Another RCA CT-100 discovery B8001599

[old_tv_nut]

Going nicely!

Edit: If you short the 820 pF you could also measure the self-resonant frequency, although you will have to correct the result due to the scope probe capacitance.

[Penthode]

Originally Posted by old_tv_nut

Going nicely!

... If you short the 820 pF you could also measure the self-resonant frequency, although you will have to correct the result due to the scope probe capacitance.

Measuring the self resonant frequency is an interesting thought, The original peaking coils are machine wound in a pattern to minimize self capacitance. My winding is by hand bunch wound which inevitably will affect self capacitance.

It would be interesting to compare a coil I make by hand with a NOS Merit machine wound coil which is first made to match resonance with the 820pF and then tested for self resonance. The peaking coil is designed to resonate with the circuit self capacitance hence my winding may skew in-circuit performance.

This will all become apparent on the final I and Q channel frequency sweeps.

[Penthode]

I knocked together a CT-100 ballast resistor replacement on the weekend. It plugs into the ballast socket.

I bought 350 ohm 50w and 800 ohm 25w chassis mount resistors and attached to a heatsink I bought on Amazon for another project. The octal male plug was from a relay and was mounted on an angle so that when plugged into the CTC2 chassis, the fins sit upward.

I used fiber glass spaghetti on heat resistant wire to the socket. The aluminum bracket attaching the heatsink to the octal plug was fashioned from a scrap aluminum plate or bracket my my scrap bin.

It will plug into the rear facing CTC2 ballast socket above the power transformer and will deflect and isolate the heat source away from the 15GP22.

The chassis is complete apart from two peaking coils. This evening with the chassis on the bench and everything connected except for the CRT, I plan to insert the tubes and provide the power up of the chassis in in over 40 years.

[Penthode]

Last night I powered up the chassis. The power supplies came up and with the new ballast resistors installed, the voltages appear correct.

The only immediate issue was no audio. I had expected the rush of noise from the speaker and traced it to a short from the plate of the 1st audio IF to ground. Unsoldered and removed the plate connection and the short went away. Reconnected and I heard the rush of noise on power up and then it disappeared. Reasoned that the RF and IF were being biased off so a slight rotation of the AGC control brought up the rush.

Connected the Zenith set top box with its modulator on Channel 3 and I can hear program audio. Looks like all my video audio IF rewiring was okay.

Have made preliminary measurements through the chassis and all seems well. The high voltage is at 22kV and the regulator is working as I can swing it up and down with adjustment. Horizontal sweep is on frequency. The photo below the chassis is running and you will see the Heathkit VTVM reading 22kV with my Heathkit probe. (I bought the Heathkit IM18 VTVM as,a kit in 1971: hard to believe I have been using it for 51 years!)

My ballast arrangement is fine. With the heat sink, the temperature levels at 55 degrees Celsius which is much lower than I had anticipated. And that is with the correct voltage drops to the two B plus branches.

I still am missing a 1mH and 1.9mH peaking coil. The 44AWG wire arrived and so I think I will have a go at that tonight.

The chassis has run about half and hour with no issues.