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Old 19th Jun 2020, 8:50 pm   #1
stand-by
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Default Tv Autovox TM 74 - ("Resurrecting a dinosaur")

https://www.radiomuseum.org/r/autovo...24tm_74_2.html

Due to circumstances that are now irrelevant, came into my hands this black-and-white television set AUTOVOX, model TM 74, 24 ”, that was made in Italy and marketed in Spain at the dawn of television in this country, at the end of the fifties in the past century.
It was a good-quality device of the time, equipped with 26 valves, two speakers, a VHF channel selector, a remote control cable connection (for volume, brightness and contrast); it weights 63 kilos and measures (71x66x62 cm.) that require a minimum of two persons for handling.

https://subefotos.com/ver/?d789c1f78...5ec74aa0fo.jpg
https://fotos.subefotos.com/7071a398...d181f07bdo.jpg
https://fotos.subefotos.com/af11094f...6cac429d7o.jpg
https://fotos.subefotos.com/3bfcf98b...fc78b933bo.jpg
https://fotos.subefotos.com/592d0134...b369c3f6co.jpg


As you can see in the photographs, it has a rather unfortunate appearance.
The grime and deterioration of the furniture, the rust of the chassis, the lack of the power transformer and some valves and in the lower part of the chassis the absence of various components, with the appearance of having been removed in scrapping plan.

This situation necessarily required having the corresponding scheme to be able to start any work aimed at its restoration. And it was not easy.
After a lot of research in different forums, on the radiomuseum and on a multitude of online schematic websites without results, I was able to casually find a Trieste seller on the Italian Ebay website who had the original, but for the price of 22 euros posted at home. ... Well, if you want it, pay it.....

https://fotos.subefotos.com/58861d58...5ba1cee2ao.jpg


Having achieved the scheme and carried out a thorough cleaning of the "dead" I began by trying to incorporate the power transformer.
After finding out all the consumptions and knowing the different voltages necessary for the supply, I took a core out of the scrap metal that fit perfectly into the hole and amply covered the required power needs, so once the calculations corresponding to the number of turns and thickness of the threads of the different windings I set out to build it with patience in my manual winder.


Fortunately, the overall condition of the valves was maintained at an acceptable emission level, so no replacement was necessary. Only two missing 5U4 rectifiers had to be added and their slightly charred sockets replaced.
All the electrolytic capacitors were changed following the camouflage method in the casings of the existing ones, and those made of paper that showed deterioration were also replaced, meeting the general standard for this type of restoration.

Some tampering was noted at the bottom of the chassis. With the scheme in front, a thorough reconnaissance of all the circuitry was made, changing several resistors that had altered their value, and the lack of the video detector diode, as well as some small value capacitors and several chokes of the video amplifier, which were incorporated again.

https://fotos.subefotos.com/bc0d15f0...0a802828ao.jpg
https://fotos.subefotos.com/3407cad6...3707ea0cbo.jpg


The coating of the image tube presented a significant chipping that just touched it fell into pieces, so it was necessary to buy a bottle of spray graphite that once applied left it looking like new.

https://fotos.subefotos.com/18810f27...2ade2163do.jpg


When I thought that everything was more or less "in order" I connected it to the network observing the normal ignition of the valves and checking that all the supply voltages were correct, until the surprise appeared: -There was no Very High Tension-
Made the pertinent checks, the cause was found in the line output transformer, with the defective High Voltage secondary coil, probably due to some short circuit between its turns.
Obtaining one of the same or equivalent was practically impossible, given its great age, and accommodating another model would be risky, especially due to the difficulty of fitting in with the characteristics of this transformer.

So I agreed to replace only the secondary coil, since it is mounted independently from the rest of the windings, and for this I used as a "donor" a Miniwat transformer, which was still in the old stock, performing the "transplant" with much careful but without major problem.

