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  #1  
Old 09-07-2016, 08:40 PM
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Zenith6S321 Zenith6S321 is offline
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Sencore VA62&VA62A Modifications

I thought it might be helpful to make a thread that collected the information about my original VA62 modifications. I also want to describe the second version of the modification that adds features, provides more frequency resolution, and is easier to use.

First maybe I should describe why I did the modification in the first place. I have limited bench space and was looking for some kind of TV testing/alignment generator that would work on all TVs. The VA62 looked like it could do a lot and I got one off of Ebay for cheap. I had to clean mine up, lube the knob shafts, and get the RF oscillator working before I could use it. I got the schematics and maint manual and got it working. The VA62 was designed to allow touching up the IF and color alignment by looking at the TV picture and an oscilloscope display of the demodulated video waveform. Unfortunately it only provides the 45.75 MHz IF output. I wanted to be able to use it for both early and later TVs. If your set is mostly working you can generate an RF antenna input and use the test patterns, but if your tuner or IF is busted or way off you may not see any demodulated video. I find it easiest to inject signals starting at the CRT grid and work my way back through the video signal chain (video amps, video IF, tuner) fixing each stage until I get it working back to the antenna input. The VA62 provides the signals to drive all the stages if it has a 45.75 MHz IF.

So I decided to read the Sencore EPROM, disassembled the code, and tried to find a way to add a user adjustable early TV low IF frequency output. That's what the first modification did for the VA62. I'll describe that mod in detail next.

Last edited by Zenith6S321; 09-09-2016 at 12:03 PM.
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Old 09-07-2016, 09:21 PM
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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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Old 09-08-2016, 07:29 PM
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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
Attached Images
File Type: jpg BackScrews-2.jpg (129.9 KB, 66 views)
File Type: jpg BottomScrews-2.jpg (48.0 KB, 64 views)
File Type: jpg RemoveBottom-2.jpg (60.4 KB, 64 views)
File Type: jpg RemoveCover-2.jpg (102.4 KB, 65 views)
File Type: jpg TopScrews-2.jpg (123.9 KB, 64 views)
File Type: jpg OpenTop-2.jpg (90.9 KB, 69 views)

Last edited by Zenith6S321; 09-08-2016 at 07:50 PM.
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Old 09-08-2016, 07:56 PM
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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.
Attached Images
File Type: jpg PryUpEPROM.jpg (87.2 KB, 69 views)
File Type: jpg BentPin-2.jpg (65.4 KB, 68 views)
File Type: jpg InstallV1EPROM-2.jpg (56.2 KB, 68 views)
Attached Files
File Type: zip VA62 CD.zip (8.3 KB, 3 views)

Last edited by Zenith6S321; 09-09-2016 at 12:09 PM.
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Old 09-08-2016, 08:21 PM
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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
Attached Images
File Type: jpg RF-IFswitchwires.jpg (71.1 KB, 69 views)
File Type: jpg SwitchDiagram.jpg (142.6 KB, 68 views)
File Type: jpg LabelWires.jpg (88.0 KB, 36 views)
File Type: jpg FrontTape.jpg (67.1 KB, 67 views)
File Type: jpg BackTape.jpg (88.9 KB, 68 views)
File Type: jpg HoleDrilled.jpg (56.6 KB, 69 views)

Last edited by Zenith6S321; 09-23-2016 at 07:30 AM.
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Old 09-08-2016, 08:45 PM
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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.
Attached Images
File Type: jpg ShavingsCaught.jpg (85.7 KB, 66 views)
File Type: jpg SwitchHole.jpg (46.2 KB, 64 views)
File Type: jpg WiredSwitch.jpg (83.4 KB, 70 views)
File Type: jpg InstallSwitch.jpg (79.5 KB, 66 views)
File Type: jpg 15MHzIFout.jpg (141.8 KB, 65 views)

Last edited by Zenith6S321; 09-09-2016 at 07:34 AM.
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Old 09-09-2016, 08:50 AM
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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, 62 views)
File Type: jpg CH42.jpg (143.9 KB, 62 views)
File Type: jpg CH42-000.jpg (128.6 KB, 61 views)
File Type: jpg CH42-025.jpg (135.2 KB, 62 views)
File Type: jpg CH42-IF3.jpg (122.0 KB, 63 views)
File Type: jpg CH42-IF3-res.jpg (47.3 KB, 66 views)

Last edited by Zenith6S321; 09-09-2016 at 10:14 AM.
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  #8  
Old 09-09-2016, 09:28 AM
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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.
Attached Images
File Type: jpg VA62EPROMv2.jpg (79.7 KB, 62 views)
File Type: jpg VA62EPROMv2-2.jpg (63.2 KB, 61 views)
File Type: jpg VA62EPROMv2-3.jpg (87.1 KB, 61 views)
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Old 09-09-2016, 01:05 PM
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Version 2 of the EPROM modification uses a 2732 4K Byte EPROM which provides room for more modification code. My goal for version 2 was to make the VA62/A be able to:
1. Not disable any original VA62/A functionality.
2. Allow you to enter four digits for the video carrier frequency to support all IF frequencies (assuming the PLL oscillator will lock).
3. Make the VA62/A provide the same functionality for the early low band IF TVs as it does for the 45.75 MHz IF TVs.
4. Be easy to use (no need to use the frequency offset stuff)

This is how to use the V2 EPROM to generate the same 25.75 MHz IF signal as the version 1 does:

1. At VA62A power on and set for no modulation, IF output, 35-50MHz you should see this normal VA62A functionality.


2. Now enter 2575 (25.75 MHz).


3. To modulate this 25.75 IF signal with the video and audio test patterns rotate the RF-IF SIGNAL switch to the PROG CABLE CH2-99 position. Then select the audio and video test patterns you want. That's it.



