Another Motorola VK-106 restoration attempt

[pdr-fml]

Happy Thanksgiving everybody. I am in process of trying to restore a 1948 Motorola VK-106 that i had found at estate sell. I have never done this before so I am on the learning curve soaking in information when I can. Mine is very similar to the one Banderson posted a few weeks ago so I wont include too many pics. When I got the set I did not know anything about the current condition whether or not tubes were all there or CRT any good. Well so far things have turned out pretty well and have not run into any project stopper problems so far I think. I do have a problem I would like to get an opinion on and I will detail this a few post down but first just a few pics of project and what I have done so far -see posts below.

Here is pic of set the day I got it and by the way I paid 50 and not 100 so not exactly a bargin.

[pdr-fml]

Here are some more pics of set. All tubes were there except for HV rectifier tube and one tube was wrong type.

[pdr-fml]

I have been busy last few weeks with this whole thing just hoping that CRT was going to come to life. My hopes were up because i found some problems that may have been reason for shelving the set that were not CRT related. Two burned up resistors and a blown fuse so I kept thinking CRT may be okay. By the way I did check crt filament and tube filaments and none were open. I restuffed two can caps and relocated two to inside chassis bottom.

[pdr-fml]

After recap job and replacing few resistors I cranked her up saw a flame up down below in chassis. Turned out to be a tapped power resistor that had one bad section that I had bypassed with new dale screw down power resistor but was still shorting out. Ended up replacing whole thing. I do have variac and was gentle with set at first. All tubes were glowing and static from speaker (no signal attached just looking for raster) but nothing at all on tube. adjusted ion trap all over the place and nothing at all. Also appeared to have high voltage using screwdriver arc test. Pulled all tubes out and let crt filament glow for a couple hours. Still nothing. I looked at voltages at crt neck and looked at schematic for orgins of voltages and started doing a little trouble shooting and pulled 12au7 video output tube. Had put everything back to normal and powered back up and still nothing. Decided to adjust ion trap again and finally got a raster. I got a replacement tube from a source and reinstalled. I am not a 100% percent about tube being problem but am going to test one day. I was really getting discouraged but now very hopeful.

Current problem I am trying to solve is shown in pic below. I got a old tv repair book and for the white vertical strip problem I am seeing as shown the book suggest either damping tube or bad deflection yoke. What do you guys think?

[Kevin Kuehn]

Very nice job on replacing those caps and resistors! I'd agree the damper tube is generally responsible for those type defects on the left side of the picture. First thing I'd try is replacing the damper tube, possibly try a few. Adjusting the horizontal drive control could also have some effect.

[pdr-fml]

Originally Posted by Kevin Kuehn

Very nice job on replacing those caps and resistors! I'd agree the damper tube is generally responsible for those type defects on the left side of the picture. First thing I'd try is replacing the damper tube, possibly try a few. Adjusting the horizontal drive control could also have some effect.

Thanks for the advice. Well I am fresh out of damper tubes -calls for a 5v4g so I guess I will place an order might get two just in case. I will try the drive control before I do that and see what happens.

[pdr-fml]

Originally Posted by Kevin Kuehn

Very nice job on replacing those caps and resistors! I'd agree the damper tube is generally responsible for those type defects on the left side of the picture. First thing I'd try is replacing the damper tube, possibly try a few. Adjusting the horizontal drive control could also have some effect.

Ok. I got rid of the white vertical lines by adjusting the horizontal drive control. When I went to adjust I found that trimmer screw was completely missing. Stole screw from another from trimmer in tuner for channel that will not be used. Unfortunately I have other problems with set and have decided that I need to test all tubes before I go any further. I know someone at work that has a tester and will try to get this done shortly.

[decojoe67]

Great job. I think you got a great bargain with that set. Those Motorola's a fairly uncommon and a classic '40's console. The CRT alone would cost at least $50.
Enjoy it!

[pdr-fml]

Originally Posted by decojoe67

Great job. I think you got a great bargain with that set. Those Motorola's a fairly uncommon and a classic '40's console. The CRT alone would cost at least $50.
Enjoy it!

Thanks. I guess I lucked out on the 10bp4 not being a dud. Have not tested it but it seems bright enough. Just to add the crt appears to be a rebuild to me due to the neck having a cut line around it.

[kvflyer]

Looking good and you are certainly on track for a complete restoration. The 5V4 could be a 5V4GA also if that is easier to find and I guess you got a HV rectifier since you have a raster.

Nice job, keep the pictures coming.

[Kevin Kuehn]

You can use a 5V4GA in place of the G. The audio guys covet those G style for their more curvy sound.

[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.