Today's Mystery Guest (RCA CTC-4)

[Phil Nelson]

I disconnected one lead of the cap for testing.



It's a little hard to see in the glare, but the eye tube's shadow is fully open, showing no leakage under 450 volts, using a new 1500-pf mica as a comparator.



I guess I will call this cap C290 after the CTC-2B schematic. Whatever it's made of, I'm calling it good enough for this application.

Now the trick will be to install it without torching everything else in the neighborhood, including parts I previously replaced.



Maybe I will install all new components and leads on the transformer first, slip the pre-wired assembly through the hole, and then connect up.

Phil Nelson

[DaveWM]

wow some of that lead dress looks really tight, with crossing paths.
I keep hoping to open this thread and see a big SOLVED beginning.

[Findm-Keepm]

Originally Posted by Phil Nelson

I disconnected one lead of the cap for testing.






Whatever it's made of, I'm calling it good enough for this application.


Phil Nelson

Definitely a mica - either silvered or mica foil. White is always Mica using the EIA code. I had to learn both the EIA and military (MIL-C-39) code.

Cheers,

[jr_tech]

Mica or Paper issue aside... Is that even the correct transformer... looks like it is configured the same as L-45, the quadrature transformer in the CT 100. Might be good to "breadboard" the circuit first to see if it will indeed oscillate without the buffer amp.

jr

[Phil Nelson]

Yes, that was my question -- should this xfmr work at all, given its component differences and the circuit differences between the earlier sets and the CTC-4? (The CT-100 has the same circuitry shown in the CTC-2B snippet I included a few posts ago.)

I sent a follow-up email to Moyer asking if they happen to have a data sheet for the Miller 6025 xfmr, which purported to be a match. They emailed me a scan of the data sheet, showing that the Miller xfmr is a match for the Meissner part -- not the CTC-4.

One way or another, I guess I'm destined to take the old transformer out. If breadboarding the new one doesn't work, then the last resort may be trying to rewind the original. But I could try the original in the breadboard circuit before attempting a rewind.

I have these parts on hand. Is this what we're talking about building?



I'm hoping I can tap the 6.3VAC filament supply and B+ supply from the power/audio chassis of my Admiral 24C15 B/W set.

Phil Nelson

[old_coot88]

Phil, there's something I was gonna suggest earlier to see if the osc can be coaxed to run, but thought the better of it until seeing the CTC-2 schematic. The CTC-2 has the osc plate clamped hard with a .01 cap.
Try disconnecting the CW xfmr's primary completely, and shunt directly across where the primary was. Bypass cap C211 is now clamped directly to the plate. This duplicates how the CTC-2 osc plate is wired.
Bring the scope probe next to L42 to see if the osc is running.
--
If not, maybe try a few higher values for R250 up to 39K to see if it makes any difference.
--
If still no go, maybe there's a conductive track inside the tube socket itself. Most likely lugs 3 and 6 (cathode and G1). If you disconnect those two lugs (no tube in socket), they should show infinite resistance to ground, verifying no conductive path.
Then (with lugs 3 and 6 still floating) power up and see that there's no positive voltage on them, verifying no leakage path from G2 or plate lugs.

If the socket shows good, I dunno what else would be revealed by breadboarding. But what the heck...

[Phil Nelson]

Originally Posted by old_coot88

Try disconnecting the CW xfmr's primary completely, and shunt directly across where the primary was.

You mean, disconnect terminals B and E, and then connect junction of C211/R250 to pin 1, right?

Originally Posted by old_coot88

If you disconnect those two lugs (no tube in socket)

Again, you mean to disconnect everything from pins 3 and 6, right?

Phil Nelson

[DaveWM]

maybe I am missing something here but is that just a amplifier that is tuned for the horz rate? maybe the issue is the tube before it? I see a cap coupling a signal to the control grid, does not look like a correction voltage but more like the signal to drive the tube.

[old_coot88]

Originally Posted by DaveWM

maybe I am missing something here but is that just a amplifier that is tuned for the horz rate? maybe the issue is the tube before it? I see a cap coupling a signal to the control grid, does not look like a correction voltage but more like the signal to drive the tube.

Which print are you referring to here?

[old_coot88]

Originally Posted by Phil Nelson

You mean, disconnect terminals B and E, and then connect junction of C211/R250 to pin 1, right?

Yes.
--

Again, you mean to disconnect everything from pins 3 and 6, right?

Correct.

[Phil Nelson]

Originally Posted by Phil Nelson

You mean, disconnect terminals B and E, and then connect junction of C211/R250 to pin 1, right?

Okay, with that lash-up, I see a regular waveform at L42 (pin 3 of the oscillator). This is with no signal to the receiver and a high sweep rate.



If I supply a color bar signal at the video amp and slow down the sweep rate, I see a more complex waveform; the amplitude of this one goes down if I turn down the Color control.



Not sure if the second trace proves anything, but it was interesting to look at.

Incidentally, with nothing connected to the primary (terminals B-E), it still measures 6.8 ohms as given in the Sams manual.

Phil Nelson

[old_coot88]

Originally Posted by Phil Nelson

Okay, with that lash-up, I see a regular waveform at L42 (pin 3 of the oscillator). This is with no signal to the receiver and a high sweep rate.

Yowie zowie! What's the actual p-p amplitude of that?
Looks like the thing is actually running.
OK (assumimg you haven't unmounted the old CW xfmr yet), just take the new one and connect its primary in place of the old one, using a couple of short leads (after removing that jumper). See if the osc still runs.