save running the battery down. The owner was doing something with the Nav kit (all switched on and displaying) when his wife - an energetic and aggressive woman - yelled at us for having the battery master "on" while using external power.

She switched off the battery master. Cue loud sequence of popping sounds as CBs and fuses blew. My test meter showed 70 volts (on a 28 volt system). All the electric instruments in the panel, all the bulbs, and the VHF Nav/Comm radios had to be replaced. He reckoned that switch change cost him over £10k.
They divorced not long after.

[mixture]

On the subject of cheap power supplies...... nicely illustrated with photographs here.....

Heroic Register reader battles EXPLODING COMPUTER ? The Register (http://www.theregister.co.uk/2012/11/26/exploding_computer_vs_reg_reader/)

[mike-wsm]

Keef

I think you may possibly be confusing the terms PSU and Charger.

[Keef]

Nope, this was labelled "Ground Power Unit 28v".

But many chargers [edit: and power supplies] will put out something around root 2 times the nominal voltage off-load.

[mike-wsm]

Ah, so you were running rectified ac rather than dc. We had one clown at bae who hooked an electronic board onto the 28V line. We got sent a whole lot of boards back from the field with exploded capacitors. The poor little things had been trying to smooth the entire generator output all on their own To give the guy his due he was ex-Concorde so he couldn't really be expected to know. :}

Your pal using the Ground Power Unit was quite right to keep the battery on line. It acts both as a zener (of sorts) and as a large capacitor. Very necessary.

[Keef]

Indeed. It's a simple rule, and an expensive one to ignore. Whenever I build a power supply for my amateur equipment, there's almost always a regulator in there to hold the output close to nominal.

For setting up the HF radio in the aircraft, his GPU was the obvious thing to use...

We did get the HF sorted, some time later. The fuse in that protected it, and no harm was done.

[Loose rivets]

Are there any reasonably priced DVMs that read true RMS?

tis why I keep my old AVO 8. Nowt like an AVO 8 :8

[tony draper]

Bro Drapers kit delivered this morning,he's back up and running
different cable modem different power supply,12 volt switched buggah this time,hung onto his old modem.
:)
Not bad service from Virgin.

[Keef]

Are there any reasonably priced DVMs that read true RMS?

tis why I keep my old AVO 8. Nowt like an AVO 8 :8

I gave away the DVM I had. My brain's analogue and can't cope with flickering numbers. I have two AVOs - a traditional 8 and one of those techie ones they didn't make many of (can't remember the number, and it's in the workshop where I'm not). I got one for free from work when they were throwing it out, and one for a £5. Brilliant bits of kit!

The daft blighters in the lab at work "had" to have their meters calibrated every year. They worked out that it was cheaper to buy a cheapish Chinese job every year than have the AVO calibrated.

[mike-wsm]

Rivets

I have never been aware of any version of the AVO8 that measured true RMS. I have just checked the schematic of the AVO8 Mk 5 (original and later versions) and it is simply a dumb old average-reading RMS-calibrated moving coil meter. The RMS calibration is true only for sinewaves and if I recall correctly the fudge factor is 1.11

True RMS meters belong to a later era, the first I am aware of was made by Advance (later Gould) at Hainault during the early seventies and rebranded in Germany and the USA.

In audio work during the seventies we generally used quasi-RMS circuits which read a combination of average and peak. I used the Analogue Devices True RMS IC in audio instrumentation designs from the mid-seventies onward.

[mike-wsm]

Yes fudge factor 1.11 being the ratio of

Vav = 2/pi x Vpk (for sinewaves)

Vrms = 1/sqrt2 x Vpk (for sinewaves)

Apologies to Analog Devices for spelling their name in the British sub-dialect of English!

[Loose rivets]

I'd assumed a perfect sinewave, but of course that is more often than not the case. I've got this distant memory of the RMS being .7 of the peak. Heck, for telly mending, we didn't have to get things more than vaguely right.

So, 240v mains X 1.42857 would be 342.857 - ish. That seems high. Perhaps it was .77, which gives an answer closer to my memory. The1950s seem a long way away sometimes. :{



(Not that our mains is 240 anymore.)

