'49 Admiral audio problem
 

All,

I'm hopefully finishing up a 1949 Admiral chassis 20B1 (Sams sheet 77-1) shortly. Complete recap, all electrolytics and wax paper caps are new. All tubes are NOS or test as such. Since these photos were taken I've centered and shrunk the picture height so that it's the classic double-D shape. The vertical linearity is set very well. Horizontal linearity is still just a tad off (you can see a drive-line just to the left of Daria's lamp -- but that's beside the point just now). The picture is rock-solid and the focus, contrast and brightness work smoothly and correctly. Channels 3 and 4 work as advertised, that's all I can test with what I've got for video sources (No VHF stations nearby).

When I run my signal generator and inductively couple the output to the TV's antenna terminals (just place the coax nearby), I can tune in very crisp horizontal bars, as shown. Tuning the signal generator to a slightly higher frequency results in fuzzy bars, and a very clear, loud tone from the speaker. If I turn the volume control up, the tone is loud to the point of risking damage to the speaker (and my eardrums). I'm convinced the audio amp circuits are golden.

When I run a VCR thru the set, matched-transformer-coupled from the VCR's coax to the antenna terminals, I get a great picture but no sound at all. Adjusting the TV's fine-tuning has no effect on the audio, normal effect on the video. If I unscrew the sound-IF coil slugs (A2 and A5 of L18 and L19) to what seems like an unreasonable distance outward, I get very minimal audio (volume control all the way up, still too muffled for comfortable listening in a quiet room, and the fair amount of hiss and a little 60Hz hum that you would expect with the volume control all the way up). This is the best I can do, regardless of where I set any other adjustments anywhere else on the chassis. I'm a little stumped at this point.

Anybody out there been through this? Any suggestions? Thanks.

This looks like one I recapped 10 years ago. I had the same problem. What you're up against is technology from the 1930's. This is what's known as the split-sound design. The sound IF stages are fed signal from somewhere near the tuner, and amplifies it at its natural IF frequency, 4.5 MHz away from the video IF signal. This differs from the later inter-carrier system, where the sound carrier signal shows up as a difference-frequency of 4.5 MHz in the video-demodulator stage, which is easily snagged and of consistent amplitude.

In your case, the AGC is probably driving the frontend gain too low --due to the strong VCR signal-- for the sound section to get enough signal. I recommend you do this: 1) review the video IF response-curve at the sound takeoff-point to see if the sound-carrier level is not down too far. 2) recheck the sound IF down to the last resistor, realign per spec, then 3) use an attenuator with the VCR or whatever you're inputting; you shouldn't have to adjust it again with the input level just past what's needed for good picture contrast.

Your video alignment looks really good from the picture! :thmbsp:

49 Admiral
 

Don't forget that in the turret tuner there is a coil for each channel that can be adjusted with a small screwdriver. Adjusting will affect both picture and sound for an individual channel. I just finished 2 of these sets earlier this year and had a concern about the old technology they used. As it turned out, I did not have to use alignment equipment. The sound on all the local channels turned out clear and plenty loud.

Einar72, your prognosis seems logical. I was thinking the audio IF Frequency was just off and I wasn't tuning the sound carrier, but your explanation makes more sense. Unfortunately I recently smoked my vectorscope -- just bought an eBay replacement last month, but it needs a recap. It's annoying when you have to fix your tools before you can fix your toys. I'm working that problem first, then I'll post a shot of the response curve.

El Predicta, thanks for the extra info. The sound loss is identical on channel 3 and channel 4. This suggests the problem is after the tuner turret, but I will still adjust those coils, last, using my signal generator, centered on each channel frequency range, for the channels other than 3 and 4. I'm doing this one for a friend, who will be hooking it up to a cable box, so channel 3 or 4 is probably enough.

Thanks for the help!

I gave this little guy another shot Monday. It was a way to rationalize not mowing the lawn on my day off. I had two anomalous resistance readings according to the Sams resistance chart. Pins 1 and 2 of the 6AL5 audio ratio detector (V11) were supposed to be infinity Ohms to ground, but were at 280K instead. I traced it down to the fact that C37 (pictured), a .005 mFd cap was not present. The wiring appeared completely original where the cap should have been. I assume it was a production variation, but I put one in anyway to obtain the infinity-ohm reading at the two pins. The set's behavior did not change. All tube-pin resistance readings are now accurate.

