Zenith 40A

[ChuckA]

Anyone in the group have any experience with a Zenith 40A radio/phono? I have been trying for a long while to find where a loud hum problem is coming from when it is in the phono mode. I looked up the schmatic in Riders and the phono input is completely different from that shown.

If I shut off the phono motor the major portion of the hum is gone, I have replaced all the shielded wire from the phono to the radio chassis, no difference. I have tried adding ground jumpers from the phono motor, the input transformer with no change. I have tried lifting one end of the shielded cables thinking maybe I have a ground loop, but again no change. If I disconnect the phono connection from the radio completely and look at the audio signal it is clean.

Any suggestions will be welcome.

Thanks

Chuck

[wvsaz]

Hi Chuck,

Are you using a high-output (1 to 2 volt) crystal cartridge?

[ChuckA]

Bill,


No, the 40A is 1929 so it uses the horseshoe style magnetic pickup.

Chuck

[wvsaz]

Does it use a transformer between the cartridge and tube grid?
There are 3 different types of magnetic pickups of that era:

Low impedance - 25 to 30 ohms (requires the transformer)
Standard impedance - 10,000 ohms
High impedance - 40,000 ohms

All have 0.5 to 1 volt output. What output voltage are you getting at the cartdige terminals?

[ChuckA]

Bill,

It's a low impedance coil, and the output of the pickup is good, I rebuilt it and recharged the magnet.

The buzz in the output is there without the pickup circuit connected.

The phono is turned on when the tuning caondenser is tuned past 1500 and flips a switch. Here's a rough drawing of the circuit.

[wvsaz]

Chuck,

Could there be some leakage from the phono motor coil to the chassis?

:mad: This type of problem can be maddening, and is usually of simple cause once discovered. I had a similar problem with a modern stereo magnetic cartridge. Turned out I had accidentally reversed the ground connections between the left and right channels at the cartridge terminals. :rolleyes: Another time I had hum being magnetically injected into the tube of a preamp because it had been mounted too close to the motor! :dunno:

[ChuckA]

What is the most maddening thing about this problem is that the unit played perfectly when I bought it. I hadn't played it in 6-8 months when I did, this buzz/hum is what I got.

Chuck

[ChuckA]

Originally posted by wvsaz
Chuck,

Could there be some leakage from the phono motor coil to the chassis?




Well, Bill it did turn out to be related to the motor. When I 'scoped the audio output with the motor on I had these large spikes in the waveform at a 60cycle rate, turn off the motor and they went away. Ended up installing .05uf cap from the motor AC to chassis ground - nice and quite now.


Chuck