[mike-wsm]

The British 240V mains is nominally 230V rms
Vpk = sqrt2 x 230V = 325V
Vav = 2/pi x 325V = 207V

If you look at your AVO8 you will see that 207V on the dc scale (which is what it is measuring) corresponds with 230V on the ac scale (fudged to indicate rms but only for sinewaves). The fudge factor 1.11 has been used when making the ac scale.

[Loose rivets]

I recall on my AVO 8 Mk5 that the scale seemed disappointingly simplified compared to my earlier meters. It only has the 1 - 10 and 1 - 3 for V and A. plus the ohms scale. Certainly not a separate AC scale.

I've just had to look at it to make sure. (only had it 30 years) The older one I have back in the UK presumably has a separate scale. I'll find out next summer.;)

His kid is bigger that this little guy was then. That's the meter I've got here, so 30years??? Time flies.

No, CORRECTION. That's the older one. Much more cluttered scale. Fooled cos it looks so new.

http://img.photobucket.com/albums/v703/walnaze/Family/Scottdoingelectronics-1.jpg

[mike-wsm]

Okay, in that case have a look at the schematic, there will be a resistor switched in somewhere to provide the 1.11 fudge factor.

[mike-wsm]

Rivets

There is a schematic here:
http://www.petervis.com/electronics%20guides/Avo%208/Avo%208%20Schematic/Avo%208%20Circuit%20Diagram.gif
I've made it a link so you can print it out full size.

You will see that the ac and dc circuitry has been separated, ac on the left and dc on the right. If you care to calculate your way through you will find that there is a 1.11 ratio in sensitivity.

However this is drawing attention away from my point. The AVO8 is not true RMS.

[Loose rivets]

There's a guy on here who is collecting records of all AVOs worldwide.


UK Vintage Radio Repair and Restoration Discussion Forum (http://www.vintage-radio.net/forum/index.php)

Too much chat is discouraged on the forum because there are not many mods. They have a Face-b thread now and show pictures of AVOs I never dreamed existed.


Last time home I sold dozens of unused movements for the VCM 163 valve (tube) tester which I happened to have in storage. They were however, made in Italy, and not of the supreme quality of the AVO 8's movement. All seems so labor-intensive now and indeed, that's why the meter is no longer made.

Just making the spring was a work of dedication, a bit like hand-painting the line on the side of Rolls Royce cars.


Rapid Electronics in Colchester were I believe, selling them until the end.

[Keef]

I picked up a job lot of old (really old) AVOs a couple of years ago. One of them had UK mil markings (the prisoner's arrow, or W ^ D). I gave it to daughter 1's FIL who had always wanted an AVO. I couldn't see which model it was.

If I really care about waveform (sometimes I do, mostly I don't) then I fetch the oscilloscope from under the desk. Most of the time, the RMS fudge factor will do me nicely.

I was told last week that the UK mains supply now has to be a minimum of 220V (not 230, nor 240) at the meter. We just had the overhead cables to the church moved underground, and the contractors were grumbling about the heavier-duty cable they had to use because of the length of the run and the load we put on it. I think that explains why the lights used to dim when we turned the heating on.

[Loose rivets]

Hee hee . . . here, if you blink just as the A/C kicks in, you think you've gone blind. We're about the 3rd house down from the nearest transformer.


Point taken about the AVO 8 not reading true RMS, but just another aside: You'll note in the schematic the movement is 37.5 micro amps. By some means or another, they make that become 50.

When I was testing the movements prior to shipping, I set my UK AVO to 50 micro amps DC current. The movements then (mostly) agreed with the AVO.

I found one of the resistance settings provided me with 70% of 50, so I could used the battery in the meter for a quick check.

I spent days packing these unites. Shipped them to five different countries.

I seem to remember this man's name. Just look at the white highlighted bit lower down! Numbers look familiar?

I had both these movements - 72, I think, but the one with the knob seems to be hiding from me. Spent hours looking for them before heading back to Texas. Might still be there, but getting a bad feeling someone from the storage place may have stood on a box and slung them out. The knob ones could make 200 quid each.

People are restoring these - even re-manufacturing the complex switches. They really do analyze the valves in great detail.

Tube Tester AVO V.C.M. 163 (http://www.radiomuseum.org/forum/tube_tester_avo_vcm_163_measurement_principle.html)