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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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Zenith6S321 Zenith6S321 is offline
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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, 206 views)
File Type: jpg BottomScrews-2.jpg (48.0 KB, 200 views)
File Type: jpg RemoveBottom-2.jpg (60.4 KB, 199 views)
File Type: jpg RemoveCover-2.jpg (102.4 KB, 200 views)
File Type: jpg TopScrews-2.jpg (123.9 KB, 198 views)
File Type: jpg OpenTop-2.jpg (90.9 KB, 214 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, 222 views)
File Type: jpg BentPin-2.jpg (65.4 KB, 213 views)
File Type: jpg InstallV1EPROM-2.jpg (56.2 KB, 214 views)
Attached Files
File Type: zip VA62 CD.zip (8.3 KB, 33 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, 219 views)
File Type: jpg SwitchDiagram.jpg (142.6 KB, 218 views)
File Type: jpg LabelWires.jpg (88.0 KB, 178 views)
File Type: jpg FrontTape.jpg (67.1 KB, 209 views)
File Type: jpg BackTape.jpg (88.9 KB, 210 views)
File Type: jpg HoleDrilled.jpg (56.6 KB, 215 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, 205 views)
File Type: jpg SwitchHole.jpg (46.2 KB, 201 views)
File Type: jpg WiredSwitch.jpg (83.4 KB, 207 views)
File Type: jpg InstallSwitch.jpg (79.5 KB, 204 views)
File Type: jpg 15MHzIFout.jpg (141.8 KB, 212 views)

Last edited by Zenith6S321; 09-09-2016 at 07:34 AM.
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  #7  
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 09-10-2016, 09:20 AM
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Thanks for posting this thread, Dave!

I got a v1 EPROM from you a while back, and it works great. Will probably upgrade it to a v2 soon using the hex file you posted.
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Old 09-10-2016, 07:26 PM
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Zenith6S321 Zenith6S321 is offline
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You're welcome, just tryin to give back a little for all the great information everybody posts here. Upgrading from V1 to V2 should be easy for you.

Dave
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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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  #13  
Old 09-10-2016, 08:31 AM
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Zenith6S321 Zenith6S321 is offline
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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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  #14  
Old 12-20-2016, 06:46 PM
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OMG! Dave and whoever authorized my inclusion in the Video Karma group, you are a Godsend! My poor little 621 and RA103 have been waiting a very long time for alignment. Now you have provided the final pieces needed. I previously ordered an e-prom burner (not arrived yet), and I had resigned myself to learn how to program in C. Now, thanks to you, I can invest that time and energy into what I enjoy most: restoring these old gems. Thank you. Dan
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Old 03-23-2017, 03:48 PM
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Well, I'm at my wit's end. I've tried and tried to burn my own eprom for my VA62 and have succeeded only in raising to blood pressure and loosing my hair. Would someone be merciful and sell me a modified eprom? I and my needy vintage sets would be eternally grateful!
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