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 Post subject: Comercial trades institute VT 20
PostPosted: Jan Thu 19, 2023 12:36 am 
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Joined: May Wed 11, 2016 1:18 am
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Location: springvalley ohio
Comercial trades institute VT 20
Anyone with a manual schematic for this?
Or know what other kit company had a unit like it?
Maybe recogncize the transformer?
Thanks
Doug


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 Post subject: Re: Comercial trades institute VT 20
PostPosted: Jan Fri 20, 2023 6:24 am 
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Joined: Jan Thu 01, 1970 12:00 am
Posts: 5863
Location: Hill City, SD 57745
While I can't help you, you have helped me by showing the only other example I've seen of something from Commercial Trades Institute. I've had these two pieces of gear kicking around for years and have never seen any mention of the company in print or online.
-Ed


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 Post subject: Re: Comercial trades institute VT 20
PostPosted: Jan Tue 24, 2023 1:50 am 
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Joined: May Wed 06, 2009 6:01 pm
Posts: 941
Location: Athens OH USA
I can't help with the schematic, but I can add to Ed's Commercial Trades Institute info. I have a copy of that TV three-position oscillator (but no power supply for it).

I also have had a Commercial Trades Institute TC-20 tube tester for some years, bought cheap because of the lack of a booklet with tube settings. I determined it was equivalent in its voltages to the Eico 625 tube tester assuming its 30 and 250 VAC windings were spot on when the line voltage preset was correct at the center of the meter. I printed out the Eico data for testing tubes on it.

I would bet that your VTVM is an Eico or Heathkit circuit clone, substituting a 6H6 for the more typical 6AL5 and an octal duo triode (6SN7 or 6SL7 or?) for the typical 12AU7. Does it use a selenium rectifier?

-- Rich


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 Post subject: Re: Comercial trades institute VT 20
PostPosted: Jan Tue 24, 2023 8:01 am 
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Joined: Jan Thu 01, 1970 12:00 am
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Location: Hill City, SD 57745
Velly Interesting! The power supply is provided by the companion Channel Selector section seen in my post. These, perhaps along with other modules unknown were likely lessons in the course which together produced a working receiver. Doubtful they were intended as a transmitter. No idea if that would be television, commercial radio or what? The frequencies would be the clue as to the intended service.

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 Post subject: Re: Comercial trades institute VT 20
PostPosted: Jan Wed 25, 2023 2:43 am 
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Joined: May Wed 06, 2009 6:01 pm
Posts: 941
Location: Athens OH USA
Ed, The pieces were used for TV alignment. The specific frequencies (44 and 42.4 MHz) were for RETMA standard television IF. The 4.5 MHz was for the intercarrier FM sound.

Maybe someday we'll find the lessons that go along with the TV servicing course. Fun stuff!

-- Rich


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 Post subject: Re: Comercial trades institute VT 20
PostPosted: Jan Wed 25, 2023 7:31 am 
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Joined: Jan Thu 01, 1970 12:00 am
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Location: Hill City, SD 57745
Thanks Rich!

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