I originally pulled each tube and tested it, if it did not test 100% I replaced it. Stupid me, I didn't check them against the schematic. Three of the tubes were replaced with incorrect replacements. The 6AC7 video amp tube had a 6AG7 installed, the 2nd sound IF amp 6au6 had a 6AG5 installed, and the 6AU6 AGC amp had a 6AG5 installed. I was pretty excited when I discovered this oversight, but installing the correct tubes made little difference. The brightness and contrast controls are more resposive with the correct video amp tube. The picture and sound quality might have come up just slightly. But the audio level is still 10% of what it should be.

If I pull the AGC tube while the set is running, the picture goes gray and the audio level comes up slightly (along with lots of buzz). I've also noticed that the audio varies slightly with the contrast control (more gray=less sound). But all of it is a few percent variance around the crappy 10% audio level.

I should also add that with no signal applied, the sound of "snow" is just as muffled -- at full volume it's down there at 10%. Is this normal? I would think the sound of snow with no signal should boom, and if so, then the AGC theory as the culprit would be questionable.

This is a 2-chassis set, the power supply and audio amp are separate. After the 6AL5 ratio detector and before the 6SJ7 audio amp there's a removeable RCA-jack & cable between the two chassis. I plugged the output from a transistor radio into that phono plug, and the sound booms.

I've double-checked every resistor in the sound-if strip. All are well within tolerance.

I recapped the Sencore PS148 Vectorscope but it has other problems. The dot only appears on the screen at max intensity, and the intensity control makes the dot wander off the top of the screen. Horizontal position control works, but vertical position control doesn't. Focus control does nothing. Inputs don't deflect anything. Not worth bothering, I'll look for a Heathkit. So I still can't do any alignment work, but I've always been able to "eyeball" an alignment pretty close. The alignment slugs on this TV seem to be doing what they're supposed to be doing. I get buzz when I should, picture tear or fuzz when I should, I'm doubting it's just an alignment issue.

Really annoying. I can't bring myself to abandon this one. Already have $150 in parts and days of labor and it's a favor for a friend (after bragging that I could fix it). So my reputation is on the line. Sure would hate to pull defeat out of the jaws of victory, when it's this close. But I'm getting pretty bummed.

Any other suggestions?

A few suggestions:

Turn to channel 6 and see if you can receive a local FM radio station at around 88.3 Mhz or so. The split sound IF should allow you to do that. Be aware that Fm radio stations use higher frequency deviation than TV sound carriers use, so the FM radio station should come in louder.

With the VCR output, try loosly coupling the signal into the antenna tterminals. Do it so the picture is snowy, see if that makes a difference.

With no signal on the antenna at all, you should get a fair amount of FM hiss, like on an FM radio between stations.

Sounds! like the FM detector is acting more like an AM detector. I'm guessing that your bar generator is just an amplitude modulated carrier at a frequency near channel 3 or such*. Video is AM modulated onto the picture carrier. An FM detector would just get quiet (hiss going away) when you get the carrier frequency dead center of the FM detector.

An AM detector will not get much signal out of an FM signal. but some if you "slope detect", ie, tune it off center. It would sound lousy though. Not that your TV set was built with an AM detector, but maybe that small electroytic on the ratio detector went bad, shorted or completely open. Try replacing it with any modern electroytic of about the same capacitence (you could just salvage one from a modern circuit board, I've done that and it works fine). Or some other part close to one of the 6AL5 diodes is bad. Or a dirty tube socket.

Also check that the 6AL5 heater is getting about 4 or more volts. Check R48, a heater voltage dropping resistor (used to reduce "contact potential" on the diode cathodes. Maybe that resistor went bad.

*And check to see that your generator can produce a clean audio tone on an FM radio set to 88MHz. That would tell you if it's producing true FM signals. If the FM radio seems to get audio slightly mistuned high and low, but quiet on center, then the generator is producing AM.

Wa2ise -

I tried loosely coupling the VCR signal on Monday -- forgot to report it. With a snowy picture, I get correspondingly weaker sound (less than 10% of normal). Sounds I would expect of a working TV, just quieter.

The "bar generator" is generating AM. It's just a standard sine-wave generator, amplitude-modulated with a fixed tone (middle-C?). The sine wave frequency is adjustable from a few kHz to many mHz. The tone is not adjustable.

The effect is bars when tuned to the video frequency, and tone when tuned a few mHz higher. It does this on all my TVs, old and new.

