Weird power transformer problem

Loss of isolation between primary and secondary ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Are all of the outputs regulated? If so, the supply is exhibiting a wide dynamic range. If your "main filter capacitor" is before any regulation, perhaps it was replaced with an incorrect WVDC rating? Is it feasible to disconnect all of the supply's outputs for further analysis and testing? How about disconnecting the transformer secondaries and running it at its rated input voltage and monitoring its temperature (shorted turns in the primary)?

Regards,

Michael

The unit appears to have a rectifier followed by a switch-mode power supply. The rectified AC probably goes into the SMPS and is supposed to have a constant low voltage output. There might/ust be a failure in the feedback circuit that sets the output of the SMPS.

H. R. Hofmann

Thanks for all the suggestions. I think I've figured out what happened, a bit far-fetched but here goes:

By taking the square-root of the primary to secondary inductance ratios, we get a step-down ratio of nearly 5, which implies a step- down ratio to 24 volts. Which is what we measure the transformer providing.

Now that tranny goes into a full-wave bridge, then thru a SCR to the

12-volt nicad pack and 16V 470F capacitor.

A completely non-sensical situation.

Now a couple more observations:

(a) The transformer doesnt smell cooked, but it does have a few droplets of varnish oozing out. (b) Somebody replaced the dual AC line fuse holders with a single fuse holder. (c) The unit arrived with no fuse caps or fuses in the AC line fuse holders. (d) The nicads, although overheated and with 1983 date codes, are in good electrical condition. (e) These 1502's tend to get very beat up, as they're meant for portable use in the field. This one looks like-new. Not a scratch or scuff anywhere.

The only explanation I can come up with is:

(1) The guys at Tek wound the transformer wrong. Twice as many turns on the secondary than required. They slapped the "0151-xxx" tek sticker on it anyway.

(2) The transformer had just enough primary resistance to limit the primary current to around one amp average.

(3) The nicad pack (remember, it looks overheated) acted like enough of a zener to clamp the voltage to about 14 to 16 volts.

(4) The rest of the power supply aged very quickly with the overvoltage, frying a transistor and a zener.

(5) ... and the unit ran just long enough for Tek to calibrate and ship it.

(6) Since then it's been blowing AC line fuses every few minutes, bad enough so someone rewired it to use just ONE line fuse.

(7) It's been used very little, probably put on a shelf as a unreliable fuse-blower. That explains the good nicads and the pristine condition.

Weird, but I don't see any other logical explanation!

You'd think Tek would have caught this, especially on such a pricey unit.

Like most everything else - Quality Control has gone to hell - too. Its all about the "buck" - never mind how you get it. Years ago - I sent in a piece of test equipment to the manufacturere - to be repaired and calibrated. I paid like $15 to ship and $75 for repairs. They "supposedly" fixed it and shipped it back. I had it a week and it did the same damned thing. So much for that. I wasn't about to shell out another $15 to ship it again. I called the company to complain and they told me how to fix it myself - to which I asked - then what did I pay you for? I finally acquired a schematic and repaired and aligned it myself.

These companies are becoming a real trip to deal with. They want top dollar for their stuff, make it cheap and expect top dollar to repair it. The consumer gets screwed. Lately it is more so the case - than not.

L.

I agree, and the way the whole world is now geared to buy and throw away IMO almost borders on the obscene.

A classic example, about 4 years ago my wife bought me quite an expensive electric razor. A year ago it stopped working and from memory the last one I looked at (a very long time ago!) consisted of an extremely simple kind of (virtually foolproof) vibrating AC powered motor which drove the cutting head to and fro.

However on dismantling this one I was surprised to find a small electric motor and a SOPS (self oscillating power supply) and I found out pretty quickly that the problem was due to an open circuit quarter watt 2.2 meg resistor, total cost to replace about two pence and suffice to say that the razor has been functioning perfectly ever since.

Now I understand perfectly well about labour costs, which would probably have ruled taking it to a service depot and having it repaired, but at same time I couldn't help wondering how many electrical items are dumped into landfills every single day, just for the sake of a couple of pennies worth of components.. probably millions on a worldwide basis, which to me begs the question, just how much longer can our profligate lifestyles be sustained?

When I was in Elementary School - they showed us "Film Clips" of "Recycling Plants" - to do paper, glass, cans, etc.... Said a few existed in the world but more to come - it was the way of the future - to recycle and save our environment. Yet - here we are - producing JUNK - and filling the landfills with it all - and as for "Recycling" - it is NOT used in all areas for the "Intended" items and certainly not for electronics. Yeah, I agree - sooner or later - we're going to have some real problems.

One of the bigger towns near me "started" to do Recycling, but like all "phases" - it passed. People are using the allocated color coded bins - for other things now - much like those milk crates you buy in a store. "Our" community has never offered recycling.

L.

I don't know if it's the same where you are, but in one of the side effects now beginning to manifest itself here in the UK is the increasingly growing numbers of ancient churches having the lead stripped off their roofs and even railways and electricity substations are also being targeted and literally put out of action because of copper cable and fittings being stolen. Apparently there is now such a famine of raw materials in China that scrap metal is becoming a very big and lucrative business.

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This used to be a third world phenomenon in the last month in the UK

80 drain covers have gone missing from the Wokingham Borough Council district.

