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 Post subject: Diagnosing Horizontal Oscillator Not Running
PostPosted: May Tue 07, 2019 12:14 am 
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Working on a Motorola 12T1 chassis TS-23B. The set has been recapped. I cant seem to figure out why the horizontal oscillator is not running. I dont hear the usual high pitched whine, and there is no high voltage.

The only place I can find a schematic: https://www.radiomuseum.org/r/motorola_ ... guage_id=2

Looking at the horizontal osc tube, all the voltages are correct except there is 70v on pin 3, where it calls for 7.5v, and -.4v on pin 1 where it calls for -6v.

I cant figure out why there is 70v when there should be 7v. Would that cause my problem?


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 Post subject: Re: Diagnosing Horizontal Oscillator Not Running
PostPosted: May Tue 07, 2019 1:04 am 
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I am looking at this schematic http://www.earlytelevision.org/pdf/motorola_ts23_sams_92-4.pdf. I see that there is a 470 ohm resistor connected from pin 3 to pin 6. And then a 1K resistor from pin 6 to ground. I suspect that the 470 ohm resistor is open. Either that or the 1K resistor is open.

Note that this schematic shows a connection from pin 6 and the 6800K resistor feeding pin 5, but someone has marked the schematic to show that there is no such connection. But the correction is very faint and you could miss it if you don't look closely.

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 Post subject: Re: Diagnosing Horizontal Oscillator Not Running
PostPosted: May Tue 07, 2019 2:40 am 
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Wow, twice in one week I've had occasion to use my Wallace's Telaids book for Motorola TVs (1946-53): it does not show any resistor added directly between pins 5 and 6 of the Hor. oscillator.

But I think that Tom may well be right that one of the cathode resistors is bad, most likely the 470 ohm, since an open 1K would make both cathodes more or less the same voltage. Note that the setting of the horizontal hold control will affect the voltage on the grid, pin 1. You might want to check the value of that pot and the series 100K resistor, too.

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 Post subject: Re: Diagnosing Horizontal Oscillator Not Running
PostPosted: May Tue 07, 2019 2:52 am 
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aj2x wrote:
it does not show any resistor added directly between pins 5 and 6 of the Hor. oscillator.

That was an error in the Sams. Two lines crossed on the schematic and they put a . there when they should shown no connection with a ) .

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 Post subject: Re: Diagnosing Horizontal Oscillator Not Running
PostPosted: May Tue 07, 2019 1:49 pm 
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Don't overlook a poor lug connection to the tube pin. Craig


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 Post subject: Re: Diagnosing Horizontal Oscillator Not Running
PostPosted: May Tue 07, 2019 2:05 pm 
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Location: Summitville IN, 46070
Ok, did some testing on it last night, went to measure the 470 ohm resistor and found that depending on how I bent the leads it would change value. Must have an intermittent connection, when bent a certain way it would go open. Replaced it and the tv is working very well.
First time I've seen a resistor do that.

Thanks!


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 Post subject: Re: Diagnosing Horizontal Oscillator Not Running
PostPosted: May Sat 25, 2019 6:13 pm 
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That is not uncommon on the small 1950s IRC resistors, the ones with the rough bodies; I had a similar problem in a 1957 Chrysler radio last year. The leads are not anchored well, and can loosen up.

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 Post subject: Re: Diagnosing Horizontal Oscillator Not Running
PostPosted: May Wed 29, 2019 5:37 am 
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Location: Belrose, NSW, Australia
I concur. IRC carbon composition resistors (the ones made at Kingsgrove in Australia in the 50s and 60s anyway) were the least reliable of all the brands and stressing the leads would make them go O/C, high resistance or noisy. Their metal film resistors were much better.

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