Circa '58 Zenith cobra-matic record player

[radiotvnut]

Here's a circa '58 Zenith Cobra-Matic hi-fi record player/radio that I picked up at the flea market. This one uses two 35C5 tubes in PP, driving an 8" speaker and two electrostatic tweeters, with seperate bass and treble controls. I think it uses an Astatic 93T plug-in cartridge. The case is in decent shape; but, when I got it, there was a big hunk of wood missing from one corner. Fortunately, the borken piece was inside the unit and I was able to glue it back in place. It still works somewhat; but, the stylus is shot and the chassis needs going over.

[DavGoodlin]

That sure is a nice portable. Two 35C5 in PP should rate about 5 watts.
Is the extra 12AV6 used as the phono amp, an extra amplifaction stage for the magnetic pick-up?

[radiotvnut]

One 12AV6 is the audio driver stage and the other is a phase inverter for the PP output stage.

I found out why the radio was cutting in and out - a cracked 220 ohm carbon resistor in the pi filter network in the power supply. There was no hum; but, the filter capacitors tested a little on the leaky side on my Eico capacitor bridge. I'm also replacing the selenium rectifier.

[Celt]

You can probably replace the "rotten egg bomb" with a 1N4007 diode.

[Jon A.]

Originally Posted by Celt

You can probably replace the "rotten egg bomb" with a 1N4007 diode.

I was thinking the same sort of thing, wondering if he was going to swap out the selenium with a modern replacement.

From the pics, I can see where this unit's name comes from. I see that the manufacturers even put little dots on the tone arm. Sweet.

[radiotvnut]

Yes, I will replace the selenium rectifier with a 1N4007 and a dropping resistor. The selenium still works; but, it's output is a little lower than what is specified on the schematic. We know it will only get worse; so, might as well change it.