Zenith Cobramatic Self-contained record Player Information needed

[Sandy G]

Aye-Yoop... That looks like its twin brother. I also remember it being quite heavy, which Bakelite stuff tends to be anyway, & Zenith items are typically "Substantial", as well.

[Electronic M]

Originally Posted by jr_tech

Sandy, was it like the one in this ‘bay listing?

https://www.ebay.com/itm/Zenith-Cobr...UAAOSw04Vcwldg

not affiliated,
jr

I wonder if that 45rpm adapter in that listing is correct for that changer? I've got that changer in a leatherette table model R/P sans adapter, and one of those adapters pictured that came in a box of odds'n'ends from a different place.

[vortalexfan]

I've cleaned it up a little bit, you could tell this unit was in a smoker's home origtinally as it was just covered in tobacco tar from top to bottom, and I was able to power this thing up briefly and surprisingly enough the amplifier had no 60 hz hum coming out of the speaker! which is amazing for a record player from the early 1950s like this unit is, it even still has its original Zenith branded tubes in it yet. with a date code of 1953.

sadly enough the speed control marker placard is coming off now, and I'm wondering if maybe the person who had this in their booth at the antique mall might have the record size marker pacard still in his possession as it seems like it might of fallen off while it was in his possession and he may of just forgot to put it with the record player for the new owner to glue back on the unit, which is why I'm going to see if I can go back to the antique mall and see if the mall owner can get ahold of the booth owner who had the record player in their booth and see if they have that placard still.

[vortalexfan]

OK so I finished recapping this record player and I hooked it back up like it was supposed to be and for some reason or another I can't get any audio to come out of the speaker for some reason.

I know it isn't the needle/cartridge because I can hear audio coming out of the cartridge/needle, the sound for some reason just isn't making it from the needle to the amplifier stage for some reason, and I checked and replaced the preamp tube (the 12AU6) and that didn't help any either, I even tried replacing the output tube and that didn't seem to help either.

So I don't know what's going on.

Anyone have any ideas as to what I could look into?

Thanks.

[zeno]

Unplug the cartridge. One of the wires is GND the other audio.
Touch the audio wire with your finger with the volume up abt 1/3.
You should get a very loud hum. If you do its the cartridge, if not
its further down the line. The wires are quite thin to the tone arm.
Easy to break or get pinched. If its in the amp you will need to
either trace or inject to find the problem usually. No big deal but you
will need a manual. Its a good learning event !

73 Zeno
LFOD !

[vortalexfan]

Originally Posted by zeno

Unplug the cartridge. One of the wires is GND the other audio.
Touch the audio wire with your finger with the volume up abt 1/3.
You should get a very loud hum. If you do its the cartridge, if not
its further down the line. The wires are quite thin to the tone arm.
Easy to break or get pinched. If its in the amp you will need to
either trace or inject to find the problem usually. No big deal but you
will need a manual. Its a good learning event !

73 Zeno
LFOD !

I figured out part of my problem, the replacement needle I ordered had the wrong contact points so in a sense I ordered the wrong needle.

But also when I touched the wires for the "Cartridge" I got a hum out of it, but it was very quiet and distorted sounding, even at 1/3 volume.

Does this sound like anything you've experienced before on one of these record players?

[vortalexfan]

UPDATE: I now know for sure that what my problem was as far as no audio goes was indeed that I had accidentally ordered the wrong needle.

The original needle was an ASTATIC 99-1 Needle which is a 1 mil sapphire tip needle for use with 16, 33, and 45 RPM Records of Microgroove vairety which that needle has the hot contact on the left side of the needle and the ground contact is a plug like contact that is on the base of the rear of the needle.

The one I ordered thinking it was the same as the original and not paying attention to the drawing of how the contacts were done or the model number on the original needle, was an ASTATIC 101D needle equivalent which has both contacts on either side of the needle which is why it didn't work and why I didn't get any audio.

But when I touched a screwdriver to the ground contact inside the "cartridge" the record player made a loud and pronounced humming noise like it was supposed to, the reason why it didn't make a loud humming noise when I tried it previously was because I was accidentally "grounding" the ground contact to the housing of the "cartridge" which essentially neutralized any hum being induced by my touching of the ground contact because it was basically functioning as if the needle was inserted inside the "Cartridge".

So it seems that my next step is to order the correct needle for this unit and I should be all set.

[vortalexfan]

Alright so I finally got the correct needle for this unit and got it installed, and I went to test it out and for some reason or another the audio coming out of this record player sound awful, its extremely distorted and sounds almost like you're playing a record on an old low-fidelity crytal cartridged record player from the early 1940s, instead of a Hi-Fi record player from the 1950s.

Any ideas as to why this thing would sound so terrible?

[Electronic M]

The 2 cobramatic changers I've owned both worked fine after a recap and getting the mech working...If you haven't recapped it yet that would be my first move. If it has fresh caps check tubes and resistors. If it still sounds bad try injecting audio and seeing if it still sounds bad. If it sounds bad with injected audio the problem is likely in the speaker or output transformer, but if injected audio is good then either the cart or the osc circuit that converts it's output to audio is bad.

