'49 Admiral audio problem

[kbmuri]

Can anybody else see my avatar or any of the photos in this thread? Did we have a server crash?

On the TV: I can put my signal generator, thru a 100pF cap, on any grid of any tube in the tuner RF or sound IF lineup, and get loud, clear sound at the speaker at low volume-control settings. Turning the volume control up is ear-splitting. Any audio tone from 30Hz to 1000Hz, at the sound-IF takeoff, volume is maximized at the 21.25 Mhz carrier (the IF). At the antenna terminals, best volume is at the expected 65 MHz.

But dead silence from the VCR. Yes, I triple quadruple checked the VCR output on the Sony handheld, it's putting out normal audio.

Gotta box this one up and wave the white flag. Freekin thing's haunted.

[3Guncolor]

If I were you I would try a different source for your channel 3 audio. Modulators in VCR's etc are very cheap and even if it works on one set it may not be up to spec.. It could have normal video carrier power but the audio could be low.
Steve

[kbmuri]

3Gun,

Thanks, you may have had a point, I've been using just the one input source. It's been adequate for the other restorations (Magnavox, Capehart).

New setup: Living-room Sony DVD player hooked up to Living-room TV via S-Video cable and primary audio-out (L and R) RCA cables. DVD Player's secondary audio and video-out going to Rat Shack 2.4 GHz rebroadcaster thru regular RCA cables. Rat shack receiver on old 1985 Commodore Amiga monitor in bedroom, plays video and sound, full fidelity. Receiver then hooked to haunted roundie. Outstanding video, same dead silence.

So video/audio source doesn't seem to be a factor.

- Kirk

[wa2ise]

Here's a thought: Maybe the local oscillator in the TV's tuner is on the wrong side of the TV channel? TV sound carrier is 4.5MHz higher than the picture carrier, and no sound carrier 4.5MHz lower than the picture carrier.

Try both the VCR modulator output, and the signal generator at the same time. Tune in the VCR video picture. Then sweep the generator (that generates the test sound carrier) at frequencies around that of the TV channels (plus minus say 10MHz). Channel 3's video carrier is at 61.25. The sound carrier is at 65.75MHz. NOT at 56.75MHz, but if you find that you can hear the generator at 56.75MHz for channel 3 and not at 65.75, then I'd start looking at the tuner's local oscillator.

Real TV channels' video signal is AM modulated onto their carriers, but the lower sideband is filtered to stop at 1.25MHz below the picture carrier, but it goes up to 4.2MHz above the picture carreir. TV modulators in VCRs don't bother to filter the lower sideband. So you might have a situation that lets you see good video from the VCR, but fuzzy video from real TV channels, over the air or from cable TV (but not from a set top converter box, but only direct from the cable).

You may also find that the sound IF may be off a little. Using your frequency counter on the generator, zero beat the VCR video carrier with the generator and see what the frequency is. Then note it down, and then sweep the generator to find what frequency it takes to get sound out of the TV. Should be 4.5MHz above the picture carrier. If it's tuned to say 4.7MHz, it will miss the sound signal from the VCR or other source. Tuning the fine tuning knob while ignoring the picture quality might find you the sound signal. Try channel 2 and channel 4 and hunt around with the fine tuning and see if you can find the sound signal. If you find channel 3's sound on channel 4, but no picture, then that could mean that the local oscillator is on the wrong side of the TV channel.

[3Guncolor]

That is a good point about the lower sideband, I forgot all about that. My test modulator has the sideband filter and all the modulators where I work have the filters. Keep trying the frequency must be off somewhere. I like Wa2ise's idea about what could be the trouble. Don't give up on the set. You have all the hard stuff working.

[kbmuri]

Sticking with the Rat Shack receiver as my Video/Audio source. Much more convenient for the source to be something the size of a coffee mug vs the size of a briefcase.

Receiver connected to matching transformer with 1 foot of coax. Transformer to antenna terminals. TV on its side, receiver placed on chassis right side (now top). DVD playing same (obnoxiously loud music) intro-screen loop. Excellent video as usual from haunted roundie. Generator connected directly to antenna terminals via alligator-clip coax probe. Counter alligator clip to chassis ground and induction-coupled to generator (alligator clip attached to coax insulation of generator probe). TV set to channel 3.

