Another Motorola VK-106 restoration attempt

[Electronic M]

Have you confirmed the DTV box RF signal quality with a different analog TV? Some cheap RF modulators (like those built into most home video sources) suck. Have you tried the set with a different modulator?...Sometimes a set happens to not like a specific modulator despite the modulator working with other sets...RF can behave like black majick sometimes.

Have you cleaned the tuner contacts, and tested the tubes in the tuner and IF? If the set has AGC have you adjusted it and verified it's operation? Is the internal tuner balun (usually mounted outside or inside the tuner box and linking the ant lead to the first tuner tube) still good?....Lightning killed a lot of tuner baluns back in the day.

Do you have a B&K 1075/76/77 series analyst? Those can modulate and inject IF carrier into the IF strip. Doing test injection with such a tool can id dead tuners and IF stages.

Have you compared socket voltages in the tuner and IF to those listed in the service literature?...If a stage has a major defect it should show up in the voltages.

These are all the easy steps to fix it or get you chasing a likely source of the problem....Beyond this lies the dark art of alignment, and or the engineering art of video injection.

[pdr-fml]

Originally Posted by Electronic M

Have you confirmed the DTV box RF signal quality with a different analog TV? Some cheap RF modulators (like those built into most home video sources) suck. Have you tried the set with a different modulator?...

Yes I did verify its operation with with my old Panasonic "flat screen" crt set I keep up in the attic now.

I just found two problems tonight while verifying actual wiring with the schematics. One problem is what looked like a small solder bridge not real sure though and the big one is I hooked up a resistor wrong that I replaced when I did cap job. The resistor was supposed to be feeding a cathode of Horiz osc/control tube off the B- supply and instead of going to B- I had it hooked up to the opposite grid on same tube. NO wonder the horizontal has always acted crazy.
I will try to repair tomorrow and check more things. Thanks for the advice.

[pdr-fml]

Originally Posted by pdr-fml

Yes I did verify its operation with with my old Panasonic "flat screen" crt set I keep up in the attic now.

I just found two problems tonight while verifying actual wiring with the schematics. One problem is what looked like a small solder bridge not real sure though and the big one is I hooked up a resistor wrong that I replaced when I did cap job. The resistor was supposed to be feeding a cathode of Horiz osc/control tube off the B- supply and instead of going to B- I had it hooked up to the opposite grid on same tube. NO wonder the horizontal has always acted crazy.
I will try to repair tomorrow and check more things. Thanks for the advice.

What I mentioned above turned out to be wrong the resistor was in correct location. In fact I have verified the proper connection of every single component and did not find anything connected wrong.

I have a few questions. I have included pic of my converter box. It is a Apex model DT250A. Does any one have any experience with this box working or not working with older set? My other question is can I feed the coax rf output (channel 3 or 4) from this box into the RF converter/mixer grid and expect a good signal or is it too weak. This would be bypassing the RF amp tuner stage where I think I may have a problem. I have successfully injected a 21.9MHz (audio IF for my set) FM modulated signal from my signal generator into the grid of the mixer and did alignment on sound IF (atleast I think I did - everything sounded good afterward). After I did the sound alignment my next check was going to be to see if I could get good audio using converter box. Well it did not work. All I got was buzzing noises. I am going to reverify my signal gen output accuracy but I think it is dead on it is a HP synthesized model. I even jacked around with the local osc adjustment for channel 3 and all I got was several FM radio stations coming thru nicely. I tried viewing signal with oscilloscope and never found anything that looked like video signal. So am I going to have to go thru tuner and RF amp?

[Kevin Kuehn]

Originally Posted by pdr-fml

My other question is can I feed the coax rf output (channel 3 or 4) from this box into the RF converter/mixer grid and expect a good signal or is it too weak. This would be bypassing the RF amp tuner stage where I think I may have a problem.

You can not run the rf channel 3 or 4 directly though the mixer because it will not provide the needed IF frequency. The correct IF frequency is generated by the rf channel frequency beating with the receivers local oscillator freq. See below for more info.

Originally Posted by pdr-fml

I have successfully injected a 21.9MHz (audio IF for my set) FM modulated signal from my signal generator into the grid of the mixer and did alignment on sound IF (atleast I think I did - everything sounded good afterward). After I did the sound alignment my next check was going to be to see if I could get good audio using converter box. Well it did not work. All I got was buzzing noises. I am going to reverify my signal gen output accuracy but I think it is dead on it is a HP synthesized model. I even jacked around with the local osc adjustment for channel 3 and all I got was several FM radio stations coming thru nicely. I tried viewing signal with oscilloscope and never found anything that looked like video signal. So am I going to have to go thru tuner and RF amp?

