Zenith Cobramatic Stroboscope - Muffled Audio?

[Dubis7]

The audio sounds like it's coming through an AM radio. I bought this thing restored a few years ago, and the amplifier has previously been recapped and was working fine. Here's what I do know:

1: About a year ago, I damaged the needle while dusting it. Literally broke the thing off of the cartridge. I was able to source a new needle and install it, but for awhile I didn't use it because there was an issue with the drive mechanism that I fixed a few weeks ago.

2: Around the same time that I replaced the needle, I discovered that I had accidentally turned the turntable off but left the amplifier running. It may have been on and cooking the tubes for over a month at the time - is it possible that I wore out one or more of the tubes doing that? I just replaced the 12AU6 today but had no change.

3: I pulled a different, restored player out of the closet and tried this same record on it. The audio was much clearer, so this is definitely something up with the player.

Example video here: https://photos.google.com/share/AF1Q...d6SlJhQkJYVEZB

[zeno]

If you changed the TT with another & it sounds good on
the Zenith amp its probably the cartridge. IIRC these had EV carts.
Look at the needle & be sure the needle is resting on a rubber thing
that goes to the cart. They break & fall off often.
This shows a magnetic but the layout is basically the same.

https://www.google.com/search?q=need...id:h5t4t5lzzA4
s
The part to look at is the "suspension".

73 Zeno
LFOD !

[Dubis7]

Oh, I should have been clearer - it was the other turntable with its built-in amp. Just confirming the record shouldn't be this muffled.

Just checked the rubber bit. It looks right, but maybe I installed the needle wrong? Tried to grab a photo to see if anything jumps out as an issue: https://photos.google.com/share/AF1Q...l1dVhpTkRJZ1B3

[zeno]

Looks OK. The other test you can try is unplug the audio from the TT
and hook it to a low level audio source like a cassette deck.

[Dubis7]

Let me see if I can figure out how to do that. Sounds like it would require actually building an adapter or something. I need to see if this turntable is soldered or just plugged into the amp.

[zeno]

Almost always the TT audio plugs in with an RCA jack. Yours is mono
so just one. If stereo two plugs. Some do use other type jack.

Zeno

[Dubis7]

Okay so I was able to work out the plugs on the bottom - it's a three prong plug similar to the one on my restored RCA suitcase player. Is that a standardized design? If so and if I knew what it was called I might could wire up something that could plug into the aux jack of a cassette player and run the audio through the amp to see if that's where my issue is.

[zeno]

Only 2 wires go to the cart. Dont know what they called the plug.
Unplug it and tack on a jack. BE SURE its not the AC power cord !

Zeno

[Dubis7]

No I can definitely see the wires from the tonearm connected to the plug on the turntable side. I think what I'll do is identify the live pins to the amplifier and then find a way to run some alligator clips or something from a radio to that. Then I can hear how the amp sounds with a different source.

[Electronic M]

Haven't seen a photo to know if this was one of the capacitive REAL cobra pickups, but if it is a normal audio signal from a normal audio source won't work.

The early cobras used a variable capacitance cartridge that NOBODY else made (and only wurlitzer bought from Zenith to use). The capacitance carts needed a tube RF osc that was modulated by the cartridge to make sound out of the variable capacitance. Zenith used this from 1948 until the late 50s when instead of making a stereo version they just used the stylized cobra tone arm with a conventional ceramic or magnetic cart.

[Dubis7]

I'm not sure what mine started off with, but I know that the cartridge was replaced with a modern, stereo compatible equivalent. It's not the radionic cartridge so I presume mine started its life with the ceramic cartridge.

Unless I'm mistaken and the radionic could have been replaced with a modern cart without significantly redesigning the electronics.

[Dubis7]

Just for sake of an update, I think I have a plan worked out. It's a bit convoluted so bear with me.

I've been wrapping up a restoration project on a Norelco EL3542. Thing's basically done, I'm just seeing if I can quiet down the takeup reel clutch before I haul it back up to my office.

A few days ago, I picked up a Wollensak T-1500. Unrestored, but it came with an adapter cable that allows a 1/4" audio jack to connect to an external source using alligator clips.

So - knowing that the Norelco is tested and sounds great, and with that cable in hand, I should be able to comfortable check the speaker on the Zenith and then work my way back to the amplifier to see if I can narrow down the source to a speaker or amp issue. Worst case, if it's the amp I can crack that open and see about sorting out the issue.