Fitting the cabinet - the kiss of death..

[stevehertz]

How many times have I restored or repaired a set, be it a vintage radio, TV, video or hifi unit, got it working properly, left it for a few hours to be sure, fitted the cabinet and then... it doesn't work! I'll tell you, lots!!

[Sideband]

I'm currently investigating why my recently restored Philips 170A has intermittent low output when fitted in its cabinet...it's worked perfectly for several years while a better cabinet was sought. Actually last night I think I discovered what might be the problem....the output transformer connections are very close to the speaker when in the cabinet and I think they touch the speaker chassis....just, and the slightest movement of the chassis is enough to make them short. Temporarily I've fitted a strip of insulation tape where the speaker chassis and the transformer connections meet.

[John10b]

Yes Steve that’s happened to me, many years ago. I remember, when serving my apprenticeship the engineer always put a blanket over the chassis for several hours before fitting it back into the cabinet.
Cheers
John

[buggies]

Ah - psychology - let it think it's safe inside a dark box when really you can get to it as soon as it plays up...

[Skywave]

Ah yes: I've had that phenomenon several times over the years. My theory is that when the chassis goes in the cabinet, the ensuing darkness encourages the electro-gremlins to emerge and do their bit. Of course, they're an intelligent lot, since when you remove the chassis from the cabinet to investigate, they usually very quickly reverse the changes they made.

Al.

[Refugee]

The best one was what my father told me about a packaging machine in the 1960s when they were very new.
The machine was a prototype and engineers had been working hard on the new magic eye safety system that shut the heat sealing bar off if you put your hand in.
Everything was working just fine and the sales rep had been shown everything working perfectly.
Come show time the covers were fitted and the customer was invited in.
When the rep showed off the new safety device the thing tried to seal his hand into a newfangled plastic bag
It was those lacquered germanium transistors. They were the early ones with clear paste in them and the boards had been handled enough to let the light in where the lacquer had rubbed off. Ouch!

[Stylo N M]

Yes I've had this quite a lot over the years, just when you think it's all done, it's as if you're starting all over again.

Paul.

[QQVO6/40]

Hello all.

One that caught me many years ago.
Voltage regulator valve that would not strike unless there was some photons of light around. It was the VR valve regulating the HT supply for the VFO in a communications receiver. When the radio was put back in the case the VFO would wander all over the place. Hard enough trying to listen to an SSB signal with an aged Comms. receiver as it is.
This radio lived at the side of my bench for a couple of years playing merrily till one day I happened to cover the power supply area with a large open book. HMMM. Thinks Book not metal but radio still played up when switched on. For so long I had been chasing an RF problem that wasn't.
Other cases where Murphy has played a part but for now I will give up on my typing practice.
Cheers all.
Robert.

[Ted Kendall]

There's a story about Joel Tall, who invented the Editall splicing block. He was called out to a transmitter whose intermittent instability had foxed others trying to fix it. He realised that the thing worked when the (metal) cabinet doors were open, but as soon as he closed them, off it went!

[lesmw0sec]

There is a similar law regarding PCB design. You can check the artwork a dozen times, but the glaringly obvious error is never apparent until viewed in hard copper!

['LIVEWIRE?']

Been there, done that, and have several Tee Shirts!! A couple of years ago I was trying to repair a Yamaha CD/Hard Drive recorder, had checked various parts and, as I thought, found & rectified the fault so I duly connected the deck up to my TV and ran it for several hours continuously. Everything was working as it should, so I put the cover back on the Yamaha and tried again. Nothing worked! IIRC I never did find the cause of that fault!!

[G8HQP Dave]

The gas fire in our lounge worked fine after installation, provided that the cast iron trim was not installed. The fitter spent ages on the phone to the maker's technical support people. It turned out that a batch of trims had been made with a piece of metal where it shouldn't be, which prevent the control lever from travelling as far as it should so after ignition the safety device cut off the gas. He had to take an angle grinder to it (in the garden!) and then it worked fine. It seems that this should have happened at the factory but our one somehow slipped through. Hopefully they have now changed the mould so new ones will not need this factory mod.

The VR valve story above (post 8) neatly explains why military versions were often radioactive. Otherwise you just need enough voltage that a passing cosmic ray will get the thing started in the dark.

[Skywave]

Originally Posted by lesmw0sec

There is a similar law regarding PCB design. You can check the artwork a dozen times, but the glaringly obvious error is never apparent until viewed in hard copper!

Same with a document that you've created and is on the screen of your PC. You've proof-read it so many times, it's now a permanent memory. And then you print it - read that printout - and discover the spelling mistakes, etc.
I've never understood just why that happens.

Al.

[Refugee]

Originally Posted by Skywave

Same with a document that you've created and is on the screen of your PC. You've proof-read it so many times, it's now a permanent memory. And then you print it - read that printout - and discover the spelling mistakes, etc.
I've never understood just why that happens.

Al.

It is covered under "Sod's law" surely.
I once spotted a menu that offered a fish dish served with torture sauce
Apply the pain before the bill and it will be easier to get the money from the customer

[mark_in_manc]

I ordered a new service dvd-rom from the proprietors of this great site last year, as I had lost the one I originally bought. When I went to stow it on the shelf where such things live, I slid it in - right next to the 'lost' one, whose presence I had sought in just that spot many times, before giving up and buying another. Oh well

[Refugee]

That is a good way of locating a lost item.
I did that with a jigsaw a few years ago.
After a search I bought one from eBay complete with a fully primed tape bomb on the power cable and restored it adding a long enough power cable to reach outside thus making it better than the lost one.
After mucking out the rubbish I spotted the lost tool where a now disposed of rubbish bag had been.
It had become hidden during clearing up after its last job before becoming lost.

[Biggles]

The glass photo-sensitive germanium transistor story reminded me of a similar tale where the service technician couldn't work out where the mains hum was coming from on a radio he was repairing. Some paint had come off an OC71 transistor and as he was working under fluorescent tube lighting, the now photo-sensitive transistor was being modulated by the flicker of the light. It would have worked just fine inside the dark case if he had re-assembled it. I remember converting OC71 type transistors into OCP71s by scraping the black paint off when I was a kid. For some reason the OCP photo-transistors were much more expensive. In reality, there may well have been a technical difference but it seemed to do the job in the days when your pocket money didn't go far.
Alan.

[Junk Box Nick]

Originally Posted by Skywave

Same with a document that you've created and is on the screen of your PC. You've proof-read it so many times, it's now a permanent memory. And then you print it - read that printout - and discover the spelling mistakes, etc.
I've never understood just why that happens.

This phenomenon was well known to that now extinct species, once of the printing industry, the typesetter. So much so that it was professional practice that proof reading was always done using a hard copy print.

[avocollector]

Then of course there is the corollary of when the cabinet is finally fitted AND it works, it is always the one unrepairable bit that then breaks. Screws that held happily for years will strip, pot metal will fragment and plastic will shatter - don't even ask what glass dials will do. At which point, if sufficiently tired, it is very hard NOT to do the adult thing of screaming mighty oaths while jumping up and down on the item until it's reduced to component parts - LOL.

[SiriusHardware]

Or, while doing that largely unnecessary 'one final tweak' on the line-up of a fully working transmitter or receiver, you hear the sickening 'crack' of a hard to get ferrite core breaking...