Today's Mystery Guest (RCA CTC-4)

[old_coot88]

Dang. That swatch of carbonization looks like it involved a half dozen turns or more. That could sure play hobb with the 'Q' factor as well as alter the inductance.

Re. the sweep test, what you'd wanta do is leave coil and cap in parallel, with the lower end grounded. Then connect both genny and scope to the top (with their cable shields grounded, of course).

A parallel-resonant LC tank (which this is) exhibits highest impedance at resonance, grounding out all other frequencies.
But a series-resonant tank (L and C in series) exhibits lowest impedance at resonance, making it useful as a wave trap to ground out an unwanted signal.

[Electronic M]

Here is a crazy thought what if there is a tin whisker or some such shorting the grid to ground forcing it to be 0V?

[old_coot88]

Is that a low capacitance scope probe? If not, it may be pulling the tank freq down too much. To minimize loading, you could add a small cap in series with the probe. Just a 'gimmick' consisting of a few inches of wire twisted together should do it.

If you need to raise the freq, try a lower value tank cap. Maybe 100 pf to start, and see which direction you have to go from there.

[Phil Nelson]

It is a plain old direct scope probe. I tried adding a 2 pf cap in series with the probe and didn't notice a significant change.

I tried substituting a 100 pf and a 82 pf cap for the 270 pf cap in the tank circuit on the workbench. 100 pf raised the peaking point somewhat, and 82 pf raised it up to roughly 3.5 mc. I substituted an 82 pf cap in the TV, and observed no change or improvement.

I don't see any whiskers or solder crumbs anywhere on the socket, which I have cleaned and re-cleaned. I have tested the socket with an adapter in place, and tried running the TV with the tube in an adapter, with and without the shield.

When I tested the old coil (L42) in a tank circuit on the workbench, it responded the same as a new one. Substituting a new coil for the old one in the TV gives me the same result.

I guess I can't rule out a socket defect such as a hidden carbon track caused by a past shorting incident. That is a hard thing to disprove without stripping everything off the socket, at which point you may as well simply replace it.

Phil Nelson

[DaveWM]

try another Xtal?

geez this is going to be great when you find it (trying for a positive spin).

sorry a little late to the party, but was the 270pf the orig cap in the circuit?

[Phil Nelson]

I have tried substituting crystals (and testing those crystals on the workbench). Also swapping among my four available 6AZ8 tubes.

The 270 pf cap is new. If you go back a page or two, you'll see that I already replaced most of the components in the oscillator circuit. Nothing left to replace in the immediate vicinity except R248, the 47K resistor, and I measure 47K resistance at pin 6, as per the resistance chart.

Yes, I'm looking forward to making this set work . . . someday. It's pretty interesting, so far. The CTC-4 is one of those stops on the road between the CT-100 and later RCA color sets that I've worked on. When it gets frustrating, I just work on something easier for a while. The TV will still be there waiting, next day -- or next month!

Phil Nelson

[Electronic M]

Originally Posted by Phil Nelson

It is a plain old direct scope probe. I tried adding a 2 pf cap in series with the probe and didn't notice a significant change.

I tried substituting a 100 pf and a 82 pf cap for the 270 pf cap in the tank circuit on the workbench. 100 pf raised the peaking point somewhat, and 82 pf raised it up to roughly 3.5 mc. I substituted an 82 pf cap in the TV, and observed no change or improvement.

I don't see any whiskers or solder crumbs anywhere on the socket, which I have cleaned and re-cleaned. I have tested the socket with an adapter in place, and tried running the TV with the tube in an adapter, with and without the shield.

When I tested the old coil (L42) in a tank circuit on the workbench, it responded the same as a new one. Substituting a new coil for the old one in the TV gives me the same result.

I guess I can't rule out a socket defect such as a hidden carbon track caused by a past shorting incident. That is a hard thing to disprove without stripping everything off the socket, at which point you may as well simply replace it.