But since Murphy's Law is almost always present, especially in these cases, it was verified once connected to the network that, although there was already a VHT voltage, it barely reached 10 KV instead of 19 KV (+/- 1) necessary, undoubtedly due to the mismatch of the new secondary in terms of the ratio of turns, making it impossible to achieve correct image illumination.
After to turn over differents ideas looking for an exit, I decided, although it was not much to my liking, to make a small “frankenstein” replacing the VHT 1B3 rectifier by a voltage doubler, consisting of two VHT diodes and a pair of capacitors type “door knob ”of 500 pf. 20 Kv. that I tried to accommodate conveniently hidden inside the VHT cage, with which the Very High Voltage was recovered and finally resolved the problem..


https://fotos.subefotos.com/bd418002...0d1e0ec6co.jpg
https://subefotos.com/ver/?3ef3f8567...2a1cc0415o.jpg


After cleaning, the furniture received severe and repeated petrol hands with diesel oil until all traces of the moth were completely eliminated. The wood was replaced in the bottom part of the front area. A silver paint was sprayed onto the cover around the screen. The brass profiles that go around the contours of the cabinet were polished and re-embedded in their lace and the whole piece of furniture, after the appropriate carpentry finishing touches, was painted and varnished until it regained its best appearance. The rear cover, in which you can see the neck protection of the CRT was made of double cardboard that, after varnishing and painting, was sufficiently rigid.

https://fotos.subefotos.com/1d94c74e...31869e799o.jpg
https://fotos.subefotos.com/212f9fc7...ccaf6bd97o.jpg
https://fotos.subefotos.com/b94a094e...86478de4do.jpg

Once the chassis and other accessories were installed inside the cabinet and the "machine" function was restored , the synchronism, width, height and horizontal and vertical linearity were adjusted, as well as the GAC levels, Brightness and Contrast.

https://fotos.subefotos.com/f91df67c...db1feac52o.jpg
https://fotos.subefotos.com/f46ba559...edf522637o.jpg
https://fotos.subefotos.com/22652898...00d6e192eo.jpg


And this is the final result using an external DTT tuner coupled to a converter with output on channel 8 VHF and applied to the antenna input of the AUTOVOX TM74 television described here, which has become part of my small personal museum.

https://fotos.subefotos.com/85e9f9cd...e90bd68f7o.jpg
https://fotos.subefotos.com/ff2193f7...1079dca6bo.jpg

Best regards
César -EA1AUI
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Old 19th Jun 2020, 10:34 pm   #2
Nickthedentist
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur"")

That's absolutely amazing work! Well done indeed!

Nick
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Old 20th Jun 2020, 8:48 am   #3
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur"")

Thank you Nick
Best regards
César
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Old 20th Jun 2020, 1:11 pm   #4
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur"")

An astonishing transformation. All credit to you for having the patience and perseverance !


Andy
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Old 28th Jun 2020, 8:10 pm   #5
vinrads
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur"")

Wow! lesser mortals would have taken a look and thought scrap, well done indeed , Mick.
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Old 29th Jun 2020, 9:30 am   #6
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur"")

What a great result from a complete wreck! Well done!
The CRT looks interesting. Is it a 21" AW53-80? John.
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Old 29th Jun 2020, 8:41 pm   #7
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur")

First say to Andy and Mick many thanks for your kind words.

John, thanks also for your nice coments and interest. The CRT is no a 21". It is a 24" type 24DP4A, working in a satisfactory way.

Thanks again and best regards.César
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Old 30th Jun 2020, 11:04 am   #8
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur")

A very interesting read about an unusual set. Detailed but a very clear and well illustrated description especially when getting around problems like only half the required EHT. Thanks for posting

Dave W
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Old 30th Jun 2020, 10:47 pm   #9
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur")

Great work César! Half the fun for me is tracking stuff down and doing the detective work to complete the project. Once it's done I tend to lose interest until the next project.

But what a great looking unusual (and heavy!) set, thanks for sharing.

Pete
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Old 1st Jul 2020, 8:51 am   #10
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Default Re: Tv Autovox TM 74 - ("Resurrecting a dinosaur")

Hello Dave and Pete. Thank you so much for your appreciated opinions.

Best regards. César
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