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Low band IF and level modification
Here are some pictures of what I ended up with to make the VA62 output an IF level modulated signal for the 15-35 MHz IF band and IF level. I found an error in the VA62 block diagram showing the wiring for the RF-IF Signal switch. The corrected wiring diagram made adding a manual RF/IF output level switch easy. One piicture is a dagram of the switch with the circled numbers that are the terminal number of the wires removed from the RF-IF Signal switch that are wired to the new manual DPDT RF/IF level switch. The pictures show before and after the switch addition.
My software modification to the VA62 EPROM uses the Prog Cable setting of the RF -IF Signal switch to change the frequency of channel number 42 to 25.5 MHz. Pressing the +/- keypad key then lets you enter an offset to shift the frequency by +/- 9.75 MHz in .25 MHz steps. There are some pictures of channel 42 with a +9.75 MHz and a -9.75 MHz offset with the output frequency displayed on a frequency counter. There are scope pictures that show the waveform and voltage level. I have not tried it with my 721TS, or tested it thoroughly, but everything else still seems to work properly. Same offer as before, send a PM and I will send a copy of the EPROM .HEX file for you to burn a copy of the modification. Or send me your 2716/2732 EPROM (keep your original safe) with an addressed, prepaid shipping, box and I can burn it for you. Dave Clarification: The numbers on the DPDT level switch diagram are the RF-IF Switch (gray one at the top of the 4th picture) numbered solder connection points. Remove the wires connected to the RF-IF switch solder connection points 8,9,10,14, and 17 and then connect those wires to the DPDT switch as shown in the diagram. The DPDT switch will then either supply power to the RF or the IF amplfiers depending on the position of the new DPDT RF/IF Level switch. Last edited by Zenith6S321; 09-08-2016 at 08:49 PM. |
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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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Testing the mods on 721TS contd.
I also tried the audio IF and it also worked, although my VA62 tones. I also found my video response rolls off at between 3.0 and 3.5 MHz using the VA62 multiburst bar sweep.
Dave Last edited by Zenith6S321; 09-09-2016 at 11:48 AM. |
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Nice to see the results clearly on the CRT.
__________________
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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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.
__________________
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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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 |
#40
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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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Audiokarma |
#41
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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 |
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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
__________________
CW 1950 Zenith Porthole - "Lincoln" |
#44
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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 |
#45
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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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Audiokarma |
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