Generator unmodulated, sweep from 50MHz to 70Mhz:
56.7 MHz-, normal video.
56.7 MHz, just notice some effect on picture quality
56.7-58.5 MHz, picture fades smoothly to black
58.5 MHz, black screen.
58.5 - 63.1 MHz, black screen (Zero-beat is in there somewhere, can I narrow this down?).
63.1 Mhz, just notice change in black screen
63.1-64.5 MHz, smooth fade back to normal video
65.5 MHz+, normal video

Tone modulation applied to generator (500Hz). Sweep back down the range:
64.9 MHz, first notice audio at 1/4th volume setting (comfortable level)
63.90 MHz, first maximum tone level, fairly loud
63.65 MHz, very noticeable lull in sound, near quiet
63.20 MHz, 2nd maximum tone level, probably louder than 1st by a little bit.
61.50 MHz, Last noticeable tone
61.60 MHz-, silence

59.27 MHz, Maximum point that tone bars on screen completely override normal video. Elsewhere in the sweep, at least some video is apparent.

[wa2ise]

Originally Posted by kbmuri

56.7-58.5 MHz, picture fades smoothly to black
58.5 MHz, black screen.
58.5 - 63.1 MHz, black screen (Zero-beat is in there somewhere, can I narrow this down?).

Try reducing the amplitude of the generator. Is there an amplitude selector switch and pot on the generator? When you get to this point, reduce the generator output, and then adjust the frequency until you get only a few bars of lighter and darker superimposed on the DVD via TV modulator image in the screen. This will not be easy to do, and won't be stable, but if you see it fleetingly, that's close enough.

[kbmuri]

I'll try reducing the generator's amplitude Friday.

One thing we know, though, is that the audio from the generator doesn't get through at the desired 65.75 MHz, regardless of its amplitude. Finally, this agrees with the fact that there's no TV-Signal audio either (assuming it's at 65.75 MHz too). Something I can grasp for a change.

Assuming the generator is AM only (I believe my 'scope bears this out), then the "lull" at 63.65 MHz is probably the dead-center point you described in post#6? If so, then is the TV mistakenly looking at 63.65 MHz for audio?

Assuming the zero-beat of the video is centered in the "black-screen" sweep, then the video and audio are only 2.85MHz apart. More likely, if the zero-beat is in fact at 61.25, then the spread is only 2.4 MHz. Not 4.5MHz.

If the TV is looking at 63.65MHz for audio, what practical steps do I take to move that up to 65.75?

I know, lotta "if"s and "assuming"s. Will verify stuff Friday. Just planning ahead.

Thanks all, for all the help to date.

[wa2ise]

Originally Posted by kbmuri

I'll try reducing the generator's amplitude Friday.

One thing we know, though, is that the audio from the generator doesn't get through at the desired 65.75 MHz, regardless of its amplitude. Finally, this agrees with the fact that there's no TV-Signal audio either (assuming it's at 65.75 MHz too). Something I can grasp for a change.

Assuming the generator is AM only (I believe my 'scope bears this out), then the "lull" at 63.65 MHz is probably the dead-center point you described in post#6? If so, then is the TV mistakenly looking at 63.65 MHz for audio?

Assuming the zero-beat of the video is centered in the "black-screen" sweep, then the video and audio are only 2.85MHz apart. More likely, if the zero-beat is in fact at 61.25, then the spread is only 2.4 MHz. Not 4.5MHz.

If the TV is looking at 63.65MHz for audio, what practical steps do I take to move that up to 65.75?

If I recall correctly, this TV set does not use the intercarrier system (using a sound IF at 4.5MHz) but has an independent sound IF normally set 4.5Mhz away from the video IF. But even if this offset is off frequency, you should still be able to play with the TV's fine tuning knob to find the sound carrier (even if the video becomes unwatchable). I assume that the fine tuning can move the tuner's local oscillator by 6 MHz.

If the sound IF is off frequency, you can "walk" it up (or down) in frequency by adjusting the signal generator first dead on where the IF is set now, then adjust the generator so it's slightly off in the needed direction. Then tweak the IF transformers to make it dead on again. Repeat multiple times.

[kbmuri]

Correct, this is not an intercarrier system.