%3A10%3A03%3A053 Hatfield and Brookmans Park and more than 200 drain covers have been taken.

More than 60 have been stolen in the past two weeks.

Some areas a few miles from me have had copper stolen from homes (plumbing, etc) being built, siding stolen off the homes being built "and" lived in. Theives went to places where stockpiles were kept - too some of it. Some were even stealing the bronze flower vases off of Cemetary markers. Was it for "China" - I don't know - but doubt it. Probably more like "drugs". Some idiot tried his luck here ripping out plumbing from a home of a "vacationer" - and let the water run - doing about $10,000 worth of damage. He was busted in a week. Our cops caught him as he was making last minute reservations to go to Florida. That was his first and last stint - here in town.

L.

It sounds like your local police are more on on the ball than ours, although I love the Simpsons, simply because IMHO it appears to reflect just how alike our two societies are.. For instance I was watching an old episode the other day when Homer rang the local police station, an answerphone cut in and said something to the effect of "The Springfield police station is now closed., if you are reporting a felony press one, If you're reporting a".. As far as our local police station is concerned it sounded about right to me!

My 85 year old mother had lead flashing stolen off her house during the day whilst she was in, a few weeks ago. The 1000 year old Saxon church in my village also had a large quantity of lead stolen from one of the roofs which form a more modern part of it. Apparently, road signs are also being stolen all over the country, and are now being replaced with glass fibre composite ones, which actually have a sticker on the back which says "NO SCRAP VALUE". Sad what the world is coming to ...

Arfa

Not to mince words with you Arfa - but I think DISGUSTING is a better term.

L.

Well, there is a sort of humorous side to this and almost in respect with your experience. See - we have a 911 system in place here. IF you have an emergency - whatever it is, you call 911, they dispatch the Police, Fire, EMS, whatever. Yet, our local police have had a couple idiots call the "Station" - to report an accident OR breakin - whatever - and with the way our police are set up no one is in the station when they're on patrol. So - these idiots wonder why the police don't show up lickity split when they call. DUH - they weren't there to get the message. If they had called 911 as they should have, the police would have got the call from 911 immediately. This 911 system has been in play since 1994. You would think they'd get it in their head by now - 2007.

As to our police being on the ball - they are - I have to give credit where credit is due. The funny thing is, many underestimate them. They look at them as being the Keystone cops. I can honestly say - not many of their crimes have went unsolved. The only one they had issues with - was a guy who "allegedly" murdered someone here - and skipped country to one who doesn't reciprocate with sending them back here. We get crime waves about once a year - usually some ass hole from out of town. I work hand in association with a couple of those guys almost daily - and they're pretty good Joe's.

L.

Well yes, you're probably right. As far as cops go, from what I've seen of U.S. police (and actually *most* officials over there), they leave ours for dead. The system of police officer / sheriff / state trooper is far superior to ours. Not so many years back, I would have said that our police force in the UK could stand with the world's finest, but as an organisation now, they are rapidly becoming a joke. This is being further added to by them replacing genuine officers with "PCSOs" - that's Police Community Support Officers. These guys (and girls) look for all the world *like* a police officer, but only have the powers to observe and report. U.S. police look the part. Their uniforms are smart, and from what I've seen around the states, always worn with a sense of pride and duty, as are those by customs and immigration officials at airports. Ours, on the other hand, now have a sort of 'scruffy' and dishevelled look about them. If you call 999 (our

911), you will get put straight through to the service you need, but with the police, you may not get an immediate response. Whereas all calls used to be taken and assessed by a police officer, if you were calling the police, you are likely now to get a civilian operator, who makes the decision how quickly to respond. Many even large police stations are now closed at night, so if you are reporting a proper crime, you may not get someone round before the next morning, or even afternoon.

On the other hand, if someone reported you making a racist or homophobic comment in public, you are likely to find yourself dragged off to a police cell within 5 minutes. It is now also very easy to end up with a criminal record for motoring offences such as speeding, and even doing your garbage recycling incorrectly, in some towns. I kid you not. Policing here now is all about meeting clear-up targets, and it's easier to do this with 'soft' offences committed by otherwise law-abiding good citizens, who still have a fear and respect for the police, and will roll over and allow themselves to be swept along by the system. Junkies, feral gangs, and career criminals are, on the other hand, laughing their heads off, and doing degrees in human rights law. I hope that the U.S. doesn't finish up going the same way, but sadly, I think it is inevitable.

All of which is a jolly long way from odd transformer problems !

Arfa

Wasn't it Mahatma Gandhi who said "There's enough in the world for every man's need, but not enough for every man's greed," how prophetic that's now turning out to be!

Ivan wrote: Homer rang the local police station, an

Erm.. that`s Morecambe police station that is! Closes at 9pm

UkuleleRon

That's ridiculous, stealing lead can't be in any way profitable.

Lead is going for about $7 a ton.

Now manhole covers are quite a bit better- Iron is going for $120- $200 a ton.

Still all it takes is a phone call from the police to scrap dealers to make them worhless.

Are you sure about that ? If so, I think you should buy up all you can, ship it over here, and resell it. According to this site

scrap lead is going for £1050 per ton. That's about $2k at the current exchange rates ...

Arfa