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 Post subject: Crosley model 179
PostPosted: Mar Thu 08, 2018 3:38 pm 
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Joined: Jul Sun 26, 2015 11:34 pm
Posts: 615
Location: Hurdle Mills, North Carolina
This is a new problem for me, working on the chassis and after recapping and checking resistors and transformers I powered the radio up. I indjected a IF signal and align the IF transformers. Next I applied a signal at the antenna post and look at the plate of the rf tube. I don't see the modulated rf signal, it is on the grid. I checked the DC voltages and all are reasonable except the screen grid of the three 58 tubes, they read about 70 volts. I set the scope to AC coupled and looked at plate voltage and find an high frequency oscillation riding on the amplitude of the DC plate voltage, never encountered this in thousands of radios I have worked on. It is by accident that I found the oscillation, my meter has a limit of 500 vdc and the plate of the IF tube was reading greater than 500 volts, because of the added voltage of the oscillation. I have gone over the circuit to check for any value or wiring errors and find none. Suggestions.

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Larry in NC


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 Post subject: Re: Crosley model 179
PostPosted: Mar Thu 08, 2018 4:49 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 7963
Location: Cleona, PA
Are all tube shields in place? Tubes can oscillate without them. Oscillation could be from improper lead dress. I believe this set has an underchassis shield also, which should be in place. Also try this: put a .1 mfd. cap across cap #28. This will act as a bypass to ground of any spurious oscillation, and is often seen on screen grids in schematics.

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Reece


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 Post subject: Re: Crosley model 179
PostPosted: Mar Sat 10, 2018 12:02 am 
Member

Joined: Jul Sun 26, 2015 11:34 pm
Posts: 615
Location: Hurdle Mills, North Carolina
Thanks for the tip, I added a .1uf to the screen grid of the output tube, that was the one oscillating. I now can receive the local station, still not terribly happy with the performance, but it does work.

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Larry in NC


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