I have a battery-powered 1-inch Sony Watchman on the workbench for reference. I tune the (variable) tuner to Ch3 or Ch4 first, using the VCR as input. Then it shows bars when the Unit-Under-Test shows bars, and sounds a tone when the UUT sounds a tone.

You are correct, the Sony's tone goes to dead silence when the signal generator is "dead on", but is nice and loud on either side. I believe the UUT was doing the same thing, but I need to verify this.

The signal generator will produce an audio tone on my Magnavox PLL digital-tuner pocket radio. I use this radio for calibrating the generator. AM and Shortwave bands sound clean, FM is distorted. I will try seeing if the radio "silences out" when an FM frequency is tuned dead on. I suspect it will.

The C5 Electrolytic is new from Mouser.com, fresh stock. It's a 4.7 mF, not a 4.0, if that makes a difference. I didn't put it on a cap checker though (even though I have one). Lazy, I guess. I'll doublecheck it.

The R48 resistor is a little low, about 4 Ohms even. I don't have any 4.7's on hand.

I'll check the voltage at the 6AL5 filament.

I've rocked all the tubes in their sockets and swapped/rotated them all. No apparent effect. Nothing seems loose/dirty.

I will try tuning to Ch.6 and trying for some FM stations. I flipped the tuner thru the cycle once or twice and don't recall any, but I wasn't looking for it.

Excellent ideas, thanks!

All the radio tests that Wa2ise recommended performed as expected. FM radio tunes to dead quiet at the very center of "loud" on either side. There is a noticeable lull but not dead silence in the TV set at "dead on" signal.

The TV cannot tune any FM stations on channel 6 around 88.3 mHz, but unfortunately neither can my FM radio (nearest station is 89.1).

The 6AL6 filament is at the same 6.3VAC as the other tubes in the sound IF strip. The Sams voltage chart says that's how it should be. The 4.7-ohm resistor R48 doesn't appear to be significant enough to drop the voltage downstream. I'd be interested in the equation that tells me what the resistor should do. I guess it limits some current to some degree. It measures 4.2 Ohms with an el-cheapo digital Ohmmeter, about 4.0 even with my old trusty analog read-the-needle one and my nearsightedness.

The Sams chart expects 0.1 VDC on pins 1, 2, and 5 of V11 (schematic 3 posts ago). With VCR input I get 0.024 on pins 1&2, 0.054 on pin 5. With my signal generator as described earlier, I can tune a maximum of 2.81 VDC on pin 5, when the tone is at its loudest. It drops off abruptly as I tune the signal generator away from "dead on" point. Hangs out around 0.025V when the signal generator is tuned well away from the target frequency.

I can understand pin 5 having DC, and varying with signal strength. If there's an AC source on pins 1 and 2, pin 5 will be the rectified component of pin 2. Smoothed by C5.

But I don't understand why pins 1 and 2 should be at a positive DC potential. Where is the source? But without understanding why, it's still troubling that I'm at 24% of the recommended daily allowance.

The Mouser electrolytic cap tests fine.

Everything sure looks wired up right and within resistance tolerance. I don't know, maybe it really is an alignment issue. There's an antique fixit shop 90 minutes from here that specializes in TV alignment. Maybe I should "know when I'm licked" and take it in.

Anybody out there have a schematic for a Sencore PS148 Vectorscope? I'm sure mine is broken in an obvious way and could be repaired easily, with a schematic.

In a sense, both diodes are in series, and as the negative end of the cap is tied to ground, the IF transformer secondary coil would tend to be at about half the voltage of what the positive end of the cap has on it.

Maybe one of the resistors have gone bad? R46 or 51 going too high in resistance could do it. Also, C37, as shown on the schematic, seems rather small. I would have expected something like 0.02 or so.

Thanks again, wa2ise, I've looked at that schematic a hundred times, and have seen pin 2 as a cathode and pin 5 as plate every time (I guess I've been looking at too many 12AX7s). So I've been imagining the top diode backwards. When you said the diodes were in series, I said, "huh?". Then I looked a little closer. Ok, so I see why the secondary coil has a DC potential from chassis ground. And, like you said, .024V is about half of the .054 on pin 5 (also at the cap +). So I guess that circuit is correct.

Like I said earlier, C37 was not present originally (solid wire intead). No sweat to try an .02 mFd.

