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Old 09-07-2016, 09:21 PM
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Zenith6S321 Zenith6S321 is offline
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Location: Charleston, West Virginia
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The first version of the VA62 EPROM modification has to work within the 2K Bytes of program stored on the VA62 2716 EPROM. That EPROM is jam packed with very few unused locations. That limited what could be changed without affecting normal VA62 functionality.

The first modification expanded the 35-50 MHz PROG position of the RF-IF SIGNAL rotary switch. I expanded the limits to allow entry of frequencies from 1 MHz to 99 MHz. The circuitry of the VA62 was not designed for this, so we only get what the components will permit. Frequencies from 15MHz to 70MHz are typical with some VA62s able to span 7 MHz to 99 MHz.

I found I could use the VA62 PROG CABLE CH2-73 position of the RF-IF SIGNAL switch to output a fixed early TV IF frequency. That position of the VA62 RF-IF switch also includes a way to enter an offset of -9.75 to +9.75 MHz. In other words, with the VA62 generating a video carrier frequency of 25.5 MHz you can enter an offset to cover from 15.75 MHz to 35.25 MHz in .25 MHz steps. Sadly this does not support some early TV IFs that have a video carrier frequency that is not a multiple of .25 MHz . That problem is fixed in version 2 of the VA62A EPROM mod .

So with that EPROM I could get the right early TV IF frequency for my 630TS (25.5 MHz + 0.25 MHz offset = 25.75 MHz) but that position of the RF-IF switch only outputs an RF level (microvolts), not the higher voltage for the first, second, and third IF stages .

Turns out the RF-IF switch controls the power supplied to the RF output amplifier and the IF output amplifier. The RF-IF switch powers either the RF or the IF amplifier depending on the switch position. So, I added a manual DPDT toggle switch to allow you to select either an RF or an IF signal level for most of the RF-IF switch positions. Flipping the toggle switch has a side effect of jarring the oscillator PLL if your VA62 doesn't want to lock to the frequency you have entered(which can happen at the limits of your VA62 components performance).

I chose overriding the PROG CABLE CH2-73 frequency to 25.5 MHz only when the user sets the channel to 42 (arbitrary choice).

A VA62 with the first EPROM modification can drive the first, second, and third early TV low band IF by setting the channel to 42, setting the offset to change the frequency to your early TV's video carrier frequency, flipping the added DPDT RF-IF level switch to the IF position, and setting the level and vernier output level pots for the right voltage levels. You can set the audio and video test patterns (or feed in an external video) and see and hear the results on your early TV (if its working...).

Next I will show some pictures of changing the EPROM and adding the manual RF-IF output level DPDT switch.

Last edited by Zenith6S321; 09-08-2016 at 06:01 AM.
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