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Old 09-09-2016, 08:50 AM
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Zenith6S321 Zenith6S321 is offline
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Now lets output an early TV 25.75 MHz low band unmodulated IF signal. I still have the frequency counter on the output to show the frequency output.

1. With the VA62 controls set as before, rotate the RF-IF SIGNAL switch to the PROG CABLE CH2-73 position. Enter 02 on the keypad if it is not already display on the left LCD. The VA62 is now putting out an unmodulated channel 2 RF signal with an IF output voltage level (since the manual toggle switch is in the IF position. The channel 2 video carrier frequency is 55.25 MHz.


2. Now enter 42 on the keypad to change to channel 42 to output the early TV low band frequency


3. Now we use the VA62 frequency offset function to get the video carrier frequency we want. Press the +/- keypad key to enter the offset frequency. The left LCD displays the current offset, which shows 0.00 MHz. The V1 EPROM mod changes channel 42 to a video carrier frequency of 25.5 MHz, so 25.5 + 0.00 offset = 25.5 MHz as shown on the frequency counter.


4. So to get the 25.75 MHz video carrier we want, we have to enter 0.25 (25.75 - 25.5 = 0.25 MHz). Enter 025 (0.25 MHz) into the keypad. The left LCD will show 0.25 and the output frequency is 25.75 MHz.


5. Here I have connected the IF output signal to the third video IF grid of my RCA 721TS with a decoupling capacitor like this


6. Now we can select the video and audio modulation that we want. The output signal will then be an early TV low band IF signal modulated with the selected audio and video patterns. Here is the VA62 multiburst pattern.


The cable from the VA62 RF-IF output should be terminated with a resistor value matching the cable (I use 50 ohm cable). Then a decoupling capacitor connected to the signal injection point.

Here is a list of the RF and IF output levels you get by using the manual RF-IF DPDT switch for each of the RF-IF SIGNAL switch positions:
PROG CABLE CH2-73-->RF and IF levels
STD CABLE CH2-73---->RF and IF levels
STD TV CH2-83-------->RF and IF levels
35-50 MHz------------->IF level only
45.75 MHz VIDEO IF-->RF and IF levels
39.75 MHz TRAP------->IF level only
41.25 MHz TRAP------->IF level only
47.25 MHz TRAP------->IF level only
4.5 MHz FM------------>RF level only
Remember that the RF-IF switch position normal sends the power to either the RF or the IF output amplifiers, but now YOU have to manually switch the RF-IF DPDT switch to power the output amplifiers. So when you switch to the 35-50 MHz position you have to have the DPDT in the IF position to get an output. If you select the 4.5 MHz FM then you have to have the DPDT in the RF position to get an output. No pain, no gain I guess. But now you do now get both RF and IF output levels from the PROG CABLE CH2-73, STD CABLE CH2-73, STD TV CH2-83, and 45.75 MHz VIDEO IF switch positions.

Sometimes the VA62 phase locked loop controlled oscillator does not lock onto the frequency you enter. This happens more often the further you are away from the standard 35-50 MHz range. I have found that flipping the RF-IF switch back and forth will often cause the oscillator to lock to the frequency you enter. So if the unmodulated RF-IF is not the right frequency, either hit the C keypad to clear the entry and then re-enter the frequency, OR flip the RF-IF DPDT toggle switch back and forth until it locks. I have not see this happen very often for the range of early TV low band IF frequencies so you may not see this problem.

That completes the description of the V1 EPROM modification. Its main limitation is that the video carrier frequency has to be a multiple of 0.25 MHz which does not handle some of the early TVs.

Version 2 of the EPROM modification fixes this and is based on the VA62A 2732 4K Byte EPROM. Turns out a one wire modification of the VA62 will allow the version 2 EPROM to work in the VA62 as well. Next I will describe the version 2 EPROM modification.
Attached Images
File Type: jpg CH2.jpg (134.2 KB, 195 views)
File Type: jpg CH42.jpg (143.9 KB, 193 views)
File Type: jpg CH42-000.jpg (128.6 KB, 190 views)
File Type: jpg CH42-025.jpg (135.2 KB, 193 views)
File Type: jpg CH42-IF3.jpg (122.0 KB, 201 views)
File Type: jpg CH42-IF3-res.jpg (47.3 KB, 201 views)

Last edited by Zenith6S321; 09-09-2016 at 10:14 AM.
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