'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.