#16
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#17
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Have you confirmed the DTV box RF signal quality with a different analog TV? Some cheap RF modulators (like those built into most home video sources) suck. Have you tried the set with a different modulator?...Sometimes a set happens to not like a specific modulator despite the modulator working with other sets...RF can behave like black majick sometimes.
Have you cleaned the tuner contacts, and tested the tubes in the tuner and IF? If the set has AGC have you adjusted it and verified it's operation? Is the internal tuner balun (usually mounted outside or inside the tuner box and linking the ant lead to the first tuner tube) still good?....Lightning killed a lot of tuner baluns back in the day. Do you have a B&K 1075/76/77 series analyst? Those can modulate and inject IF carrier into the IF strip. Doing test injection with such a tool can id dead tuners and IF stages. Have you compared socket voltages in the tuner and IF to those listed in the service literature?...If a stage has a major defect it should show up in the voltages. These are all the easy steps to fix it or get you chasing a likely source of the problem....Beyond this lies the dark art of alignment, and or the engineering art of video injection.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#18
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I just found two problems tonight while verifying actual wiring with the schematics. One problem is what looked like a small solder bridge not real sure though and the big one is I hooked up a resistor wrong that I replaced when I did cap job. The resistor was supposed to be feeding a cathode of Horiz osc/control tube off the B- supply and instead of going to B- I had it hooked up to the opposite grid on same tube. NO wonder the horizontal has always acted crazy. I will try to repair tomorrow and check more things. Thanks for the advice. |
#19
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digital converter box
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I have a few questions. I have included pic of my converter box. It is a Apex model DT250A. Does any one have any experience with this box working or not working with older set? My other question is can I feed the coax rf output (channel 3 or 4) from this box into the RF converter/mixer grid and expect a good signal or is it too weak. This would be bypassing the RF amp tuner stage where I think I may have a problem. I have successfully injected a 21.9MHz (audio IF for my set) FM modulated signal from my signal generator into the grid of the mixer and did alignment on sound IF (atleast I think I did - everything sounded good afterward). After I did the sound alignment my next check was going to be to see if I could get good audio using converter box. Well it did not work. All I got was buzzing noises. I am going to reverify my signal gen output accuracy but I think it is dead on it is a HP synthesized model. I even jacked around with the local osc adjustment for channel 3 and all I got was several FM radio stations coming thru nicely. I tried viewing signal with oscilloscope and never found anything that looked like video signal. So am I going to have to go thru tuner and RF amp? |
#20
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It's always a good idea to use a coupling cap in series with you generator output just in case you accidentally hook into some high voltage source. Generally you'll be feeding the signal into the the grid of each stage. Last edited by Kevin Kuehn; 04-01-2018 at 08:10 PM. Reason: Removed some confusion(hopefully) |
Audiokarma |
#21
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Phil Nelson Phil's Old Radios https://antiqueradio.org/index.html |
#22
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When I aligned the sound IF I did as you said and turned tuner to blank position like the instructions say to do in the manual. When I hooked convertor box to mixer grid I then set the tuner to channel 3. Wouldn't this enable the local osc? You gave some great ideas for testing video IF. My signal generator I got should be able to do this no problem. I discovered though it is quite limited for sweeping functions. I disabled all the high voltage and removed the crt for what I am doing right now so I will try that later when I reconnect everything. I did verify my signal generator accuracy by using my Sony multiband radio using the SW band and tuning in to generator.
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#23
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#24
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[edit] After looking at your sets tuner circuit more thoroughly, I believe your injecting the rf channel 3 or 4 directly into the mixer grid will load down the oscillator signal feeding that same point through the 1.5 pf cap. I believe you would need some type of isolation or impedance matching circuit between your signal generator and the grid of the mixer. Even then I think without the gain provided by the rf amp, it would not mix correctly with the oscillator, or at best it would provide a greatly reduced amplitude IF signal. Keep in mind that your HP generator likely has a 50 ohm output impedance. It would be great if one of the EE engineering types hanging around here could explain these type circuits so that everyone(myself included) could better understand their operation. I only know from experience that injecting an rf signal at a critical mixing point tends to load down the circuit and create errors. Last edited by Kevin Kuehn; 04-02-2018 at 02:41 PM. |
#25
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I think after I check a few more things I am going to look over my antenna input section real good to make sure there is no damage there. Someone said it is possible it could have been damaged by lightning which makes sense. If I can give that section a thumbs up then I will hook my converter box to antenna input and not have to worry about injecting into mixer.
On a side note I just noticed locally on Craigslist someone if trying to sell a Motorola VT 107 which has same or nearly same chassis as the one I'm working on right now. According to the ad it has been in storge since the 60s. They are asking too much right now but I will grab it if the price is right. I guess I've got the bug. |
Audiokarma |
#26
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Progress
Have made some pretty good progress last few days with my VK-106. My reception problem looks like it was due to a bad antenna lead connection. I did not realize it until I removed the antenna tuning unit but the twin lead goes to a pin and socket type connector and I think one side was not making up good. I found this out by pushing in real hard and voila my audio came in much clearer. I still have crt disconnected. I resoldered the antenna wires to the pins and squeezed the sockets with some pliers for a tight fit. In response to the reception problem I thought the worse and have been spending last couple weeks learning how to do an alignment or at least how to attempt to do an alignment. I think I the sound IF aligned nicely since I had the equipment to do it with -got a pretty nice S curve. My HP8643a signal generator which was a impulse ebay purchase is a great piece of equip but is limited in its sweep capabilities. It will only do a 600kHz sweep width in its "phase continuous" mode. I got a tip off one off bandersons vids which suggested you could plot curve out on piece of graph paper and that is what I did for the video IF alignment. I will try to post a pic of that maybe tomorrow.
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#27
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IF alignment plot
Here is the result on graph paper after my IF alignment. I know it has some issues but going on some info I've read from different sources it isn't too bad. I think I will purchase a sweeper sometime in the near future so I can more easily touch it up. I plotted every 100kHz.
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#28
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Last edited by Kevin Kuehn; 04-13-2018 at 12:09 PM. |
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Finally
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#30
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Sounds good. So long as you're getting a respectable picture and sound at the same location on the dial, that's really all that matters.
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Audiokarma |
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