Packard Bell 17VT1 Audio

[Winky Dink]

I thought this project was finished some time ago, but after reinstalling the chassis some new issues appeared. After putting on a test pattern I could see the vertical linearity needed adjustment. Also I couldn't control the vertical roll. My first concern, however, is an audio problem.

Audio sounds normal when the set is first turned on, but after a few minutes the sound deteriorates to a low-tone rumbling "Gro-o-nk, Gronk, Gro-o-o-nk."

I'm looking at tubes first because any work with the chassis electronics requires removal of the CRT, which I dread. But which tubes might adversely affect the audio?
Here's a link to a block diagram from Rider's:

https://flic.kr/p/2nGpcxk

And a link to the full schematic:

https://flic.kr/p/24Z7cYP

Other than snagging all the tubes that say "audio," I don't know which ones to be concerned with. Can anyone help me with that?

Thanks.

From Gary Larsen's The Far Side.

[jr_tech]

Does the volume control change the noise ?

jr

[Winky Dink]

I dunno. I'll set up the usual gear and play a DVD.

[Winky Dink]

Yes, the distortion increases with volume. It's not bad until midway, but with the side-facing speaker you'd have to turn it up pretty high for several viewers in the room. It's not so bad if you point the speaker at your ear from 12 inches away.

[jr_tech]

Does the gronk gronk noise stop if you pull the 6AU6 sound if tube?

jr

[Winky Dink]

With v1 Sound IF removed there is no audio, no noise.
I checked the speaker last night as if the cone was off-center, but that didn't eliminate the noise/distortion.

[jr_tech]

So the noise sounds more like a dragging speaker? That’s usually a griticha gritcha noise rather than a gronk gronk Can you substitute another speaker?

jr

[Winky Dink]

It's not gritcha gritcha (good description!). I have recentered a cone, and it doesn't really sound like that. I do have a box of old speakers, so I''ll look for a substitute.

[Winky Dink]

Tried a 3.2 ohm 2.3 W Sony speaker (matches the original speaker impedance) and half a dozen 3-,4-, 6 inch 8-16 ohm impedance, 1-2 watt speakers. All sound the same or worse. I'll try tonight to made a (audio) video so you can hear the sound rather than rely on a description. Thanks for your help.

[zeno]

Audio probs boil down to this
Hiss in audio & maybe its weak = audio IF
Buzz or ticking that changes with scene changes = audio detector
Hums, distortion etc = audio amp & out

How the volume control effects it is very important. A signal
detector is a nice thing to have for probs like this.

Test SPKR can be almost anything. Keep in mind almost all schematics, flow charts & block diagrams go from left to right. ( L input, R output ).
Dont go nuts changing stuff, we can help you narrow it down to a stage
and a few parts.

enuf fer now Zeno
LFOD !

[Winky Dink]

Thanks for weighing in, Zeno. I do have a Heathkit T-4 signal tracer. I don't really know how to use it, even after reading the documentation. However, I did try it when I was trying to build an AM transmitter and it did pick up audio signal, but no RF because either I wasn't using it properly, the tracer was defective, or there was no RF signal. Advice on what to do next?

[zeno]

You may not have a detector probe on it. Usually has a
switch on it.

[Winky Dink]

The probe does have an AF/RF switch. But like I said, I never detected anything with the RF.

For your entertainment, here's about 28 seconds of audio/video. First 10 seconds are mid-volume, then the volume increases a lot, but not to the max. Sounds much worse if there's music in the background.

https://flic.kr/p/2nGTHre

The horizontal lines at the top appeared only after I adjusted vertical linearity, but I have to save that for later.

[jr_tech]

What is the history of the set...has it been re-capped?
I hear distortion (perhaps leaky audio coupling cap C37) plus what sounds to me like buzz from the vertical sweep (perhaps tired filter cap). Does the buzz change when the vertical rolls?

jr

[Winky Dink]

History: Prior to my acquisition the CRT had been removed once, but no other evidence of repairs. I replaced all paper caps, all electrolytics, all out-of-spec caps and resistors, ensured a good set of tubes, checked for continuity and resistance through all coils, and other stuff I can't remember.

The vertical does not roll, but there was a condition briefly in which, although the image was stable, a light-colored band repeatededly moved from the bottom to the top of the screen in synch with the movement of that band the horizontal lines at the top of the screen appeared and disappeared and a buzz came and went.