I received a Conar Model 250 from a friend of mine for free and surprisingly everything worked as soon as i got it. This scope had been sitting in a moist shed for probably at least a decade or two. I was very anxious about even plugging it in, let alone turning it on. This scope is also my first venture into pre-transistor electronics, so i was pretty scared about it not being grounded and something arcing internally. I've had older men tell me that old electrical stuff was unsafe, so i had an unnecessary amount of fear about it. Well i plugged it in and crossed my fingers, even after reading that its best to use a Variac to warm these old things up after sitting for a long time and to have it hooked to an ohmmeter to see if there is a big amperage spike indicating a shorted capacitor. I had no such devices so i had to just hope for the best and expect the worst. After the tubes fully warmed and after i cycled it on and off out of fear of exploding caps, i finally got a trace. I was getting a weak sine wave on the screen that almost fully went away when i touched bare metal on the case, so it had a ground problem. All was well until i shut it off after letting it sit on for a little bit to hook it up to a car audio amplifier to see if we could get a clean sine wave on the screen and to set the gain on that particular amp. I turn it back on and the screen is arcing in the back inside of the crt and then it pops up front, so we sort of panic and we shut it off. we try to turn it back on and see if we can find a source of either set of sparks. No luck there but the sparking in the screen stopped in a few seconds. I was worried so i did a little research and found that my CRT definitely has air in it, indicated by the spots(not sure what they are called) near the base of the CRT are totally white. After i got it back to my house, i tested every circuit i could and everything checks out to be in working condition. All the voltages are within the operating ranges of the tubes and even the HV's are where they should be. I only know from reading the data sheets for the 6 types of tubes in the unit and testing accordingly. I have a working 5up1 on its way here that should be here before the end of this week. I am convinced that after my voltage checks that its safe to put it on and everything should work. There was no burn marks or anything to be worried about anywhere on the unit other than those white spots. I am having trouble finding any documentation for this scope though.

This website has data for CONAR and a lot of other stuff:
http://bama.edebris.com/manuals/

They show a model 255 which or may not be close to your 250.

Edit: Just saw on another site (Nostalgic Kits) that the 255 is solid state.

As for running older equipment without any rework, you will probably find that this doesn't work out too well in the long run.

RRM

I have actually thought of modernizing it a little bit by replacing all the capacitors with new ones and retrofitting a 3 prong plug on to it. I'd like to do small things like that to make it that much better, but i really don't know where to start. I've also cleaned all the potentiometers so that they have smooth operation and they seem to be working properly now from what ohms reading i'm getting while turning them. I don't want to update anything to solid state though, i feel that would take away from the experience.

You should consider replacing all the capacitors, they're probably leaky by now. The ones in the HV section could leak enough to kill the HV and maybe the rectifier too.

A 3-wire cord is not a bad idea, although it's poo-poohed by the purists.

I've tested most of the resistors on either of the boards inside and all the ones around the pots and switches up front and they all are at the right resistance still so i'm not worried about those. I've left it on for about 30-45 minutes recently and i didnt have any glowing plates on any tubes so all of them are getting at least close to the correct bias. i'm wanting to find a tube tester so i can really make sure everything is in working condition.

You won't see any glowing plates, a scope normally runs the output tubes at their peak dissipation, any off biasing will make them run cooler, with a higher plate voltage and lower current, or more plate voltage and less current.

But leaky caps can cause all kinds of weird symptoms, consider changing any old wax or plastic tubular caps.

haha. Thats funny you would say that. They are all either wax or plastic except for two of them, one is a cardboard cased one and the other is the modern aluminum type, so i know its not original.

I found a video of a Conar 250 in use on Youtube for anyone who wants to see what it looks like. They said it's probably circa 1964:

http://www.youtube.com/watch?v=LOU3fCvG_Xg

Defnitely change the wax and paper caps before attempting to use again.

K7MCG wrote back in 2010 that it's from NRI (National Radio Institute:

http://www.antiqueradios.com/forums/viewtopic.php?t=142111

The link that was given there for the schematic did not work for me.

caps are cheap so i'll definitely do that. and i emailed the owner of that site last night asking if he still had those documents. if he sends them to me, isn't there a way i can upload them to this site in the resources?

Send me a PM with your email and I will send the schematic and brochure.

Dave

do i need to replace the caps in it with axial ones like those that are in it now or will radial ones do? and i've done recapping before, so should i follow the same general rule that the higher max voltage rating the better?

EDIT: i sent you a PM dave.

I would change axial to axial and radial to radial. Me personally, I wouldn't change it to the other type unless I couldn't find the same kind (axial->axial or radial->radial). It's just where the leads come out. I think people on here have said sometimes repositioning parts can cause noise or other problems sometimes.

I'd replace the caps with like or slightly higher voltages. There isn't any reason to change a 16 volt cap with a 1600 volt cap, for example. One reason to go to a higher voltage is if the original one was not sufficent in the original design and was known to be the cause of problems.

I'm sure others can give better reasons to do or not to do any of these things.

thats what i figured. i could get radial ones all day for next to nothing. axial seems like less choices. and i'm wanting to use good quality low-esr caps but they aren't too common. maybe i'm just not looking hard enough. i have a short list of brands that i trust not to fail as well just to complicate things.

I have used thousands of radial caps, never had a problem with either the electrolytic or mylar types.

are tantalum or metal film types any better than electrolytic? and i'm assuming i wont really need to replace the ceramic ones in my unit?

i'm having a little trouble here. i go on digikey.com and try to filter my results to .1 uf 400v and nothing exists. unless i'm missing something. are these not rated in microfarads? or are they rated in millifarads?

Your caps will be micro farads or micro-micro farads (pico farads now days). Vendor search engines leave a lot to be desired. You often have to try every variation you can think of to specify parts. Try mf or mfd instead of uf.

RRM

note to self and others who use digikey, everything is in uf, so if you need .1 mfd like me, its 100 uf. just for one example. no big deal, but it makes it easy to use one of the online converters so you make sure you get the right decimal place. should 100 uf 400v axial caps cost 8 dollars a piece? thats the price i got for a nichicon one and that was basically my only choice. thats direct replacement for one of the caps in it.

one major question, what do i need to look for in a replacement capacitor? obviously i need to meet or exceed the original voltage rating and have the same capacitance, but should i be concerned with finding ones that are considered high ripple? or some are 105C rated and some of those high temperature ones are designed for long life as well, and some other ones are low esr (sometimes labeled as impedance). obviously size isn't an issue, since the case is practically empty anyways and everything is spaced so far apart. i'm really wondering if i should get a capacitance meter and just check all of them and only replace the ones that need it, after seeing that 8 dollar nichicon cap. if thats a normal price for what i need, then this could potentially become an unwise project if i decide to totally re-do all of them.

'note to self and others who use digikey, everything is in uf, so if you need .1 mfd like me, its 100 uf. just for one example. no big deal, but it makes it easy to use one of the online converters so you make sure you get the right decimal place. should 100 uf 400v axial caps cost 8 dollars a piece? thats the price i got for a nichicon one and that was basically my only choice. thats direct replacement for one of the caps in it."

mfd and uf are one in the same. Don't know how you converted a .1mfd to a 100 mfd (uf). Just buy a .1 mfd (uf) for about 40 cents. Use one of equal or higher voltage rating to the original.