You can set your generator to the video IF frequency and check the mixer and IF stages, similar to how you did the sound IF alignment. Just make sure you have the channel set to a blank position between 2 and 13, to disable the local oscillator. You may also use your signal generator as a video test bar generator by AM modulating the IF frequency with 1200hz. If 1200hz is not possible it's OK to use 400 or 1000hz. 1200hz in theory will give you 20 horizontal test bars on the CRT screen. The AM modulation beats with the receivers vertical sync oscillator and provides black and white test bars on the CRT screen. Normally you would start at the Video IF closest to the detector and work your way back towards the antenna input. You can even set the frequency generator to any channels video carrier(61.25mhz for ch 3) and connect it to the antenna input(same AM modulation will produce bar pattern). In this instance you do need to tune in the channel so that the local oscillator will beat with the video carrier freq and produce the correct IF.

It's always a good idea to use a coupling cap in series with you generator output just in case you accidentally hook into some high voltage source. Generally you'll be feeding the signal into the the grid of each stage.

[pdr-fml]

Originally Posted by Kevin Kuehn

You can not run the rf channel 3 or 4 directly though the mixer because it will not provide the needed IF frequency. The correct IF frequency is generated by the rf channel frequency beating with the receivers local oscillator freq. See below for more info.

When I aligned the sound IF I did as you said and turned tuner to blank position like the instructions say to do in the manual. When I hooked convertor box to mixer grid I then set the tuner to channel 3. Wouldn't this enable the local osc? You gave some great ideas for testing video IF. My signal generator I got should be able to do this no problem. I discovered though it is quite limited for sweeping functions. I disabled all the high voltage and removed the crt for what I am doing right now so I will try that later when I reconnect everything. I did verify my signal generator accuracy by using my Sony multiband radio using the SW band and tuning in to generator.

[Kevin Kuehn]

Originally Posted by pdr-fml

When I aligned the sound IF I did as you said and turned tuner to blank position like the instructions say to do in the manual. When I hooked convertor box to mixer grid I then set the tuner to channel 3. Wouldn't this enable the local osc?

The problem is that the local oscillator can't generate it's proper frequency when you inject your channel 3 rf directly into the grid of the mixer(see below edit). If you feel your rf amplifier is the defective stage, then I would suggest you make voltage and resistance checks around that area per the SAMs info. I'd like to point out that starting at the antenna end is sort of working the problem backwards, as it creates a lot of unnecessary confusion. The logical way to troubleshoot these things is to inject a signal as close to the output stage as possible. Once you have a picture or sound, you work your way back through the stages towards the antenna input. When you lose the signal, it's at that point that you have found the problem area.

[edit] After looking at your sets tuner circuit more thoroughly, I believe your injecting the rf channel 3 or 4 directly into the mixer grid will load down the oscillator signal feeding that same point through the 1.5 pf cap. I believe you would need some type of isolation or impedance matching circuit between your signal generator and the grid of the mixer. Even then I think without the gain provided by the rf amp, it would not mix correctly with the oscillator, or at best it would provide a greatly reduced amplitude IF signal. Keep in mind that your HP generator likely has a 50 ohm output impedance.

It would be great if one of the EE engineering types hanging around here could explain these type circuits so that everyone(myself included) could better understand their operation. I only know from experience that injecting an rf signal at a critical mixing point tends to load down the circuit and create errors.

[pdr-fml]

I think after I check a few more things I am going to look over my antenna input section real good to make sure there is no damage there. Someone said it is possible it could have been damaged by lightning which makes sense. If I can give that section a thumbs up then I will hook my converter box to antenna input and not have to worry about injecting into mixer.
On a side note I just noticed locally on Craigslist someone if trying to sell a Motorola VT 107 which has same or nearly same chassis as the one I'm working on right now. According to the ad it has been in storge since the 60s. They are asking too much right now but I will grab it if the price is right. I guess I've got the bug.

[Phil Nelson]

Originally Posted by pdr-fml

I even jacked around with the local osc adjustment for channel 3 and all I got was several FM radio stations coming thru nicely.

The FM radio band is located between channels 6 and 7. If you turned the channel 3 oscillator to receive FM radio, that's pretty far off (i.e., somewhere between channels 6 and 7) from where you want it to be.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html

[pdr-fml]

Originally Posted by Phil Nelson

The FM radio band is located between channels 6 and 7. If you turned the channel 3 oscillator to receive FM radio, that's pretty far off (i.e., somewhere between channels 6 and 7) from where you want it to be.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html

This was only after I started to adjust it thru its limits just to see what would happen. I eventually got it back roughly to where it was originally. I guess the fact that I was able to tune to anything tells me the local osc is working. I am thinking my video signal was not strong enough to pick up the audio channel but then how is that not strong enough yet I am pulling in FM radio stations off the air? I will get it figured out one of these days hopefully before I break something. I sure do appreciate the advice I get from you guys.