The Set: Pete Deksnis's Site about the CT-100
Restoring a Vintage Color Television Set
7-17-99 It was one day in the summer of 1966 that power to THE SET was last turned off. It was fully operational then.
This weekend I hope to return to operation five of its functional areas -- the tuner, video IF, audio IF, ratio detector, and audio voltage amplifier. The speaker and its integral audio output transformer are missing, so eliminating the current-hungry class A audio power amplifier from this test will conserve resources. My 1960-vintage variable power supply is rated at just 100-mA continuous current (125-mA intermittent current).
The five sections will have filament power applied first, then I will slowly bring up the 275-volt bus, which normally powers all these areas except the tuner, and just maybe observe some signal processing.
If successful, it will be the first time in 33 years that anything is functional. The tuner high voltage, which normally is drawn from the 285-volt bus, will be connected instead to the 275-volt bus. The 285-volt bus still has issues pending. Hopefully, my next report will be in past perfect!
7-18-99. Sound! At two this afternoon THE SET spoke. Actually, it was some sports guy announcing a golf match or something, but it was really THE SET and it looked good. "Looked" good because I hooked a scope to pin 7, the control grid, of the disabled (for this test) 6AQ5 audio power output stage. I tuned an LCD pocket TV to channel seven and confirmed that the trace on the scope was actually channel 7 audio. All seven New York City channels could be received indicating a good tuner, video IF, second detector for sound, audio IF, and so forth.
Earlier in the day...
First, with the filaments cold, I checked the 275-volt bus one more time for 5 mA of quiescent current (and the tuner not yet connected). Next, with the filaments hot, 105 mA were drawn at 275 volts. This was without external AGC applied. When minus 6 volts was applied to the AGC bus at the hot end of C74 (1C161 on the RCA schematic), the quiescent current dropped to 85 mA.
Finally, with the tuner connected to the 275-volt bus (clip the red wire on the tuner and connect the lug to the 275 bus), the bias supply on, and the filaments hot, I cranked up the variable supply. At about 150 volts there was the beginning of audio on the scope. It gets solid at 200 volts. At 275 volts, the current drain (without an operational class A audio power stage) is 110 mA. Of course, if the bias is driven toward zero, the current increases.
As good as it feels, that was the easy part. I'll check the video detector with the scope for a waveform (W1) that Sams says should be at the grid of the first video amplifier. After that, I'll take some time to savor the milestone and plan my next move.
7-19-99. It's Monday and it's summer. So I called the boss and left a message saying I was taking today off... then formulated my next step with THE SET. Here it is.
Restore the horizontal sync separator and the AGC keying circuit.
Not very exciting I know, but here's why. It requires the main power supply to be operational, and it clears up some basic utilitarian circuits that, if I don't restore now, I'll wish I had later when the color circuits come on line.
The remaining circuits in THE SET use these remaining bus voltages -- 380, 375, and 285. The AGC keying and horizontal sync separator circuits use all three voltage levels and are locked together with dc-coupling, which is in itself a bit of a challenge.
So, now that (1) the aforementioned first video amplifier does have a typical video signal on its control grid, and since (2) a composite sync signal is tapped from its plate using (3) a dc-coupled circuit to the horizontal sync separator, which (4) is dc-coupled to the AGC keying stage, which (5) requires a 375-volt positive pulse from the flyback for the plate of the keying pentode (1/2 a 6AN8), and since (6) the vertical sync separator and sync amp circuits are right next door and are a probable piece of cake to restore, and (7) since my variable power supply is already maxed-out current wise, I thought it would be a nice and logical next step since (8) the main low-voltage doubler supply will be needed to power the 380/375 bus, and (9) something will have to generate a 375-volt positive pulse synchronized to the horizontal line frequency of the received signal.
7-25-99 Quick update: Spent time this weekend isolating the 375-volt bus and slowly applying the full 375. After taking about 30 minutes to ramp the voltage up to 375, the quiescent current ran 25 mA. This is, as before, without filament voltage applied.
So far, I've corresponded with one collector who has two CT-100s; neither one is functional at this time. More to come... later.
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