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 Post subject: 12lp4 sub for 12up4
PostPosted: Jun Fri 30, 2017 12:03 am 
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Joined: Jan Sat 16, 2016 10:03 pm
Posts: 312
Hi everybody,

I have a quick question:

I am replacing a 12up4 crt in one of my Zenith Portholes with a NOS 12lp4 crt. When I opened the 12lp4 crt box, the Aquadag coating had flaked off and was at the bottom of the box.

Now this is the question.

Do I necessarily need to recoat the 12lp4 crt with Aquadag since the unit has a doorknob cap?

In addition to the recoating, I will need to fabricate a grounding spring with a contact mounted to the chassis that will ground to the Aquadag. But if I don't need the Aquadag because of the doorknob cap why disturb the original chassis.

Any feed back would be greatly appreciated.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jun Fri 30, 2017 4:34 am 
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If you wanted to remove the remaining coating, you could likely get away without it. It's not all that difficult to recoat the tube though. A bigger issue is whether or not your chassis was cut from the factory to accept the very different bulb shape of the 12LP4. If not, it's no fun to cut out so a glass CRT will fit in place of the metal one. Been there done that.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jun Fri 30, 2017 2:12 pm 
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Joined: Jan Sat 16, 2016 10:03 pm
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The chassis is factory "U" cut to accommodate the 12lp4 crt.

Also, this type of chassis is outfitted with some type of insulator between the tope plate of the chassis (where the tubes are) and the bottom part of the chassis. Don't know why Zenith did this.

Anyhow if I attached a spring grounding mechanism to the top of the chassis, I will be bypassing/eliminating this chassis insulator.

I will post a pic of this insulator as soon as I can.

Thanks for your response.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jun Fri 30, 2017 3:09 pm 
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Location: Chicago, IL USA
If I recall correctly, this is a hot chassis set and that insulator serves a purpose. Do not bypass it.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jun Fri 30, 2017 3:25 pm 
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Exactly. That seems like a useless feature to include an insulator between the two halves of the chassis when someone could still physically touch the part of the chassis which is connected to the power line. They must have had a valid reason for doing it that way or it wouldn't have been part of the design.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jun Fri 30, 2017 7:32 pm 
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Joined: Jan Sat 16, 2016 10:03 pm
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I've been working on this chassis for sometime now and it is not a hot chassis. Here are two pics of the two chassis with the insulator (brown in color). Maybe its not an insulator because I get a 90K ohm reading between the top chassis and bottom chassis. Maybe what I'm calling an insulator is actually resistance material. Any thoughts?

I'm leaning toward applying Aquadag (which I have - carbon conductive coating by MG Chemicals) to the 12lp4 and gluing a small plastic insulator to the top chassis and fabricating a spring grounding contact that will be affixed to the plastic insulator and ground the spring to the bottom chassis (the bottom chassis is the ground). I have plenty of crt grounding springs from my collection of chasses I can rig to accomplish this.


Attachments:
23G22.jpg
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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jun Fri 30, 2017 7:33 pm 
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Attachment:
23G22 #2.jpg
23G22 #2.jpg [ 95.66 KiB | Viewed 1461 times ]


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jun Fri 30, 2017 10:30 pm 
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Location: Chicago, IL USA
That's definitely an insulator. Some Zenith porthole chassis like the 23G22 and 24G23 are indeed hot even though they have a power transformer. The power transformer is only used for B+ and the tube filaments are series strung right off the AC line.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jul Sat 01, 2017 1:07 am 
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Joined: Jan Sat 16, 2016 10:03 pm
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So the top would be hot while the bottom is the ground. Am I thinking correctly.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jul Sat 01, 2017 2:37 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Lafayette, CO
You will find the top is chassis ground, the sides are floating except for a modest 1/2 meg resistor and cap between them. The top can be directly connected to the power line, depeding on which way the plug is in the wall. I had a 16-inch one which had a separate chassis for the power transformer and audio output. That power transformer was an autotransformer, by that alone it was hot. If l were you, l would consider both to be hot.... use the one-hand rule so you don't get zapped. Craig


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jul Sat 01, 2017 9:18 pm 
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analog.tv wrote:
You will find the top is chassis ground, the sides are floating except for a modest 1/2 meg resistor and cap between them. The top can be directly connected to the power line, depeding on which way the plug is in the wall. I had a 16-inch one which had a separate chassis for the power transformer and audio output. That power transformer was an autotransformer, by that alone it was hot. If l were you, l would consider both to be hot.... use the one-hand rule so you don't get zapped. Craig


What's odd is I've touched the top and bottom chassis with the unit on (with raster) and did not get jolted or even feel anything.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jul Sat 15, 2017 5:36 pm 
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clsoca wrote:
What's odd is I've touched the top and bottom chassis with the unit on (with raster) and did not get jolted or even feel anything.

You will not get a jolt unless you are also touching something that is grounded and the power cord is plugged in the correct way so as to make the chassis hot. Some people will feel a sort of vibration if they lightly brush their fingers across a hot chassis. If you want to try that you will have to try it with the power cord plugged in both ways and you will have to be very careful that you are well insulated from anything that may be grounded.

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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jul Mon 17, 2017 2:45 pm 
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Tom Schulz wrote:
clsoca wrote:
What's odd is I've touched the top and bottom chassis with the unit on (with raster) and did not get jolted or even feel anything.

You will not get a jolt unless you are also touching something that is grounded and the power cord is plugged in the correct way so as to make the chassis hot. Some people will feel a sort of vibration if they lightly brush their fingers across a hot chassis. If you want to try that you will have to try it with the power cord plugged in both ways and you will have to be very careful that you are well insulated from anything that may be grounded.


IMHO better to check chassis hotness with a DMM than your body...Though I've done both on accident.


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 Post subject: Re: 12lp4 sub for 12up4
PostPosted: Jul Tue 18, 2017 6:27 am 
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Joined: Nov Fri 30, 2012 3:35 am
Posts: 151
Location: Phoenix, AZ
From page 3 (general information/introduction) of the service manual for the Zenith 23G22/24G22:

"All chassis are constructed of a top plate and a wrap-around base. The two sections are insulated from each other and are interconnected with only a 100K resistor. All electrical circuits including the AC line return to the top chassis plate only. DO NOT CONNECT SHIELDED TRANSMISSION LINES, COAXIAL CABLES OR ANY GROUNDS TO THE CHASSIS. Always use a 200MMFD 900V AC test capacitor between the transmission line shield and chassis.

"The service man should bear in mind when testing the receiver to use a 300 to 500 watt isolation transformer and to CONNECT TEST EQUIPMENT COMMON LEADS TO THE TOP CHASSIS PLATE AND NOT THE WRAP-AROUND BASE." (Capitalization in the original.)

R/ John


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