A little Surprising, CT100 Fans - Page 6 - Videokarma.org TV - Video

ChrisW6ATV · #76 10-10-2011, 12:17 AM

Quote:

Originally Posted by Steve D.

HUH! I think Murphy needs to brush up on his math. Or am I missing something here?

-Steve D.

Murphy has no problem with his math at all. The rest of us, yes, we have BIG problems with Murphy's math!

Steve D. · #77 10-10-2011, 01:02 AM

Quote:

Originally Posted by ChrisW6ATV

Murphy has no problem with his math at all. The rest of us, yes, we have BIG problems with Murphy's math!

I'm familiar with Murphy's law. I break it all the time. I guess I never heard it expressed in those terms.

-Steve D.

***Eric H*** · #78 10-10-2011, 12:47 PM

Sounds a lot like Drag Racing law, it takes 90% of the money to get that last 10% of performance.

Dave S · #79 10-10-2011, 08:12 PM

Quote:

Originally Posted by John Folsom

Its Alive! For a HV probe, I recommend one that plugs into your DMM, not one with a meter built into the handle.

Hi John,

You've got me curious: why do you recommend one over the other? I've never heard that before.

--Dave Sica

John Folsom · #80 10-10-2011, 10:46 PM

The HV probes with the built in meter have a tiny scale which goes from 0 to 30 or 40KV; not much resolution. The ones which plug into a DMM have much better resolution.

miniman82 · #81 10-20-2011, 04:51 PM

Took John's advice, ended up with a Coline 40Kv probe (model HV40B) from Ebay for around $70. Nice unit, even has spring loaded covers that slide over the terminals when it's unplugged. I was able to verify that I have a solid 19.5Kv coming out of the HV rectifier, and the regulator looks like it's doing it's job as well. But another problem surfaced: if I tried to get the picture into focus, all I got was some very pissed off sounding arcing and a bad smell...

Focus pot appears to have bought the farm, probably as a result of years of wear or maybe the old convergence transformer taking a dump. As predicted by John, it only arced when near the high potential side of the pot. On visual inspection there is a good big of gouging in the carbon element, it only reads 3.2 meg now vice the 5 meg it should be according to the schematic. Once it's replaced I should be able to get a decent picture on the screen, so off to the interwebs I go in search of a new one.

miniman82 · #82 10-26-2011, 01:10 AM

I had a go at replacing the pot with a standard type, which actually still had the same part number called out in the Sams for this set but with a much smaller element (think volume control). I can now confirm that a modern Clarostat A47-5MEG-S will arc right accross all the terminals with focus voltage applied to it, so it's back to the drawing board.

Phil Nelson · #83 10-26-2011, 02:21 AM

Can you temporarily substitute a couple of fixed resistors for this pot? A workaround might let you make progress on other issues while you hunt for a replacement.

Just a thought.

Phil Nelson

Steve McVoy · #84 10-26-2011, 07:18 AM

I have used a smaller value pot with fixed resistors on each side (you have to experiment with them to find where you get the proper focus range). This way there is less voltage across the pot, therefore less chance of arcing.

Electronic M · #85 10-26-2011, 01:05 PM

Another possible temporary option would be to get one of those giant, old, high-watt, ceramic, wire wound resistors (I used to have a bunch, but have not seen them since the last move) some had a side of the wires exposed and a slidable center tap thus makeing them a (very annoying to adjust) slide potentiometer.

EDIT: There is a surplus store in Orlando called Sky Craft that has some of the type I mentioned above and may have the right value with an adjuster. These resistors have no insulation on the center adjuster so you would have to jury rig one or keep the set off when adjusting it.

miniman82 · #86 10-26-2011, 07:55 PM

Quote:

Originally Posted by Steve McVoy

I have used a smaller value pot with fixed resistors on each side (you have to experiment with them to find where you get the proper focus range). This way there is less voltage across the pot, therefore less chance of arcing.

The thought had occurred to me, but I don't have any 2 watt megohm value resistors lying around to experiment with. If I can get my hands on some, I suppose it would be possible to use a 1 meg pot so it's dropping less voltage across it.

Steve McVoy · #87 10-26-2011, 08:19 PM

You don't need 2 watt ones. If you use several, the heat dissipation will be through all of them, and 1 or even 1/2 watt ones can be used.

miniman82 · #88 10-26-2011, 10:06 PM

All I have is 1/4 watt, lol!

Time to experiment I guess.

miniman82 · #89 11-14-2011, 02:25 AM

Thanks for the input Steve, you were right! I was sure I'd have the same arcing issues across a 1-meg pot as I did with the 5-meg, but that didn't happen at all. I ordered a 2w 1-meg pot and some 1-meg fixed resistors, to try and get the correct range for this CRT. I spent the last few days playing around with it and ended up with a trio of 1-meg resistors before the pot (on the high potential side), then the pot with the original 8.2-meg on the other end of it going to ground. That got me to the point where I can dial it in exactly, but there's very little range. Not that it matters, so long as it's sharp at some point. Didn't even have to take the metal back off the pot to make it work either, no zapping whatsoever. Result is below, though there's still some issue with the color circuits. I can't get any color on it no matter what I try, so I suspect the reference oscillator is not working or something. I'll dig into it next chance I get, hopefully the next pic I post is in glorious 1954 COLOR!

miniman82 · #90 11-15-2011, 01:59 AM

I was right, the local 3.58Mhz oscillator is not running. Actually it starts out running when power is applied and tube starts to warm up, but the output quickly shrinks to nothing. Curious...

Voltage readings on the osc tube (V29b in Sams) are all wrong:

Pin 1 (plate): 50 volts, should be 95
Pin 2 (grid): 0 volts, should be -9
Pin 3 (cathode): 0 volts, which is normal

In other words instead of oscillating, the waveform collapses for some reason and the tube is left biased on which drags down the plate voltage. I'm not versed in oscillators, so I tried the following:

Different 6AN8: no change
Different crystal: no change
Removed the quadrature amp tube, in case it was messing with the oscillator: no change
Removed both phase detector tubes: no change

So the question now becomes: what's causing the local oscillator to be unstable? I attached a video, so you can see exactly what I'm talking about. This is the scope attached to pin 8 of the quadrature amp, V28a in Sams.

View My Video