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 Post subject: Grebe CR-12 Questions
PostPosted: Sep Sun 12, 2021 8:09 pm 
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I've just started to work on a Grebe CR-12. While I've worked on a few Grebe receivers and amps before, this one had a couple of things that were new to me. Why the dual element, single control rheostats? Why all the individual on/off switches? First thing I'm looking into is a one volt drop in filament voltage between the 2nd AF (5.45v) and all the other tubes (4.45v), all 01A's.


Attachment:
Grebe CR-12 Schematic- High Resolution.jpg
Grebe CR-12 Schematic- High Resolution.jpg [ 495.1 KiB | Viewed 575 times ]

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Last edited by atwaterkent1 on Sep Sun 12, 2021 8:27 pm, edited 1 time in total.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Sun 12, 2021 8:22 pm 
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The double rheostat elements allowed the user to use 5-volt O1A type tubes or 3-volt 299 type tubes by switching the active element.

Norman

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Sun 12, 2021 8:27 pm 
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So that's what all the switches are for, thanks Norm.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Sun 12, 2021 9:57 pm 
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I noticed the rubber on the fine tuning vernier's needs replacing, typical for the Grebe's. I thought I'd do a quick replacement of them both but they look larger than the replacement rubber I've always used in the past. Just holding the fine adjust in place with my thumb it looks like almost nothing is sticking up above the surface of the face plate. I know its not supposed to be much, but before I put it all back together, does anyone remember if they used a different size rubber ring on the CR-12? Its hard to get an accurate thickness measurement with the distortion of the original.
Here's a photo of the old one and the replacements I've used in the past. The replacements are size 203.

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SAM_4121.JPG
SAM_4121.JPG [ 296.32 KiB | Viewed 556 times ]

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Sun 12, 2021 10:59 pm 
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I would not expect that any 1920's era "O" that is application specific to comply with 2020's standards, search for a new size that does the job...

Chas

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Sun 12, 2021 11:13 pm 
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I have one of these and I replaced the O rings. Maybe I still have the package of extras somewhere.

Do you have the instruction card inside the lid? It tells how to set the small switches.

Anybody have an extra knob for the small pull switches? I am missing one.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Mon 13, 2021 12:27 am 
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I do have the instruction card inside the radio. I'll need the knobs for the antenna and on/off switch, which are the standard Grebe cap nuts for connection posts. When did this thread become an AFT Classified? :lol:

If you find the rubber rings Jeff, see if you can get a thickness measurement please. I don't want to buy several packs of 100 O-rings trying to find the right one. :(

The replacement rings I've been using are about .135 thickness measured with digital calipers and a light as possible touch.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Mon 13, 2021 5:04 pm 
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The O-rings I have seem to be working fine. Just have to remember to put a little pressure on the knob when putting it back on the shaft to get good contact with the O-ring, easier to do it that way then trying to put the little vernier control shaft back on last.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Mon 13, 2021 5:24 pm 
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Here is what I did. The O-rings came in a small assortment from the plumbing department.
Attachment:
DSC04094.JPG
DSC04094.JPG [ 513.09 KiB | Viewed 481 times ]

Attachment:
DSC04095.JPG
DSC04095.JPG [ 410.64 KiB | Viewed 481 times ]

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Mon 13, 2021 7:15 pm 
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Can you measure the thickness of your o-rings Jeff?

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Mon 13, 2021 8:20 pm 
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Location: Keystone Heights, FL, USA 32656
That O-ring measures 0.145" thickness. It protrudes from the front panel about 0.090". It would work better if it was a bit thicker, slips a little in places.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Tue 14, 2021 12:35 am 
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Okay, thanks for the info Jeff.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Thu 16, 2021 2:23 am 
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Filament voltage problem was caused by the rheostat elements being out of place causing some shorting. I loosened the brackets and re-aligned the elements to make sure the wipers don't short on the bracket that holds them. Radio is now working but I'm getting some crackling and I'm not sure if its the radio or some signal interference, so I'll test it again tomorrow. Like Jeff, I am getting a little slipping of the fine adjust wheels, but for the most part they are working. I started to remember, back when I was restoring a CR-9, how the o-rings I bought were the only ones I could find that were close to the originals. I wish they were about 1/64 thicker.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Sat 18, 2021 11:47 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Phoenix, Arizona
My 3 cents:

I have a couple of "O" ring selections and I have usually found a size that works. You can get the tray of "O" rings from an auto supply.

It is easy to adjust the filament rheostats, they seem to get loose with age and slide out of contact.

I have a CR-14 almost perfect as soon as I find / make the 1/2 inch flat nuts that go in the battery compartment. I have a parts CR-9 that may have the knob you need

There are folks that reproduce the card for the lid and the one down in the battery compartment.

I haven't tried a CR-12 but my CR-9 with an RF amp (RORN) works like a charm. The CR-14 play very well considering just 3 tubes (V-99).

Jim


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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Sun 19, 2021 2:26 am 
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Quote:
I wish they were about 1/64 thicker.

Dan, you can wrap the the u-channel of the wheel with one or two layers of black electric tape cut to proper width before you put the o-ring on. That brings it to the correct thickness. It works like a charm for me on several Grebe radios.

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 Post subject: Re: Grebe CR-12 Questions
PostPosted: Sep Mon 20, 2021 7:45 pm 
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Since the slippage is minor I'm going to leave it as is, thanks for your suggestions though. Radio is finished and back in cabinet.

Attachment:
SAM_4151.JPG
SAM_4151.JPG [ 460.28 KiB | Viewed 205 times ]

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