Zenith Zenette 4G800

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Zenith Zenette 4G800

Post by Valentino Bergamo Filho on Sat Aug 12, 2017 3:56 pm

Connecting a scope to the oscillator grid I find there is no oscillation. Looking at the schematics diagram there is one side of L2 (point "E") connected to a 2K2 resistor and the other side is loose. Is there a problem with the schematics? Somebody had the same problem? Valentino (from Brazil)

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Re: Zenith Zenette 4G800

Post by jukeboxman on Sun Aug 13, 2017 2:00 am

Hello welcome to TRF --i don't have an answer for you, but there a lot of smart people here
i am sure you will have help soon

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Re: Zenith Zenette 4G800

Post by Chas on Sun Aug 13, 2017 11:03 am

With a dry battery tube portable there is not a lot of drive in the local oscillator. The 'scope probe and the input impedance may be killing the oscillator.

Take a pocket portable transistor radio and turn it on, tune it to the middle of the BC bans or 1mhz. Place it next to the Zenith and turn on the Zenith, be sure batteries are up to required voltage. Slowly sweep the Zenith tuning through the BC band, around 1455 or so and 545, there should be a disturbance to the pocket portable from the local oscillator. Try setting the pocket portable to 950 or 1050 to be sure the mixed frequency captures the signal radiating from the Zenith.

If no joy, try replacing the oscillator tube, this tube is also the mixer in the Zenith if the emission is down the oscillator may not turn on or the mixing may not happen...

If still no joy, measure the resistances of the oscillator coil with a VOM, to be sure no winding is open. Clean the variable capacitor rotor wiper blades. It there are signs of damage to the oscillator coil it will either have to be rewound or replaced. There may be a mica coupling or padding capacitor, it is remotely possible that it is defective. Try to lift one lead carefully and measure for capacitance and leakage. Replace with one of same characteristic if proven defective.

Sometimes excessive moisture absorbed through the wax on the coil can kill the "Q", use a heat gun and gently warm the oscillator coil until the wax becomes molten. Avoid letting the wax run off. The heat will drive out any moisture, the softened wax will let it escape, and as the wax cools the moisture will stay out and the coil will reseal itself.

Sanity check, tune zenith to known radio station, inject a CW unmodulated signal by inductive coupling near the oscillator coil of a frequency of the radio station plus the IF frequency. Try also IF minus the desired station. If the station is received then the oscillator is dead.

GL

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Re: Zenith Zenette 4G800

Post by mr_ed01 on Sun Aug 13, 2017 1:10 pm

Schematic is correct. Common type of oscillator coupling to the mixer tube.

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Re: Zenith Zenette 4G800

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