Zenith 25MC33 Set

[bhegges]

Quick update: I purchased a replacement 6JS6C horizontal output tube from eBay and was surprised that a different tube brought the current down 20ma. The tube was listed as tested good and I lucked out, NOS tubes are priced crazy no thanks to transmitters like the Yaesu FT-101 transmitter

On my B&K 747 the tube I was using measures 111 from a scale of 0 - 120 with good starting at 65, I thought was actually very good. The new tube measures above the meter range (120+) so maybe it was NOS. I have a bucking transformer I plan to use which drops the line voltage down to 106vac and has the horizontal output current running about at 190-195ma. I would suspect as the horizontal tube ages the current will rise.

Next is to run through a full setup and put this set into use.

[bhegges]

I tackled the cataract tonight. The tube is a rebuilt Sylvania 21FJP22A/21GVP22 type which tests strong, not sure how translatable one CR70 is to the next but all 3 guns measure the same at 12.1v on the scale. To remove the safety glass I pulled a guitar wire through, it pulled very hard but slow and steady got the job done, also wore heavy overalls & coat, helmet, and thick gloves. Acetone and lots of paper towels to remove the sealant. Tomorrow I will work on sealing it all back up.

[Yamamaya42]

Originally Posted by bhegges

I tackled the cataract tonight. The tube is a rebuilt Sylvania 21FJP22A/21GVP22 type which tests strong, not sure how translatable one CR70 is to the next but all 3 guns measure the same at 12.1v on the scale. To remove the safety glass I pulled a guitar wire through, it pulled very hard but slow and steady got the job done, also wore heavy overalls & coat, helmet, and thick gloves. Acetone and lots of paper towels to remove the sealant. Tomorrow I will work on sealing it all back up.

I can smell that from here!

[Electronic M]

Hopefully that 12.1V is the emission measurement, and not the heater setting.

[bhegges]

Originally Posted by Electronic M

Hopefully that 12.1V is the emission measurement, and not the heater setting.

Yes, that would be bad, when I had the tester out and made notes I guess without thinking used the heater voltage scale as the reference. But hey at 12.1 volts I should have excellent emissions, longevity might suffer a bit

[bhegges]

I am trying to minimize new threads but if needed I can start a new thread, also note this post is a duplicate to a thread on Antiqueradios that I started in regards to injecting a composite signal. Finally note this is the same set that I just finished a full alignment on so I believe everything is working correctly.

To summarize the picture will not sync when injecting at test point C1 and am looking for help.

• I built an op-amp circuit to amplify the signal to match the voltage at C1 which is approx. -2.5 volts and 4 volts P-P. Note I added batteries to drive the signal down negative to -2.5 volts.
• Also used a Sencore VA62 to feed signal into the injection point. The VA62 allows you to adjust the drive voltage positive or negative. A positive signal is inverted and the color bars are wrong, a negative signal is correct but again I can not get the picture to sync. I set the VA62 to 4 volts P-P, note measuring at C1 shows no DC voltage, and again no picture sync.
• Finally tried moving back the injection point by lifting the anode of the video diode and then keeping L5, C18 in circuit, this made no difference.

Relevant parts of the schematic with Injection point and switch


Op-Amp


Composite jack and switch

[bhegges]

Did some more testing and while I initially thought the polarity was correct it might not be. From the bits and pieces of out of sync image I see inverted colors.

Assuming the negative drive on the VA62 implies an inverted output this aligns with what I am seeing but I would expect the signal to then be in sync. Also tried to feed the VA62 inverted signal through the op-amp to add the DC offset but that did not help.

Pulling the 3rd IF tube seemed to help and could almost get the picture to sync. Is there something related to AGC or other signals from the IF that are needed for the sync circuit?

[old_tv_nut]

I want to make sure of your terminology of a positive signal or negative signal from the VA62.

It looks to me like the polarity of the video detector diode means the signal at C1 should have negative-going sync (casually called a "positive signal"). So, a signal from the Sencore VA62 with negative-going sync should be the proper polarity, it seems.

I wonder if you are having a problem due to the AC coupling in your amplifier (even though you are inserting an average DC with the battery).

Can you check with a scope set to DC coupling or maybe just with a voltmeter whether the VA62 output runs negative from ground (like the regular C1 voltage) or floats around zero?

[bhegges]

Originally Posted by old_tv_nut

Can you check with a scope set to DC coupling or maybe just with a voltmeter whether the VA62 output runs negative from ground (like the regular C1 voltage) or floats around zero?

In short there is no DC voltage offset from the VA62. As I look over the schematic it looks like everything that gets signal from test point C1 is capacitively coupled so does the DC offset even matter? Now for more details I Took some measurements from both the VA62 feeding the RF generator into the tuner, and then also the VA62 injecting composite video (currently at L5 & C18 with the diode lifted).

Using the tuner:

• C1: -2.97 Volts DC
• C1: 0.25 Volts AC
• 

Injection:

• C1: 0.043 Volts DC
• C1: 0.25 Volts AC
• Note I did adjust the VA62 signal amplitude up to match the 0.25 Volts AC
• Note the scope pictures are with DC coupling and no changes were made other than adjusting the trigger to get a clear waveform.
• Note with the tuner still tuned to the RF generator which is sending a blank signal I can kind of get the picture to sync, when I turn the tuner to an empty channel the sync from injecting is very scrambled.
• Note due to the camera shutter speed the below image looks way better
• 

[bhegges]

Interesting, see below for the signal diagram from Zenith for this set. Still studying the schematic to see how the sound detector signal feeds the sync circuit, but this would explain the sync issue from just feeding signal at the video detector alone. Maybe lift a lead on L14 and additionally inject video at test point C2.

[bhegges]

Duplicate, see below post.

[bhegges]

Looking at the signals on C1 and C2 to me they look the same, with just the signal at the sound detector attenuated some. How important is the DC offset for each of these? Would it work to splice in the same amplified video signal from the op-amp to C1 but then include a dropping resistor to connect to C2? Is there a need for coupling capacitors between these?



Here is what I measured:

• C1: 0.23 VAC
• C1: -2.98 VDC
• C2: 0.10 VAC
• C2: -1.14 VDC

Also the scope traces look like the waveforms are the same:

[old_tv_nut]

I think the DC coupling from C2 to the AGC/sync circuits is crucial.
I think normal operation is that AGC sets the (negative ) sync peaks to the desired level, thus setting the overall amplitude and the DC level of the half point of the sync pulses for sync slicing. It looks like the two sync/AGC tube sections are DC coupled, meaning that they depend on the signal going negative from ground to put the sync slicing point at the correct level.

I think what might work is to build a DC restorer/amplifier circuit for the input that puts the bottom of sync tips at the correct negative voltage at C2 (about minus 4.5 volts?) while amplifying the signal to 4 V p-p. You will need a negative supply for this.

[bhegges]

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