I'm willing to replace all the resistors in this circuit, just in case one is failing under a load more extreme than my Ohmmeter. Although the whole kit-n-kaboodle is operating in the 0.1-Volt neighborhood, so it's hard to imagine they're particularly stressed out.

If I'm understanding well, the DC potential at the + side of cap C5 could be said to vary in proportion to the amplitude of the AC signal as seen at the secondary of the IF transformer. Demonstrated by the fact I could get pin 5 to read nearly 3 Volts with the signal generator applied. If so, then I suppose a low DC reading at pin 5 might still be a symptom of weak signal from further up the line. If so, then I'm still just as "in the dark" as I ever was. The Sams isn't clear whether the 0.1V expected at pins 1 & 2 are with or without a signal applied.

The capacitor symbol connecting across the IF transformer secondary has no part number and doesn't appear to exist on the TV chassis. I've been assuming it's physically inside the IF transformer's shell (or a measurable property of the transformer itself, perhaps?). Could this have gone bad? I'm reluctant to pull the transformer and start digging inside it on a "maybe". Just throwing it out for debate.

You may have noticed I can only work on the set on the weekends. Other pressing tasks. So any suggestions and comments during the week are more than welcome, even if it takes a little time to get to them.

Or does the DC at C5 vary with the frequency of the AC at the transformer secondary? This is FM, right?

Sounds like the detector is probably working, and the problem may be a dead sound IF stage
That's not likely to be the problem, then. Just leave the wired connection.

If they read reasonably correct (20%) with the ohmmeter, then they should be okay and not need replacing.
That cap is inside the IF can, and is not likely to be bad. Though if it did go open circuit, it would screw up the resonant circuit frequency. Not likely.
thats okay :-). I think you said that you did check the circuits in the sound IF strip (do you get voltages on tybe plates, and do you see a volt or so of voltage across cathode resistors of those tubes?). A broken IF stage could ruin the sensitivity of the sound detector. Maybe an IF transformer went open circuit.

Here's some info from the Admiral service manual. If all else fails pop out your 6SQ7 and replace it with a 6SL7 and rewire with two stages of audio.

I was forced to mow the lawn last weekend so I couldn't do much with the TV. More this weekend.

Thanks 7"estatdef for the photos. The 6SL7-swap idea is genius, and if I have to go there, I will.

I don't notice much sound-level change from adjusting A9, but it does alter picture quality as expected, so I imagine it's working. I can try some of the other mods suggested in the photos, but will stop short at finding new IF transformers. Probably unobtanium, and a redesign is a lot of work for fixing something that ought not be broken.

Wa2ise, The voltages at the sound IF tube plates are correct per the Sams voltage chart. I don't have the data in front of me, but if I recall correctly, the first stage's 6AU6 plate was at 75 Volts, the other was at 150. I didn't check voltages across the cathode resistors, but will this weekend. The resistors themselves check ok though. Resistance checks of the IF transformers say they're not open-ciruited anywhere. Shorted, perhaps. They're pretty much 0 ohms at DC. I assume they put up their struggle at IF frequencies.

I put a 'scope on the sound-takeoff point and see some slight signal there. The load of the 'scope drags the sound level down even farther (half again?), so maybe a small observation there is normal. The takeoff is just a single loop of thick soft wire from ground, once-around the first video IF transformer, then about ten loops of tuning slug, then to the grid of the first 6AU6 sound-IF amp. So at DC, it's grounded, but I guess at sound-IF frequencies a small signal is induced. How much I should see on the 'scope, I'm not sure. I should probably restore another split-sound-system set, working, for reference. I have an RCA on deck that's split-sound too. I hate having 2 sets on the bench at the same time though.

I'll shoot a few pics of 'scope readings. Maybe something will click.

Thanks for all the help.

If the TV sound is just weak, but is undistorted and noise free when you do turn it way up, then adding an extra audio amp stage with the 6SL7 would be a sensable thing to do.

I got off of work a little early today so in my spare time I rewired the audio amp to use a 6SL7. Before the volume was low. In a quite room it was ok but add in kids or a wife and it was tough going. Not the case after the 6SL7 was added. Now it has pleasnt room filling sound and that's with the knob turned half way up. I did change a few values of the original caps to boost the high fq responce.
As is note in the service manual it seems as though the original has some flaws but this puts them to rest. I'll also mention that there's a good article that discribes a vertical blanking circiut for your set. I refasioned it to work on my set and it too works great.No trace of retrace! Happy to pass it along I got it off the web.
Terry

Yes, please forward me the vertical-blanking circuit mods. PM if you need an email address.