4. A Teletone TV-149 video carrier is 37.30 MHz so


5. Want to set the TV-149 sound trap? Rotate the RF-IF SIGNAL switch to the 41.25 (sound trap for a 45.75 MHz IF) and the mod calculates the sound trap frequency and sets it for you.


The RF-IF switch positions for the 45.75 MHz IF traps are all set to the proper frequencies that correspond to the video carrier frequency you entered. So the V2 EPROM modification provides all of the functionality for early TV low band IFs as the 45.75 MHz sets.
Attached Images
File Type: jpg V2-3500.jpg (142.4 KB, 55 views)
File Type: jpg V2-2575.jpg (142.4 KB, 53 views)
File Type: jpg TT-3730.jpg (56.6 KB, 49 views)
File Type: jpg TT-3280.jpg (144.6 KB, 50 views)
File Type: jpg 630TS-2575Modu.jpg (56.4 KB, 50 views)

Last edited by Zenith6S321; 09-09-2016 at 02:11 PM.
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Old 09-09-2016, 02:31 PM
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The ability to set the video carrier frequency means you can generate a channel 1 modulated output. So here is the VA62A driving a 45.25 MHz (channel 1 video carrier freq) into an RCA 621TS antenna input with the multiburst test pattern selected.




This was a basket case 621TS from an ETF annual auction.

The EPROM modification code is enabled by entering any frequency other than the default 35.00 MHz into the 35-50 MHz PROG function. So you can disable it by entering 3500 with the RF-IF SIGNAL switch in the 35-50 MHz PROG position. Or you can just turn the VA62/A off and back on again, as that value is the default at power on.

If you have an EPROM burner and want to burn your own V2 modification EPROM the attached zip file contains three file formats for the EPROM image.


Dave Perdue
Attached Images
File Type: jpg 621TS-CH1-VA62A.jpg (55.3 KB, 54 views)
File Type: jpg 621TS-CH1.jpg (51.5 KB, 57 views)
Attached Files
File Type: zip VA62A-V2-EPROM.zip (12.0 KB, 7 views)

Last edited by Zenith6S321; 09-09-2016 at 02:41 PM.
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Old 09-09-2016, 08:38 PM
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Wow, Dave, this is great work that you have done. Thank you for sharing your efforts and describing everything here.
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Old 09-09-2016, 09:16 PM
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Zenith6S321 Zenith6S321 is offline
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You're welcome. I wish I could say I have mastered using the VA62 to perform quick easy alignments but I'm still learning.
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Old 09-09-2016, 09:25 PM
Crist Rigott Crist Rigott is offline
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Ditto what Chris said! Nice job with the instructions.
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Old 09-09-2016, 11:21 PM
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IIRC the VA62 does not have sweep/marker capability....So I have no clue how it could be used to do a proper TV alignment.
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Old 09-10-2016, 08:31 AM
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As you say, the VA62 does not have the sweep and marker functions that provides an image of the bandpass response on an oscilloscope. But the VA62 manual does provide instructions on doing alignments for modern (45.75 MHz) color sets. The procedure uses either an oscilloscope displaying the demodulated video or using the image on the CRT to determine the affect of adjustments. The VA62 MULTIBURST BAR SWEEP test pattern provides 10 frequencies that are displayed as vertical black and white lines on the CRT. The frequencies are 0, .5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 MHz. So instead of a continuous sweep of frequencies like the traditional sweep and marker, you get the 10 discrete frequencies that can be viewed in the demodulated video on a scope or the TV CRT. You view the amplitude of each frequency burst on the scope or the contrast ratio (magnitude of the difference between each bursts black and white vertical bars) of the burst. Viewing the unmodulated VA62 "VCR STANDARD" video output on the scope shows all of the bursts at equal amplitude. Viewing the sets detect video shows what frequencies are being passed through the tuner and IF stages. This is helpful if you have a color set that shows a black and white picture but has poor color performance. You can see if the demodulated video color frequencies (3.0-4.5 MHz) are not getting through the tuner and IF stages. Understanding of the IF frequency that corresponds to the demodulated video frequency allows you to pick the IF stage that needs adjustment. So the sets video carrier frequency (45.75 for modern sets) is 0 MHz (the low frequency spectrum of the sync signals) while the audio carrier frequency is 4.5 MHz (45.75 - 4.5 = 41.25 MHz). Stagger tuned IF stages frequencies fall between the video and audio carrier frequencies. Effectively the stagger tuned IF stages are high gain amplifiers each tuned to different frequencies that in total provide a bandpass response to reject adjacent channel signals and amplify the tuned channel from the antenna input microvolts to the volts input to the video detector. Other than the Sencore VA62 manual, the book "Practical Troubleshooting With the Advanced Video Analyzer by Robert Goodman" is a guide on using the VA62 for alignments. Somewhere (probably here on VK) I downloaded a scan of the Video IF/detector section of the book. The book is still available on Amazon and other places.

Last edited by Zenith6S321; 09-10-2016 at 08:52 AM.
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