[DavGoodlin]

It would not hurt to connect the tonearm wires to a different amplifier just to make sure the cartridge compliance is not hardened like I have found on some RCA carts.
Lots of noise touching and movement of the tone-arm but lousy sound is a familiar symptom.

I have a few cobras around and we might have to get crafty to refit a modern ceramic cart to get decent sound.

[vortalexfan]

Originally Posted by Electronic M

The 2 cobramatic changers I've owned both worked fine after a recap and getting the mech working...If you haven't recapped it yet that would be my first move. If it has fresh caps check tubes and resistors. If it still sounds bad try injecting audio and seeing if it still sounds bad. If it sounds bad with injected audio the problem is likely in the speaker or output transformer, but if injected audio is good then either the cart or the osc circuit that converts it's output to audio is bad.

it has brand new capacitors in the amplifier, the only capacitors that havent been changed were the power supply filter caps which are still working. The thing I have noticed on it is that it seems that it has a grounding issue betweem the cartridge and the amp because it has a hum to it that goes up and down with the volume control and isn't the same hum as the hum you get with bad filter caps.

Also this Cobra-matic changer uses an Astatic model 99 Powerpoint "cartridge" (which is basically a needle and cartridge in one unit that plugs into the tonearm) so the only thing that could be bad would be the needle/cartridge assembly itself which could always be possible seeing as it was a NOS Needle/Cartridge assembly I had bought for this unit.

I've changed out the tubes several times with other known good 50C5 and 12AU6 and 12BA6 tubes and it doesn't sound any better, and the speaker is still good it doesn't have any damage that I can see of.

I do have a Sonotone 2T Cartridge and needle assembly that I could try and install in this record player in place of the original Powerpoint cartridge assembly but unfortunately the tracking weight would be different as the Sonotone cartridge tracks lighter than the original cartridge does by a couple of grams and I don't know how to adjust the tracking weight on these old cobra tone arms.

[Electronic M]

Originally Posted by vortalexfan

it has brand new capacitors in the amplifier, the only capacitors that havent been changed were the power supply filter caps which are still working. The thing I have noticed on it is that it seems that it has a grounding issue betweem the cartridge and the amp because it has a hum to it that goes up and down with the volume control and isn't the same hum as the hum you get with bad filter caps.

Also this Cobra-matic changer uses an Astatic model 99 Powerpoint "cartridge" (which is basically a needle and cartridge in one unit that plugs into the tonearm) so the only thing that could be bad would be the needle/cartridge assembly itself which could always be possible seeing as it was a NOS Needle/Cartridge assembly I had bought for this unit.

I've changed out the tubes several times with other known good 50C5 and 12AU6 and 12BA6 tubes and it doesn't sound any better, and the speaker is still good it doesn't have any damage that I can see of.

I do have a Sonotone 2T Cartridge and needle assembly that I could try and install in this record player in place of the original Powerpoint cartridge assembly but unfortunately the tracking weight would be different as the Sonotone cartridge tracks lighter than the original cartridge does by a couple of grams and I don't know how to adjust the tracking weight on these old cobra tone arms.

Speakers can look fine and still sound like crap. The cone can rub, the suspension fail, the voice coil can have defects. When a problem is not obvious that is the time to test everything carefully.

If that someone is a conventional magnetic or ceramic type instead of the capacitive type most cobras used originally then you could take the output of another Phono with a cart that puts out similar voltage and see if the amp circuits in your unit sound good with that.... I've used that trick a few times.

[vortalexfan]

Originally Posted by Electronic M

Speakers can look fine and still sound like crap. The cone can rub, the suspension fail, the voice coil can have defects. When a problem is not obvious that is the time to test everything carefully.

If that someone is a conventional magnetic or ceramic type instead of the capacitive type most cobras used originally then you could take the output of another Phono with a cart that puts out similar voltage and see if the amp circuits in your unit sound good with that.... I've used that trick a few times.

Well it seems that I spoke too soon about the power supply caps, as they just failed when I powered it on just now, there was a slight hum that started growing louder and then it started squealing and when it started squealing I quickly turned it off but now when I go to turn it on the tubes wont light up.

So I'm thinking the power supply filter caps must of suddenly failed catastrophically as it was working fine hum free for the most part up until just now.

So I wonder if that might of been part of the reason why it was putting out bad audio, because the filter caps were starting to go out and I didn't realize it.

[Electronic M]

It is now the right place to start. You probably lost the heater of the rectifier tube when the lytics died.

Generally anything made before the mid 60s the lytics will be living on borrowed time. I thought about advising you to change them earlier but I didn't want to debate the merrit of the obvious.

[vortalexfan]

Originally Posted by Electronic M

It is now the right place to start. You probably lost the heater of the rectifier tube when the lytics died.

Generally anything made before the mid 60s the lytics will be living on borrowed time. I thought about advising you to change them earlier but I didn't want to debate the merrit of the obvious.

No, you're right I should of thought of that myself, but it didn't even cross my mind because I've never heard of electrolytics failing in that manner before.

This unit uses a 4 section electrolytic which will be tricky to replace because the amp doesn't have much room underneath to stick new caps inside as it is.

The original filter cap is of the can type that's fitted with a cardboard tube on top, so I might be able to gut and restuff the original can but not sure seeing as its a 4 section can and not sure if 4 radial lead caps will fit inside that can.