Here you go... Read about it at:

http://groups.google.com/group/rec.a...e39.0207011623.

Diagram below

After rereading this info I think I know why my picture is slightly smaller on the right side (abt 1/4") I'll bet I need ato adjust that shaping network.
Terry

ok, so the 6SN7 swap is really a good idea for this set, but as wa2ise hinted at, the extra stage of amplification just amplified some pretty rotten sound, louder. Hum and tinny, unacceptable.

So I did a complete IF alignment, by the book. With my crude tools, but I'm sure I nailed it. My Knight am signal generator made sine waves, and I set probe A of my tektronix dual-trace scope to monitor them, setting the sweep to 0.1 microseconds/div (or 1 microsecond per full frame of 10 divisions). Counted the waves by hand to make sure the 2nd-hand Knight generator was dead on. 21.25 Mhz where needed, 23.5, 25.3, 22.0. Then I used probe B to measure voltage deflection at test points. Had to build a detector probe per the Sams schematic and a resistor-capacitor filter test point and an insulated tube shield with test-clip loop (for injecting signals inductively into the 1st tuner tube). Quite a time-consuming affair.

Every slug in the sound IF strip was way off base. Every slug responded marvelously, every one is set perfectly now (maximum deflection where called for, minimum deflection where called for, zero-reading where called for). It was really exciting to see everything come into tune, and I was pretty optimistic about the whole process. I even got the languishing .025-volt discriminator output back up to where it belongs. Sure thought it would do the trick.

After removing the filter test point and reassembling channel 13 of the turret tuner and replacing the tube shield, I re-applied VCR signal. Lost all my sound now, and made crap out of my crystal-clear picture.

How exhausting is that?

If this set were my own property, I'm pretty sure I'd part it out now. It got the best of me. Best I think I can do now is take it to a pro, and hope he's not too judgemental of my recap and willing to work on something that's been worked on by an amateur.

Or I suppose I will sleep on it one more time...

Counting waves isn't really that an accurate way to measure frequency. But it does sound like all the circuits fundamentally are working, but at a somewhat wrong frequency.
Been there at times... VCRs usually create a signal on either channels 3 or 4. I presume you didn't try to use channel 13 (I've made similar silly mistakes myself... :-) )

I wish I had a professional's lab, but for most things what I have is adequate. This is the first job I've done that's required some real precision. I admit counting waves on my scope assumes that when the scope says it's sweeping at 1 mHz, and I count exactly 22 peaks from leftmost grid to rightmost grid, that my generator is broadcasting at exactly 22Mhz. A leap of faith, no doubt. So ok, I probably perfectly aligned the set to the wrong specs. This is a learning deal for me, this one.

I've got this coming in the mail next week: http://cgi.ebay.com/ws/eBayISAPI.dll...m=170122318458 which may or may not be calibrated either so I guess I still need some absolute reference. But at least it'll be another opinion. The price was very right, and nixie tubes are cool, but I may want to look into buying something newer and more likely to be using modern technology. Anybody out there have a strong preference/recommedation?

Sorry, I guess I didn't mention the the Sams alignment procedure called for removing the channel 13 turret coils and then tuning to channel 13, to disable the local oscillator "to prevent erroneous indications". Reinstalling them was the last step after the IF alignment procedure. Of course I tuned back to channel 3 to view the VCR output.

I'll give it another shot before parting it out...

The images in the first post are very, VERY cool :yes:

Yeah, and I got there after only one evening. This audio thing has been nothing short of miserable, and it's weeks later. And right now, even the cool images in post 1 are gone. Like I said, this one's giving some tough lessons. If I beat it, I'll be more of a "pro" for later on. Worthwhile in that respect, but I wish the dam thing would go away.

A way to test the counter is to get a crystal oscillator using a known frequency crystal (like a color subcarrier crystal, 3.579545MHz). And see what the counter says. But counters are a bit fussy to work with, they need a minimum level to count accurately. Another and easier way is to connect your signal generator to an antenna and the counter, mess with the levels so you get a reading that looks about what you'd expect. Set the frequency of the generator to match that of a local mid signal strength AM radio station of known carrier frequency and zero beat the signal generator as heard on an Am radio in the room. And see what the counter says. AM radio station carriers are quite precise to something like 0.1 part per million. If it yields a correct (or very close) number, it should give accurate readings for the alignment frequencies for the TV work.

