Sencore VA62 adventure

[Notimetolooz]

This will have to be a multi-post story.
The VA62 came out in about 1985 and I think it was made for less than a decade. It combines several TV test instruments in one box, making it very complex.
A 1 Amp 0-30volt power supply. A voltmeter that can monitor the variable supply voltage and current, measure external voltages, monitor various signal level the unit puts out. It has sweep drive output signals similar to a B&K 1077B but without the flying spot scanner. Instead of a flying spot scanner the VA62 has a digital video pattern generator. It will generate video modulated RF channel frequencies and 45.75MHz IF frequency, also audio modulated 4.5MHz sound IF signal. In addition it has a programmable 30-50MHz modulated IF range. (This by the way should be able to be used to generate the early obsoleted TV channel 1.) One thing it can't do is output the older 21-25MHz IF signal. Although the VA62 can insert some marker frequencies into the generated video, it really isn't the best thing to be used for alignment. It can be used to confirm and compare the performance of TV sets.

I obtained my VA62 several years ago. A short time later I found out about a modification kit by Zenith6S321 in this forum that would allow the operation in the 21-25MHz IF range, so I got a kit in 2018. I didn't do much with the Sencore or that kit until recently.

[Notimetolooz]

I installed the modification kit into my VA62 before I had checked out the basic operation completely. There were problems, so I put the original EPROM back in. The problems were still there so it wasn't the EPROM and I rechecked the added switch modification and it was fine.
Luckily the kit had included a CDROM with a lot of info including schematics and a maintenance document. Also Phil Nelson on this forum had scanned the schematics and some documents.
I started by checking the internal main power supplies. I found some voltage were out of spec and some have more hum than spec. It became evident that someone had worked on the unit before, some resistor were not the value in the schematics and parts list. I corrected that. I changed a voltage regulator IC and replace some filter caps. The meter circuits had their own power supplies that I check next. I replaced a regulator there also. Then I made a mistake re-connecting a cable. Many cables use a pin header on a circuit board and the mating plastic block connector on the cable. If you are not careful you can plug the cable to the board with the pins offset from the right place. That had happened and the wrong voltage was put on a large IC that ran the display on one side. That chip was messed up afterwards. The IC was obsolete long ago. I found one on ebay in Germany. I waited weeks for it to arrive. I contacted the seller who said he would send another. More waiting, it didn't arrive either. I contacted the seller and got my money back. Then I found the IC at a company in the US and get it in three days.

[Notimetolooz]

With an oscilloscope, meter and frequency counter I started to look at my VA62
outputs in detail.
The audio output was distorted and had hum, in fact most of the drive outputs had too much hum. The 3.58MHz output from the separate front jack was distorted and noisy. The 30Hz (VCR servo) front panel output wasn't working either. The programmable 30-50MHz IF output was off frequency and drifting.
One handy thing was that the RF/IF switch modification allowed a higher output than usual for the RF output, that allowed my frequency counter to work.
Troubleshooting one output at a time I worked through the problems.
After replacing the op-amp IC that was used to generate the audio tones (no change), I discovered that someone had maxed out the oscillator signal levels. This caused the outputs to clip. Someone had worked on the unit that didn't know what they were doing, I was going to have to check every adjustment on the sections that were giving me problems.
Next I looked at the drive board. I found evidence that a lot of work had been done. There were signs that spots on the board had overheated and some part replacements were suspicious. I ended up replacing all the electrolytic caps, a couple of resistors, two zeners and a transistor.

[Notimetolooz]

Along the way I discovered that the -30V supply to the drive board had more hum than when I checked it earlier unloaded. This put hum on just about every output of the drive board. Going back to the power supply board, making measurements, checking regulator specs and doing some calculations I realized that one on the regulators was voltage starved. One of the resistor values was shown wrong on the schematic and parts list! The 2.7K value shown on the schematic had been replaced with a 1.2K when I first got it and that value worked in the circuit. I put a 1.2K back in but this time I knew it should have been a higher wattage due to my calculations. Changing that resistor solved the driver output hum problems.
Next the 3.58MHz (color reference) and 30Hz (VCR servo) outputs, they used the same output module. When I opened the module cover I got a surprise! The small board was burnt in the center almost all the way through it. Three transistors were also toast. I never smelled anything burning so the damage was done before I got the VA62. Since I don't plan on working on VCRs and seldom work on color TVs, I decided to just disconnect the power to the module.
To be continued.

[Notimetolooz]

I was able to connect a good TV to the video out as well as the standard TV channel RF output (tried separately) and the pattern generator seems to work fine.
Lastly I moved on to the programmable 30-50MHz IF output which was off frequency and drifting. By the way the VA62 frequency display is the intended output not the actual measured output. The board that contains the microcontroller for the RF and IF outputs is called the PLL (Phase Locked Loop) board. The board in my unit has a small piggyback (daughter) board that subs for a 8 pin op-amp IC. The small daughter board contains Surface Mounted Devices, one of which is an op-amp. This op-amp was running very hot, I suspected it was shot. I ordered a new one and installed it but it did not solve the frequency problem. PLL circuits are tricky to troubleshoot because, when they are working right, much of the circuit is in a self correcting feedback loop.
One good way is to "open" the loop. I cut the wire that fed the voltage output of the op-amp I replaced to the VCO (voltage controlled oscillator). I used a set of molex pins and some heat shrink tubing to make a connector to re-connect the wire later. By opening the loop I found that the VCO was not responding to the voltage. The VCO is in a small module on the RF board. You have to remove two screws and un-solder a wire that comes up in the middle of the board in order to remove the VCO. Everything I could test on the VCO tested OK. I cleaned the five position connector and re-installed the VCO, re-connecting the ends of the wire I has cut. That fixed the problem! The connector had been dirty or oxidized.

[Notimetolooz]

After changing the EPROM to the modified version again the Programmable 30-50MHz IF does allow setting the frequency below 30MHz. However I noticed that the frequency is off somewhat and drifts.
I watched a video where Bob Andersen uses a modified EPROM and if you look closely at the frequency on the scope, it does seem off a bit.
https://www.youtube.com/watch?v=Y6WOjtaBqLM
The video was made nine years ago with an earlier version from Zenith6S321. Bob didn't seem to notice the frequency discrepancy.
I noticed that all the documentation I could find concerned the VA62 like Bob and I have while Zenith6S321 unit is a VA62A. Maybe there is some important difference.
I'll add a link to an earlier thread about the modification kit for the VA62.
http://videokarma.org/showthread.php?t=273989

Zenith6S321 has not been active on this forum since 2018.
I recently took the liberty to upload the info on the kit CDROM to the BAMA site.
https://bama.edebris.com/manuals/sencore/va62/
To paraphrase Jimmy Durante, "Good night to all of ya, and good night to Mr. Zenith6S321, where ever you are."
https://www.youtube.com/watch?v=EWN97q7xtDE

[ChrisW6ATV]

Thank you for your detailed discussion and notes!