I need an adult! - Videokarma.org TV - Video

miniman82 · #1 01-19-2011, 01:49 AM

I figure after 2 weeks of the CTC-7 kicking my ass all over the place, it's time to ask for a little help.

Here's the deal:

About a month ago, the screen would randomly start going blank. Sometimes after a little playing, sometimes right away on startup. Then one day, there was no getting it back. 2 weeks ago, I decided to pull the chassis and do some digging. As I suspected I had no signal through the video amp, causing a blank raster. First thing I did was inject a signal right into the 6AW8 first video tube to see if there was any trouble with the video amp itself, but it passed the signal just fine all the way to the CRT connector. I did this by lifting the E terminal on the board, and attaching the NTSC gen through a coupling cap.

Next, I reconnected the wire at point E and read voltages. Come to find out, the video amp tube is being completely cut off any time the E terminal is connected to the IF strip! I get -22 grid volts where I should have +21, and the cathode is sitting at 2 volts instead of 23.

I left the E terminal disconnected and looked into the IF strip, where more funky readings showed up. 1st IF has nearly 200 volts plate where it should be 110, 2nd has same, 3rd is at 80 when it should be 150. Similar story with the screens. I can't figure it out! It's like the first 2 IF's are cut off, 3rd is on hard, then 1st video is cut off. No explaination either, every single part I sub/check comes back fine. It's gotta be something in common with all of them, right? It's not bad grounds, I've resoldered all of them in desperation.

miniman82 · #2 01-19-2011, 09:59 AM

Well, I woke up with a clear head and decided to try a few more things. I ended up finding that I had already fixed the problem by replacing a few bad resistors and the video detector diode, but without AGC the strip was overloading with such a strong signal. Put all the tubes back in and fired it up with the pix tube, and everything's just fine again. Well, except for a noisy video amp tube socket which I'll clean later today.

DaveWM · #3 01-19-2011, 10:08 AM

so you did not have all the tubes in when testing? or was one just not making good contact.

miniman82 · #4 01-19-2011, 05:03 PM

When I run the chassis outside the set, I pull the vert and horiz outputs. Horiz is obvious because I don't want HV when I'm working that close to the chassis, but I also find it necessary to pull the vert tube too or a resistor starts overheating. I found out the video diode was bad when it wouldn't pass a signal, but I never put it back in the set to verify because all the other readings were so out of whack I assumed something more was wrong. Note to self: don't assume.

DaveWM · #5 01-19-2011, 05:36 PM

Oh I see. yea no horz no keying pulse for the AGC.

miniman82 · #6 01-20-2011, 09:38 PM

Back to normal now, set looks great!

ctc17 · #7 01-20-2011, 10:34 PM

video detector diode: another one of those 'never goes bad' parts that do. i have been down that road a few times, i had one set that acted like the alignment was off, seemed like the fine tuning was always off. turned out to be a leaky detector.

i got some 1n60p diodes i want to try as a video detector. its a schottky equivalent of the 1n60 germanium. probably wont work because the capacitance is much higher, 1n60 is like .6pf and 1n60p is 60pf. the schottky shouldn't have the organic failure thing the ge diodes do.