Philips G11 earthed chassis by mistake

[simpsons]

Help needed to bring back to life a G11 which, under test, I accidently earthed the chassis when an aerial lead brushed against it. I could kick myself.

The 3.15 fuses blew and there is now a short to chassis on both fuses.

So that I don't reinvent the wheel are there any other suggested replacement components? Here I'm thinking of in addition to possibly the rectifiers and thyristors.

Chris

[MALC SCOTT]

Should have damaged nothing more than the bridge recs, Malc.

[Red to black]

We have all done similar at one time or another oh Buger !

[toshiba tony]

Our apprentice once did it, with a scope. We told him to use an isolation transformer, but he didn't. Just took two diodes out. But here's a better one, our senior engineer swore a variac provided mains isolation. Has anyone on here seen one that does? I haven't, and the final one, what about scoping the primary side of a SMPS without a transformer, BANG!!!

[Jac]

Originally Posted by toshiba tony

our senior engineer swore a variac provided mains isolation. Has anyone on here seen one that does? I haven't

Oh yes, most of the time I use a variac with mains isolation.
Philips has made variacs with mains isolation.
Please see attached photos of two of them, the symbol that indicates the insulation, and the schematic diagram of the first pictured variac.

Jac

[Brigham]

I had wrongly assumed that a Variac was an isolation transformer, because, until I had one go wrong, and found a brush in it, I had thought it was a giant variometer!

[Hybrid tellies]

I have seen this a few times on G11's once at college, once in the workshop and once in a customers house.
If you had just repaired the set and plugged the aerial in and as you did so the outer case of the aerial plug touched the chassis you would get a loud bang as two of the rectifier diodes went short circuit taking out the mains fuse.
I think this only happened if the outer screen of the aerial coax cable was earthed probably via the aerial distribution amp.

[simpsons]

Many thanks for the replies. I can breath a sigh of relief that hopefully all I need are a couple of BY diodes.

Thank you also for the isolating transformer advice. For my sins I have one, but it has remained in my "to do" list since I built my shack 10 years ago. I've found time to insulate it; reinforce the shelves; installed srip power supplies; Digital and FM aerials; broadcast monitor video to give me a reference to work from; too good a pair loudspeakers so the mains hum from radios on test sounds louder than it it would in the radio; a Freeview source with RF output; a BBC ref colour bar source etc and, most importantly, a remote bell so that my wife can tell me that she is either leaving me for good or food is on the table.

One thing I haven't got round to is installing the isolating transformer which is still in the box!

Watch this space.

Chris

[simpsons]

Malc Scott, you were spot on, 2 of the diodes have in fact short circuit. The snag is I can't find the spec to source an equivalent.

So, whilst I could buy BYW 95c which sound just about right, at 3amps 600vdc its only a guess.

Could a better informed BVWS member help me.

Chris

[Restoration73]

This is a more recent ultrafast equivalent

https://www.rapidonline.com/dc-compo...-diode-47-3170

the UF5408 is also suitable

[MALC SCOTT]

I used BY133 the last time replaced them, Malc.

[Heatercathodeshort]

I used to use BY127's. Worked fine and 10p a dozen. J.

[Sideband]

Use BY127, they should be fine or even 1N4007 and peanuts to buy. I STRONGLY SUGGEST you replace all four.

[simpsons]

Thank you all for coming to my rescue.

Funny world isn't it when in ignorance, until I found a copy of the Television Magazine article on the G11, you're wondering if a 200ns avalanche diode will work in place of a 95ns one. The article explained how the power supply circuit worked and that the original part needed beefing up as it was prone to failure.

What was recommended? A BY127 or its stable mate BY133!! No mention of an avalanche diode to be found.

In they will go and I'll get on with finding out why there is a convergence error bottom left hand side which just spoils an otherwise perfect one. If I can't find the answer........

Thanks again

Chris

[Welsh Anorak]

Hand up here too! I was in a customer's house to fix a dry joint on their G11, switched the TV off and touched my iron on the frame - BANG! The mains switch was shorted on the live but still worked via the neutral.... Didn't do too much for my credibility, but four BY133s and two fuses later - and a mains switch - and all was well.
Odd how only two fail, but I always used to replace all four, except when there were only three in the field spares box...

[its ur aerial]

Once did a silly thing with a G11 chassis, I held the coax plug from the Communal aerial system which was obviously at earth potential, with one hand and then grabbed the chassis with the other, result, half mains potential across my Heart, just couldn't let go, nearly resulted in me having half a day out with the Co Op.

[ntscuser]

Until very recently I mistakenly assumed all transistor-only chassis were fully mains-isolated, it's a wonder I wasn't killed. It's one of those things that people working in the trade took for granted but hobbyists like myself didn't have a clue about.

[Mooly]

I remember setting up our workshop at Norweb (remember them) and of course we had isolation transformers for every bench and the working environment had to be earth free, but not only that, we also had ground lifts on all test equipment which meant you could couple the scope to the SMPS primary and still touch the chassis with the soldering iron.

Being Norweb (as you might imagine) meant that someone periodically came round and checked all electrical appliances, sockets and light fittings attaching labels to these as they were tested. Can you imagine the reaction on finding no earth continuity on all outlets in the work area, and all grounds being lifted on the equipment.

They just couldn't grasp it... so a demonstration was held showing what would happen.

We never saw them again, maybe it was something we said.

As to live chassis in the customers house, well I think all techs at some point have fallen foul of this. If its not the loose coax plug falling out and brushing chassis its the damp carpet and kneeling down on it (we've all done it).

Also never trust a mains switch to be OFF. Someone might have been there before and shorted the neutral side out. Why is it always the neutral that goes OC.

[G4_Pete]

I was keeping quiet as I thought I was the only one that at some point had fallen foul of directly powered mains TV,s!

Was not thinking and put the scope earth on the remote control IR detector shield on a Philips colour set , the set had a 13 amp fuse which due to the circuit layout blew the 13 amp fuse by pulling the current through the 5 Volt logic. Very spectacular even blew out the LED channel indicator segments.

Life lesson learnt never to be forgotten. Sill thing was I turned the set of at the mains first for "safety". But that ment at turn on the connection was solid until the fuse blew.

Still did not have to worry about the remote not working any more. Wife not impressed.

Pete

[G8vsjDave]

The Rank Z718 chassis also sits at mains potential, and during the development, we had several instances of accidental earthing of the chassis resulting in the 2.5 amp AT fuse exploding with one hell of a bang and showing glass fragments all over the place. After a few dead-end attempts to contain the exploding glass, a 5 amp quick blow fuse was added in series with the 2.5 Amp AT fuse. In the event of a short to ground, the 5 amp fuse went O/C almost instantly and without fuss.
Dave