Back to the VT-71H

[Chris K]

Got it. I received my usb/laptop oscilloscope this evening and I’ll trace it out tomorrow night. Thanks Bob

[bandersen]

Between the detector and the CRT.
Check the signal at the grid of the video amp.
Check the signal at the plate of the video amp.
Check the signal at the cathode of the CRT.

Note that the sound is picked off the screen of the 6AU6 video amp while the video comes off the plate. So I'm thinking it's possible that circuit is screwed up in some way that allows the signal off the screen to be OK, but not from the plate.

[Chris K]

Understood. Thank you very much!

[Chris K]

Keeping in mind, I just got this thing and my skills with it are very, VERY limited, I took pictures of the waveforms of the signals going into the video amp pin1, and out the video amp pin 5. The scope has an auto trigger and an auto adjustment feature that creates the best display parameters. First in pin 1 input from the detector. The second is pin 5 video output. Am I doing this trace correct so far? I've never looked at a video signal on an oscilloscope before. I'm feeding an RF modulated video into it and I have sound but no image information on the CRT...just a raster.

[bandersen]

No. Look at the frequency. It's 60Hz. You're just seeing noise from the AC line.
Amplitude is pretty big too. What do you have the ground of your scope connected to?



Get that frequency dialed in to more like 45kHz or higher so you can see the horizontal sync pulses.

It should look something like this.

[Chris K]

Ground is connected to the chassis. I thought I was seeing AC when I saw that. The 60Hz pulse is what is going into the amp pin 1 from the detector.

[bandersen]

No, the chassis is not ground - it's floating. Connect your scope to B-. Same place the negative leads of your electrolytics caps are going.

[bandersen]

and make sure you have the TV running on an isolation transformer!

[Chris K]

I don't have one...just a Variac but I ordered one so I'll keep away until it's safe!

[bandersen]

Yikes! Remember B- connects directly to one side of the AC plug. If you connect that to ground on your scope - POW!

[Chris K]

Originally Posted by bandersen

Yikes! Remember B- connects directly to one side of the AC plug. If you connect that to ground on your scope - POW!

Scope is OK. I did try it (), grounding it to B-, but the scope still works ok on my calibration generator. That's my issue with understanding parts of this hobby and being out to lunch with other things. I'm a science guy...a biology professor and researcher, but there are things about electronics where I have no idea what I'm doing. Feel pretty stupid at times working on these things...but I just learned B- connects directly to one side of the AC line!!!!!

[Electronic M]

You should also check continuity of the ground lead of your scope probe....The positive path can be OK at the same time the ground is open and the probe will still work on the scopes internal calibrator because they share an internal ground, but connect it to a working TV chassis and you'll get hum and weird behavior from the open ground...Ask me how I know!

[Chris K]

I used an external frequency generator atom

[Chris K]

Tom. Not atom!!!!

[Electronic M]

I am atomic! Lol. You can edit your posts if you want to there should be an edit button next to quote if you are reading this via a web browser.

It's encouraging that it was an external generator, but you can still get same false reassurance from an external generator if both the generator and scope have 3 prong power cords...Most test equipment that have 3 prong power tie the signal negative/ground to building ground and test equipment plugged into the same outlet can sometimes use that in lieu of the signal cable ground when it goes open.