VT-73 Motorola Sound IN video?
 

Hello all. I have recapped a VT73 recently and am having some difficulty getting things playing properly. When I fine tune the set I can get either decent sound OR decent video. But never both. If I attempt to align the set I can get both together but the sound has a terrible buzz (more than normal) and with volume set high it trashes the video. (It modulates the video). If I back off on the audio the video returns to normal. But then I cannot hear the audio. I think my problem is alignment related. But I cannot seem to get past the first alignment step using the Motorola service manual. The waveforms just don’t look at all like what I would expect so I just decided to align by eye and ear and have video peaked nicely but the audio is missing. Or vice versa.

Alignment by ear/eye might work for a radio where the idea is simply to tune for max signal, but seems to me like a disaster for video.

By loud volume, do you mean using the volume control, or tuning for loud volume?
If the video distortion comes when turning the volume control to max, it sounds like a power supply problem to me.

Hello Old_TV_nut,

I will check the power supplies and see if they fade when cranking up the volume.

Yeah I figured I'd get a rise out of someone about that TV alignment by ear comment.

When I align the audio limiter and ratio detector following the Rider instructions, the end result is still poor audio. Weak and misaligned with the video.

The video is clear and strong. Like I said, it feels like a bad alignment issue.

When I adjust things for the best audio, at full volume it is nice and loud and clear (no distortion).

Maybe a short video clip will help??

Audio problem - Video clip of the TS-4J Chassis
 

Hope this helps with troubleshooting my "sound modulated video trouble."

https://youtu.be/EqhdXZkK_WM

You don't seem to have 60 Hz hum, so the main power supply caps seem OK. You need to look at the B+ filter caps for the audio, sync, and sweep stages, like C38, 60 microfarad in the SAMS schematic.

C38 is good. I had replaced it along with all the other electrolytics and papers. But I double checked this one this morning. I just bought a B&K 415, so I’ll fire it up and see if I can get the IF aligned. Hope this makes a difference. Any other thoughts? Btw, the demodulation trimmer adjustment does not seem to make any difference. What function does that coil serve in that demod canister?

Unfortunately you wont be able to align this set with a B&K 415. The 415 is for the more modern 44 MHz IF, while the VT-71 uses the older 21 MHz IF.

Are you referring to the coil in the canister with the 1N34 detector diode ? It corresponds to A4 in the Sams alignment info. It affects the video IF curve along with A5 and A6. You'll need a sweep generator with markers and scope to properly adjust those coils.

I use a Wavetek 1002 for my alignments.

Any progress on finding the sound in pic problem?

If you have a scope, you can look for sound waveforms on pin 6 of V11B, 2nd sync clipper (second half 12SN7). If there is sound there, then it must be a problem with C38 or the connections to C38.

I also see a weird three-way connection to the bottom of R29, with two leads going to the same electrical point in the sound section above. I wonder if this is a mistake in the SAMS. Maybe the horizontal line should be broken between these two leads and not to the right of them.

I really appreciate your followup. I will poke around on V11B and see if I have sound there. Like I said, I can get a nice picture OR I can get decent sound, but not both at the same time.

I was able to get a Heathkit T-3. Wasn't aware that the B&K 415 would not drop low enough to work on these pre 50 sets. I am new to aligning TV, so its a learning curve for me in that regard. Thanks for the info on the coils. I have spent considerable time on your channel and appreciate the investment you have made for the hobby. I was hoping to find a basic alignment step by step on one of these older sets. If you have a video like that let me know - I may have missed it.

Alignment procedure varies from model to model. Get the Sam's or riders for your set and follow the alignment procedure written there.

Not just pre 50s, but some TVs into the mid 50s used the 25mhz IF frequency as well.

I have a 1077B B&K Analyst that uses a flying spot scanner to reproduce a test pattern from a transparency at any IF frequency the dial is set for (infinitely adjustable).

For S&Giggles, I'm going to try an eyeball IF alignment on a 25mhz IF using the 1077B in the near future. If I screw it up I do have an old marker generator I can use to get it back.


John

I followed your instructions. What initially threw me off was that the cap is C3 B (not 38). So I scoped that connection on pin 6 and had a lot of noise and audio on the pin. I decided to replace the new cap. That fixed it. Good work. Now I can do an alignment. I have the sams and instructions, so this weekend I will try again. I have it pretty close, but I am still getting some "vertical buzz" in the video. I know this is typical of many early sets. I can find a happy medium of audio gain and near elimination of the buzz. Maybe the alignment will address some of this. Thanks so much. Another lesson on questioning your assumptions ("I replaced that cap already...").

:banana:

Here is the video I made featuring this set. Thanks again for your advice.
https://youtu.be/MVpgbdzLlKA
Kent