|
|
|
#121
03-07-2014, 03:44 PM
|
||||
|
||||
|
It's really not over engineered. Take a look at any RCA or Admiral from the same year and they're pretty much the same. What you experienced were sets with inter-carrier sound where both video and audio go through a common IF. The audio is picked off after the detector and a trap keeps it out of the final video stage.
This set uses a separate IF for video and audio like many sets of the late 40s - early 50s. Here the audio trap is right after the tuner and two are amplified independently. The reason I had so much trouble with aligning the set was bad mica caps and damaged video IF transformers. I wish I could have replaced them, but spare parts are hard to come by these days. 
|
|
#122
03-07-2014, 09:25 PM
|
|||
|
|||
|
Quote:
Its being 'approximately' correct is probly good enuff in most cases.  In a TV with split IFs, you definitely want the sound IF to be correct for the sound to track correctly with the pic.
|
|
#123
03-09-2014, 06:44 AM
|
|||
|
|||
|
Hi Old Coot,
I'm talking 5 maybe 10 Khz here. I must try a mathematical analysis some day. I know I set the local oscillator to the low end and tuned the trimmer at the high end, so the dial was set correctly. Let me give this a think. Bob, I never worked on a double IF strip set. I hope the engineer who came up with the single strip got a good raise that year. Did you try turning those two adjustments on the top of the tuner? -Steve (old fart)
|
|
#124
03-09-2014, 07:10 AM
|
|||
|
|||
|
Bob,
In "Admiral 24C16 restoration part 5 (we have sound!)" at 3:30, you adjust a coil on the tuner which fixes the audio problem. Specifically, it adjusts the output IF frequency of the audio band to properly go through the audio IF stage which you just aligned. You do not re-adjust the fine tuning of the station and presumably the video remains clear (the video IF strip still gets what it needs from the tuner output, frequency-wise). How can this be without two mixers in the tuner? See my previous post about two mixers. Steve
|
|
#125
03-09-2014, 11:12 AM
|
||||
|
||||
|
Quote:
No, I didn't touch the tuner. Quote:
Last edited by bandersen; 03-09-2014 at 12:00 PM.
|
| Audiokarma |
|
#127
03-10-2014, 05:39 PM
|
||||
|
||||
|
Yes, I think so. Here the response curve of a typical tuner. Ideally, one hump is the sound carrier and the other the video carrier. That's how you get away with one mixer. The response is wide enough to include both, but not so wide it picks up adjacent channels.
|
|
#128
03-11-2014, 03:34 AM
|
|||
|
|||
|
Got it.
I wonder is there a way to show the bandwidth of the tuner? Can you sweep across, say, channel 3 frequencies with any of your signal generators? That might be an interesting trace, and a way to check tuner performance. I do not know how low the levels are coming out of the tuner. -60dB? But maybe the input signal could be pretty hot to boost it a bit. One might need a sensitive spectrum analyzer for this test. Steve
|
|
#129
03-11-2014, 01:11 PM
|
||||
|
||||
|
The marker blips in the photo are 4.5 MHz apart. If I went out to the -3db point it would be around 5.5 - 6.0 MHz bandwidth.
Sure, I have sweep generators that can cover that easily. I can already tell you what you'd see though. It would be the image above squeezed horizontally.
|
|
#130
03-11-2014, 09:21 PM
|
|||
|
|||
|
Squeezed or stretched? The tuner output has to be at least as wide as the IF strip. You wouldn't need the IF skirts so much if the tuner were set at 6 MHz wide. Maybe I am misunderstanding your meaning.
Actually, the tuner could be very wide, say 18 MHz. The IF strip just picks out a 6 MHz window as the tuner slides the spectrum back and forth in front of the IF window. I have seen sets actually tune to adjacent stations when the fine tuning is adjusted far off to the extreme ends of its range. That means that three channels can come through with the channel selector is set to one channel. This doesn't necessarily mean that the bandpass of the tuner is very wide. But with a good IF, there isn't a need to design tight passbands into tuner mixers and tuner RF amps. I suspect this is the cheapest way to build a tuner and IF strip combination. And it explains why there is rarely a need to re-align a tuner. It is so wide that it doesn't ever go out of spec as part drift out of tolerance. And it explains why the IF strip is so critical. The tuner doesn't do anything to limit the bandwidth. The IF does all the work. Food for thought. Steve I wish you had a pricey spectrum analyzer that could look at the raw tuner output.
|
| Audiokarma |
|
#131
03-11-2014, 09:42 PM
|
||||
|
||||
|
I meant if I expanded the width sweep generator the response curve would become squeezed on the scope. All I can tell you is that in all the late 40s TVs I've worked on the tuner had about as wide a response as the IF.
I'm not sure what would be gained by a spectrum analyzer that you can't see with a sweep generator and scope.
|
|
#133
03-12-2014, 01:40 PM
|
||||
|
||||
|
Well yes, that's what the photo is above - the actual response curve of a tuner.
I connected a sweep generator to the antenna terminals with proper termination and connected my scope to a test lug they provide on the tuner. It's all explained in the alignment instructions. Any decent scope will have enough sensitivity. That being said, it's not something I recommend messing around with. It's very easy to screw up the tuner and they rarely need adjustment.
|
|
#134
03-12-2014, 04:02 PM
|
|||
|
|||
|
Quote:
|
|
#135
03-12-2014, 06:16 PM
|
|||
|
|||
|
Quote:
|
| Audiokarma |
 |
|
|
Linear Mode
