Sencore VA62&VA62A Modifications

[Zenith6S321]

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.

[Zenith6S321]

Replacing the Sencore EPROM with another EPROM requires unplugging the VA62 and
1. Remove the four cover screws from the back


2. Remove the four bottom screws


3. Remove the bottom


4. Remove the cover


5. Remove the top screws


6. Open the top

[Zenith6S321]

Then
1. Pry up the EPROM


2. Insert the EPROM but be careful not to end up with a bent pin like this shows


3.The installed V1 mod EPROM should look like this


If you have a 2716 EPROM burner you can unzip the attached zip of EPROM image files and burn your own. It include a .bin, .hex, and .s format of the EPROM. Version 1 of the EPROM modification was made for the VA62. The EPROM will also work in the VA62A if you burn it to a 2732.

You can put the case back together if you do not want to put in the manual DPDT switch RF-IF signal level switch. But then the low band early TV IF output comes out at an RF level (microvolts). I recommend doing the DPDT switch mod. This is described next.

[Zenith6S321]

Installing the manual RF-IF output level DPDT toggle switch requires moving wires from the RF-IF switch to the DPDT switch. Here is how
1. Locate the back of the RF-IF switch. It looks like this


2. You need to wire the DPDT toggle switch like this


3. Remove these wires and label them. ONE AT A TIME.


4. Put some heavy tape (I used red duct tape) onto the front panel between the AUDIO and the RF-IF LEVEL switches


5. Put some tape on the back to catch the drill shavings


6. GENTLY drill a hole for the DPDT switch

[Zenith6S321]

Next

1. Remove the duct tape from the back, it should have caught some shavings


2. And remove the duct tape from the front


3. Now wire the switch. The labels really help.


4. DOUBLE AND TRIPLE CHECK THE WIRING! Get this wrong and you WILL short the different RF and IF amplifier supply voltages together and MESS UP YOUR VA62! Don't blame me, I warned you. Now install the switch into the front panel


5. Apply a water transfer decal to label the switch and give it a try. You did wire the switch right? RIGHT? What could go wrong?
Hopefully it did not blow up. Set the RF-IF SIGNAL switch to 35-50 MHz PROG and enter 1500 to output a 15.00 MHz signal. Set the VIDEO PATTERN to External Modulation (with no signal driving that input) and set the AUDIO switch to off. You should get a clean sine wave (as well as the components will allow) out of the RF-IF OUT with its level controlled by the setting of the manual RF-IF DPDT toggle switch you added.


Next we will try to output an early TV low band IF signal.

[Zenith6S321]

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.

[Zenith6S321]

Even though the version 2 of the EPROM modification was developed for the VA62A, it can be used in the VA62. So before I describe the operation of version 2 of the EPROM modification I want to show the installation of the version 2 EPROM into the VA62. Here is how

1. Open the VA62 as described before and remove the EPROM. Solder a small wire (I used a 30 gauge wire-wrap wire) to the pin of the CPU as show in this picture.


2.


3. Bend the pin of the EPROM shown up, so that it will not be inserted into the EPROM socket. Solder the other end of the new wire to this pin. You may want to bend the pin, with the wire soldered to it, so that it does not interfere with the socket so that the EPROM will fully seat into the socket.


If you have a VA62A, then do not add the wire as it is already part of the circuit board. For a VA62A you just plug the version 2 EPROM into its socket.

Next I will describe the operation of the version 2 EPROM modification.