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 Post subject: Got caught out trusting an electrolytic!
PostPosted: Feb Tue 06, 2018 4:44 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
Last night I was tasked with getting a late 60s B&W TV (HMV PU series chassis) running for a TV production that is re-creating family life in the 70's. They will be shooting pictures off its screen...
Hard to find, late 60's B&W TVs, all in landfill years ago!
Anyway, no picture, no HV. Flyback had been replaced back in the 70s with a non-genuine part. Strange, these flybacks never fail in my experience....
-60 volts drive on G1 of 6CM5, 135v on G2, 0 volts on B+Boost, either side of the cap.
A half-watt 27 ohm resistor in series with the 6AL3 damper plate had burned up (because of the previous bad flyback?)
Replaced it, now have HV and a bright horizontal line on the screen.
I shorted the vert osc plate to ground, line jumps up and down on the screen. So OP stage is OK.
Vert osc not running, plate at 23 volts.
Checked blocking osc transformer (yes it has one!!) measured OK.
Measured height pot slider, 23 volts. Hang on, that's the same as the output stage cathode voltage!
There is a 2uF electro from height pot slider to cathode. No!! electros don't go S/C!
Measured it, ZERO OHMS!
Replaced it, adjusted picture geometry etc. Nice bright sharp picture from original Thomas CRT. A bit of cramping at the top of the raster. Vertical hold pot up one end.
Replaced the original 1967 vintage 6GV8. Perfect linearity, hold pot in middle of range.
Helped Producer to load it into his van, alongside the period-correct and working Victa lawnmower!

Now I've got to get my 70s color set going in time for the shoot on Friday.

There's one good thing about "restoring" a late 60's TV - you generally don't have to! No paper caps and by then they had learned how to make good electrolytics (well usually!!)


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 Post subject: Re: Got caught out trusting an electrolytic!
PostPosted: Feb Tue 06, 2018 5:52 am 
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Joined: Dec Fri 16, 2016 2:58 pm
Posts: 63
G mate sounds like that HMV is a drama! Glad i did not give you that one!
So the 660 has to be done by friday, thats going to be a rush ,have you got the parts?
All the best Pete

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Vintage Tv's ! When Television was worth watching !


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 Post subject: Re: Got caught out trusting an electrolytic!
PostPosted: Feb Tue 06, 2018 5:54 am 
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Joined: Mar Sun 01, 2009 10:27 pm
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Location: Ann Arbor, MI
irob2345 wrote:
There's one good thing about "restoring" a late 60's TV - you generally don't have to! No paper caps and by then they had learned how to make good electrolytics (well usually!!)

And then by the late 90s they forgot again! Well, there were those counterfeit Chinese caps that looked just like the ones made by legitimate manufactures.

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Tom


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 Post subject: Re: Got caught out trusting an electrolytic!
PostPosted: Feb Tue 06, 2018 6:06 am 
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Joined: Dec Fri 16, 2016 2:58 pm
Posts: 63
Thats 4 sure Tom,
I have some crts from the 90s and i just put a new board in!
Much easier, particularly with my limited knowledge.
Pete

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Vintage Tv's ! When Television was worth watching !


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 Post subject: Re: Got caught out trusting an electrolytic!
PostPosted: Feb Tue 06, 2018 11:35 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 250
The Great Capacitor Plague of the 1990s is an interesting story.
Major Japanese manufacturer leaked a document that had a deliberate error in the formula and Chinese manufacturers picked it up.


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