Please don't part the TV out, someone here at AK will probably be interested in it. As it looks like all the major hard to find replacements for parts look to be working well (the CRT, flyback, yoke, power transformer and such).

It isn't mine to part out. A friend was very excited to have found it for 50 dollars at a flea market in Shipshewana, Indiana. She knew I had some experience in repairing them, so I said ok. It would have scored me some points to get it back to her in 2 or 3 days. As it is, I'm starting to look bad.

I bought these items on Monday. I should have them by Friday.
http://cgi.ebay.com/ws/eBayISAPI.dll...m=200119280910
http://cgi.ebay.com/ws/eBayISAPI.dll...m=200061373963

I'll put the Heathkit online too, so between the generator, counter, and Heathkit counter, I'll have 3 opinions of my signal frequency. If they all match, hooray.

I'll zero-beat 1190 kHz (WOWO Fort Wayne) for an absolute reference. I would like to try to do the same for WWV Fort Collins Shortwave at 20.00 mHz because that's a lot nearer the frequencies I'm targeting. 20.00 doesn't come in too often around here though. 10.00 often. I think WWV also transmits some reference audio, accurate to some parts per billion. It'll be a fun "science project" late-night insomniac event.

Just glad I'm not being paid by the hour. I'd have been fired some time ago.

Got my test gear. This is a cool photo. Both counters are extremely accurate. Was able to zero-beat WWV on the digital radio at 15mHz (20mHz doesn't come in often). Both counters were dead on. Also did WOWO 1190 Khz, same results. The radio's top shortwave frequency is 21.75 mHz but no broadcast there. Still was able to tune everything to that and tone-modulate the generator and hear it there on the radio and see 21.750 on the green counter. The Heatkit scrambles its brain around 20.000 so I suspect that's its advertized maximum. So I'm glad I got the heatkit for show, for cheap, and the new one for doing useful work. Even if I can't fix this audio problem, it will be nice to know how to maximize performance on my other sets. Fun stuff.

First thing I did was check my 'scope against the new tools. At 15.00 MHz and zero-beating WWV, my 'scope set at 0.1 microsec/div, expecting 15.0 sine waves to be displayed from left grid to right, I got 16-1/2 instead. At 25 MHz I had 27-1/2. So ok the scope is out of calibration and the previous IF alignment was a waste of time. It's a Tektronix 465B that I paid pretty good money for & works perfectly otherwise. I'll probably get the motivation to recalibrate it sometime. Now that I have a good reference.

I repeated the Admiral's IF alignment procedure at the accurate frequencies. Got the crystal-clear picture back. Audio is dead silence again. Right back to post-recap square-one.

I snagged an apparently-working replacement Sencore PS148 so I can get my sweep setup going. http://cgi.ebay.com/ws/eBayISAPI.dll...m=250136626040

Hopefully the sound isn't making it out of the tuner, somehow. Next week maybe...

To get a rough idea of where the bad part of the sound IF chain might be:
Do you get a fair amount of "FM hiss", like that you hear on an FM radio tuned between stations? If I recall correctly (IIRC) you were getting some detected audio, but very weak. If the sound IF is operational but on a wrong frequency, you would be able to get good sound but lously or no picture, or good video and no sound. Do you get much FM hiss with the TV tuned to an empty channel? If not, there may be a not working late sound IF stage that's losing you the sound signal strength. Or a weak link from the TV tuner to the sound IF, as you mentioned.

With your generator set to the proper sound IF frequency, do the alignment. But you should be able to set the output of the generator level to lower levels as you step thru the IF from the detector to the tuner. (you'd do this by injecting the generator output, via a small capacitor like 100pF @ 300V or higher rating, at various points (tube grids, tube plates) in the sound IF stages. You should be able to make it to the plate circuit of the tuner mixer tube (thru a capacitor) with a very low output setting on the generator. If you lose it, or have to jack up the level, you've found a problem area.

No hiss at all on empty channels. I'm sure it's down there with the rest of the missing audio. I only got weak audio by drastically misadjusting a couple of the tuning slugs at the very end of the IF strip (nearest the 6AL5). I have no audio at all, presently.

I don't know if the new signal generator is modulating FM. One would think that would matter. I'll play with it some more and find out.

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.

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

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

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.

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.

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.

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.

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.

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.

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.