Packard Bell 17VT1 Audio
 

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.
https://live.staticflickr.com/65535/...76406c87_z.jpg

Does the volume control change the noise ?

jr

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

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.

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

jr

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.

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

jr

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.

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.

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:smoke:
LFOD !

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?

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

Here's what it sounds like.
 

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.

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

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.

The lines at the top are the Copyguard protection from your video source. They prevent videotaping the signal. Old TVs without power vertical retrace blanking will show them. This is the bane of vintage TV restorers. If you are using a separate player and modulator, you can find a device to purchase that you insert in the video feed to remove them (or a schematic to build your own). A professional video proc amp (generally more expensive) that reinserts clean sync and blanking also does the job.

Thanks for that explanation. It also explains why I didn't previously have this problem--until now I had been using a combination VCR/DVD player with a direct connection to the TV antenna. I might go back to that, but a name or description of a copyguard defeater so that I can look for one?

It's been a long time since I acquired one, so maybe I was optimistic about availability of a built unit. Search around to see what you can find.

Correction to my previous quick reply: The combination VCR/DVD player also produces the copyguard lines. I'll look around for a device.

The copy guard system was called macrovision. Look for a macrovision remover.

Macrovision wouldn't just mess with retrace it would often also mess with AGC which would change contrast/brightness of the video and on some tube sets mess with the audio as you're experiencing...If the AGC misadjusts the gain till the IF goes into clipping it can create buzz at the audio detector.

I bought one of these products a long time ago. It looks to be the grey one.
I think that was before they had so many products. They must have increased the prices. Maybe twice as expensive now. Looks like the Red versions are discontinued. I seem to remember I got it through Amazon.
http://www.xdimax.com/common/default.html

There must be similar things cheaper now.

As it turns out, the Macrovision/copy guard lines are not a problem for me. The DVDs I intend to use are all cheap collections of 1950s television. So far I haven't found any of them to have any type of copy prevention.

However, Dimax (or XDimax) makes the "GREX" which is available from the XDimax website (www.xdimax.com) for $98 (plus shipping, I guess) or from Amazon for $98.99 free shipping for prime.

Regarding a possible leaky C37 coupling--C37 is a new .020 film cap, and I couldn't see any defects with V9 - 6AS5 Audio Out or V8 - 6T8 AF Amp. The filter caps are all new electrolytics.

So, while watching Roy Rogers, I couldn't hear any buzz, but low frequency audio is distorted and seems louder than it should be at any volume and increasing the volume distorts a higher range of frequencies. Normal dialog at mid-volume sounds normal.

Any suggestions as to what I should look at next?

Thanks,
The Eternal Novice

Have you tried probing the audio stages with your signal tracer? You might be able to locate the stage where the distortion starts.

jr

Thanks. Makes sense to me. I'll try that, but it'll take some time because I have to pull the CRT for unfettered access to the circuitry.