Phil Nelson

You could always ohm out the grid pin with respect to ground and surrounding pins to look for abnormalities....

[DaveWM]

one more, confirm the heater voltage.

[dtvmcdonald]

Based on similar previous threads: the crystal is always suspect unless
tried and working in an identical circuit. Known good crystals that were
not 100% identical replacements have been known to not work.

[old_coot88]

Here's another (way out) idea, since Phil now has an extra 12uh coil, and assuming he has an old 10-365 pf tuning condenser. But first I'd like to run it by the collective wisdom of the group and see if it has any merit.

Cut the crystal and R248 out of circuit as shown, and replace them with a tank consisting of the 12uh coil and the tuning cap. Sweep the cap and see if it'll hit a resonant spot where the osc will start and run.
The xtal is primarily for freq stability, but the osc should free-run OK on the LC circuit (shouldn't it?)

[dtvmcdonald]

The circuit should not run without a crystal. It is the tank circuit.

[old_coot88]

Originally Posted by dtvmcdonald

The circuit should not run without a crystal. It is the tank circuit.

Sorta what I figgered. Just wanted to toss the idea out there before nixing it.

Phil wrote,

Not sure what qualifies as "considerably lower"....but I believe I have measured anywhere from +249V to +270V at pin 1, maybe a little more.

You indicated about 249V, which is down from the spec'd 280V, showing the tube is dropping about 30V across the 1K resistor (R250). That's .9 watt and the resistor would be running quite hot. If it is, you might want to sub a 2 watt resistor during the troubleshooting period.

[Phil Nelson]

Originally Posted by old_coot88

You indicated about 249V, which is down from the spec'd 280V, showing the tube is dropping about 30V across the 1K resistor (R250). That's .9 watt and the resistor would be running quite hot. If it is, you might want to sub a 2 watt resistor during the troubleshooting period.

I just remeasured those voltages. Today I measure +267.5V at the junction of R249 and R250. I measure +255V at pin 1 of the oscillator tube. R250 seems to be holding up.

The Sams and RCA schematics give somewhat different voltages at various points. In Sams the voltage at junction of R249 and R250 is given as +270V. In RCA it is given at +285V. Both give a nominal line voltage of +117VAC and I set my variac at approximately that level, but it's not a laboratory grade instrument and I'm using a DMM.

The Sams and RCA schematics agree in showing a 5V drop across R250 (either 270-265 or 285-280). Today it looks like I am dropping 12.5V (267.5-255).

I remeasured voltage at pin 2. It is +107V (Sams calls for +85V and RCA calls for +110V).

While I was away, I ordered a couple of new 6AZ8 tubes and tried subbing them out of superstition. No change.

I also installed two very short leads, coming out from pin 6 of the oscillator tube and ground, that I can use to substitute crystals more easily. I swapped all four of my crystals with no change. Still measuring zero volts at pin 6.

I removed the can from T115 (the 3.58 mc CW transformer) to replace the two 1000pf caps, C219 and C220. Now that I look closely, it looks pretty toasty.



The resistance on the primary should measure 6.8 ohms according to the Sams schematic. I measured it at 6.8 ohms. When the caps were off, I measured the other coils; both of them have continuity and they measure at .2 ohms. I just noticed that the Sams parts list has a note stating, "R-Y windings = .2 ohms."

I tried the TV again after those cap replacements and saw no change.

I'm not sure how much faith to put in those resistance measurements. I'd feel a lot better if these coils looked brand spanking new.

Phil Nelson

[Phil Nelson]

I have ordered a replacement for transformer T115. A while back (post #179), I did a test that seemed to show that T115 is at least partly functional, but I suspect that didn't tell the whole story, and those coils look they overheated in the past.

If the new transformer doesn't improve the situation, I'm not sure what else to try, except to replace the tube socket.

I wonder if there's a way to do a bench test of the old transformer after I take it out?

Phil Nelson

[zenithfan1]

Wow! You got an amazing deal on that. Welcome to the Director 21 owners club lol!