Back to the VT-71H

[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.

[Chris K]

Originally Posted by 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.

They are both run off USB cables off non-grounded plug in transformer chargers...thank God

[timmy]

So when are we going to see a nice clear picture on that Motorola.

[Chris K]

Just got the isolation transformer yesterday so I’m going to start back up again this weekend!

[Chris K]

I give up. It's going back on the shelf again. All thumbs with the oscilloscope. Grounded to B- and the TV plugged into an isolation transformer, I get no signal off pin 5 of the video amp despite getting audio off pin 6. I've tried 4 different tubes, all tested good. I clip on the probe and the auto-read function gives me nothing...no signal. Before anyone wastes their time trying to help me go further with this, I'm going to put it up on the shelf and start working on something else. I wish there were some gurus around Philadelphia who would take it on and get it running for me. Tim and Chuck A. are in Northern NJ but I really don't know of anyone else who would be able to fix this and Chuck keeps his contact information very private as he should. I was so impressed with his take on the business of restoration during the February online meeting. Bob A.'s too! Anyway, thanks to everyone who tried to help this clumsy novice through this. Sorry if I frustrated you.

[Yamamaya42]

If you ever get back into working on the set, you always have the option of trying to direct feed video into the grid of the 6AU6, far side of point D on the SAMS, making very sure your video source is protected and isolated, ( TV on isolation transformer, video coupled via capacitor), according to the service manual, signal level out of video detector is 1.8v PtP, standard composite video level should be close enough to this to see SOMETHING W/O any amplification.

[Chris K]

Originally Posted by Yamamaya42

If you ever get back into working on the set, you always have the option of trying to direct feed video into the grid of the 6AU6, far side of point D on the SAMS, making very sure your video source is protected and isolated, ( TV on isolation transformer, video coupled via capacitor), according to the service manual, signal level out of video detector is 1.8v PtP, standard composite video level should be close enough to this to see SOMETHING W/O any amplification.

Thank you so much...yes I will try that. At the least, it will give me a reference as to what can be ruled out. Also, it will be so wonderful to see an actual picture on this!!! A few questions...what value capacitor...should the ground for the composite input be to the chassis or B- and, I have the variac set to 117 volts plugged into the isolation transformer so the setup is IT-Variac-Television. Is that OK? I know it makes sense to me but I trip over so much stuff in this hobby.

[timmy]

Originally Posted by Chris K

I give up. It's going back on the shelf again. All thumbs with the oscilloscope. Grounded to B- and the TV plugged into an isolation transformer, I get no signal off pin 5 of the video amp despite getting audio off pin 6. I've tried 4 different tubes, all tested good. I clip on the probe and the auto-read function gives me nothing...no signal. Before anyone wastes their time trying to help me go further with this, I'm going to put it up on the shelf and start working on something else. I wish there were some gurus around Philadelphia who would take it on and get it running for me. Tim and Chuck A. are in Northern NJ but I really don't know of anyone else who would be able to fix this and Chuck keeps his contact information very private as he should. I was so impressed with his take on the business of restoration during the February online meeting. Bob A.'s too! Anyway, thanks to everyone who tried to help this clumsy novice through this. Sorry if I frustrated you.

Hey Chris I’m up here in ferndale ny upstate probably 2-3 hours from philly if you give up take a road trip.

[Yamamaya42]

Well since C31 is a .1, most likely a film capacitor of the same size will work for coupling, but the value is not super critical, .1,.01 should not matter TOO much.

What's important is taking the same safety measures that were in place when the oscilloscope was used, MY guess would be to use chassis gnd, but someone who has worked on the set, and knows it a bit more may know more info on how it's to be done.

[timmy]

I have 6 of those Motorola sets and had one that had audio but no video had my share of problems with those sets. I can get a picture on your set I’m sure. A lot easier then a color set lol….

[Chris K]

Thanks Timmy. My guess is you have a good 7JP4 to use? Unfortunately, you're 3 hours away by car. PM me and let's see if we can work something out!

[timmy]

Originally Posted by Chris K

Thanks Timmy. My guess is you have a good 7JP4 to use? Unfortunately, you're 3 hours away by car. PM me and let's see if we can work something out!

I have a 3fp7 same base as a test tube and a numbers matching with box 7jp4.

[bandersen]

Originally Posted by Chris K

Thank you so much...yes I will try that. At the least, it will give me a reference as to what can be ruled out. Also, it will be so wonderful to see an actual picture on this!!! A few questions...what value capacitor...should the ground for the composite input be to the chassis or B- and, I have the variac set to 117 volts plugged into the isolation transformer so the setup is IT-Variac-Television. Is that OK? I know it makes sense to me but I trip over so much stuff in this hobby.

Ground for the composite source connects directly to B-. Anything 0.01 and up for the connection between the composite signal and tube grid as Yamamaya42 said.

So what I'm thinking is that the 6AU6 plate is being shorted to something and the screen is acting as the plate to get a signal for the audio. What voltage do you have on the plate and screen (pins 5 and 6)? How about the 6.8K, 4.7K resistors and 20uF cap?

Good luck!

[Chris K]

Originally Posted by bandersen

Ground for the composite source connects directly to B-. Anything 0.01 and up for the connection between the composite signal and tube grid as Yamamaya42 said.

So what I'm thinking is that the 6AU6 plate is being shorted to something and the screen is acting as the plate to get a signal for the audio. What voltage do you have on the plate and screen (pins 5 and 6)? How about the 6.8K, 4.7K resistors and 20uF cap?

Good luck!

Thanks Bob and I will check those values and post later tonight. Do you want to know voltage on the resistors? They have been replaced already with 5% value resistors. I did that after I started troubleshooting issues from the video amp to the CRT, and the originals were spot on anyway. Timmy has graciously offered to get it going and we've worked out a shipping scenario but I'm willing to give it one more shot before bubble wrap, Styrofoam and a corrugated box!

[dtvmcdonald]

There are other tests you can do! First, get a .01 to .047 uF film cap (greater than 250 volts) and connect the 6.3 volt filament line to pin 2 of the CRT (point C on Sam's) with it. This should generate a substantial hum bar on the screen.

You could also try connecting the 6.3v through that cap to pin 1 of V6 (video amp) as that should product a really strong hum bar.