The fine tuning on this set, unfortunately, is pretty lame. Unless there's something broken about it. It's just a piece of phenolic or bakelite, spirally tapering, floating in front of the tuner. Too hard to describe, I'll have to shoot a photo.

I doubt it can move the local oscillator 6MHz. The effect from full counterclockwise to full clockwise is noticeable, but "feels" like turning a typical fine tuning only a few degrees left and right of tuned.

There are coarse-tuning slugs per channel that I haven't tampered with yet. I'll give channel-3's a spin Friday too, see if I can find any sound that way. Presumably the picture will go south at that point though. Need to get that 4.5 MHz separation.

[kbmuri]

With the generator's amplitude turned down, I'm clearly zero-beating the video at exactly 61.25 MHz. This makes sense to me because the TV is obviously processing the 61.25 MHz video signal perfectly, so it must be tuned there exactly. The picture is crystal clear. The zero-beat is when the generator perfectly matches the video signal -- the TV under test isn't really part of THAT experiment. We just demonstrated (again) that my frequency counter is very nicely accurate.

The fine tuning knob doesn't change the local oscillator 6 MHz, but the oscillator tuning slug A14 sure does. I was able to put the Ch3 video on channel 2 and channel 4 with it. I watched the video signal come down from the topside to Ch2, and up from the bottomside to channel 4, and all across the slug's range on channel 3. Volume at max, not one peep of audio signal. I even turned the generator's tone modulation on, set it at exactly 65.75 MHz, and tuned the slug for maximum sound. Turned the generator off, STILL no audio. The generator tone is apparently only getting thru because it's AM. Completely assuming the audio of my Rat Shack receiver is accurate, at 65.75 MHz, and present. Just doesn't get thru the system. Only difference is AM vs. FM.

I didn't see any point in messing with the coarse-tuning slugs in the tuner turret. Those are for adjusting a single channel once A14 gets most of them right, if one or two is off. The sweep of A14 ruled that out to my satisfaction.

Still stumped.

[wa2ise]

Originally Posted by kbmuri

Volume at max, not one peep of audio signal. I even turned the generator's tone modulation on, set it at exactly 65.75 MHz, and tuned the slug for maximum sound. Turned the generator off, STILL no audio. The generator tone is apparently only getting thru because it's AM. Completely assuming the audio of my Rat Shack receiver is accurate, at 65.75 MHz, and present. Just doesn't get thru the system. Only difference is AM vs. FM.

Another thought, using another TV set, see if you can "jam" the sound carrier with your signal generator, at 67.75MHz. Only a few dB stronger signal on the sound carrier should be enough to achieve "capture" of the FM detector, thus blotting out the normal sound signal. You should hear only a faint detection of the generator's AM modulated signal on this other TV set. And if you have the set under repair also receiving the same TV channel (or the VCR RF output) and see the same video and hear the generator's modulated AM, I'd suspect that the FM detector is somehow misadjusted or broken and can only hear AM and no FM. That would be the final sound transformer. If you can tweak the detector for max AM detection, and the VCR sound carrier is jammed on the other TV, try turning off the generator and try tweaking the detector to see if you can get some FM sound.

[kbmuri]

I bought a couple of B&K Television Analysts with the intent of fixing one of them.

http://cgi.ebay.com/ws/eBayISAPI.dll...m=140136583787
http://cgi.ebay.com/ws/eBayISAPI.dll...m=170128981748

Reading the manual, I gather it's a pretty amazing tool. Seems as though it can inject a signal at just about any point in any (vintage) TV and give a go/nogo indication for that stage.

I'm putting the Admiral aside while I restore one (or both) to working, then maybe I can learn more with it to fix the Admiral. The manual claims any seasoned idiot can easily diagnose any TV problem with it. Including audio. We'll see if it's true.

[kbmuri]

What's the proper local-oscillator frequency for channel 3?
The Admiral from Hell uses a 6J6 as the oscillator/mixer tube (it's a twin-triode). Side A is the mixer, side B is the oscillator. The plate of side B is pin 1, the control grid is pin 6. I put my frequency counter on pin 6, and also thru a 100pF cap to pin 1. The results were troubling.

[Einar72]

Try a loop or two of wire around the tube. VHF circuits get very unhappy when a big chunk of stuff comes a-connectin' to easily-detuned or easily-loaded-down nodes. Hopefully your counter has enough gain to deal with the lower input level.