Buzz in sound from video - a porthole

[kramden66]

If someone could please take a quick look at the schematic for this porthole , the 24g24/25 figure 42 , page 39

http://www.earlytelevision.org/pdf/Z...anual_TV-5.pdf

problem is a buzz in the sound from the video , adjustments of the sound seem to make very little difference only a little improvement , buzz control doesn't either , what does is if you turn the agc , the higher you go the more the buzz , if you go down to very grey the buzz is almost gone , if there are some capacitors in that circuit that would make a difference please let me know , i'm stumped and don't want to test every capacitor there , the paper / wax ones were replaced the discs haven't , the micas haven't , the ones that look like resistors haven't etc etc since they are normally good but i get the feeling one isn't
anyway thanks
mike

[old_coot88]

First of all, reduce the input signal to a level that's consistent with a snow-free picture. Adjust AGC for good contrast with no sync tear.

Adjust the quadrature (sound detector) coil L64 in conjunction with the buzz control for minimum buzz. You may have to juggle them a few times.
The IF input coil L63 may need a slight peaking for best volume.

The L64/buzz control adjustments are quite critical and may need re-tweaking as the set warms up, so set them with the set warm.

[wa2ise]

What an unusual power supply, an autoformer to get B+, series string heaters, and thus a hot chassis. Guess the cost of isolating a hot chassis was less than the additional cost a full power transformer would have cost...

[DaveWM]

try a new 6BN6 too.

[kramden66]

tried the adjustments as stated , the quad really doesn't do too much , the way it is now if you set to good contrast , not too much it has a loud buzz , if you adjust it so it is grey with just barely enough contrast it is tolerable , if you adjust the agc to wear it is very grey and almost no more signal the buzz is pretty much gone , maybe the cap on the quad is shot
tried more then one 12at7 with no luck

[old_coot88]

The quad coil should show a sharp peak at resonance. Check it for continuity if it doesn't.

[kramden66]

old coot thanks i didn't think of testing to see if the quad coil was open / ok , if not where the hell do i find one ?

[dtvmcdonald]

What kind of signal are you using? Does the buxx vary between a
black and a white screen? If it is bad only in white, the signal
many be overmodulated.

That's a really cheesy IF response for four IF amplifiers!
And does it really use a local osc that is on different sides
for the two VHF bands?

[zeno]

Be sure you are not turning up the AGC too much to
make up for a problem causing weak video.
With no signal you should get a nice clean snow ( dots of snow
no streaking etc) with very good contrast & a nice waterfall
sound. Anything in the tuner, IF, video, CRT & AGC can
give weak video. Start by subbing KNOWN good tubes,
dont trust a checker.......

73 Zeno

[kramden66]

yes i get nice snow , the problem is not weak video it is a buzz caused by the video signal , it is not over modulated , tried a vcr , rf modulator and another rf modulator , all the same , what i said was if you turn the agc to wear it is weak and is so grey you are starting to lose sync the buzz is gone , increase the agc just a little and buzz is there but fain , increase to where it starts to be less gray and more black and white and buzz .
perhaps this means something , the buzz control when turned hits a point where the sound goes out and comes back , it acts more like a coil , perhaps the quad coil isn't right because it really has no effect on the sound yet it really should , or the little resistor looking cap on it is no good.

[old_coot88]

So does the quad coil have continuity as measured with an ohmmeter (on the RX1 scale) or not?

[wa2ise]

Originally Posted by old_coot88

So does the quad coil have continuity as measured with an ohmmeter (on the RX1 scale) or not?

It should measure around 10 to 30 ohms, I'd think. Definitely less than a few hundred ohms.

If it shows an open, check for corrosion of the coil leads at the terminals (inside the transformer can).

IIRC, any coil and cap that can be adjusted to resonate at 4.5MHz would work as a replacement. This kind of FM detector is very common in tube TV sets, so finding a replacement should not be hard to do.

[Rcr1961]

Mike, try a 6ba6 for v7, this worked in the 24g26 I got from you.
This substitution totally eliminated the buzz without affecting the picture !

Bob R.

[kramden66]

Bob R thanks , whats supposed to be v7 , what is originally there ?
v7 according to that service manual i posted a link to ?
this too is a 24g26 , i don;t remember a buzz in the one you got from me but i guess there was.

thanks old coot and the rest , i haven;t had time to do anything with it yet
mike

[Penthode]

Have you checked the i.f. alignment? If the audio carrier level is too low at the detector, the buzz will be a problem.