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And a few more pictures of the modification.
Last edited by Zenith6S321; 09-08-2016 at 08:50 PM. |
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Testing the mods on RCA 721TS
Although the scope and frequencies look right, the best test is to try the 25.75 MHz modulated video and audio on an old set. Attached are some pictures. First I hooked the VA62 up to the antenna leads and sent video and audio on channel 3, the first two pictures.
Then, in the last three pictures, I hooked the modified VA62, set to 25.75MHz, up to the third, second, and first picture IFs and set the VA62 levels accordingly. It worked! Last edited by Zenith6S321; 09-09-2016 at 11:48 AM. |
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Nice to see the results clearly on the CRT.
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Chris Quote from another forum: "(Antique TV collecting) always seemed to me to be a fringe hobby that only weirdos did." |
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Very impressive, Dave. Now I want to see someone do a bar sweep IF alignment on a vintage set.
They explain the Sencore bar sweep alignment method in my 1980's vintage school text book by Kiver. They're using the older VA-48 but I'm sure the process is the same. You really need to have a solid grasp on how the IF response curve works in order to relate that to what's showing up on the sets CRT. At my basic level of alignment experience, it just seems very confusing. |
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I have a VA-48, and also Milton Kiver's excellent book about the B&K 1077 (Television Analyzing Simplified). Can you tell me what book you have with Kiver explaining the Sencore device? Thanks.
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Chris Quote from another forum: "(Antique TV collecting) always seemed to me to be a fringe hobby that only weirdos did." |
Audiokarma |
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Chris,
I was off on the year. It's a 1973 revised text that we used in school. Basically an update of the older Kiver works. If fact much of it reads the same as my 1949 edition, only this is updated for the newer sets and test equipment. I can try to scan the VA-48 pages. I think there's about 8 pages. If interested pm me an email. |
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Quote:
Dave |
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Something tells me Bob probably has the burst bar sweep alignment method mastered by now. He's been very quiet.
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LOL, nope. We've had crazy stretch of very warm weather and I've been doing some refinishing.
I'll be getting back to TVs and alignments soon though |
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Further fiddling with VA62 code
Just an update. Since the 30-50MHz RF-IF SIGNAL switch setting has AM modulated audio tones I was hoping to be able to get the VA62 to output an AM radio band RF signal but the PLL in my VA62 will not lock reliably below about 15MHz even by changing the programmable PLL loop gain. Although it can be persuaded to lock down to about 8MHz by repeatedly flipping the RF/IF level select switch I added. Here is the RF/IF Level switch functionality you get in all the RF-IF SIGNAL switch settings:
PROG CABLE CH2-73 RF and IF levels (mod gives CH42=25.5MHz +/-9.75MHz in .25MHz steps) STD CABLE CH2-73 RF and IF levels (mod gives CH42=25.5MHz) STD TV CH2-83 RF and IF levels 30-50 MHz IF level only (mod gives 1-99MHz circuit limited to about 8-99MHz on mine) 45.75MHz VIDEO IF RF and IF levels 39.75MHz TRAP IF level only 41.25MHz TRAP IF level only 47.25MHz TRAP IF level only 4.5MHz FM RF level only Dave |
Audiokarma |
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Dave, thanks again for the work you are doing with the VA62. Can't wait to get that EPROM from you to try in my Sencore VA62.
I've never aligned a TV before but with a mod like this perhaps I can figure it out. BTW, anyone have a YouTube video on TV alignment or a thread showing the intricate details? Like to try aligning my GE Model 50 Locomotive TV once I finish recapping it -- the IF is 26.3 Mhz. Carl
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CW 1950 Zenith Porthole - "Lincoln" |
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Bandersen has many amazing restoration youtube videos at http://www.youtube.com/user/bandersentv
which include full alignments of the older TVs. I find his explanations very helpful! Dave |
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Thanks. I think this is the best I've done so far on aligning a vintage stagger tuned IF: http://youtu.be/VH0yXo6GAuw
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#14
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VA62 EPROM mod. clarification
I edited my post #31 to clarify the wiring of the DPDT RF/IF Level switch. Attached is a picture of the VA62 block diagram showing the RF-IF Switch wiring with its numbered wire connections. There was a mistake in the schematic that I corrected in pencil.
Last edited by Zenith6S321; 09-09-2016 at 11:48 AM. |
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Dave's reprogramming of the uP for the Sencore VA62 is ingenious.
I received his EPROM yesterday and installed it this morning. My VA62 can now go down to 12.0Mhz but at that low frequency the amplitude is subdued and waveform a little malformed. But, that is a non-issue. Don't need for it to go that low frequency anyway. My VA62 produces a nice waveform and good amplitude down to 15.0Mhz: Here it is at 18.5Mhz: Here at 26.30Mhz: Here at 42.75Mhz: Have not installed the toggle switch yet. Need to visit Radio Shack for a nice looking switch. Think I will put masking tape on the inside to capture metal filings. Carl
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CW 1950 Zenith Porthole - "Lincoln" Last edited by cwmoser; 04-17-2012 at 09:17 AM. |
Audiokarma |
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