Hi I just finished restoring a Motorola VT-71 television. I replaced the electrolytics, all wax/paper capacitors, selenium rectifiers and all resistors that were out of tolerances. The picture looks great, but when running the tv through a digital converter box for off air antenna stations, there is a buzzing sound coming through the speaker all the time. When using the fine tuning, the buzzing gets worse when the picture looks the best, but when the picture is not as good the buzzing isn't as bad. With more contrast, the buzzing is more profound. If I disconnect the digital converter box from the tv the buzzing goes away; so definitely the converter box is overloading the audio on the tv. Is there anything that can be done to correct this problem. I don't know if this is part of the problem; after the tv is turned on the filaments on the glass ballast tube glow very bright, but some are still glowing lightly when all the tubes have lite up and the tv is operating. Other than that I'm real happy with the restoration. Any help would be greatly appreaciated.

What is likely happening is the IF amps momentarily going into cutoff or saturation, usually happening when there is white lettering in the video. If the video IF goes into cutoff or saturation, the sound subcarrier and or the video carrier gets obliterated for a short while, pattern repeating at the vertical rate. In the intercarrier system, the video detector sees the sound carrier as being 4.5MHz away from the video carrier, and converts it down to 4.5MHz. If the video carrier goes away, there's then no converted sound carrier, and the picture looks peak white. Thus the sound detector ends up with interrupted sound at a 60Hz (vertical) rate, which sounds like buzz. If the picture looks to be blooming white (like an overexposed photograph) you're losing the carrier (peak white on video happens at minimum amplitude modulation of the picture carrier). You may want to check that the voltages on the IF tubes are correct, or maybe too low.

You can probably get rid of the sync buzz by touching up the audio alignment, it has always worked for me.
The attached describes a simple way to do this without a signal generator.

BTW, if you want to make a 'better ballast' for your VT71, see my thread
http://videokarma.org/showthread.php?t=249434

Thanks Wa2ise and Polyphase for responding to my question pertaining to the audio buzz coming from my VT-71 tv during operation. Wa2ise, the picture is great and does not look to have the overexposed photograph affect that you mentioned, so I haven't checked the voltages on the IF tubes. Polyphase, I'll do the audio alignment that you listed tomorrow. Thank you both for your quick responses to my question. I'll keep you posted.

Hi Polyphase, I aligned my tv today to the list you sent me. It's much better. The only thing is when I'm watching it and a sceene changes, sometimes it will buzz. Even when it's good there is always a little buzz. Sometimes worse than other times. I adjusted the tv with the best picture (clear & sharp) and with the contrast at full. I did this, as before when there was a lot of contrast the tv would buzz. I did this to try to eliminate this problem. Is this what I should expect from this tv or should I be able to eliminate the buzz altogether? Also, I just thought of this. I installed the electrolytic capacitors underneath the chassis, but 2 capacitors in parrallel for (C1 140uF) are touching the T6 audio transformer and are very close to the tuning coils for tuning the various stations. Could this be causing my buzzing problem? Thanks again for all you help!

Hi.. The remaining buzz could just be strong modulation from your source. The contrast control is basically a picture volume control and when turned up can overload the video amp (V5) which also amplifies the FM subcarrier.
However, it could be a weak V5 (6au6) too. An easy way to check this is to measure the voltage across the 6800ohm plate resistor(R138). It should be minimum 80V, normally around 100V.
Finally, assuming you checked the resistors around the ratio detector circuit,
imbalance in the 6AL5 double diode tube could be a cause.

BTW, for best overall performance the B+ buss should be close to 120V or half way between B++ and gnd. If needed I can give you a couple of ideas on how to correct.

Hi I substituded video amp V5 (6AU6) tube with a known good tube and I also tried a known good 6AL5 ratio detector tube and there still was a buzz. I checked the voltage across 6800 ohm resistor (R138) and it reads 93V. I haven't checked the resistors around the ratio detector circuit yet. I did check the B+ and the B++ voltages and the voltages were B+ a little low at 100.7V and B++ a little high at 256.4V. I don't know if this is ok or not. Thanks for all your help, I'm learning as I go along, as this is my first restoration for either a radio or tv. Thanks again!

Is this what I should expect from this tv or should I be able to eliminate the buzz altogether?
Many vintage TVs have some degree of buzz in scenes with extreme contrast, such as white letters on a black background, or vice versa. You can reduce it, but perhaps never eliminate it completely, especially when you have a strong signal source, as noted.

If you have other sources -- a VHS or DVD player, whatever -- it might be interesting to see how the audio behaves when using them. My old TVs respond differently to various sources.

This problem is more annoying when watching contemporary TV programming, where the bottom of the screen is often plastered with high-contrast crawling text, animated previews, network logos, etc. That junk wasn't present 50 or 60 years ago.

Phil Nelson