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Cobra 25 unhacking project
- Thread starter 338_MtRushmore
- Start date
Smart phone, tone generator app, old cardboard box, toilet paper tube + 1khz works for me. I have found that a Pringles creates weird reverberations 
I was browsing the bay and a tektronix 475A has caught my eye. Out of curiosity I looked at oscilloscope on wiki, imagine that, the first pic was a 475A. Seems like a sign to me?
I am guessing it doesn't work. The pics show the power on, but the screen isn't lit up. Better pass on that one
I am guessing it doesn't work. The pics show the power on, but the screen isn't lit up. Better pass on that one
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If it doesn't work; then they have to say that in the description. I use a 465, and it is enough for my needs. The 475 is rated up to 200mhz/max, while the 465 is rated at 100mhz/max.
The reason people dump these scopes, is mostly either they don't need it or use it anymore. The other reason is that the new digital scopes are much smaller and they upgraded to that. In some ways, the old analog scopes are better. Seen a couple of these scopes on ebay for around $150 -$175. So long as it hasn't been beaten or abused, it probably is still one of the better scopes that one can have to use. Mine has been xlnt; traded a A99 antenna for it.
Unfortunately, very few listing say that they work. Those that do start north of $300. The problem is that there are truly untested units, an "its broke but I can list it as untested so I'm not responsible". What do you do with a $150 anchor? Resell it on ebay.
Guess I was lucky. Perhaps you may get lucky too. Depending on whether you live in a small town or a city, the chances of finding a working unit changes. Better chance in a city; but then you might just get lucky in a small town. Put a want ad in your local paper; or even try Craigslist. Bargain with them; but ~$100 is a fair price.
Make sure whatever 'scope you decide to buy has at least around 60 Mhz of bandwidth, 40 Mhz is too low imho. The higher the better but anything at 100 Mhz or above bandwidth is stellar for CB. I'll have to find a source for you to read, you want "(x2) the limit" of what you are sampling. So for CB (27 Mhz) use, you'd want to buy a 60 Mhz bandwidth 'scope.
I bought my oscilloscope (2336YA) for $125 Shipped (eBay), it was truly untested.
Plugged it in first time, got a trace after a little brief warm up time, then the thing started smoking so I unplugged it. Replaced one charred resistor & one failed styroflex (Polystyrene) cap, & boom, vintage Tektronix up and running!
The same units that had been tested by the seller and had 30 day warranty had a asking price of around $300!
Sometimes a guy buys a whole warehouse and finds hundreds of these from old schools, so they don't always have time to test every unit.
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I am working on a 148gtl 412, and this choke with the yellow wire looks like it was added to the dynamite ground for the echo board. Maybe it is one of the 2 grounds for the old 5 pin socket? I am removing the echo, and installing a 4 pin socket. What do I do with this choke?
good luck on changing it to a 4 pin. might work and might not. if you get a squeal when keyed and talking your radio did not like this change. the 5 pin 148 radios use 2 grounds. one to isolate the keyup ground and the audio ground from each other.. some work with the mod and some do not. Nomad will more than likely chime in if he sees this. he had built a special board for this conversion years ago because some radios work and some do not. I tried one years ago and all it would do was squeal. had to convert it back.
A thought occurred . . .
The newer 148GTL NW/ST radios are four pin. Since the schematic is nearly identical to a five pin version - except for a few instances - perhaps that might provide a guideline to make that conversion work?
What do you think, sonoma?
I tried it years ago and never did get it to work right. I even used the schematic that Rick Jackson drew up and put it on cbtricks, it is a 4 pin also, and followed it. some thing about it is different and did not find the difference in it. Nomad explained it very well. he has a write up about it on cbtricks some where.
Do the 5 pin to 4 pin mic adapters not work either? I tried using the 412 schematic that was redrawn on cbtricks that shows a 4 pin. I think it may have squealed before I removed that choke from the other ground. Maybe this is why there was a 4 pin jack half wired in when I got it.
That picture of the choke with the yellow shrink wrap should go to one of the pins on the mic plug board. You can see it on the schematic. Echo boards and roger beeps use the incoming mic wires, as I am sure you know well by now. That choke looks like an OEM part for that board.
I think the adapters do work, but I don't know; I have been advised to use one instead of attempting a 4 pin conversion.
Someone started a conversion and didn't finish it? Wow, maybe they had issues w/ squealing. I'm sure it can be done, how much is involved IDK.
Someone started a conversion and didn't finish it? Wow, maybe they had issues w/ squealing. I'm sure it can be done, how much is involved IDK.
with a cobra 148 or 2000 you have a 50- 50 shot of them working with a 4 to 5 pin adapter. I have no idea why some work and some do not. a lot could be with how much audio the radio puts out. I know the one I tried it on had lots of audio plus a echo board and people would not believe me when I told them it was a 148 radio. they all claimed it had to be a galaxy 99 or some thing like that with a lot of audio.
there has been a few guy's try the mod on cbtricks forum and some it would work and some it would squeal. just try it and see if it works or not. it is just time to do it and time to put it back if it did not work.
there has been a few guy's try the mod on cbtricks forum and some it would work and some it would squeal. just try it and see if it works or not. it is just time to do it and time to put it back if it did not work.