1956 Zenith Cobra-Matic Stroboscope Changer Transition

[JoshDenty]

That would require a non-cobra cartridge though, wouldn't it? I don't think they made stereo cobra cartridges, so I'd have to change all the mounting hardware and whatnot.

I'm trying to preserve this in a factory new state, so if that precludes stereo, it just won't be a stereo machine. I'm okay with that.

[William]

They actually did make a stereo version using the Cobra tone arm but the Cobra tone arm was installed on a VM changer. I have that version, just the changer, no cabinet or electronics that was all to far gone for a restoration.

Bill

[TC Chris]

Just so everybody is on the same page, let's be clear that the Cobra tone arm is one with the snake-like head. At one time, they carried Zenith's Radionic cartridge. It used a moving vane to modulate an RF frequency and demodulated it for audio. Zenith called their modem stage a "preamplifier" but it was really what we'd call a modem (MOdulator/DEModulator). I've only seen mono versions. I have one in my 1948 Wurlitzer jukebox. At some point they put a ceramic stereo cartridge in the Cobra arm, for a while.

Chris Campbell

[William]

The ceramic cartridge, whether mono or stereo, showed up when Zenith started using VM changers with a custom Cobra tone arm. And yes, it had the snake like head.

Bill

[JoshDenty]

The ceramic cartridge, whether mono or stereo, showed up when Zenith started using VM changers with a custom Cobra tone arm. And yes, it had the snake like head.

Bill

I think there was some overlap, actually. If I'm understanding my history correctly, the VM changers are a fairly obvious departure from the Zenith-made true cobra changers visibly. Both have the tonearm with the cobra styling, yes, but Zenith changers have the two analog slide controls while the VM changers have two dials. Unless VM was making a complete copy of the original cobra mechanism for a bit, mine is definitely a Zenith-made unit designed to take a ceramic cartridge. It's a 1953 build date, and the amplifier is in the original configuration which lacks the additional oscillator tube for a radionic cartridge.

I'm guessing from what I've seen, but I do think it's correct to say that the Zenith-made changers never handled stereo. The timeline seems to be that the original units used the radionic cartridge (which I've read don't really go "bad" in the traditional sense, but the needles can wear out and you can't replace just the needle on any of these) and for a brief period in the early 1950s Zenith updated them to use a ceramic cartridge that supported hi-fi, which in rare occasions supported two needles so you didn't have to swap cartridges to play 78s or LPs. I don't know the exact date, but it seems like they moved to VM changers some time after that and never looked back.

[William]

My apologies, Josh. It was not my intention to override your topic about your Zenith Cobramatic changer. You know way more about the Zenith Cobramatic than I do and I just learned a bunch from your post. I was just trying to add that Zenith did use a ceramic stereo cartridge in a Cobra tone arm mounted to a VM changer. And you are so right, it was VM from that point forward until they tried their hand at making changers again when they add the 45 adaptor to the platter.

Bill

[JoshDenty]

Don't apologize, Bill! I'm sorry if I came across as feeling spoken over. I only clarified that point because I've seen that hard cutoff like you mentioned come up quite a bit, and it's been a point of confusion for me. I'm throwing my own guess into the air based on what I've seen on my unit, but I could also be completely wrong so please feel free to correct me.

By the way - I'm still working with the tone arm. Years ago I did some work on an RCA 4 speed portable changer of similar vintage and remember that unit having plugs that allowed the mechanism to easily disconnect from the amplifier. I had thought this might be the same case, but it looks like I may have to actually remove the entire amplifier to get at the tone arm I want to remove from my parts unit. This is much less service friendly than I had expected.

On the bright side, at least my replacement cartridge should be here in a few days.

I don't want to turn this thread into a repair topic for my own unit, since I'm already well overstaying my welcome on someone else's discussion, but I'll mention a few notes on the machine I'm repairing. I purchased it back in 2018 or 2019 if memory serves, and it was fully restored at that time. However, it's since developed two issues that I'm trying to resolve - the speed control is flakey at best and tends to stop altogether when I try to play it at 16rpm, and the cartridge has started sounding like you're listening to the music on the other side of a thin wall. I had taken this over to a local repair shop and they fiddled with it for a year before giving up. I'm thinking I'll probably end up finding a trusted expert and have them do a total rebuild, but I want to at least get the cartridge hardware swapped out. That might fix one of my two issues, since the amplifier had already been recapped.

[William]

All good Josh, and may I suggest you start your own thread on your unit so we can continue to follow your repairs. With it being in your name it will be easy for us to find.

Bill

[JoshDenty]

Wholeheartedly agreed. This has ballooned.

[JoshDenty]

One last post just to close the loop on this. I'm putting it here since I had mentioned adding it in the past and don't want anyone who finds this thread in the future to come away empty handed.

I'm attaching a few photos of the tonearm mechanism. I'm glad I didn't have to recreate it. From what I can tell, the switch and cartridge socket are one piece of plastic. The socket has two metal strips inside that make contact with the cartridge and are connected to the tonearm wires, and there is some sort of metal prong attached to the cartridge on either side. Those metal prongs press against a curved metal arm that's screwed onto the bottom of the tonearm and allow it to slide back and forth for the different positions. It's a unique design. I'm not sure I've seen any other turntable use it.

Anyway, I'm going to eventually get this tone arm cleaned up and swap it onto my player. If anyone needs references in the future, I should still have mine available.

[William]

Thanks, Josh, and just for a refresher, what does the little knob/switch do on top of the snake head?

Bill

[TC Chris]

That sure does look like a Radionic cartridge. I assume it tilts back & forth to accommodate stylii for 8 microgroove?

Chris Campbell

[JoshDenty]

Bill - it's a switch between the two different needle types. Most cobra arms only accepted one needle at a time, but this variant is unusual in that it has a provision for a dual needle cartridge. The rarity is why I'm returning mine to its original setup.

Chris - I'm almost certain they intentionally designed these cartridges to look the same. It probably saved on development costs for new mounting hardware. It's definitely ceramic, though. The amplifier is missing the oscillator tube for the radionic setup. You're correct about the tilting mechanism.

[William]

Thanks, Josh, and please keep us posted as you progress with your project.

Bill